Unravelling the genetic cause of life-threatening infections in children

Despite the progress of the last century in living conditions and medical care, some previously healthy children are still affected by lethal or potentially lethal infections with pathogens that only cause mild illness in most people. Hypothesizing that monogenic defects of immunity underlie the unusual infectious phenotype in these children, we apply an unbiased diagnostic approach with next generation sequencing to identify possible causes of immunodeficiency and proceed to analyze the identified mutations to characterize their pathophysiological mechanism. First of all, we study a patient from a consanguineous family who suffered from disseminated vaccine-strain measles and another patient with viscerotropic vaccine-strain yellow fever. Whole exome sequencing (WES) identifies previously not reported biallelic variants in the gene encoding the type I interferon receptor 1 (IFNAR1), involved in anti-viral innate and intracellular responses. A complete functional analysis of the variants is performed and demonstrates their pathogenicity at the gene, protein and cellular level. In particular, we show that patients' fibroblasts are highly susceptible to viruses, including the vaccine-strain measles and yellow fever. Therefore, autosomal recessive complete IFNAR1 deficiency results in life-threatening complications of live attenuated viral vaccinations in previously healthy children, but apparently not other severe infections. This study provides new insight into the specific function and potential redundancy of a pathway of innate immunity and highlights the fact that a severe/unusual course of infection in an otherwise healthy child should always be considered as a potential immunodeficiency. Subsequently, we expand our current knowledge of STAT2 deficiency, another autosomal recessive disorder of the type I interferon response. The affected patients present with a characteristic, although not fully penetrant, risk of disseminated disease and encephalitis after inoculation with live measles-mumps-rubella (MMR) vaccine. In contrast with IFNAR1 deficiency, they also seem to be susceptible to other viral illnesses and show a more severe phenotype, with a mortality of 19% in childhood. Analyzing sixteen patients from seven unrelated kindreds with STAT2 deficiency, we find a broader infectious and non-infectious phenotype than previously described, centered on viral susceptibility. Moreover, the functional assessment of the different mutations hints at a possible incomplete form of STAT2 deficiency, where type I interferon responses are partially preserved in vitro. As with other innate immunity defects, once patients reach adulthood they seem to be protected from severe infections. Finally, we tackle another inborn error of immunity presenting with a broader phenotype than previously described. A young woman manifesting intellectual and growth delay, dysmorphisms, macrothrombocytopenia, camptodactyly, and structural brain abnormalities, suffered from immunodeficiency, severe lung infections, immune dysregulation and systemic inflammation. WES identified a de novo mutation in CDC42, causing Takenouchi-Kosaki syndrome. The extended immunophenotyping of the patient showed B cell lymphopenia with increased naïve B cells, decreased naïve T cells, and a global defect of CD8+ T cell activation. Most interestingly, the patient had signs of immune dysregulation and autoinflammation, including myelofibrosis and upregulation of inflammatory cytokines in her blood. These findings expand the phenotypic spectrum of Takenouchi-Kosaki syndrome and link autoinflammation with other CDC42 mutations than those in the C-terminal domain, recently identified as a cause of neonatal inflammation and hemophagocytic lymphohistiocytosis.

Publication year:2020

Accessibility:Embargoed