Munc18-2 deficiency causes familial hemophagocytic lymphohistiocytosis type 5 and impairs cytotoxic granule exocytosis in patient NK cells

Abstract

Familial hemophagocytic lymphohistiocytosis (FHL) is a genetically heterogeneous autosomal recessiveimmune disorder characterized by the occurrence of uncontrolled activation of lymphocytes and macrophages infiltrating multiple organs. Disease-causing mutations in the perforin (PRF1; also known as FHL2), Munc13-4 (UNC13D; also known as FHL3), and syntaxin-11 (STX11; also known as FHL4) genes have been identified in individuals with FHL. These genes all encode proteins involved in the cytotoxic activity of lymphocytes. Here, we show that the gene encoding syntaxin-binding protein 2 (Munc18-2; official gene symbol STXBP2) is mutated in another subset of patients with FHL (designated by us as "FHL5"). Lymphoblasts isolated from these patients had strongly decreased STXBP2 protein expression, and NK cells exhibited impaired cytotoxic granule exocytosis, a defect that could be overcome by ectopic expression of wild-type STXBP2. Furthermore, we provide evidence that syntaxin-11 is the main partner of STXBP2 in lymphocytes, as its expression required the presence of STXBP2. Our work shows that STXBP2 deficiency causes FHL5. These data indicate that STXBP2 is required at a late step of the secretory pathway for the release of cytotoxic granules by binding syntaxin 11, another component of the intracellular membrane fusion machinery.

Syntaxin 11 is required for NK and CD8+ T‐cell cytotoxicity and neutrophil degranulation

“…This indeed is not altered upon STX11 deficiency, and our data are perfectly in line with the findings reported from STX11-deficient patients [15].…”

“…Recently two groups have shown a direct interaction of STX11 and MUNC18-2 in regulating granule exocytosis, by demonstrating that FHL type 5 is caused by mutations in MUNC18-2 (STXBP2 encoding syntaxin binding protein 2) that affect its binding to STX11, mutated in FHL-4 [15,16]. MUNC18 proteins are involved in the regulation of intracellular trafficking and in the control of SNARE complex assembly and disassembly [17].…”

Perforin deficiency and susceptibility to cancer

“…66 Consistent with the role of SNAREs in subcellular trafficking, unstimulated NKs from FHL4 and FHL5 patients had no cytotoxic activity and were unable to degranulate. 64,65 However, IL-2 stimulation considerably restored cytotoxicity. 66 These observations are significant as they are the first to implicate NK cell deficiency as a specific trigger for FHL, although the mechanism is not fully understood.…”

“…Recently, mutations in the STX11 gene encoding a t-SNARE protein, syntaxin 11 (STX11), and the partner protein Munc18-2 (STXBP2) 64,65 have also been associated with type 4 and 5 FHLs, respectively. Although FHL2 and FHL3 phenotypes are easily identifiable through the loss of IL-2-stimulated peripheral blood mononuclear cell (LAK) cytotoxicity, FHL4 and FHL5 patients display a differential phenotype, in which only NK cells appear to be functionally impaired.…”

Reconciling the regulatory role of Munc18 proteins in SNARE-complex assembly

“…However, a recent NMR study on full length Sx1a, including its TMD, reported a higher proportion of the open Sx1a conformation than was observed using the soluble cytosolic domain of Sx1a (Dawidowski & Cafiso, 2013 Although its precise role remains controversial, the importance of SM proteins in disease has been well documented. For example, mutations in Munc18a have been linked with early infantile epileptic encephalopathy (Saitsu et al, 2008(Saitsu et al, , 2010 and Munc18b mutations can result in familial hemophagocytic lymphohistiocytosis type 5 (Cô te et al, 2009;Hackmann et al, 2013). This link between SM proteins and disease has also been confirmed by mutagenesis studies in mice, where deletion of Munc18-1 in mice leads to a complete loss of neurotransmitter secretion from synaptic vesicles throughout development (Verhage et al, 2000).…”