Chris Balak
email: cbalak@tgen.org

Marianne Benard

Elise Schaefer

Sumaiya Iqbal

Keri Ramsey

Michèle Ernoult-Lange

Francesca Mattioli

Lorida Llaci

Véronique Geoffroy

Maité Courel

Marcus Naymik

Kristine K Bachman

Rolph Pfundt

Patrick Rump

Johanna Ter Beest

Ingrid M Wentzensen

Kristin G Monaghan

Kirsty Mcwalter

Ryan Richholt

Antony Le Béchec

Wayne Jepsen

Matt De Both

Newell Belnap

Anne Boland

Ignazio S Piras

Jean-François Deleuze

Szabolcs Szelinger

Hélène Dollfus

Jamel Chelly

Jean Muller

Arthur Campbell

Dennis Lal

Sampathkumar Rangasamy

Jean-Louis Mandel

Vinodh Narayanan

Matt Huentelman

Dominique Weil

PhD Amélie Piton
email: piton@igbmc.fr

Balak
email: cbalak@ucsd.edu

Al

Balak Al

Rare De Novo Missense Variants in the RNA Helicase DDX6 cause Intellectual Disability and Dysmorphic Features and Lead to P-Body Defects and RNA Dysregulation

Keywords: Intellectual disability, RNA helicase, processing bodies, p-bodies, mRNA metabolism, missense variants, DDX6, DEAD helicase, RecA domain, neurodevelopmental disorder

ABSTRACT

The human RNA helicase DDX6 is an essential component of membrane-less organelles called Processing Bodies (PBs). PBs are involved in mRNA metabolic processes including translational repression via coordinated storage of mRNAs. Previous studies in human cell lines have implicated altered DDX6 in molecular and cellular dysfunction but clinical consequences and pathogenesis in humans have yet to be described. Here we report the identification of five rare de novo missense variants in DDX6 in probands presenting with intellectual disability, developmental delay, and similar dysmorphic features including telecanthus, epicanthus, arched eyebrows, and low-set ears. All five missense variants (p.His372Arg, p.Arg373Gln, p.Cys390Arg, p.Thr391Ile and p.Thr391Pro) are located in two conserved motifs of the RecA-2 domain of DDX6 involved in RNA binding, helicase activity, and protein-partner binding. We use functional studies to demonstrate that the first variants identified (p.Arg373Gln and p.Cys390Arg) cause significant defects in PB assembly in primary fibroblast and model human cell lines. These variants' interactions with several protein partners were also disrupted in immunoprecipitation assays. Further investigation using complementation assays included the additional variants p.Thr391Ile and p.Thr391Pro which, similarly to p.Arg373Gln and p.Cys390Arg, demonstrated significant defects in P-body assembly. Complementing these molecular findings, modeling of the variants on solved protein structures showed distinct spatial clustering near known protein binding regions. Collectively, our clinical and molecular data describe a neurodevelopmental syndrome associated with pathogenic missense variants in DDX6. Additionally, we suggest DDX6 join the DExD/H-box genes DDX3X and DHX30 in an emerging class of neurodevelopmental disorders involving RNA helicases.

INTRODUCTION

Intellectual disability (ID) is a result of abnormal neurodevelopment and affects at least 1% of the population. ID typically presents in the first few years of life and is characterized by impairments in cognition and adaptive behavior. It is often accompanied with further delays in language and motor skills (developmental delay, (DD)), as seen in many neurodevelopmental disorders (NDDs). Given the complexity of central nervous system development, the genetic contribution to ID is quite heterogeneous. Pathogenic variants in several hundred genes are now involved in monogenic forms of ID with each being responsible for a subset of individuals [START_REF] Vissers | Genetic studies in intellectual disability and related disorders[END_REF] .

The widespread use of high-throughput sequencing has allowed for a considerable increase in the identification of these pathogenic variants. Large-scale studies utilizing whole exome sequencing (WES) or whole genome sequencing (WGS) continue to be performed looking for pathogenic de novo variants involved in autosomal dominant forms of ID and NDD, but have focused more on loss-of-function (LoF) variants. Hence, we can expect that most genes sensitive to LoF events will be identified in the next few years. On the contrary, identification of missense variants which lead to ID/NDD will continue for some time due to the difficulty in defining pathogenicity from large-scale sequencing data. In silico analyses such as spatial clustering studies will help to identify novel genes with clustered de novo pathogenic missense variants [START_REF] Lelieveld | Spatial Clustering of de Novo Missense Mutations Identifies Candidate Neurodevelopmental Disorder-Associated Genes[END_REF] , but there remains a necessity for variant-specific functional studies to confidently demonstrate cellular dysfunction.

The hundreds of genes with pathogenic variants implicated in ID encode proteins involved in different neuron-specific or ubiquitous cellular processes: synaptic function and architecture, cytoskeleton remodeling, regulation of transcription/chromatin remodeling, or mRNA posttranscriptional regulation such as maturation, export, degradation, translation, etc [START_REF] Vissers | Genetic studies in intellectual disability and related disorders[END_REF][START_REF] Bardoni | Intellectual disabilities, neuronal posttranscriptional RNA metabolism, and RNA-binding proteins: three actors for a complex scenario[END_REF][START_REF] Sartor | Mutation of genes controlling mRNA metabolism and protein synthesis predisposes to neurodevelopmental disorders[END_REF] .

In eukaryotic cells, this post-transcriptional regulation of gene expression is essential to proper neurodevelopment. A large variety of RNA-binding proteins (RBPs) control mRNA localization, translation, storage and decay in the cytoplasm, thus enabling the spatio-temporal adjustment of protein synthesis depending on cellular needs. One of the most illustrative examples of how alterations of mRNA metabolism can lead to NDD is Fragile-X syndrome, caused by pathogenic variants in the gene FMR1 [START_REF] Oberlé | Instability of a 550-base pair DNA segment and abnormal methylation in fragile X syndrome[END_REF] , a gene encoding the FMRP protein, which is involved in regulation of mRNA transport and translation [START_REF] Hagerman | Fragile X syndrome[END_REF] .

In this study we have assembled a cohort of children with ID and other similar features who harbor rare de novo variants in a single exon of DEAD-box helicase 6 (DDX6, MIM: 600326), a gene involved in RNA metabolism. The DDX6 gene encodes a RNA helicase within the Helicase Superfamily 2 (SF2) [START_REF] Jankowsky | RNA helicases--one fold for many functions[END_REF] . SF2 helicases function on RNA in an ATP-dependent fashion. They are characterized by the presence of two RecA-like globular domains; ancient protein construction modules used in many motor-type proteins that transit on or remodel RNA/DNA, connected by a flexible linker that together forms a cleft for ATP and RNA binding. Within SF2, the DExD/H-box helicases (DEAD, DEAH, DExH and DExD) are a subfamily sharing at least eight conserved motifs and slight variations of the signature helicase motif II for which they are named. There are 50 DDX/DHX helicases in humans [START_REF] Abdelhaleem | The human DDX and DHX gene families of putative RNA helicases[END_REF] . While most of them are functionally uncharacterized, the data available so far indicate that they have highly specific functions.

DDX6 in particular is a DEAD-box protein characterized in part by its Asp-Glu-Ala-Asp (DEAD) [START_REF] Jankowsky | RNA helicases--one fold for many functions[END_REF] motif and is involved in the regulation of mRNA decay and translation [START_REF] Ozgur | Structure of a Human 4E-T/DDX6/CNOT1 Complex Reveals the Different Interplay of DDX6-Binding Proteins with the CCR4-NOT Complex[END_REF] . It is also essential for forming cytosolic membrane-less ribonucleoprotein (RNP) granules called Processing Bodies (P-bodies, or PBs) [START_REF] Ayache | P-body assembly requires DDX6 repression complexes rather than decay or Ataxin2/2L complexes[END_REF][START_REF] Minshall | Role of p54 RNA helicase activity and its Cterminal domain in translational repression, P-body localization and assembly[END_REF] which are involved in RNA metabolism through the coordinated storage of mRNAs encoding regulatory functions [START_REF] Hubstenberger | P-Body Purification Reveals the Condensation of Repressed mRNA Regulons[END_REF][START_REF] Standart | P-Bodies: Cytosolic Droplets for Coordinated mRNA Storage[END_REF] . P-bodies are observed in all eukaryotes. Like other RNP granules, such as stress granules, germ granules, and neuronal granules, their formation relies on RNA-protein networks, multivalent low-affinity interactions and liquid-liquid phase separation. Nevertheless, each RNP granule is distinct in its composition and function. Currently, no neurological diseases have been associated with P-body defects. We present here the clinical phenotypes of individuals harboring rare DDX6 missense variants and functional studies which indicate these variants affect DDX6 function in terms of mRNA decay and PB assembly, leading to a neurodevelopmental DDX6 syndrome.

SUBJECTS AND METHODS

Subjects

Whole blood, saliva, buccal cells, skin fibroblasts, and/or photos were obtained from research participants subsequent to informed consent. The research on individuals within this study was performed according to research protocols approved by the institutional review boards or local ethics committees of the Translational Genomics Research Institute (TGen) / TGen's Center for Rare Childhood Disorders (C4RCD) (Subjects 1 and 4), Strasbourg University Hospital (Subject 2) and Groningen University Medical Center / Radboud University Nijmegen Medical Center (Subject 5). All research participants and research groups, excluding Subject 5, were connected through GeneMatcher [START_REF] Sobreira | GeneMatcher: a matching tool for connecting investigators with an interest in the same gene[END_REF] .

DNA sequencing and bioinformatic processing

This study involves subjects and data from five unrelated families at different institutions in the US and Europe. Subject 1 and parents underwent WES at TGen. Genomic DNA was extracted from peripheral blood then isolated in a CLIA lab using the DNeasy extraction kit (Qiagen). Libraries were prepared with the Hyper DNA Prep Kit for Illumina Platforms (Kapa Biosystems). Exome capture was performed with the SureSelectXT Target Enrichment Platform using Clinical Research Exome baits (Agilent Technologies). Sequencing was performed by 100 base pair (bp) paired-end sequencing on a HiSeq4000 instrument (Illumina, Inc.). Reads were aligned to the human genome (Hg19/GRCh37) using the Burrows-Wheeler Aligner (BWA mem v.0.7.8) [START_REF] Li | Fast and accurate short read alignment with Burrows-Wheeler transform[END_REF] . PCR duplicates were identified using Picard MarkDuplicates v1.79. Base quality recalibration and indel realignment were performed using the Genome Analysis Toolkit (GATK v3.5-1) [START_REF] Mckenna | The Genome Analysis Toolkit: a MapReduce framework for analyzing next-generation DNA sequencing data[END_REF] . Variants were jointly called with HaplotypeCaller and recalibrated with GATK [START_REF] Van Der Auwera | From FastQ data to high confidence variant calls: the Genome Analysis Toolkit best practices pipeline[END_REF] .

Quality controls were conducted using FASTQC v0.11.5. Called variants were annotated with SnpEff v3.0a 18 against Ensembl GRCh37.66 and filtered against dbSNP137 1000 Genomes Project (minor allele frequency <0.05), SnpEff Impact: High + Moderate, GATK quality score >300, and known genes. Prediction scores from dbNSFP (Database for Nonsynonymous SNP's Functional Predictions) and an internal annotation tool were used for filtering. Subject 2 underwent a simplex WES (Dijon Hospital, Dijon, France) as previously described [START_REF] Thevenon | Diagnostic odyssey in severe neurodevelopmental disorders: toward clinical whole-exome sequencing as a first-line diagnostic test[END_REF] . When no pathogenic variant was found, trio WGS was performed at the Centre National de Recherche en Génomique Humaine (CNRGH, Evry, France). Libraries were prepared and sequenced using the Illumina TruSeq DNA PCR-Free Library Preparation Kit and sequenced (3 lanes/genome, paired-end 100 bp) on a HiSeq2000 platform from Illumina (Illumina Inc., CA, USA) to obtain a depth of 30X for each sample. Reads were aligned on the human genome (GRCh37) using bwa software [START_REF] Li | Fast and accurate short read alignment with Burrows-Wheeler transform[END_REF] , duplicate sequences were filtered out using Sambamba tools and an additional step of realignment was performed using GATK programs (IndelRealigner). Variants were identified using 4 programs (UnifiedGenotyper and HaplotypeCaller from GATK, Platypus 20 and Samtools). Single Nucleotide Variations (SNV) falling in a coding region or in a genomic region of a known ID gene were annotated using Varank 21 including information about the inheritance (de novo vs inherited variants). Comparison of SNV identified genome-wide in the proband and his parents resulted in a list of potential de novo variants. Structural variants were detected using SoftSV [START_REF] Bartenhagen | Robust and exact structural variation detection with pairedend and soft-clipped alignments: SoftSV compared with eight algorithms[END_REF] and copy number variants using CANOES [START_REF] Backenroth | CANOES: detecting rare copy number variants from whole exome sequencing data[END_REF] and then annotated by AnnotSV [START_REF] Geoffroy | AnnotSV: an integrated tool for structural variations annotation[END_REF] . For Subject 4, genomic DNA from the proband and respective parents was sequenced at GeneDx (Gaithersburg, Maryland). The exonic regions and flanking splice junctions of the genome were captured using the IDT xGen Exome Research Panel v1.0. Massively parallel (NextGen) sequencing was done on an Illumina system with 100bp or greater paired-end reads. Reads were aligned to human genome build GRCh37/hg19 and analyzed for sequence variants using a custom-developed analysis tool. Additional sequencing technology and variant interpretation protocol has been previously described [START_REF] Tanaka | De novo pathogenic variants in CHAMP1 are associated with global developmental delay, intellectual disability, and dysmorphic facial features[END_REF]

Variant prioritization and analysis

Candidate variants were categorized into inheritance patterns including de novo, recessive, compound heterozygous, and X-linked, and prioritized by various methods including variant frequency (less than 2% or absent) using the Genome Aggregation Database (gnomAD) [START_REF] Lek | Analysis of protein-coding genetic variation in 60,706 humans[END_REF] . Further filtering of candidate genes and variants was performed from prediction of damaging effects using in silico tools such as gnomAD v2.1.1 probability of loss-of-function intolerance score (O/E, pLI) and missense Z-score, Combined Annotation Dependent Depletion (CADD) [START_REF] Kircher | A general framework for estimating the relative pathogenicity of human genetic variants[END_REF] , Genomic Evolutionary Rate Profiling (GERP) [START_REF] Cooper | Distribution and intensity of constraint in mammalian genomic sequence[END_REF] , SIFT [START_REF] Ng | SIFT: Predicting amino acid changes that affect protein function[END_REF] , Polyphen 2 30 , SNAP2 [START_REF] Hecht | Better prediction of functional effects for sequence variants[END_REF] , Envision [START_REF] Gray | Quantitative Missense Variant Effect Prediction Using Large-Scale Mutagenesis Data[END_REF] , biological relevance, and association with human disorders with a neurodevelopmental phenotype in the literature. Candidate variants were interpreted using the guidelines published by the American College of Medical Genetics and Genomics (ACMG) [START_REF] Richards | Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology[END_REF] .

RNA sequencing and gene expression analysis of fibroblasts

Immunoprecipitation and western blot analyses

For immunoprecipitation, HEK293 cells transfected with FLAG-DDX6-HA plasmids were grown in 100 mm Petri dish, washed twice in PBS, scrapped in 2 mL PBS and pelleted at 360 g. Pellets were resuspended in 0.5 mL lysis buffer (50 mM Tris pH 7.5, 125 mM NaCl, 0.5% NP40, 1mM EDTA, 1mM EGTA, 1mM DTT, 5% glycerol) supplemented with RNAseA (10 µg/mL) and 2x EDTA-free protease inhibitor cocktail (Roche Diagnostics, Meylan, France).

After 30 min incubation at 4°C, nuclei were pelleted at 500 g for 10 min at 4°C, and proteins from the supernatant were immunoprecipitated as follows. 1-3 mg of the cytoplasmic extracts was incubated at 4°C for 2 h with anti-FLAG M2 magnetic beads (Sigma-Aldrich, Saint-Quentin Fallavier, France). After washing, bound proteins were eluted with SDS loading buffer.

Immunoprecipitated proteins were migrated along with 30 µg of control cytoplasmic lysate and analyzed by western blot as described below. Signals were quantified from scanned X-ray films using ImageJ, normalized to the signal obtained with FLAG M2 antibody in the same experiment, and expressed as a relative percentage of binding proteins using the wild-type FLAG-tagged DDX6.

For western blot analyses, cytoplasmic extracts were prepared as described [START_REF] Ernoult-Lange | Multiple binding of repressed mRNAs by the P-body protein Rck/p54[END_REF] , except that soluble and insoluble proteins were separated by centrifugation at 500 g for 10 min at 4°C. After immunoprecipitation, proteins were separated on a NuPage 4-12% gel (Invitrogen, Thermo Fischer Scientific, Carlsbad, CA, USA) and transferred to a PVDF (poly-vinylidene fluoride) membrane (Perkin Elmer, Courtaboeuf, France). After blocking in PBS containing 5% (wt/vol) nonfat dry milk for 1 h at room temperature, the membrane was incubated with the primary antibody overnight at 4°C, rinsed in PBS, and incubated with horseradish peroxidase-conjugated secondary antibody for 1 h at room temperature. After washing in PBS, proteins were detected

RESULTS

Identification of rare de novo missense variants in DDX6 in individuals with ID

Individual analyses of the five families did not reveal any known pathogenic variants in genes with associated syndrome(s) closely overlapping the clinical features of the probands.

Subject 1 (S1) WES study did reveal a de novo missense variant in a gene not previously associated to human disease, DDX6; NM_004397.5; c.1118G>A (p.Arg373Gln)(Table 1). Data also showed compound heterozygous missense variants of unknown significance (VUS) in S1). A key feature of males with AHDS is abnormal triiodothyronine (T3) and thyroxine (T4) levels. Repeat thyroid studies in S4 including dosages of T3 and T4 (free, total, and reverse) in S4 were all normal. During the final stages of this study, a fifth individual (Subject 5, S5) with a single de novo missense variant c.1115A>G (p.His372Arg) in DDX6 was identified by WES. For all individuals, no other variants except the DDX6 missense changes described above were classified pathogenic or likely pathogenic according to ACMG guidelines.

DNAH11 (MIM: 603339) (

Clinical features of individuals carrying de novo missense variants in DDX6 exon 11

We could retrieve clinical information for four of the individuals carrying de novo missense variants in DDX6. These probands are from unrelated families in both the USA and Europe. The affected individuals present with NDD of unknown etiology and range between the ages of 4 to 13 years at last clinical assessment. Parents are noted to be without any significant health problems. Phenotyping of the four affected probands showed overlap of clinical features and minor variation in severity across individuals (Table 2 2A). The specific variants identified in probands lead to changes at conserved positions and are predicted to be deleterious by in silico analysis tools (Table 1, Figure 2B). The QxxR motif containing Arg373 is conserved in 33/35 DEAD-box family members, with the varying residues being structurally and chemically similar (Glutamine). In Motiv V, residue 390 maintains small, uncharged residues throughout the DEAD-box protein family (35/35), and Thr391 is conserved in every member (Figure S1, Figure S2). Further, mapping of the variants on a 3D structure of DDX6 in complex with the CNOT1 MIF4G and 4E-T CHD domains [START_REF] Mathys | Structural and biochemical insights to the role of the CCR4-NOT complex and DDX6 ATPase in microRNA repression[END_REF] showed proband variants spatially cluster near or at the protein surface in a region depleted of missense variants in gnomAD populations (Figure 2C-D) and close to the 4E-T interaction surface, suggesting that this region is of functional importance for DDX6.

Mutant DDX6 proteins are defective in PB assembly

As DDX6 is known to be involved in the formation of cytoplasmic PBs, we first analyzed the presence of PBs in fibroblasts obtained from S2 carrying the p.Cys390Arg change and from an unrelated age-matched control subject. Cells were analyzed by immunofluorescence using antibodies against DDX6 and a second PB marker, EDC4 (Figure 3A). While most fibroblasts from the control subject showed numerous PBs detected by both DDX6 and EDC4 antibodies, only some of the fibroblasts obtained from S2 contained PBs. However, Western blot analyses indicated that similar amount of DDX6 protein was present in these cells (Figure S3A, 3C), suggesting a functional defect in this variant. Fibroblasts were also obtained from S1 carrying the p.Arg373Gln variant and from her parents. Again, Western blot analysis showed similar levels of DDX6 protein in the three cell samples (Figure S3A), while immunostaining indicated that the proband's cells contained fewer PBs than the mother's ones (Figure S3B-S3C). The father's cells grew slowly however, and possibly related to that, had an intermediate number of PBs: lower than the mother's, but higher than S1's cells (Figure S3B, S3C). To confirm the specific defect of PBs in S1, we therefore submitted the three cell cultures to a mild cold shock at 30°C for 2 hours, since we showed previously that such a treatment results in increased number and size of PBs in established cell lines [START_REF] Ayache | P-body assembly requires DDX6 repression complexes rather than decay or Ataxin2/2L complexes[END_REF] . With this treatment, 90% of the parents' cells contained numerous PBs, while 70% of S1's cells remained devoid of PBs (Figure 3B,3C). Moreover, in the 30% of cells with PBs, their numbers remained low, thus confirming a specific PB defect in the proband cells.

We then investigated the ability of the different variant-containing DDX6 proteins to assemble PBs in a model human cell line [START_REF] Minshall | Role of p54 RNA helicase activity and its Cterminal domain in translational repression, P-body localization and assembly[END_REF] . Plasmids encoding the four DDX6 variants initially identified (p.Arg373Gln, p.Cys390Arg, p.Thr391Ile, and p.Thr391Pro) fused to FLAG and HA tags were transfected in HeLa cells (Figure S4A). The variant proteins localized in PBs, as assessed by immunofluorescence using LSM14A as a P-body marker (Figure S4B). However, they dramatically affect PB assembly as tested by the following complementation assay (Figure 4A). Briefly, cells were depleted of endogenous DDX6 by using siRNA targeting the 3'UTR of DDX6 mRNA (Figure 4B), which resulted as expected [START_REF] Minshall | Role of p54 RNA helicase activity and its Cterminal domain in translational repression, P-body localization and assembly[END_REF] in PB disassembly (Figure 4A,4C). 24 h later, cells were transfected with wild-type or mutant FLAG-DDX6-HA plasmids and analyzed 40 h later for the presence of PBs (Figure 4A,4C). Whereas the wild-type DDX6 restored PB assembly, no or very few PBs were observed using the mutant proteins, with less than 5% efficiency for p.Arg373Gln, p.Cys390Arg and p.The391Ile variants as compared to the wild-type protein, and 17% for the p.Thr391Pro variant (Figures 4A,4C). Similar results were obtained using EDC4 as a PB marker (Figure S4C). This indicated that the mutant DDX6 proteins were unable to replace the endogenous DDX6 for PB formation.

Mutant DDX6 proteins are defective in interactions with protein partners key to PB assembly.

Previously, we showed that three factors, DDX6, LSM14A and 4E-T, are required and their interactions essential for PB assembly [START_REF] Ayache | P-body assembly requires DDX6 repression complexes rather than decay or Ataxin2/2L complexes[END_REF][START_REF] Minshall | Role of p54 RNA helicase activity and its Cterminal domain in translational repression, P-body localization and assembly[END_REF][START_REF] Kamenska | The DDX6-4E-T interaction mediates translational repression and P-body assembly[END_REF] . Mutating the DDX6 FDF-binding pocket (Mut1: four substitutions introduced between residues 320 and 331 [START_REF] Ozgur | Structure of a Human 4E-T/DDX6/CNOT1 Complex Reveals the Different Interplay of DDX6-Binding Proteins with the CCR4-NOT Complex[END_REF][START_REF] Tritschler | Similar modes of interaction enable Trailer Hitch and EDC3 to associate with DCP1 and Me31B in distinct protein complexes[END_REF][START_REF] Sharif | Structural analysis of the yeast Dhh1-Pat1 complex reveals how Dhh1 engages Pat1, Edc3 and RNA in mutually exclusive interactions[END_REF] ) prevents the binding of LSM14A and 4E-T and strongly reduces PB formation [START_REF] Ayache | P-body assembly requires DDX6 repression complexes rather than decay or Ataxin2/2L complexes[END_REF] . Since the variants identified in DDX6

are located close to the Mut1 mutations, we hypothesized that they could also alter interactions between DDX6 and these protein partners. To address this question, plasmids encoding wildtype and mutant DDX6 (p.Arg373Gln and p.Cys390Arg) fused to FLAG and HA were transfected in HEK293 cells after depletion of the endogenous DDX6 protein, immunoprecipitated 48 hours later with FLAG antibodies, and protein complexes were analyzed by western blot (Figure 4D). First, both mutant DDX6 proteins showed reduced binding to 4E-T and LSM14A, as compared to wild-type DDX6, and the defect was stronger for p.Arg373Gln than for the p.Cys390Arg mutant. This is consistent with the inability of these mutants to assemble PBs in the complementation assay. A binding defect was also observed for other DDX6 partners: LSM14B binding was as affected as LSM14A with both mutants, while other bindings were defective only in the case of the p.Arg373Gln mutant. This included a strong defect in PAT1B binding and some impairment of EDC3 and EDC4 binding as well. Overall, the p.Arg373Gln mutant protein was the most defective for interacting with its partners, especially those acting as translation repressors (LSM14A, 4E-T, PAT1B).

p.Cys390Arg variant cells accumulate mRNAs involved in translational regulation, DDX6

mRNA targets, and PB-excluded mRNAs.

Transcriptomic (RNAseq) analysis was performed from skin fibroblasts obtained from S2

(carrying the p.Cys390Arg variant) and compared to eight unrelated individuals with other neurodevelopmental or sensorineural conditions unrelated to DDX6 mutation. More than one thousand genes were found to be significantly (p-adjusted <0.05) differentially expressed (DE) (Figure 5A) including 493 up-regulated and 979 down-regulated protein-coding genes (Table S2). Enrichment analysis using DAVID [START_REF] Huang | DAVID Bioinformatics Resources: expanded annotation database and novel algorithms to better extract biology from large gene lists[END_REF] demonstrated that the set of genes significantly upregulated was enriched for Gene Ontology Biological Process and Molecular Function terms related to protein translation such as "SRP-dependent cotranslational membrane targeting" (GO:0006614, fold enrichment FE=12.5, adjusted p-value=2.0e-18) or "formation of translation preinitiation complex" (GO:0001731, FE=11.3, adjusted p-value=3.3e-02), or "structural component of the ribosome" (GO003735, FE=5.4, adjusted p-value=1.5e-9) (Figure 5B). This enrichment was confirmed by analyzing KEGG pathways ("ribosome", map03010 FE=7.9, adjusted p-value=7.2e-15). More specifically, 4 out of the 14 expressed eIF3 translation initiation factor subunit mRNAs and 28 of the 85 expressed ribosomal protein mRNAs were significantly up-regulated in S2, whereas none were significantly depleted (Table S3). A subset of the DE genes encode RNA-binding proteins also known to be involved in ID (CNOT3, HNRNPH2, DDX59, RPL10, etc.) (Table S4).

To investigate relationships between S2's transcriptomic profile and DDX6 function, we took advantage of two datasets available from the ENCODE project, both from the human erythroid cell line K562: 1) a DDX6 cross-linking followed by immunoprecipitation (CLIP) We have recently shown that mRNAs regulated by DDX6 at the level of decay are localized out of PBs [START_REF] Hubstenberger | P-Body Purification Reveals the Condensation of Repressed mRNA Regulons[END_REF] . Accordingly, comparing S2's transcriptome to the mRNA content of purified PBs showed that mRNAs which are enriched in S2's fibroblasts are excluded from PBs (Figure 5D).

In the same study we showed that the mRNAs targeted to PBs, in contrast, are regulated at the level of translation. While translation regulation is not accessible in our transcriptome analysis, the fact that PBs are drastically reduced in S2 strongly suggests that DDX6-dependent translation repression is also affected in these cells. Finally, the mRNAs accumulating in S2 have a high GC content compared to other human genes, whereas the mRNAs depleted are rather AU-rich (Figure 5E). This is consistent with our recent study showing that DDX6 is preferentially involved in the decay of GC-rich mRNAs. Such a high GC content increases the frequency of optimal codons and is therefore observed in actively translated mRNAs.

DISCUSSION

In this study, we describe five rare de novo missense variants in the RNA helicase DDX6, identified in individuals with intellectual disability (ID). Clinical, genetic, and functional data demonstrated that these missense variants, inducing deficits in DDX6 functions, cause an autosomal-dominant form of intellectual disability associated with mild dysmorphic features.

These deficits include DDX6 ability to assemble PBs -constitutive cytoplasmic RNP granules which act in mRNA storage -resulting in their drastic reduction in primary cells of affected individuals as well as when modeled in human cell lines. Furthermore, co-immunoprecipitation assays showed the pathogenic variants affect DDX6 interactions with protein partners involved in PB assembly, such as LSM14A and 4E-T, suggesting a mechanism for PB dysfunction and subsequent deregulation of mRNA storage. Interactions with other DDX6 protein partners (PAT1B, EDC3, and EDC4) were differently impacted, depending on the variants. The surface of DDX6 that interacts with these different proteins was reported to be distinct, though overlapping. Therefore, we did not expect that different variants of DDX6 would alter all interactions to the same extent. Additional defects included DDX6 dysfunction in the control of mRNA stability, since transcriptomic analysis of S2 revealed an enrichment of mRNAs that are regulated by DDX6 at the level of decay [START_REF] Courel | A unifying model for mTORC1-mediated regulation of mRNA translation[END_REF] (Figure 5F). This includes in particular mRNA encoding subunits of the translation initiation factor eIF3 and ribosomal proteins. Many also contain a TOP motif in their 5' UTR responsible for their control by the mTORC1 kinase (25 out of the 94 TOP mRNAs previously reported 46 were enriched in S2 cells). Their deregulation could have consequences on the control of cell proliferation/function inside and outside of the CNS 46,[START_REF] Costa-Mattioli | mTOR complexes in neurodevelopmental and neuropsychiatric disorders[END_REF] . Taken together, these findings highlight crucial roles for DDX6 and/or PBs in proper neurodevelopment.

The missense changes identified in affected individuals cluster in two pairs of amino acids in conserved motifs of the DDX6 C-terminal RecA domain: residues 372-373 which comprise the second half of the QxxR motif, and 390-391 which lie in motif V [START_REF] Sengoku | Structural basis for RNA unwinding by the DEAD-box protein Drosophila Vasa[END_REF] (Figure 2A, Figure S1). The substitutions p.Arg373Gln (S1) and p.His372Arg (S5) cause significant alterations to residue properties and likely the local structure/function. Electrically-charged surface residues like Arg373 are known to promote proper protein folding through solvent interactions, and reactive histidines are most common in protein active sites. The QxxR motif is also involved in interdomain interactions with motif Ia in vitro in the yeast DEAD-box protein Mss116p [START_REF] Bifano | Structure-guided mutational analysis of a yeast DEAD-box protein involved in mitochondrial RNA splicing[END_REF] , which was proposed to help form a continuous RNA binding surface and stabilize bound RNA. The substitution of Cys390 (S2) to a large electrically-charged arginine, and the conversions of Thr391 (S3,S4) from a polar threonine to non-polar, unreactive residues (proline/isoleucine), also suggest destabilizing regional changes to the RecA-2 domain. Our coimmunoprecipitation and complementation assays support these predictions with results showing dysfunction of the variant DDX6 proteins to bind partners through this region and to assemble PBs, respectively. Since the proband fibroblasts have a strong PB deficiency despite the presence of one wild-type allele, it raises the possibility that the DDX6 variants could have some dominant-negative effect on PB assembly. We have previously characterized a clearly dominantnegative DDX6 change in DDX6 (motif VI mutant HRIG) in an experiment similar to the one described here (Figure S4B) and found that its expression completely suppressed PBs in the presence of the endogenous DDX6 protein [START_REF] Minshall | Role of p54 RNA helicase activity and its Cterminal domain in translational repression, P-body localization and assembly[END_REF] , which is not the case for the probands' variants.

We would therefore favor the hypothesis that the low PB assembly results from haploinsufficiency of the wild-type allele.

DDX6 is not the first gene encoding a RNA helicase involved in monogenic forms of NDD/ID. The involvement of DHX30 in ID was uncovered in 2017 with several de novo missense variants also in the RecA domains reported in twelve individuals with ID, speech impairment and gait abnormalities [START_REF] Lessel | De Novo Missense Mutations in DHX30 Impair Global Translation and Cause a Neurodevelopmental Disorder[END_REF] . Pathogenic variants including de novo truncating and missense changes in RecA domains were have also been recently reported in the DDX3X gene [START_REF] Snijders Blok | Mutations in DDX3X Are a Common Cause of Unexplained Intellectual Disability with Gender-Specific Effects on Wnt Signaling[END_REF] , located on the X-chromosome, in females with ID, hypotonia, movement disorders, and epilepsy.

DDX3X now represents one of the genes most frequently mutated in ID; involved in ~0.5% of ID cases and ~2% of female cases [START_REF] Wang | Phenotypic expansion in DDX3X -a common cause of intellectual disability in females[END_REF] . Interestingly, after sequence alignment of DDX6 and DDX3X, the Arg373 position found mutated in our cohort corresponds to the Arg480 position recurrently mutated in the DDX3X cohort of affected individuals, reinforcing the functional importance of this amino acid and the QxxR motif in DEAD-box proteins. LoF function variants in another member of this helicase family, DDX59, have also been reported to cause an autosomal recessive form of ID with features of oral-facial-digital syndrome [START_REF] Shamseldin | Mutations in DDX59 implicate RNA helicase in the pathogenesis of orofaciodigital syndrome[END_REF][START_REF] Salpietro | A loss-of-function homozygous mutation in DDX59 implicates a conserved DEAD-box RNA helicase in nervous system development and function[END_REF][START_REF] Faily | Confirmation that mutations in DDX59 cause an autosomal recessive form of oral-facial-digital syndrome: Further delineation of the DDX59 phenotype in two new families[END_REF] .

Interestingly, the phenotypic traits observed in our DDX6 cohort align well with the syndromes described in individuals with pathogenic variants in DDX3X and DHX30 [START_REF] Lessel | De Novo Missense Mutations in DHX30 Impair Global Translation and Cause a Neurodevelopmental Disorder[END_REF][START_REF] Snijders Blok | Mutations in DDX3X Are a Common Cause of Unexplained Intellectual Disability with Gender-Specific Effects on Wnt Signaling[END_REF][START_REF] Wang | Phenotypic expansion in DDX3X -a common cause of intellectual disability in females[END_REF] exclude seizures as an absent feature. Our cohort also exhibits a degree of variability in their clinical presentations. Individuals S1 and S5 present with mild abnormalities on brain MRI (corpus callosum hypoplasia) and appear to be slightly more affected than the others. Their amino acid changes (p.His372Arg and p.Arg373Gln) lie in the QxxR motif suggesting variants in this motif elicit a slightly more severe phenotype than variants in the motif V region. In support of this, co-immunoprecipitation studies demonstrated more drastic defects of protein binding caused by the p.Arg373Gln variant. Since differences in the proband's genetic backgrounds are also likely to contribute to phenotypic variability, more individuals with pathogenic DDX6 variation will be required to fully answer this question.

DDX6 is widely expressed throughout tissues with some of the highest levels in the cerebellar hemispheres. Its expression is timely regulated in the mouse neocortex during development [START_REF] Telley | Sequential transcriptional waves direct the differentiation of newborn neurons in the mouse neocortex[END_REF] , and it has been found to be expressed in both mouse [START_REF] Nicklas | The RNA helicase DDX6 regulates cell-fate specification in neural stem cells via miRNAs[END_REF] and human neural stem cells (NSCs) (data extracted from previous works [START_REF] Quartier | Genes and Pathways Regulated by Androgens in Human Neural Cells, Potential Candidates for the Male Excess in Autism Spectrum Disorder[END_REF] ). DDX6 is further involved in the CNS as it increases the activity of Let-7a, a microRNA important in neuronal differentiation. DDX6 overexpression in mouse NSCs leads to an increase in neuronal differentiation whereas loss of DDX6 function inhibits the formation of new neurons [START_REF] Nicklas | The RNA helicase DDX6 regulates cell-fate specification in neural stem cells via miRNAs[END_REF] . DDX3X was also found to be essential for neuronal differentiation and migration, and its inactivation in mice disturbs cortex development and leads to abnormal distribution of neurons in the cortical plate [START_REF] Lennox | Pathogenic <em>DDX3X</em> mutations impair RNA metabolism and neurogenesis during fetal cortical development[END_REF] . It is possible the brain anomalies present in DDX6 individuals S1 and S5 result from impaired processes in NSC specification. While some evidence supports this idea, the exceedingly heterogeneous nature of corpus callosum anomalies requires further in-vivo work to validate how these pathogenic variants would alter proliferation/differentiation/migration of neuronal precursors and synaptic functioning, and also to identify which mRNAs are deregulated in this cell type.

We have shown that our DDX6 variants result in failure to form PBs efficiently and lead to a decreased number of these RNP granules in fibroblasts of affected individuals. Lessel et al.

recently showed that DHX30 missense variants, in addition to altering DHX30 ability to bind RNA or to exert its ATPase activity, increase the formation of another significant type of RNP granules, stress granules, which is associated with a decrease of global protein translation [START_REF] Lessel | De Novo Missense Mutations in DHX30 Impair Global Translation and Cause a Neurodevelopmental Disorder[END_REF] . The missense variants identified in DDX3X were also shown to affect ATPase helicase activity and lead to protein accumulation into RNP granules not well characterized but harboring some translational activity [START_REF] Lennox | Pathogenic <em>DDX3X</em> mutations impair RNA metabolism and neurogenesis during fetal cortical development[END_REF] . RNP granule dysfunction in neurons specifically could relate to their polar, asymmetric nature. Indeed, there are a variety of neuronal RNP granules, mostly described in dendrites. Several types of dendritic granules contain PB components, and were initially called P-body-like structures [START_REF] Cougot | Dendrites of mammalian neurons contain specialized P-body-like structures that respond to neuronal activation[END_REF] or P-bodies [START_REF] Zeitelhofer | Dynamic interaction between P-bodies and transport ribonucleoprotein particles in dendrites of mature hippocampal neurons[END_REF] . They are important in regulating local translation at the synapse, which is essential for synaptic plasticity [START_REF] Kiebler | Neuronal RNA granules: movers and makers[END_REF] . However, further attempts to define these granules with expression of DDX6 and decapping enzymes suggested that these structures were more diverse in neuronal processes than in other cells [START_REF] Miller | Combinations of DEAD box proteins distinguish distinct types of RNA: protein complexes in neurons[END_REF] . In the neuronal soma, RNP granules may be closer to fibroblast PBs in terms of composition and function, but they have not been well characterized. Repression by miRNAs also plays a major role in the control of signaling pathways regulating several steps of cortical development [START_REF] Lennox | RNA on the brain: emerging layers of posttranscriptional regulation in cerebral cortex development[END_REF] , therefore we can speculate that the different cytoplasmic foci that play roles in the storage of miRNA targets are important for a proper development of the brain.

The present study has taken effort to demonstrate rare changes in DDX6 cause PB and mRNA metabolism dysfunction. These DDX6 dysfunctions were observed not only by overexpressing DDX6 in immortalized cell lines but also directly in affected individual's primary fibroblasts (individuals S1 and S2). The use of these non-neuronal cells may only Pdistantly reflect the actual disease pathogenesis in affected individual's brains and other organs.

Further transcriptomic and proteomic studies in neuronal cells derived from primary fibroblasts or in-vivo animal models will shine more light on the specific mechanisms of DDX6-associated pathogenesis in neurodevelopment.

In conclusion, our findings implicate rare de novo missense variants in the second RecA domain of DDX6 in a neurodevelopmental disorder (NDD) with ID. These variants significantly affect DDX6's ability to assemble PBs, resulting in their drastic reduction in cells, and, in our affected individual with transcriptomic data, the significant alteration of the transcriptome landscape. We suggest rare variants in DDX6 be considered for pathogenicity in pediatric cases of NDDs with ID and this RNA helicase be added to the growing list of ID/NDD genes. Lastly, together with the recent involvement of DDX3X and DHX30 in ID and similar NDDs, these results also highlight an emerging class of NDD involving RNA helicases.

SUPPLEMENTAL DATA

Supplemental Data include four figures, four tables, and expanded clinical descriptions of the subjects.

ANR grant 14-CE09-0013-01ANR, and the Laboratory of Excellence GENMED (Medical Genomics) grant no. ANR-10-LABX-0013 managed by the National Research Agency (ANR) part of the Investment for the Future program. The authors want also to thank all the people from IGBMC sequencing platform (Bernard Jost, Christelle Thibaut-Charpentier, Céline Keime, Damien Plassard) for their technical and bioinformatics support. The authors would like to thank all donors who have contributed and participated in TGen's Center for Rare Childhood Disorders

Center. Lastly, the authors thank all of the families for their participation in this study.

DECLARATION OF INTERESTS

IW, KGM, KM are employees of GeneDx, Inc.

WEB RESOURCES

The URLs for online tools and data presented herein are:

Clinvar

Subject 2

He was born in 2011, first child of non-consanguineous parents with European descent. His father presented with bilateral clubfoot put in plaster in infancy. The mother had gestational diabetes during the pregnancy, treated by diet. The birth was accelerated due to alteration of the fetal cardiac rhythm and the patient presented transitory respiratory distress (APGARs were 4, 6, 10). At birth, the weight was 2650 g, the height 49 cm and the head circumference 33 cm. Hypotonia with sucking difficulties was noticed during the two first days of life. Abdominal and cerebral ultrasound, skeleton radiographs and otoacoustic emissions were normal.

Echocardiography found atrial septal defect and patent foramen ovale. The patient also exhibited dysmorphic features with right plagiocephaly, small palpebral fissures, hypertelorism, bilateral epicanthus and asymmetric ears with overfolded superior pinna. He had a unilateral cryptorchidism, a syndactyly between the 2d and the 3th toes on right foot and a supernumerary nipple. The patient was operated of inguinal hernia in the first months of life. The growth was regular at -2.5 SD for the weight, -1 SD for the height and -2 SD for head circumference.

Psychomotor retardation was noticed at 9 months old with a sit station delayed. Walk was acquired at 26 months and toilet training at 5 years.

The patient also exhibited language delay and comprehension and concentration troubles. He needed specialized education. He has no stereotypy or behavioral disorders but transitory sleep disturbance. Medical check-up was made for the weight retardation and did not find any medical trouble. Only masticatory troubles were highlighted. Ophthalmologic examination found strabismus (operated at 5 yo), canthi dystopia, hypermetropia and normal fundus. ENT evaluation found normal hearing but the patient did many ENT infections leading to tonsillectomy. Cerebral MRI found no cerebral malformation. CT scan showed craniostenosis of the metopic suture without chirurgical gesture necessary. Genetic testing including karyotype, array CGH, fragile X, high-throughput sequencing of 220 genes implicated in ID were performed with normal outcomes. At the last clinical examination at 6 years, the patient weighted 18 kg (-2.5SD) for 115 cm (-1 SD) and had a head circumference at 50 cm (-1.8 SD). He made simple sentences, wrote his name, fed himself, and bicycled (with 4 wheels). Pathologic excessive sociability was noticed.

Subject 4

This is an 11-year old boy of non-consanguineous parents of German and Irish descent. He was born at 36 3/7 weeks to a 28 year old G1P0->1 mother. Prenatal complications included gestational diabetes, infection (unknown type), and bleeding. He required resuscitation after a traumatic delivery and had hypoglycemia, but did not require a NICU stay. He was large for gestational age with a birth weight of 7 lbs 8 oz, birth length of 53.3 cm, and head circumference of 34.5 cm. He was seen by Ophthalmology at 2 months of age due to physiologic anisocoria and facial asymmetry. He was later seen by Orthopedics at 2.5 years of age due to concerns of toewalking; diagnoses included femoral retroversion, external tibial torsion, and flat feet. He has a history of abdominal pain and constipation felt to be due to IBS, and at one point was found to have unexplained elevation of liver function tests. He has been noted to have autistic features, but does not meet criteria for a diagnosis of autism spectrum disorder. Other neurodevelopmental diagnoses include communication disorder, stereotypic movement disorder, motor coordination disorder, learning disorder, anxiety, and inattention. He had a full scale IQ of 70 using the Wechscler scale. Genetic tests including karyotype, microarray, and fragile X testing were all normal. He was evaluated by Endocrinology at 7 years of age due to concern about penile length, and possible anosmia/ hyposmia was noted. He had normal thyroid studies, growth hormone level, and bone age at that time, and was referred to Genetics and Urology for further evaluation. Of note, Urology did not feel that he has micropenis, but does have a buried penis and adhesions which should be re-evaluated at puberty. He was first seen by Genetics at 8 years of age and Kallmann syndrome was suspected, but a 13-gene testing panel was normal. Following this evaluation, there was concern for seizure activity due to a staring event and jerky movements in sleep, as well as persistent toe-walking and gait instability. An EEG, brain MRI, and lumbar spine MRI were normal. He was also seen extensively by Dermatology for dermatitis, juvenile spring eruption and allergic contact dermatitis. Biopsy of skin revealed spongiotic dermatitis. He was seen for follow-up in Genetics at 10 years of age, with new clinical features of mild truncal obesity, severe eczema of the hands, generalized sun sensitivity (erythema and pain after minimal exposure), and progressively worsening striae on thighs and lower legs. Urine porphyrins and whole exome sequencing were recommended. Urine porphyrins were normal.

Whole exome sequencing revealed one variant of uncertain significance in DDX6 (c.1171A>C), as well as a variant of uncertain significance in the SLC16A2 gene (c.985G>A). Repeat thyroid studies including T4, T3 (free, total, and reverse) were normal, so we are less suspicious for the SLC16A2 variant. He previously had a normal 24-hour free cortisol, but was re-evaluated by Endocrinology due to his striae. He had an elevated cortisol and ATCH, but it was later noted that this was following a course of high dose prednisone for his skin rash. Repeat ACTH was within normal limits; and cortisol was low. They recommended further evaluation to rule out iatrogenic adrenal insufficiency due to slight drop in growth velocity and history of treatment with prednisone. His bone age was advanced (2.9 SD above the mean); cortrosyn and growth hormone stimulation tests have not yet been completed.

Subject 5

She is the third child of non-consanguineous parents. She was born after an uncomplicated pregnancy and delivery at 41 weeks gestation. She was born with a single umbilical artery and a large slowly closing fontanel. There was prominent hypotonia, constipation, and feeding problems during the first years of her life. Her psychomotor development was severely delayed. On evaluation at age 1.5 years she had a high narrow prominent forehead, a large fontanel, strabismus, telecanthus, bilateral epicanthus, and a short nose with a broad tip and anteverted nares. She spoke her first words after the age of 3 years and was able to walk without support after the age of 5 years. She still has trouble walking. She walks instable like a young child, falls easily and cannot walk long distances. During recent years she also developed a scoliosis. She has no seizure and normal hearing. She attends a special school for children with severe learning problems. An ophthalmological evaluation showed prominent hypermetropia and right sided amblyopia. She further had mild bilateral hip dysplasia and an echocardiogram, performed because of a heart murmur, showed a patent foramen ovale. Renal ultrasound was normal, and a brain MRI showed non-specific abnormalities, including wide central and peripheral liquor spaces, a thin corpus callosum and a mildly delayed myelination. At age 11 years, her height was 149 cm (-0.11 SD), her weight 53 kg (+2.19 SD; weight for height) and her occipital head circumference was 52 cm (-0.76 SD). Sha had a narrow prominent forehead, deep set eyes, strabismus, epicanthic folds, broad nasal tip, short philtrum, and thin vermilion of the upper lip. She further had tapering fingers, overriding toes and pes plano valgus with joint hyperlaxity.

  experiment identifying mRNAs that bind to DDX6 and 2) a transcriptome after induction of a stably-transfected DDX6 shRNA for 48 hours identifying mRNAs differentially expressed after DDX6 silencing. If the DDX6-Cys390Arg variant leads to a defect in its function in mRNA decay, we would expect an accumulation of these DDX6 mRNA targets. Indeed, we first observed that significantly enriched mRNAs in S2 fibroblasts were also enriched in ENCODE's DDX6 CLIP experimental data (Figure5C, left panel), indicating their direct binding to DDX6.Second, S2-enriched mRNAs also tended to be enriched in the DDX6-silencing dataset (Figure5C, right panel). Thus, in spite of the differences between the two cell types under comparison (dermal fibroblasts producing a p.Cys390Arg +/-variant DDX6 versus an immortalized erythroid cell line transiently depleted for DDX6); there was a common signature of enriched mRNAs between the two. Further, the enrichment was even stronger for the mRNA subset that was enriched in both the CLIP experiments and the DDX6 silencing experiment (Figure5C, right panel), indicating that the mRNAs enriched in the cells of S2 were likely decayed by a DDX6dependent mechanism that involved direct DDX6 binding.

Figure 1 .

 1 Figure 1. Individuals with intellectual disability and developmental delay and rare de novo missense variants in DDX6. Pictures of individuals with rare de novo missense variants in DDX6 (Subjects S1, S2, S4, and S5).

Figure 2 .

 2 Figure 2. Localization of the different de novo missense variants in DDX6 (A) Linear representation of the DDX6 protein (NP_004388) with highlighted functional DExD/H-box motifs. De novo missense variants identified in individuals with ID are indicated by red dashed lines, and missense variants observed in the gnomAD population are plotted in orange with an indication of the number of individuals carrying them. Motif residue numbers and colors are as follows: motifs Q (115-123) and GG (201-203) in yellow; motifs I (140-146) and II (DEAD) (246-249) in dark green; motifs Ia (171-181) and Ib (222-225) in orange, motif IV (338-340) and V (395-399) in light green; and motifs III (277-279), QxxR (370-373), and VI (419-427) in grey. Motifs Q, I, II, V and VI are involved in ATP binding and motifs GG, QxxR, Ia, Ib, IV and V are involved in RNA binding. Motif III is involved in intramolecular interactions 13 . (B) Heatmap spanning the linear DDX6 protein representing in silico impact (effect) predictions of all possible amino acid substitution mutations in DDX6 from the SNAP2 classifier. Dark red indicates a strong signal for effect, white indicates weak signals, and blue a strong signal for

Figure 3 .

 3 Figure 3. Fibroblasts from individuals S1 and S2 contain a low number of PBs (A) Fibroblasts from an unrelated, age-matched control individual and from S2 carrying the DDX6-p.Cys390Arg variant were immunostained with DDX6 and EDC4 antibodies. Nuclei were stained with DAPI. Arrows point to some selected PBs within cells. Scale bar = 20 µm. (B) Fibroblasts from S1 carrying the DDX6-p.Arg373Gln variant and her healthy parents were analyzed as in (A) except that cells were grown 2h at 30°C before fixation. Scale bar = 20 µm. (C) Quantification of fibroblasts having PBs. PB-containing cells were counted and plotted as a percentage of total cells (98 to 126 cells from 3 and 4 independent experiments for S2 and S1, respectively). Error bars, SD; t-test: p<0.0001 for both probands, as compared with their respective control.

Figure 4 .

 4 Figure 4. Rare missense variants in DDX6 affect its interaction with protein partners and PB formation

Figure 5 .

 5 Figure 5. DDX6 targets and mRNA-encoding proteins involved in translation initiation are enriched among genes upregulated in individual S2. (A) Biotype distribution (according to Ensembl) of the differentially-expressed (DE) genes identified in S2. (B) Pathway enrichment analysis revealed a significant enrichment of GO terms Biological Process and Molecular Function related to protein translation, ribosome and RNA processing, and of KEGG pathway "ribosome" among the upregulated genes. (C) The mRNAs up (orange boxes) and down-regulated (green boxes) in Subject 2 (S2) were analyzed for their enrichment in a DDX6 CLIP experiment (left panel) and their differential expression after DDX6 silencing (right panel), both in K562 cells. The analysis was performed using various p-value thresholds for the differential expression in S2, as indicated. The distribution of the whole dataset (all) is given for comparison (grey boxes). The whiskers indicate the 10-90 percentiles. Upregulated mRNAs in S2 are preferential DDX6 targets and preferentially up-regulated after DDX6 depletion in an erythroid cell line. (D) Up-regulated mRNAs in S2 tend to be excluded from PBs. The mRNAs up and down-regulated in S2 were analyzed for their enrichment in purified PBs, as in (C). (E) Up-regulated mRNA in S2 tend to have a high GC content. In panels C to E, two-tail Mann-Whitney tests were performed with respect to all mRNAs: ***, p<0.0005; **, p<0.005.

  . The general assertion criteria for variant classification

	are	publicly	available	on	the	GeneDx	ClinVar	submission	page

(http://www.ncbi.nlm.nih.gov/clinvar/submitters/26957/). Subject 5 and parents underwent trio WES. Sanger sequencing was used to confirm DDX6 variants in all affected probands.

Molecular modeling of DDX6 and missense variant impact prediction

	check that the distributions are centered around the zero line and as tight as possible.
	Comparisons to the eight aforementioned individuals with other neurodevelopmental or
	sensorineural conditions unrelated to DDX6 mutation were performed using the statistical
	method proposed by Anders and Huber 35 . The Wald test was used to estimate the p-values and
	they were adjusted for multiple testing with the Benjamini and Hochberg method 36 . Significant
	differentially-expressed (DE) genes were analyzed using the Database for Annotation,
	Experiments were performed in duplicate from two different fibroblast pellets obtained Visualization and Integrated Discovery (DAVID 6.7). Biological processes and molecular
	from S2. Total RNA was extracted using the RNeasy mini kit (Qiagen, Valencia, CA, USA) functions of Gene Ontology Consortium (GO), as well as pathways from KEGG were used for
	including a DNase treatment. RNA levels and quality were quantified using a Nanodrop the functional annotations. The list of genes known or suspected to be involved in ID was
	spectrophotometer and a 2100 Bioanalyzer (Agilent, Santa Clara, CA, USA). mRNA libraries of extracted from the SysID database (http://sysid.cmbi.umcn.nl/). Genes with 10 or more reads
	template molecules suitable for high throughput DNA sequencing were created using TruSeq™ detected (on average) were considered as expressed in fibroblasts.
	RNA Sample Preparation v2 Kit from 200 ng of total RNA. S2 was prepped with eight other
	individuals affected by Bardet Biedl syndrome or other causes of ID not related to DDX6
	mutation, and the subsequent libraries were sequenced on one run with the Illumina Hiseq 4000 High-resolution crystal structures for human DDX6 in an open conformation (PDB ID:
	sequencer as paired-end 100 base reads. Image analysis and base calling were performed using 4CT5) and closed conformation in complex with the CNOT1 MIF4G and 4E-T CHD domains
	RTA 1.18.61 and CASAVA 1.8.2. Reads (403,718,870 and 299,879,030) were mapped onto the (PDB ID: 5ANR) were obtained from Protein Data Bank originally submitted by Mathys et al 37
	hg19 assembly of the human genome using TopHat 2.0.14 and the Bowtie 2-2.1.0 aligner. The and Ozgur et al 9 . Three-dimensional (3D) mapping of gnomAD v2.1.1 missense variants and
	raw sequencing data generated in the course of this RNA study are not publicly available but DDX6 variants p.His372Arg, p.Arg373Gln, p.Cys390Arg, p.Thr391Pro and p.Thr391Ile were
	more details are available on demand. Gene expression was quantified using HTSeq-0.6.1 34 and performed using PyMOL v2.1.1 and the UCSF Chimera package 38 .
	gene annotations from Ensembl release 75. Only uniquely-mapped and non-ambiguously
	assigned reads were retained for further analyses. Read counts were then normalized across
	libraries with the median-of-ratios method proposed by Anders and Huber 35 . To check if the
	normalization was correctly performed, Relative Log Expression (RLE) plots were drawn to

Cell Culture, siRNA, plasmids and transfection

	Thr391Pro_F: 5'-TTGTTTGCCCTGATCTGTTTACCCGAGGTATTGATATACAAGCTG-3'
	Thr391Pro_R: 5'-GATCAGGGCAAACAAGATTGCGGCATAAGCCATTTCGGAAATCAT-
	3'. Cells were harvested 60 h after plating.
	Immunofluorescence
		Cells were grown on glass coverslips in 35 mm Petri dish and fixed in methanol for 3 min
	at -20°C. Cells were blocked 10 min in 2% BSA (bovine serum albumin) in PBS (phosphate-
	buffered saline), incubated 1 hour with the primary antibody, rinsed with PBS, incubated 1 hour
	with a fluorochome-conjugated secondary antibody and rinsed in PBS. Slides were mounted in
	Citifluor (Citifluor, London, UK). For complementation assays, microscopy was performed on a
	Leica DMR microscope (Leica) using a 63 × 1.32 oil immersion objective. Photographs were
	taken using a Micromax CCD camera13 (Princeton Instruments) driven by the Metamorph
	software. For fibroblast imaging, epifluorescence microscopy was performed on an inverted
	Zeiss Z1 microscope (Zeiss) equipped with a motorized stage using a 63x1.4 oil immersion
	objective and running under the Zen software. Photographs were obtained with an Axiocam 506
	mono camera (Zeiss). Images were processed with ImageJ. To quantitate PBs, we used the
	plugin	Spot	Detector	of	the	open	bioimage	informatics	platform	Icy
	(http://icy.bioimageanalysis.org 39 ). Primary antibodies were rabbit DDX6 (Novus Biologicals,
	Bio-Techne, Lille, France), rabbit LSM14A (Merck-Millipore (Molsheim, France), mouse EDC4
	(Santa Cruz Biotechnology, Heidelberg, Germany) and rabbit HA and mouse FLAG M2 (Sigma-
	Aldrich, Saint-Quentin Fallavier, France). Secondary antibodies were purchased from Jackson
	Fibroblasts from Subject 2 and one sex-matched control individual, as well as fibroblasts ImmunoResearch Laboratories (Suffolk, UK).
	from Subject 1 and her parents were maintained in 50% low glucose DMEM and 50% nutrient

Table S1

 S1

). DNAH11 is associated with autosomal recessive Ciliary Dyskinesia, Primary, 7, with or without situs inversus (CILD7) (MIM: 611884) predominately through LoF mutations. Clinical features of CILD7 do not include ID or resemble S1's phenotype. Further, DNAH11 possesses significantly more missense variation than expected in gnomAD (Z-score = -5.61) and ClinVar contains no confirmed pathogenic missense variants. Thus, the VUS was excluded as a candidate. Subject 2 (S2) first underwent an ID gene panel sequencing, singleton WES, and finally WGS of the nuclear family. Among the 146 putative de novo variants identified in the proband, only one occurred in a coding region: a missense change c.1168T>C (p.Cys390Arg) in DDX6. No other candidate variant(s) were identified in coding or genomic regions of genes known to be involved in ID. Two additional de novo missense variants in DDX6, c.1172C>T (p.Thr391Ile)(Subject 3, S3) and c.1171A>C (p.Thr391Pro)(Subject 4, S4), were identified by trio WES in unrelated individuals with developmental disorders (GeneDx). Subject 4 carries an additional hemizygous missense VUS in the gene SLC16A2 (MIM: 30095) associated with Allan-Herndon-Dudley syndrome (AHDS, MIM: 300523), but this diagnosis does not fit with his clinical history (Table

DDX6 missense variants found in affected individuals are absent from population databases, evolutionarily conserved, and spatially clustered within protein

	and genitourinary abnormalities (3/4). Other features present include mild-moderate MRI
	anomalies, small head circumference (<-2 Standard Deviations SD), congenital hydronephrosis
	with vesicoureteral reflux, mild obesity, and autistic traits. The probands also share common
	facial dysmorphic features including a high forehead, a bulbous nasal tip, widely-spaced eyes,
	arched eyebrows and low-set ears (Figure 1). Expanded summaries can be found in
	Supplemental Data: Case Reports.
	indicated by a gnomAD Z-score of 3.93 (observed/expected ratio of 0.34 [0.29-0.41]) suggesting
	selection pressure against amino acid changes 26 . In particular, exon 11 (aa 371-391) was found
	highly depleted of nonsynonymous variations in gnomAD (Figure

). Clinical features include ID (4/4), DD (4/4), mild/moderate cardiac anomalies (3/4), hypotonia (3/4), gait instability (3/4), hand/feet All de novo missense variants identified in affected individuals are located in exon 11 of DDX6. This exon is present in all DDX6 isoforms and codes for conserved motifs QxxR and V within the second RecA-2 domain of the helicase core. The variants have not been observed in the gnomAD database, which contains genetic data from ~140 thousand individuals and is depleted of severe pediatric disorders. DDX6 is highly intolerant to LoF variants as indicated by gnomAD's (v2.1.1) observed/expected LoF ratio of 0.04 (confident interval [0.01-0.17]), with only one heterozygous LoF change reported in the penultimate exon of its canonical isoform (NM_004397.5). DDX6 was also found to have significant intolerance to missense change

Table 1 .

 1 Pathogenic DDX6 variants in individuals with ID.

Table 2 .

 2 Clinical features of individuals with de novo pathogenic variants in DDX6.

	DDX6 variants in individuals with ID
	Table 1. Pathogenic

Table 2 .

 2 Clinical features of individuals withShe is a 3-year old girl of non-consanguineous parents with Hungarian/Puerto Rican descent (mother) and Hispanic/Mexican descent (father). Regarding family history, mother was noted to have a half-brother diagnosed with autism. She presented to TGen's Center for Rare Childhood Disorders exhibiting several conditions including global developmental delay, microcephaly, dysmorphic features, bilateral vesicoureteral reflux with hydronephrosis, and hypoplasia of the posterior corpus callosum. Pregnancy was uneventful up until the discovery of oligohydramnios at five months gestation. During this time, a fetal ultrasound revealed a low-lying kidney and renal reflux. Following 34 weeks of gestation, the patient was born via C-section with a birth weight of 4 pounds and 14 ounces. APGARs are not known. She was admitted to the neonatal intensive care unit to address feeding problems. During that time, she required nasogastric feedings and received oxygen by nasal cannula. After 3 weeks, the patient was discharged with bottle feedings. At 16 months, she underwent ureteral re-implantation surgery to correct the reflux. Neuroimaging studies were performed at 27 months of age. A brain MRI study revealed hypoplastic posterior corpus callosum. The CT scan of the brain appeared normal. Further diagnostic testing including karyotype, MECP2, fragile X, PWS methylation, CMA, and plasma acylcarnitine were performed with normal outcomes. Hearing tests conducted at that time were also normal. Developmental delays had been observed since birth without history of regression of developmental milestones. Rolling over was accomplished at 12 months and crawling at 16 months. At the time she was referred to the center, she was able to feed herself using both hands. She exhibited language delays, not yet communicating with words although she is able to vocalize and follow very simple commands. To overcome these delays, she participates in ST, PT, and OT and continues to make slow developmental progress. She is now able to walk independently, but requires assistance with dressing and toileting.There is no history of seizures, pulmonary, ophthalmologic, and gastrointestinal problems. Physical observations were noted at her four year physical exam. The patient had a height of 101.8 cm (44%), weight of 15.6 kg (34%), and head circumference of 46.2 cm (2 SD below mean for age). Dysmorphic facial features included epicanthal folds, hypertelorism, and almond shaped eyes. Her ears were low set and posteriorly rotated with overfolded superior pinna. Furthermore, she had anteverted nares and a broad flat nasal root. Her hair was well grown for her age with two posterior hair whorls 2 cm apart. Her philtrum was prominent and she had a high arched palate. Her neurological examination revealed an alert, smiling, and interactive female. She responded to her name, vocalized, but did not use any words. Her face was symmetric and she made eye contact. A motor exam demonstrated mild axial hypotonia with normal strength and tone in upper and lower extremities. Deep tendon reflexes were present in upper and lower extremities. She was able to walk on her own and sit independently. She is noted to be quite socially outgoing.

		Subject 5	Female / 13 yrs	c.1115A>G p.(His372Arg)	de novo	+	+	+	-	wide liquor spaces, thin corpus	callosum, mild delay of myelination	-	+	trouble with walking, using	wheelchair for longer distances	tapered fingers, overriding toes	high bossing forehead, bitemporal	narrowness, telecanthus, epicanthus	strabismus; hypermetropia	overfolded upper helix	-	patent foramen ovale	-	+
	de novo pathogenic variants in DDX6	Subject 1 Subject 2 Subject 4	Sex / Age at last exam Female / 5 yrs Male, 6 yrs Male / 10 yrs	DDX6 Variant (NM_004397.5) c.1118G>A (p.Arg373Gln) c.1168T>C (p.Cys390Arg) c.1171A>C (p.Thr391Pro)	Inheritance de novo de novo de novo	Intellectual disability + + +	Developmental delay + + +	Speech delay + (nonverbal) + +	Small head circumference (<2SD) + + -	MRI brain anomalies hypoplastic posterior corpus callosum (27mo) --	Seizures --h/o staring spell, normal EEG	Hypotonia mild, axial + -	Gait walked w/ support till 27m, many falls from balance issues -instability/ delayed coordination, toe-walking	Digit abnormalities / arthrogryposis overriding second toes bilaterally syndactyly II-III of the feet tapered fingers, hypermobile flat feet	Facial dysmorphy fissures, bilateral epicanthus, arched hypertelorism, small palpebral hypertelorism, small palpebral fissures, bilateral epicanthus eyebrows arched eyebrows	Eyes strabismus, hypermetropia strabismus, hypermetropia slight anisocoria	Ears low set, petite and rotated. overfolded superior pinna low set -	Mouth high arched palate, smooth prominent philtrum --	Heart abnormalities patent foramen ovale, aberrant rt subclavian artery, cardiac murmur atrial septal defect with spontaneous closing no imaging	Congenital hydronephrosis / + vesicoureteral reflux --	h/o abdominal pain/ constipation,	Feeding difficulties / GERD + (G-tube) + unexplained elevated LFTs-	resolved

ACKNOWLEDGEMENTS

The authors thank the families for their participation to the study. The authors also thank the Foundation Jerome Lejeune and Fondation Maladies Rares for their financial support, as well as the Association Paul and Liba Mandel and the CREGEMES. This study was also supported by the grant ANR-10-LABX-0030-INRT, a French State fund managed by the Agence Nationale de la Recherche under the frame program Investissements d'Avenir ANR-10-IDEX-0002-02, the

 S3. Eukaryotic translation initiation factor 3 (eIF3) and ribosomal protein family genes differentially-expressed in Subject 2.