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
Authors: Nathalia Benavides, Waldo A Spessott, Maria L Sanmillan, Marcelo Vargas, Mylynda S Livingston, Nissa Erickson, Tamara C Pozos, Margaret E McCormick, Emilia Scharrig, Yoav H Messinger, Claudio G Giraudo
Journal: Front Immunol (2020)
Keywords: g6pd deficiency, snares, stxbp2, cytotoxic t-lymphocytes (ctls), familial hemophagocytic lymphohistiocytosis−5 (f-hlh), lytic granule exocytosis, natural killer cells
MeSH terms: Exocytosis / genetics, Glucosephosphate Dehydrogenase Deficiency / diagnosis, Glucosephosphate Dehydrogenase Deficiency / genetics, Lymphohistiocytosis, Hemophagocytic / diagnosis, Lymphohistiocytosis, Hemophagocytic / genetics, Munc18 Proteins / genetics, Mutation, Case Reports, Research Support, N.I.H., Extramural, Alleles, Amino Acid Sequence, Amino Acid Substitution, Apoptosis / genetics, Apoptosis / immunology, Biomarkers, Cytotoxicity, Immunologic, Disease Susceptibility, Gene Expression, Genetic Association Studies, Glucosephosphate Dehydrogenase Deficiency / complications, Humans, Infant, Newborn, Killer Cells, Natural / immunology, Killer Cells, Natural / metabolism, Lymphohistiocytosis, Hemophagocytic / complications, Male, Membrane Proteins / genetics, Membrane Proteins / metabolism, Models, Molecular, Munc18 Proteins / chemistry, Munc18 Proteins / metabolism, Protein Conformation, Qa-SNARE Proteins / genetics, Qa-SNARE Proteins / metabolism, Structure-Activity Relationship, T-Lymphocytes, Cytotoxic / immunology, T-Lymphocytes, Cytotoxic / metabolism, Biomarkers, Membrane Proteins, Munc18 Proteins, Qa-SNARE Proteins, STX11 protein, human, STXBP2 protein, human, UNC13D protein, human

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.