HOMOZYGOUS DELETION IN STXBP2 CAUSATIVE OF FAMILIAL HEMOPHAGOCYTIC LYMPHOHISTIOCYTOSIS TYPE 5 (FHL-5)

Purpose: Hemophagocytic lymphohistiocytosis (HLH) is a rare, life-threatening immunedeficiency, characterized by a hyperinflammatory syndrome. Familial forms are caused by mutations in genes associated with lymphocyte granule-mediated cytotoxicity. Four subtypes are defined by mutations in the following genes: PRF1 in FHL2, UNC13D in FHL3, STX11 in FHL4, and STXBP2 in FHL5. STXBP2 codes for Munc18-2 protein which is involved in regulation of SNARE-mediated membrane fusion events. FHL5 has been reported to account for up to 20% of cases with FHL in the German series. Since 2010, 47 different mutations of STXBP2 have been described, mainly missense/nonsense, frameshift and splicing. Here we describe a large deletion causative of FHL5. Methods: A female baby, aged 47 days, second child from related parents of Egyptian origin, developed a clinical syndrome fulfilling the Histiocyte Society diagnostic criteria for HLH and was directed to functional study and molecular analysis of FHL related genes. Results: Flowcytometry analysis showed normal perforin expression but almost undetectable degranulation capacity. Direct sequencing showed no mutations in UNC13D and STX11, while the analysis of STXBP2 gene showed no amplification of the last three exons. To test the hypothesis of an homozygous deletion, we used Syber Green RT-PCR, which revealed the loss of the genomic region. The deletion was confirmed by CGH-array. Western blot analysis documented the absence of expression of STXBP2 protein in this patient. Conclusion: Identification of biallelic mutations in children with a clinical diagnosis of HLH is of paramount clinical relevance as it confirms the diagnosis of FHL, thus giving the opportunity to bring the patient to early hematopoietic stem cell transplantation. The finding of a large homozygous deletion in STXBP2 gene extends the spectrum of possible causative mutations in FHL-5 and requires adaptation of the strategy of analysis to identify complex defects.