UNIQUE MONOCYTE EXPRESSION SIGNATURE IN A STXBP2 MUTATION CAUSING INFANTILE ONSET VEO-IBD AND HLH

Abstract

Abstract Mutations in syntaxin-binding-protein-2 (STXBP2) can result in NK cell dysfunction leading to a hyperinflammatory state including familial HLH and infantile onset inflammatory bowel disease (IBD). While the function of STXBP2 in NK cells and cytotoxic T cells has been well described, it is highly expressed in monocytes with unclear relevance. We report the case of an infant who presented with severe infantile onset diarrhea with an identified pathogenic mutation in STXBP2. We sought to utilize scRNA-Seq to study the impact of STXBP2 deficiency on monocyte transcription, with the goal of generating insight into the pathogenesis of disease. METHODS: Trio whole exome sequencing was performed. PBMCs were collected from the described patient (case), and two control groups: 3 patients with VEO-IBD, and 3 age matched healthy controls. Single cell RNA-Seq was performed using the 10x platform and data analysis was done using Seurat in R. RESULTS: A male infant presented at 1 week of age with diarrhea and metabolic acidosis. Endoscopic evaluation at 4 weeks of age showed histologic findings of colitis and marked epithelial cell changes, including cell rounding. WES identified a homozygous pathogenic mutation in STXBP2. Due to significant risk of HLH, he underwent evaluation for stem cell transplant. However, in the interim he developed severe HLH requiring ICU level care. He recovered following aggressive treatment and underwent HSCT at 4 months of age and retransplant for graft failure at 11 months of age. He is currently in endoscopic remission off therapy. We analyzed PBMCs from the case patient using single cell RNA-Seq, as compared to patients with VEO-IBD and age-matched controls. STXBP2 expression was most enriched in monocyte populations and reduced in the case patient compared to controls. 231 and 187 genes were differentially expressed in the case monocytes as compared to both controls. The case patient had a paucity of non-classical monocytes. Within the classical monocytes, differentially expressed genes showed enrichment of host defense, cytokine processes, and apoptotic processes. Within the intermediate monocytes, differentially expressed genes showed enrichment of chemotaxis pathways, antigen presentation and T cell activation processes. CONCLUSIONS: Mutations in STXBP2 are a life-threatening cause of fHLH and infantile onset IBD. We report a case of infantile onset IBD due to a homozygous mutation in STXBP2 with resolution of disease post HSCT. Transcriptional analysis of PBMCs from this patient reveals a paucity of non-classical monocytes in the peripheral blood, and differential regulation of genes implicated in host defenses, antigen presentation, chemotaxis and apoptosis. These results may suggest the role of monocyte function in pathogenesis of STXBP2 and in other forms of IBD.