STXBP2-R190C Variant in a Patient With Neonatal Hemophagocytic Lymphohistiocytosis (HLH) and G6PD Deficiency Reveals a Critical Role of STXBP2 Domain 2 on Granule Exocytosis

Abstract

Neonatal hemophagocytic lymphohistiocytosis (HLH) is a medical emergency that can be associated with significant morbidity and mortality. Often these patients present with familial HLH (f-HLH), which is caused by gene mutations interfering with the cytolytic pathway of cytotoxic T-lymphocytes (CTLs) and natural killer cells. Here we describe a male newborn who met the HLH diagnostic criteria, presented with profound cholestasis, and carried a maternally inherited heterozygous mutation in syntaxin-binding protein-2 [ STXBP2 , c.568C>T (p.Arg190Cys)] in addition to a severe pathogenic variant in glucose 6-phosphate dehydrogenase [ G6PD , hemizygous c.1153T>C (Cys385Arg)]. Although mutations in STXBP2 gene are associated with f-HLH type 5, the clinical and biological relevance of the p.Arg190Cys mutation identified in this patient was uncertain. To assess its role in disease pathogenesis, we performed functional assays and biochemical and microscopic studies. We found that p.Arg190Cys mutation did not alter the expression or subcellular localization of STXBP2 or STX11, neither impaired the STXBP2/STX11 interaction. In contrast, forced expression of the mutated protein into normal CTLs strongly inhibited degranulation and reduced the cytolytic activity outcompeting the effect of endogenous wild-type STXBP2. Interestingly, arginine 190 is located in a structurally conserved region of STXBP2 where other f-HLH-5 mutations have been identified. Collectively, data strongly suggest that STXBP2-R190C is a deleterious variant that may act in a dominant-negative manner by probably stabilizing non-productive interactions between STXBP2/STX11 complex and other still unknown factors such as the membrane surface or Munc13-4 protein and thus impairing the release of cytolytic granules. In addition to the contribution of STXBP2-R190C to f-HLH, the accompanied G6PD mutation may have compounded the clinical symptoms; however, the extent by which G6PD deficiency has contributed to HLH in our patient remains unclear.

Case Report: Characterizing the Role of the STXBP2-R190C Monoallelic Mutation Found in a Patient With Hemophagocytic Syndrome and Langerhans Cell Histiocytosis

“…The significant differences in degranulation between the mother and the patient and the fact that the mother has not experienced any HLH/MAS events during her lifetime led to the speculation that additional factors (e.g., age at episode onset, trigger, genetic background, and environment) are involved in the development of HLH. This is consistent with other publications (1,5,9) in which relatives of patient cases who were carriers of monoallelic variants had not yet manifested with HLH despite reduction in NK cell function.…”

“…Thus, we speculate that the interaction of the STXBP2/ STX11 complex (not affected by the STXBP2-R190C mutation) may not be adequate when a higher demand is required (e.g., viral infections, LCH). This hypothesis is in line with the descriptions of a possible novel binding mode of the domain 2 of the STXBP2 complex with other factors that could be truncated by the mutation (5).…”

Secondary Leukemia in a Patient With EBV-HLH Carrying Heterozygous STXBP2 Variant

“…However, some studies suggested heterozygous variants of genes in cytolytic pathway impaired degranulation and cytotoxic activity by arresting soluble N-ethylmaleimidesensitive factor attachment protein receptor mediated membrane fusion. 14,15 Consistently, heterozygous variants of genes in cytolytic pathway also have been suggested in malignancy. Chang et al 16 reported 1 patient with a heterozygous UNC13D variant who progressed to acute monoblastic leukemia with an MLL-AF9 fusion gene, without any use of known leukemogenic agents.…”

Familial hemophagocytic lymphohistiocytosis in a neonate

“…[8,9] Here, we reviewed 9 cases of neonate FHL (onset within 28 days after birth) (Table 2). [10][11][12][13][14][15][16][17][18] This table shows the clinical and laboratory features of 9 infants. These cases were all identified as FHL because of positive family history or the discovery of related gene mutations.…”

“…Benavides et al (2020) [18] 1DOL hepatosplenomegaly Hyperbilirubinemia, thrombocytopenia, anemia, high ferritin, elevated sIL2r, elevated liver enzymes, degranulation defect of NK cells the disease was in remission. However, the patient presented again with fever, pulmonary pneumocystis, elevated IFN-g and IL-10, which resulted from the re-activation of HLH (Table 1).…”