Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory disorder. HLH can be considered as a threshold disease depending on the trigger and the residual NK-cell cytotoxicity. In this study, we analyzed the molecular and functional impact of a novel monoallelic mutation found in a patient with two episodes of HLH. A 9-month-old child was diagnosed at 2 months of age with cutaneous Langerhans cell histiocytosis (LCH). After successful treatment, the patient developed an HLH episode. At 16 month of age, the patient went through an HSCT losing the engraftment 5 months later concomitant with an HLH relapse. The genetic study revealed a monoallelic mutation in the STXBP2 gene (.pArg190Cys). We transfected COS7 cells to analyze the STXBP2-R190C expression and to test the interaction with STX11. We used the RBL-2H3 cell line expressing STXBP2-WT-EGFP or R190C-EGFP for degranulation assays. Mutation STXBP2-R190C did not affect protein expression or interaction with syntaxin-11. However, we have demonstrated that STXBP2-R190C mutation diminishes degranulation in the RBL-2H3 cell line compared with the RBL-2H3 cell line transfected with STXBP2-WT or nontransfected. These results suggest that STXBP2-R190C mutation acts as a modifier of the degranulation process producing a decrease in degranulation. Therefore, under homeostatic conditions, the presence of one copy of STXBP2-R190 could generate sufficient degranulation capacity. However, it is likely that early in life when adaptive immune system functions are not sufficiently developed, an infection may not be resolved with this genetic background, leading to a hyperinflammation syndrome and eventually develop HLH. This analysis highlights the need for functional testing of new mutations to validate their role in genetic susceptibility and to establish the best possible treatment for these patients.

噬血细胞性淋巴组织细胞增生症 (HLH) 是一种危及生命的炎症性疾病。根据触发因素和残留的 NK 细胞细胞毒性，HLH 可被视为阈值疾病。在这项研究中，我们分析了在 HLH 两次发作的患者中发现的新型单等位基因突变的分子和功能影响。一名 9 个月大的儿童在 2 个月大时被诊断出患有皮肤朗格汉斯细胞组织细胞增生症 (LCH)。成功治疗后，患者出现 HLH 发作。在 16 个月大时，患者进行了 HSCT，5 个月后移植失败，同时 HLH 复发。遗传研究揭示了 STXBP2 基因 (.pArg190Cys) 中的单等位基因突变。我们转染 COS7 细胞以分析 STXBP2-R190C 表达并测试与 STX11 的相互作用。我们使用表达 STXBP2-WT-EGFP 或 R190C-EGFP 的 RBL-2H3 细胞系进行脱粒测定。突变 STXBP2-R190C 不影响蛋白质表达或与 Syntaxin-11 的相互作用。然而，我们已经证明，与用 STXBP2-WT 转染或未转染的 RBL-2H3 细胞系相比，STXBP2-R190C 突变减少了 RBL-2H3 细胞系中的脱颗粒。这些结果表明 STXBP2-R190C 突变作为脱粒过程的调节剂，导致脱粒减少。因此，在稳态条件下，一份 STXBP2-R190 的存在可以产生足够的脱粒能力。然而，很可能在生命早期，当适应性免疫系统功能未充分发育时，这种遗传背景可能无法解决感染问题，导致过度炎症综合征并最终发展为 HLH。该分析强调需要对新突变进行功能测试，以验证它们在遗传易感性中的作用，并为这些患者建立可能的最佳治疗方法。