Elena Sieni

Valentina Cetica

Alessandra Santoro

Karin Beutel

Elena Mastrodicasa

Marie Meeths

Benedetta Ciambotti

Francesca Brugnolo

Udo Zur Stadt

Daniela Pende

Udo Zur Stadt

Lorenzo Moretta

Gillian M Griffiths

Jan-Inge Henter

Gritta Janka

Maurizio Aricò
email: m.arico@meyer.it

Maurizio Aricò Direttore

GENOTYPE-PHENOTYPE STUDY OF FAMILIAL HEMOPHAGOCYTIC LYMPHOHISTIOCYTOSIS TYPE 3

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Hemophagocytic lymphohistiocytosis (HLH) is a genetically heterogeneous disorder characterized by a hyperinflammatory syndrome with fever, hepatosplenomegaly, cytopenia, and frequently also central nervous system involvement. [START_REF] Aricò | Hemophagocytic lymphohistiocytosis. Report of 122 children from the International Registry. FHL Study Group of the Histiocyte Society[END_REF] Bone marrow aspiration shows haemophagocytosis by activated macrophages. In most cases the natural course of HLH is rapidly fatal within a few weeks, unless appropriate immune suppressive and cytoreductive treatment by agents including corticosteroids, cyclosporine, etoposide, anti-thymocyte globuline, can obtain transient disease control. [START_REF] Henter | Treatment of hemophagocytic lymphohistiocytosis with HLH-94 immunochemotherapy and bone marrow transplantation[END_REF][START_REF] Mahlaoui | Immunotherapy of familial hemophagocytic lymphohistiocytosis with antithymocyte globulins: a single-center retrospective report of 38 patients[END_REF] So far, haematopoietic stem cell transplantation appears to be the only curative treatment. [START_REF] Jabado | Treatment of familial hemophagocytic lymphohistiocytosis with bone marrow transplantation from HLA genetically nonidentical donors[END_REF][START_REF] Horne | Haematopoietic stem cell transplantation in haemophagocytic lymphohistiocytosis[END_REF][START_REF] Baker | Unrelated donor hematopoietic cell transplantation for hemophagocytic lymphohistiocytosis[END_REF][START_REF] Cooper | The use of reducedintensity stem cell transplantation in haemophagocytic lymphohistiocytosis and Langerhans cell histiocytosis[END_REF][START_REF] Cesaro | Hematopoietic stem cell transplantation for hemophagocytic lymphohistiocytosis: a retrospective analysis of data from the Italian Association of Pediatric Hematology Oncology (AIEOP)[END_REF] Differential diagnosis of HLH may be difficult. [START_REF] Aricò | Haemophagocytic lymphohistiocytosis: proposal of a diagnostic algorithm based on perforin expression[END_REF] To this purpose, diagnostic guidelines for HLH have been established by the Histiocyte Society. [START_REF] Henter | Diagnostic guidelines for hemophagocytic lymphohistiocytosis. The FHL Study Group of the Histiocyte Society[END_REF][START_REF] Henter | HLH-2004: Diagnostic and therapeutic guidelines for Hemophagocytic lymphohistiocytosis[END_REF] In particular, demonstration of frequent association with common pathogens, together with evidence of impaired natural killer cytotoxic activity, provided the rationale for considering HLH as a selective immune deficiency. [START_REF] Perez | Impaired natural killer activity in lymphohistiocytosis syndrome[END_REF][START_REF] Aricò | Natural cytotoxicity impairment in familial haemophagocytic lymphohistiocytosis[END_REF][START_REF] Schneider | Hemophagocytic lymphohistiocytosis is associated with deficiencies of cellular cytolysis but normal expression of transcripts relevant to killer-cellinduced apoptosis[END_REF] Starting from the original report by Farquhar et al. in 1952, [START_REF] Farquhar | Familial haemophagocytic reticulosis[END_REF] autosomal recessive inheritance was proposed and then confirmed as the common mode of inheritance for the familial form of HLH (FHL).

Linkage analysis identified a candidate genomic region on chromosome 9q21. 3-22 (FHL1, OMIM 603552). [START_REF] Ohadi | Localization of a gene for familial hemophagocytic lymphohistiocytosis at chromosome 9q21.3-22 by homozygosity mapping[END_REF] However, the causative gene responsible for the disease has not yet been identified. A simultaneous report established a linkage with another region, 10q21-22, [START_REF] Dufurcq-Lagelouse | Linkage of familial hemophagocytic lymphohistiocytosis to 10q21-22 and evidence for heterogeneity[END_REF] in which the perforin (PRF1) gene was identified as responsible for a relevant proportion of cases of FHL (FHL2, OMIM 603553). [START_REF] Stepp | Perforin gene defects in familial hemophagocytic lymphohistiocytosis[END_REF] In patients with FHL2, PRF1 mutations reduce or abolish the synthesis of the perforin protein, resulting in an impairment of the granule-mediated cytotoxic machinery of NK and T cells. [START_REF] Clementi | Six novel mutations in the PRF1 gene in children with haemophagocytic lymphohistiocytosis[END_REF][START_REF] Risma | Aberrant maturation of mutant perforin underlies the clinical diversity of hemophagocytic lymphohistiocytosis[END_REF][START_REF] Ueda | Characteristic perforin gene mutations of haemophagocytic lymphohistiocytosis patients in Japan[END_REF][START_REF] Feldmann | Functional consequences of perforin gene mutations in 22 patients with familial haemophagocytic lymphohistiocytosis[END_REF] In 2003 a third locus, 17q25, was reported in linkage with FHL (FHL3, OMIM 608898). [START_REF] Feldmann | Munc13-4 is essential for cytolytic granules fusion and is mutated in a form of familial hemophagocytic lymphohistiocytosis (FHL3)[END_REF] The involved gene UNC13D encodes a protein named Munc13-4 which is thought to contribute to the priming of the secretory granules before they fuse into the plasma cell membrane.

Mutations in this gene impair the delivery of the effector proteins perforin and granzymes into the target cells, resulting in defective cellular cytotoxicity and a clinical picture that appears identical to that associated with PRF1 mutations. On the basis of a genome-wide screening in a highly consanguineous Kurdish family with FHL, a fourth chromosomal region (6q24) has been reported (FHL4, OMIM 603552). [START_REF] Zur Stadt | Linkage of familial hemophagocytic lymphohistiocytosis (FHL) type-4 to chromosome 6q24 and identification of mutations in syntaxin 11[END_REF] Mutations of the syntaxin 11 gene, mapped in this region, are thought to alter intracellular vesicle trafficking of the phagocytic system. [START_REF] Bryceson | Defective cytotoxic lymphocyte degranulation in syntaxin-11 deficient familial hemophagocytic lymphohistiocytosis (FHL4) patients[END_REF] Very recently, zur Stadt et al. have allocated a novel FHL type, FHL-5 (OMIM 613101), to a 1 Mb region on chromosome 19p using high resolution SNP genotyping in eight unrelated FHL patients from consanguineous families. They have identified mutations in STXBP2, encoding syntaxin binding protein 2 (Munc18-2) a protein involved in the regulation of vesicle transport to the plasma membrane. The 12 patients with FHL-5 originated from Turkey, Saudi Arabia, and Central Europe. [START_REF] Zur Stadt | Familial Hemophagocytic Lymphohistiocytosis Type 5 (FHL-5) Is Caused by Mutations in Munc18-2 and Impaired Binding to Syntaxin 11[END_REF] Almost simultaneously, a similar report was provided by Côte et al. [START_REF] Côte | Munc18-2 deficiency causes familial hemophagocytic lymphohistiocytosis type 5 and impairs cytotoxic granule exocytosis in patient NK cells[END_REF] The exact contribution of the various mutations in FHL-related genes can only be evaluated in large cohorts. A genotype-phenotype study of FHL2, due to PRF1 mutations, was reported a few years ago by an international consortium for the Histiocyte Society. [START_REF] Trizzino | Histiocyte Society HLH Study group. Genotype-phenotype study of familial haemophagocytic lymphohistiocytosis due to perforin mutations[END_REF] Nothing comparable has been performed so far for FHL3, the other large subgroup of this disease. For this purpose, we pooled data from three European referral centres, allowing collection of the largest series of patients with FHL3.

METHODS

Data collection

The consortium established between Italy, Germany and Sweden pooled in a common database data on ethnicity, family history, presenting features, mutations and cytotoxic function from individual patients with FHL3 diagnosed on the basis of documented biallelic UNC13D mutations. Hemophagocytic Lymphohistiocytosis was defined by the diagnostic criteria established by the Histiocyte Society. [START_REF] Henter | Diagnostic guidelines for hemophagocytic lymphohistiocytosis. The FHL Study Group of the Histiocyte Society[END_REF][START_REF] Henter | HLH-2004: Diagnostic and therapeutic guidelines for Hemophagocytic lymphohistiocytosis[END_REF] Central nervous system (CNS) involvement was defined as the presence of at least one of the following items: neurologic symptoms, CSF pleocytosis (≥5 cells/mm 3 ), elevated CSF protein (≥30 mg/dl), MRI alteration. All data were stored in a common database and analysed. The completeness of the data was: ethnicity, 95%; consanguinity, 95%; age at diagnosis, 98%; persistent fever, 96%; splenomegaly, 96%; central nervous system (CNS) disease, 95%; haemoglobin 85%; neutrophils, 89%; platelets, 92%; triglycerids, 75%; fibrinogen, 70%; ferritin, 67%; haemophagocytosis, 96%; CD107 expression, 36%; Munc protein expression, 16%; NK activity, 53%.

Informed consent for the genetic study and the data collection was obtained from the parents or legal guardian at the participating centres. The study was approved by the local IRB at all the participating centres.

UNC13D gene analysis

Genomic DNA was isolated from peripheral blood samples using BioRobot® EZ1 Workstation (Qiagen, Jesi Italy). Some samples were retrieved from our DNA library of patients previously diagnosed. [START_REF] Aricò | Hemophagocytic lymphohistiocytosis. Report of 122 children from the International Registry. FHL Study Group of the Histiocyte Society[END_REF][START_REF] Clementi | Six novel mutations in the PRF1 gene in children with haemophagocytic lymphohistiocytosis[END_REF][START_REF] Trizzino | Histiocyte Society HLH Study group. Genotype-phenotype study of familial haemophagocytic lymphohistiocytosis due to perforin mutations[END_REF][START_REF] Santoro | Novel Munc13-4 mutations in children and young adult patients with haemophagocytic lymphohistiocytosis[END_REF][START_REF] Marcenaro | Analysis of natural killer-cell function in familial Hemophagocytic lymphohistiocytosis (FHL): defective CD107a surface expression heralds Munc13-4 defect and discriminates between genetic subtypes of the disease[END_REF] The 32 coding exons and exon-intron boundaries of Munc13-4 gene were amplified and directly sequenced, in both directions, with the BigDye® Terminator Cycle Sequencing Ready Reaction Kit (Applied Biosystems, Foster City, CA, USA). Amplification reactions were performed with 60 ng of DNA, 10 ng of each primer, 200 µM dNTPs, 1x PCR reaction buffer, and 2.5 U Taq polymerase in a final volume of 25µl; primers are available upon request. Sequences obtained using an ABI Prism® 3130XL Sequence Detection System (Applied Biosystems) were analyzed and compared with the reported gene structure (gene number: 201294, NCBI) using the dedicated software SeqScape® (Applied Biosystems). Mutations were confirmed in the parents.

In silico analysis

All mutations were searched in dbSNP (http://www.ncbi.nlm.nih.gov/snp/). Unknown mutations were tested by bioinformatic facilities in order to predict whether an amino acid substitution will have functional effect. We used two web query tools: SIFT (Sorting Intolerant From Tolerant: http://blocks.fhcrc.org/sift/SIFT.html) which, is based on sequence homology, defines tolerated or not tolerated protein changes with a score ranging from 0 to 1 (the amino acid substitution is predicted to be damaging if the score is ≤0.05, and tolerated if the score is > 0.05); and POLYPHEN (prediction of functional effect of human nsSNPs: http://genetics.bwh.harvard.edu/pph/) which considers phylogenetic and structural informations and defines a substitution to be "probably damaging" with a score >2, "possibly damaging" 1.5-2 and "benign" <1.5. Cryptic splice sites were predicted by NNsplice software (http://www.fruitfly.org/seq_tools/splice).

Functional analyses

Peripheral blood mononuclear cells (PBMC) from FHL3 patients and healthy donors were isolated by Ficoll gradient centrifugation. NK cells were also purified using the RosetteSep method (StemCell Technologies, Vancouver, British Columbia, Canada) following manufacturer's instructions. NK cells were cultured on irradiated feeder cells in the presence of 2 μg/mL phytohemagglutinin (Sigma-Aldrich, Irvine, UK) and 100U/mL rIL-2 (Proleukin, Chiron Corp., Emeryville, USA) to obtain high numbers of polyclonal activated NK cell populations. To analyse the cytolytic activity in 4-h 51 Cr-release assays, PBMC were tested against K562, while activated NK cells were tested against the HLA-class I -B-EBV cell line 721.221, demonstrated to be suitable effector/target combinations to reveal cytolytic defect of FHL3 patients. [START_REF] Marcenaro | Analysis of natural killer-cell function in familial Hemophagocytic lymphohistiocytosis (FHL): defective CD107a surface expression heralds Munc13-4 defect and discriminates between genetic subtypes of the disease[END_REF] E:T ratios ranging from 100:1 to 1:1 were used for PBMC as effector cells, while from 8:1 to 0.5:1 for activated NK. Lytic units (LU) at 30% lysis were calculated. PBMC and activated NK cells were also tested in degranulation assay quantifying cell surface CD107a expression upon co-culture with K562, as previously described. [START_REF] Marcenaro | Analysis of natural killer-cell function in familial Hemophagocytic lymphohistiocytosis (FHL): defective CD107a surface expression heralds Munc13-4 defect and discriminates between genetic subtypes of the disease[END_REF] All reagents were from BD Biosciences (Oxford, UK). Briefly, anti-CD107a-PE mAb was added prior to incubation of 3 hours at 37° C in 5% CO2.

Thereafter, the cells were stained with anti-CD56-APC and anti-CD3-PerCP mAb and analysed by flow cytometry (FACSCalibur, Becton Dickinson). Surface expression of CD107a was assessed in the CD3 -CD56 + cells. Results are reported as ΔCD107a (i.e. % CD107a + cells of stimulated -% CD107a + cells of unstimulated sample).

Western blot analysis

Western blot analysis of Munc13-4 protein was performed as previously described. [START_REF] Santoro | Novel Munc13-4 mutations in children and young adult patients with haemophagocytic lymphohistiocytosis[END_REF]

Genotype-phenotype correlations

To explore the correlations between different mutations and clinical, laboratory and functional parameters, UNC13D mutations were classified according to their functional impact. The group of disruptive mutations included the nonsense, frameshift and all splice errors, except c.952-1G>A and c.2626-1G>A (Santoro A, Aricò M., data not shown), that did not alter the frame; furthermore, the single nucleotide substitution c.1847A>G, based on our previous findings of its capacity to induce a splicing error only in selected clones, [START_REF] Santoro | Mutations affecting mRNA splicing are the most common molecular defect in patients with familial hemophagocytic lymphohistiocytosis type 3[END_REF] was not included in the group of disruptive mutations. Based on this, the 84 patients were classified into 3 groups: group 1, defined by missense mutations only (n=17); group 2, including patients with one disruptive and one missense mutation (n=18); group 3, including patients with biallelic disruptive mutations (n=49).

In order to try to define differences between FHL2 and FHL3, data from the current cohort of patients with FHL3 were compared with available data from the previously reported cohort with FHL2. [START_REF] Trizzino | Histiocyte Society HLH Study group. Genotype-phenotype study of familial haemophagocytic lymphohistiocytosis due to perforin mutations[END_REF]

Statistical analysis

Statistical significance of the differences between the ages at the disease onset and the quantitative evaluation of the cytoxicity (in lytic units) between groups of patients with different mutations, was calculated by Student t test; to compare the differences in the proportions of CNS involvement, Chi-Square test was used. P value ≤0.05 was considered as significant. Statistical analysis of the functional data comparing FHL3 and healthy donors was performed using one-way ANOVA followed by Bonferroni's Multiple Comparison test, by GraphPad Prism 5 Software.

RESULTS

Study population

A total of 84 patients from 69 unrelated families were reported from the following reference centres: Florence, Italy, n=54; Hamburg, Germany, n=18; Stockholm, Sweden, n=12. They had been diagnosed between 1981 and 2009. Some of these patients had been previously reported as part of single-centre series. [START_REF] Santoro | Novel Munc13-4 mutations in children and young adult patients with haemophagocytic lymphohistiocytosis[END_REF][START_REF] Marcenaro | Analysis of natural killer-cell function in familial Hemophagocytic lymphohistiocytosis (FHL): defective CD107a surface expression heralds Munc13-4 defect and discriminates between genetic subtypes of the disease[END_REF][START_REF] Santoro | Mutations affecting mRNA splicing are the most common molecular defect in patients with familial hemophagocytic lymphohistiocytosis type 3[END_REF][START_REF] Stadt | Mutation spectrum in children with primary hemophagocytic lymphohistiocytosis: molecular and functional analyses of PRF1, UNC13D,STX11,and RAB27A[END_REF][START_REF] Rudd | Spectrum, and clinical and functional implications of UNC13D mutations in familial haemophagocytic lymphohistiocytosis[END_REF] The ethnic origin was as follows: Caucasian, n=58; Turkish, n=10; Asian, n=7; Hispanic, n=4; African, n=3 (not reported, n=2). Male:

Female ratio was 1.2:1.

Presenting features

Main presenting features are summarized in Table 1.

The median age at the diagnosis was 4.1 months, with a range of 1 day to 18.8 years (quartiles: 2.5, 4.1 and 18.4 months). Consanguinity was reported in 28 of 80 (35%) patients.

The current diagnostic criteria for HLH were fulfilled -i.e. they had at least five of the eight items proposed [START_REF] Henter | HLH-2004: Diagnostic and therapeutic guidelines for Hemophagocytic lymphohistiocytosis[END_REF] -by 58 of the 84 (69%) patients; of the remaining 26 patients, 15 had a positive family history, 4 had related parents, 4 had incomplete data, and 3 fulfilled less than 5/8 criteria.

In particular, among those for whom the information was available, fever was present in 89% of patients, splenomegaly in 96%, bi-cytopenia in 88%, hypertriglyceridemia and/or hypofibrinogenemia in 95%, haemophagocytosis in 78%, hyperferritinemia in 77%, and low or absent NK cell activity in 98% (table 1).

The combination of fever, splenomegaly and thrombocytopenia was present in 65 of 77 patients (84%) evaluable for these three parameters; of them, 55 had information on ferritin level, which turned to be elevated in 39 (71%); otherwise, 59 of the 78 had information on fibrinogen level, which turned to be reduced in 33 (56%); finally, all of the 78 had information on hemophagocytosis, which turned to be present in 54 (69%) (Figure 1). CNS involvement was reported in 60% of the patients: 30/80 (37%) had neurologic symptoms, 35/51 (69%) had ≥5 cells in the CSF, 22/31 (71%) elevated CSF protein, and 15/25 (60%) MRI alterations.

UNC13D mutations

A total of 54 different mutations were observed in the 84 patients from 69 families.

Nineteen were missense mutations, 14 deletions or insertions, 12 nonsense and 9 splice errors (Fig. 2).

A total of 23 different mutations were observed at the homozygous state in 42 individuals from 36 unrelated families; their presenting features are summarized in table 2. The remaining 42 patients from 33 unrelated families were compound heterozygous. The most frequent mutations are described below.

The c.2346_2349delGGAG mutation, causing a frame shift at p.R782, was the most common mutation; it was identified in a total of 19 patients from 15 families, all Caucasian but one Turkish (not reported, n=1). In three homozygous patients the ages at the diagnosis were 2, 3 and 3 months; only one of them had CNS involvement. NK activity was reduced in the only patient analysed.

The c.753+1G T mutation, resulting in a splice error, was found in 19 patients from 11 families (15%), 9 Caucasian, one Asian and one Hispanic. Nine homozygous patients had a median age at the diagnosis of 3 months (range, 2 to 12); 4 of them had CNS involvement. CD107a expression was absent in two and reduced in two patients analysed, NK activity was absent in three and reduced in three patients tested. The p.E616G missense mutation, resulting from the c.1847A>G nucleotide change, was found in 8 individuals from 6 families of Caucasian origin. Four patients carried the mutation at the homozygous state: age at diagnosis was 99 and 139 months in two unrelated subjects, and 6 and 226 months in two siblings. Three of the four had CNS involvement. CD107a expression was reduced in the only patient tested; NK activity was reduced in all the three patients tested.

The p.R928C missense mutation (c.2782C>T) was identified in 8 patients from 7 unrelated families, 6 Caucasian and one Hispanic. This mutation falls in the C2B domain; its predicted impact is controversial, i.e. it may be tolerated according to SIFT, or potentially deleterious according to POLYPHEN. It was observed in three patients who also carried two additional mutations; one patient with this as second mutation has defective NK activity.

The nonsense mutation p.R273X (c.817C>T) was identified in six patients, including one pair of homozygous Caucasian siblings, both diagnosed at 2 months, and two additional heterozygous pairs, one Hispanic and one Caucasian. NK activity was absent in one patient tested.

The splice error c.1389+1G>A was identified in 4 unrelated patients (3 Caucasian, 1 origin not reported). All were compound heterozygous with different frameshift (n=3) or nonsense (n=1) mutations.

Novel mutations

Fifteen novel mutations were identified in 15 patients; three were nonsense and five deletion/insertion mutations (Figure 2, tables 2 and 3), while the remaining seven were missense mutations. The p.I140T missense mutation (c.419T>C) falls in the C2A domain and is predicted to be damaging. The p.S398L missense mutation, resulting from the c.1193C>T nucleotide change, falling outside the functional domains and predicted to be

Functional study

Granule release capacity was significantly reduced in 29 of the 30 patients tested. Of them 17 had quantitative evaluation, analysing activated NK cells upon co-culture with K562, with a mean value of 14.5% of ΔCD107a + cells (SD 11.7, SE 2.8). This value was significantly inferior to that of healthy controls, both adults and infants (p<0.0001).

Reduced or absent NK cytolytic activity was found in 44 of the 45 patients analysed (98%).

Of them 17 had quantitative evaluation of the lytic units (LU), analyzing activated NK cells against 721.221, with a mean value of 36.4 (SD 30.4, SE 7.3). This value was significantly inferior to that of healthy infant controls (p<0.005).

Missense mutations falling in the functional domains

Overall, one third of the mutations were missense: of them, 11 within the functional domains of the protein, while the remaining 8 fall outside. Only three patients had biallelic missense mutations falling outside the domains: UPN 257 homozygous for c.1193C>T, UPN GE095 homozygous for c.1208C>T, and UPN 483 heterozygous for c.175G>A and c.2180G>A. Their ages at diagnosis were 69.6, 2.6 and 0.7 months respectively, with no significant difference compared to the remaining patients with missense mutations.

Genotype-phenotype correlations

Patients in group 1 (missense mutations, n=17), in group 2 (mixed mutations, n=18) and patients in group 3 (nonsense mutations, n=49) were compared to identify possible differences in their disease manifestations. Patients in group 3 were diagnosed at a significantly younger age than patients in groups 1 and 2 (p<0.0001) (Fig. 3); moreover group 2 developed the disease at a significantly younger age than patients in group 1 (p=0.01).

No major differences in the symptoms present at diagnosis were detected between the patient groups. The combination of fever + splenomegaly + thrombocytopenia + ferritin elevation was present in 8 of the 13 (61%) evaluable patients in group 1 versus 27 of 33 (81%) in group 3 (p=0.28). The combination of fever + splenomegaly + thrombocytopenia was present in 13 of the 16 (81%) evaluable patients in group 1 versus 39 of 43 (90%) in group 3 (p=0.58). The frequency of CNS involvement also was not statistically different between the two groups (29/46, 63% in group 3 vs. 10/17 59% in group 1; p=0.98).

Granule release capacity was significantly reduced in 29 of the 30 patients tested. Of them 17 had quantitative evaluation, analyzing activated NK cells upon co-culture with K562, with a mean value of 14.5% of ΔCD107a + cells (SD 11.7, SE 2.8). This value was significantly inferior to that of healthy controls, both adults and infants (p<0.0001).

Analysing separately patients of group 3 from those of groups 1 and 2, they did not differ from each other and both were significantly lower than healthy controls (Figure 4, panel A).

Reduced or absent NK cytolytic activity was found in 44 of the 45 patients analysed (98%). Of them 17 had quantitative evaluation of the lytic units (LU), analyzing activated NK cells against 721.221, with a mean value of 36.4 (SD 30.4, SE 7.3). This value was significantly inferior to that of healthy infant controls (p<0.005). Mean LU value was not significantly lower in patients from group 3 than those in groups 1 and 2 (Figure 4, panel B).

Comparison between FHL2 and FHL3

The median age at the diagnosis of patients with FHL3 (4.1 months) was not significantly different from that previously observed in patients with FHL2 (3 months). [START_REF] Trizzino | Histiocyte Society HLH Study group. Genotype-phenotype study of familial haemophagocytic lymphohistiocytosis due to perforin mutations[END_REF] When the patients are classified according to the functional impact of the mutations, the age at diagnosis also is not significantly different for patients with missense (group 1) or with mixed mutations (group 2). Yet, when we focused on patients with biallelic disruptive mutations only, patients with FHL2 had a significantly younger age at the diagnosis, with a median age of 2 months compared with 3 months in patients with FHL3 (Figure 5; p=0.001).

CNS involvement was found in 49/81 (60%) patients, a frequency which is significantly higher than that reported in FHL2 (n=31/86; 36%; p=0.001). [START_REF] Trizzino | Histiocyte Society HLH Study group. Genotype-phenotype study of familial haemophagocytic lymphohistiocytosis due to perforin mutations[END_REF]

DISCUSSION

This is the largest series of patients with FHL3 reported so far. Since UNC13D mutations have been found worldwide, the predominance of Caucasian patients in this series only reflects the reporting bias of the European location of the contributing centres. However, due to increasing immigrations, a minority of families originating from Asia and Africa are also included.

FHL is usually considered a disease of early infancy. In this series, the median age at the diagnosis was 4.1 months, three-quarters of the cases being diagnosed within 18 months. When the patients are classified according to the functional impact of the mutations, patients with disruptive mutations developed the disease at a significantly younger age than patients with at least one allele bearing a non-disruptive mutation. This finding is in keeping with what previously observed in FHL2, in which the median age at diagnosis (3 months) was slightly younger. [START_REF] Trizzino | Histiocyte Society HLH Study group. Genotype-phenotype study of familial haemophagocytic lymphohistiocytosis due to perforin mutations[END_REF] The age at diagnosis of patients with FHL2 or FHL3 having at least one missense mutation is comparable. Yet, when we focus on patients with biallelic disruptive mutations, those with FHL2 have a significantly younger median age at the diagnosis than those with FHL3 (2 vs. 3 months). This might suggest that absence of perforin induces a disruption of the cytotoxic machinery which is even more deleterious than the priming impairment induced by a Munc13-4 defect. The onset of FHL is a conditional disease, [START_REF] Crozat | Jinx, an MCMV susceptibility phenotype caused by disruption of Unc13d: a mouse model of type 3 familial hemophagocytic lymphohistiocytosis[END_REF] triggered by a viral pathogen. Since we have no evidence that patients with FHL3 had a later exposure to community acquired pathogens, this might support the hypothesis that patients with complete perforin defect behave like being fully unable to cope with any common pathogen, whereas the degranulation defect induced by defective Munc13-4 may have some residual/redundant function, allowing residual defensive activity, at least against selected pathogens. Indeed, analysis of cytotoxic activity of activated NK cells, revealed a more striking defect of FHL2 than FHL3 NK cells when tested against various tumor cell lines or EBV-infected cells. [START_REF] Marcenaro | Analysis of natural killer-cell function in familial Hemophagocytic lymphohistiocytosis (FHL): defective CD107a surface expression heralds Munc13-4 defect and discriminates between genetic subtypes of the disease[END_REF] However, FHL3 NK cells, also tested upon activation, displayed a significantly lower degranulation capacity and cytotoxic activity as compared to healthy controls (Fig. 4), while for other diseases (e.g. FHL4, FHL5 and GS2) the defects can be prevalently detected using resting NK cells. [START_REF] Bryceson | Defective cytotoxic lymphocyte degranulation in syntaxin-11 deficient familial hemophagocytic lymphohistiocytosis (FHL4) patients[END_REF][START_REF] Zur Stadt | Familial Hemophagocytic Lymphohistiocytosis Type 5 (FHL-5) Is Caused by Mutations in Munc18-2 and Impaired Binding to Syntaxin 11[END_REF] This allows to perform functional assays even using NK cells derived from patients studied many years ago, since these cells are polyclonal, activated, expanded in vitro and cryopreserved.

The age at the diagnosis of HLH has been reported to be usually consistent within individual families, although some exceptions are known. [START_REF] Aricò | Hemophagocytic lymphohistiocytosis. Report of 122 children from the International Registry. FHL Study Group of the Histiocyte Society[END_REF] Yet, in this series, two siblings were found to be homozygous for the c.1847A>G (p.E616G), a missense mutation which we previously documented to disrupt splicing and result in absence of the Munc13-4 protein. [START_REF] Santoro | Mutations affecting mRNA splicing are the most common molecular defect in patients with familial hemophagocytic lymphohistiocytosis type 3[END_REF] Both ultimately developed FHL3, although at a different age, i.e. 6 months and 18.8 years. This is the most striking case of age discrepancy at the diagnosis of FHL3 in this series (Figure 6). This observation further supports the hypothesis that an external trigger may be necessary to induce symptoms on the basis of a genetic predisposition.

Whether or not additional disease-modifier genes are involved, we are not able to document, at present. All the above should be taken into account when counseling asymptomatic family members with documented genetic defect.

The diagnostic criteria for HLH have been originally defined by the Histiocyte Society in 1994, and then revised in 2004. [START_REF] Henter | Diagnostic guidelines for hemophagocytic lymphohistiocytosis. The FHL Study Group of the Histiocyte Society[END_REF][START_REF] Henter | HLH-2004: Diagnostic and therapeutic guidelines for Hemophagocytic lymphohistiocytosis[END_REF] In the present series, the combination of 5 of the 8 items, i.e. the required standard for the diagnosis of HLH, was fulfilled in 69% of the patients. It was recently proposed that these criteria might be simplified. [START_REF] Filipovich | Hemophagocytic lymphohistiocytosis (HLH) and related disorders[END_REF] In the attempt to contribute to this debate, we have explored different combinations. The combination of fever, splenomegaly and thrombocytopenia, three clinical findings immediately available, was present in 84% of our patients. If, in addition, bone marrow aspiration, which is indicated to rule out the more frequent differential diagnosis "leukemia", demonstrates hemophagocytosis, the diagnosis is even more supported. In this series, the combination of these 4 findings was present in 69% of the patients evaluated also for bone marrow morphology. Alternatively, when the level of ferritin --an acute phase reactant widely available bedside -was investigated, it turned to be elevated in 71% of patients with fever, splenomegaly and thrombocytopenia. Thus, we suggest that the combination of fever, splenomegaly, and thrombocytopenia represents the initial clinical background to raise the suspicion of FHL3; when associated with evidence of increased ferritin level, these features may be considered as a very sensitive tool to address the diagnostic work-up already during the first few hours from admission. The sensitivity of this combination of clinical features for the diagnosis of other genotypic subsets remains to be evaluated.

It has been proposed that Munc13-4 deficiency may be associated with a higher rate of CNS manifestations. Although many proteins of the Munc family have specific roles in the central nervous system, Munc 13-4 is not expressed in the brain [START_REF] Ménasché | Primary hemophagocytic syndromes point to a direct link between lymphocyte cytotoxicity and homeostasis[END_REF] rendering a specific impact of the defective protein unlikely in the pathogenesis of CNS disease in FHL3. CNS involvement was found in 60% of the patients. While only one third of the cases had neurologic symptoms at the diagnosis, 69% had ≥5 cells in the CSF, 71% had elevated CSF protein, and 60% had MR alterations. These data suggest that a Munc13-4 defect induces a higher proportion of CNS involvement than a perforin defect, with only 36% of FHL2 patients showing CNS involvement. [START_REF] Trizzino | Histiocyte Society HLH Study group. Genotype-phenotype study of familial haemophagocytic lymphohistiocytosis due to perforin mutations[END_REF][START_REF] Horne | Frequency and spectrum of central nervous system involvement in 193 children with haemophagocytic lymphohistiocytosis[END_REF] UNC13D mutations are scattered over the entire gene, without any apparent clustering. This confirms that the strategy of analysis cannot rely on any preferential region(s). [START_REF] Cetica | Molecular basis of familial hemophagocytic lymphohistiocytosis[END_REF] A total of 54 different mutations were collected, suggesting that no founder effect is evident in FHL3. In contrast to FHL2, we did not observe any correlation between ethnic groups and specific mutations. It may be worth mentioning that the c.1596+1G>C mutation was reported to be the most common UNC13D mutation in Japan, accounting for 70% of the patients. [START_REF] Cetica | Molecular basis of familial hemophagocytic lymphohistiocytosis[END_REF] In addition, recently Yoon et al. reported that c.754-1G>C accounts for the majority of UNC13D mutations (58%) in Korean patients with FHL3. [START_REF] Yoon | UNC13D is the predominant causative gene with recurrent splicing mutations in Korean patients with familial hemophagocytic lymphohistiocytosis[END_REF] These mutations were not found in our series, which yet did not include any Japanese or Korean patient.In this series we report 15 novel mutations. Two mutations, c.2346_2349delGGAG and c.753+1G>T, accounted for 26 of the 69 unrelated families (38%). As already reported, 46% of patients in FHL3 have at least one mutation responsible for a splicing error, [START_REF] Santoro | Mutations affecting mRNA splicing are the most common molecular defect in patients with familial hemophagocytic lymphohistiocytosis type 3[END_REF] as expected because of the complex structure of UNC13D, with its high number of exons, which is a well-known predisposing factor for the occurrence of this type of aberrations.

Overall, one third of the mutations were missense, including 7 novel ones. Their pathogenic role may be questionable. The missense mutation c.1193C>T (p.S398L) was found at the homozygous state in a Caucasian patient who presented at the age of 70 months. Of the four patients with homozygous p.E616G mutations, three presented at 99, 140 and 226 months. We have previously proven that this mutation is associated with a splice error and protein defect. [START_REF] Santoro | Mutations affecting mRNA splicing are the most common molecular defect in patients with familial hemophagocytic lymphohistiocytosis type 3[END_REF] Missense mutations p.L647P, p.R608P and p.R928P were associated with reduced cytotoxic function, protein expression and later onset, suggesting a hypomorphic effect.

In patients with suspected FHL appropriate treatment should be started promptly, since about 20% of the patients may die within the first few weeks even despite specific treatment. Although mutation analysis remains the gold standard for diagnosis of FHL, results are not readily available. We have found defective degranulation in almost all of the evaluated FHL3 patients. We therefore recommend that patients with a robust clinical suspicion based on evidence of fever, splenomegaly, thrombocytopenia and haemophagocytosis, or high ferritin level, should have a functional screening including perforin expression and cytotoxic lymphocyte degranulation assays by flowcytometry [START_REF] Marcenaro | Analysis of natural killer-cell function in familial Hemophagocytic lymphohistiocytosis (FHL): defective CD107a surface expression heralds Munc13-4 defect and discriminates between genetic subtypes of the disease[END_REF][START_REF] Wheeler | A novel assay for investigation of suspected familial haemophagocytic lymphohistiocytosis[END_REF][START_REF] Kogawa | Perforin expression in cytotoxic lymphocytes from patients with hemophagocytic lymphohistiocytosis and their family members[END_REF] at national reference laboratories. All patients fulfilling the diagnostic pattern and with degranulation defect should be considered eligible for immediate empiric treatment, followed by directed mutation analysis to support indication to HSCT as a curative therapeutic program. Given the high frequency of mutations leading to splicing errors, the strategy of analysis must include the RNA study. a In the degranulation assays, CD107 expression was considered "absent" if below 5% or 10% in resting or activated NK cells, respectively; "reduced" if significantly (p<0.05) lower than controls. b In the cytotoxicity assays, NK activity was considered "absent" if below 5% or 30% lysis at the highest E:T ratio examined using either resting or activated NK cells, respectively; "reduced" if significantly (p<0.05) lower than controls.. c Munc13-4 was tested by western blot.

LEGEND TO FIGURES

. Distribution of the frequency of some diagnostic parameters for HLH and pattern of their association.

  not tolerated, was identified in a 70-month Caucasian girl (UPN 257), with reduced NK activity. The p.L647P missense mutation (c.1940T>C) falls in the MHD1 domain and is predicted to be damaging. It was found in a 168-month male (UPN 397) presenting with typical FHL, reduced CD107 expression and NK-activity. The p.R727Q missense mutation (c.2180G>A) falls outside any functional domain and is predicted to be tolerated by SIFT and benign by Polyphen (score 0.353). It was observed in a compound heterozygous patient with defective degranulation. The p.A859T missense mutation (c.2575G>A) falls in the MHD2 domain and is predicted to be not tolerated. The p.E1017K missense mutation (c.3049G>A) falls in the C2B domain and is predicted to be not tolerated. The p.L1058P missense mutation (c.3173T>C) falls within the C-terminal domain and is predicted to be damaging. None of the missense mutations resulted in cryptic splice sites according to NNsplice software. Analysis of control subjects None of the following novel mutations were found in 100 healthy Caucasian control subjects: c.403insC, c.419T>C, c.1193C>T, c.1387C>T, c.1822del12, c.1940T>C, c.2057C>A, c.2180G>A, c.2212C>T, c.2437_2439delAACinsT, c.2477_2480delTCAC, c.2575G>A, c.3049G>A, c.3082delC, c.3173T>C.

Figure 2 .

 2 Figure 2. Details of 54 different UNC13D mutations observed in 84 patients with FHL3 from 69 families. Mutations are spread over the entire gene. Disruptive mutations are indicated in bold.

Figure 3 .

 3 Figure 3. Age at onset of FHL3 depends on the functional impact of the UNC13D mutations. Patients with two disruptive mutations had a significantly younger age at diagnosis than patients with two missense mutations. Each point represents one person, lines indicate mean values.

Figure 4 .

 4 Figure 4. Impaired degranulation and cytotoxicity by activated NK cells from FHL3 patients. Polyclonal activated NK cell populations derived from healthy adult and infant donors, and from FHL3 patients, considered separately as group 1+2 and group 3, were analysed for surface expression of CD107a after stimulation with K562 (data are expressed as % ΔCD107a, panel A) and for cytolytic activity against 721.221 (data are expressed as LU at 30% lysis, panel B). Each point represents one person, lines indicate mean values. *P<0.05; **P<0.001; ***P<0.001; n.s., not significant.

Figure 5 .

 5 Figure 5. Age at onset of FHL in patients with two disruptive mutations is significantly younger in patients with FHL2 than in patients with FHL3. Each point represents one person, lines indicate mean values.

Figure 6 .

 6 Figure 6. Comparison of age at onset in 14 families with FHL3 and at least two affected siblings.

Table 1 .

 1 Presenting features in 84 patients with familial hemophagocytic lymphohistiocytosis and UNC13D mutations

	Number / total (%)

Table 2 .

 2 Main clinical features and initial laboratory evaluation of 42 patients with familial hemophagocytic lymphohistiocytosis type 3 and homozygous mutations

	Mutation	UPN	E t h n i c g r o u p	A g e a t d i a g n o s i s	( m o n t h s )

Table 3 .

 3 Main clinical features and initial laboratory evaluation of 11 patients familial hemophagocytic lymphohistiocytosis with compound heterozygous novel mutations

	Allele 1 Mutation	Allele 2 Mutation	UPN	Ethnic group	Age at diagnosis	(months)	Fever	Splenomegaly	CNS disease	Anemia	Neutropenia	Thrombocytopenia	Hyperferritinemia	Hypertrigliceridemia	Hypofibrinogenemia	a	C D 1 0 7 e x p r e s s i o n	b	N K a c t i v i t y	M U N C 1 3 -4	c	e x p r e s s i o n
	c.403insC	c.627delT
	(p.Y135Fs)	(p.T209Fs)	481	Caucasian 28	+	+	+	+	+	+	+	+	-Reduced Reduced	NA
	c.419T>C	c.2782C>T
	(p.l140T)	(p.R928C)	349	Hispanic	106 +	+	-	+	+ NA +	+ NA	NA	NA	NA
	c.1387C>T
	(p.Q463X)	c.753+1G>T	183	Caucasian 3	+	+	-	+	-	+ NA NA NA	NA	NA	NA
	c.1387C>T
	(p.Q463X)	c.753+1G>T	182	Caucasian 1	+	+	-	+	-	+ NA NA NA	NA	NA	NA
	c.2180G>A	c.175G>A
	(p.R727Q)	(p.A59T)	483	Caucasian 1	-	-	-	-	+	+	-	-	+ Reduced	NA	NA
	c.2212C>T	c.3173T>C	GE
	(p.Q738X)	(p.L1058P)	268	Caucasian 6	+	+	-	+	+	+	+	+	-Reduced Reduced	NA
	c.2437-	c.2346-
	2439delAACinsT	2349delGGAG
	(p.N813Fs)	(p.R782Fs)	472	Caucasian 1	+	+	-	-	-	+	+	+ NA	NA	NA	NA
	c.2477-	c.2346-
	2480delTCAC	2349delGGAG
	(p.L826Fs)	(p.R782Fs)	532	NA	3 NA NA NA NA NA NA NA NA NA	NA	NA	NA
	c.2575G>A	c.1225delC
	(p.A859T)	(p.L409Fs)	245* Caucasian 96	-	-	+	+	-	+ NA NA NA Absent	Absent	NA
	c.3049G>A	c.2782C>T
	(p.E1017K)	(p.R928C)	208	Caucasian 58	+	+	NA	+	-	+	+	+	+	NA	Absent	NA
	c.3082delC	c.2782C>T
	(p.L1028Fs)	(p.R928C)	277	Caucasian 119 +	+	-	-	+	+	+ NA NA Reduced Absent Absent

NA: Not Available. Novel mutations are in bold. * This patient also carried the c.2782C>T (p.R928C) mutation.

ACKNOWLEDGEMENTS

This work was partly supported by the following sources: European Union 7 th Framework Program under Grant agreement No. HEALTH-F2-2008-201461; "Antonio Pinzino -Associazione per la Ricerca sulle Sindromi Emofagocitiche (ARSE); "Noi per Voi per il Meyer Onlus"; Italian Ministry of Health, Bando "Malattie Rare 2008"; A.O.U. Meyer; Swedish Children's Cancer Foundation, the Swedish Research Council, the Cancer and Allergy Foundation of Sweden, the Swedish Cancer Foundation, the Karolinska Institutet, and the Stockholm County Council.