Genetic predisposition to immune cell malfunction and inflammation

Abstract: Our genome contains all the instructions to control the synthesis of proteins, the development of cells and the interaction between body systems. We are armed with an immune system, made up of cells, chemical compounds and physical barriers, that protects us from invading pathogens. The fields of human genetics and human immunology noticeably intertwin with each other when faults in one system are phenotypically manifested as diseases in the other one. Primary immunodeficiencies represent a diverse group of genetically determined disorders characterized by functional defects in the immune system that display a wide range of clinical and immunological phenotypes. Among these phenotypes, hemophagocytic lymphohistiocytosis (HLH) involves a group of hyperinflammatory illnesses characterized by life-threatening, toxic activation of the immune system caused by mutations in diverse genes that impact the killing capacity of cytotoxic lymphocytes. In this thesis, we have investigated and characterized novel gene variants associated with HLH or hyperinflammatory phenotypes in case reports (Papers I, II, IV, V and VI) and as well as in one comparative study (III). In Paper I, we identified a novel hemizygous SH2D1A variant in a 21-yearold man that succumbed to Epstein-Barr virus (EBV)-HLH/lymphoma diagnosis. Interestingly, the patient had three healthy siblings carrying that variant, suggesting incomplete penetrance for this variant in X-linked lymphoproliferative disease type-1. In Paper II, a patient with a disease-causing SH2D1A variant displayed central nervous system damage without ongoing EBV infection, supporting the notion that EBV might be dispensable for the observed central nervous vasculitis. In Paper III, we compared two UNC13D isoforms with distinct transcription start sites that presented different N-terminal domains and we confirmed that although their expression was differentially regulated, they were equally recruited to the immune synapse and similarly contributed to cytotoxicity. In Paper IV, we uncovered a novel homozygous RAB27A variant identified in a patient with adult-onset HLH. Interestingly, this study presents the first RAB27A variant with enhanced binding to Munc13-4, a key player of cytotoxicity. In Paper V, we identified two novel MVK variants in a newborn manifesting systemic inflammation and exaggerated mevalonate accumulation. In this case, we established an association between severe MVK deficiency and HLH phenotype. In Paper VI, we reported a novel homozygous ISG15 variant in a patient with viral susceptibility phenotype. Remarkably, ISG15 deficiency has been previously associated with mycobacterial susceptibility and excessive type I interferon responses, but not with viral immunity. Thus, our study underscores the role of ISGylation for viral protection in the human setting. Overall, these studies expand our understanding of HLH and related hyperinflammatory disorders at the genetic level, since new gene variants and new associations were identified; at the protein and pathway interaction level, due to novel protein properties described, and at the phenotypic level since both uncommon and new manifestations were reported.