Investigating single-gene disorders of childhood infectious disease

A common feature of infectious diseases, including an infection with Neisseria meningitidis (Nm), is that only a small proportion of the individuals exposed to the same strain of the bacteria suffer from the clinical disease. Host genetics has long been considered to be an important determinant of both predisposition to and severity of outcome from invasive meningococcal disease (IMD). The human complement system is central to protection against IMD. It is well established that individuals with terminal or alternative complement deficiencies are predisposed to invasive, often recurrent meningococcal infections. However, the occurrence of these putative genetic deficiencies is rare, such that complement deficiencies account for less than 3 % of the disease cases. The current study sought to uncover novel genetic aetiologies of IMD, by employing WES and GWAS, in conjunction with molecular functional characterisation assays. Firstly, genetic analysis of six familial IMD exomes revealed a novel mutation in the SPLUNC1 gene. The encoding protein has been shown to play an important role in innate immune defence against a number of Gram-negative bacterial infections. The characterisation assays undertaken in this work suggest that the protein encoded by SPLUNC1 is also implicated with host innate immune defence against Nm infection, by providing protection against nasopharyngeal colonisation of a pathogenic Nm strain. The results further suggest that harbouring rare pathogenic mutations that impact the function of the encoding protein is associated with reduced host defence activities in the resulting protein, which in turn may possibly lead to increased susceptibility to IMD in the carriers. Furthermore, a large-scale GWAS was performed to define common polymorphisms underlying host susceptibility and severity of IMD, using 1236 individuals with confirmed disease and over 5000 controls. In this work, efforts were made to understand the biological plausibility of the genetic associations identified through the GWAS analysis.Open Acces

A Rare Mutation in SPLUNC1 Affects Bacterial Adherence and Invasion in Meningococcal Disease

BACKGROUND Neisseria meningitidis (Nm) is a nasopharyngeal commensal carried by healthy individuals. However, invasive infections occurs in a minority of individuals, with devastating consequences. There is evidence that common polymorphisms are associated with invasive meningococcal disease (IMD), but the contributions of rare variants other than those in the complement system have not been determined. METHODS We identified familial cases of IMD in the UK meningococcal disease study and the European Union Life-Threatening Infectious Disease Study. Candidate genetic variants were identified by whole-exome sequencing of 2 patients with familial IMD. Candidate variants were further validated by in vitro assays. RESULTS Exomes of 2 siblings with IMD identified a novel heterozygous missense mutation in BPIFA1/SPLUNC1. Sequencing of 186 other nonfamilial cases identified another unrelated IMD patient with the same mutation. SPLUNC1 is an innate immune defense protein expressed in the nasopharyngeal epithelia; however, its role in invasive infections is unknown. In vitro assays demonstrated that recombinant SPLUNC1 protein inhibits biofilm formation by Nm, and impedes Nm adhesion and invasion of human airway cells. The dominant negative mutant recombinant SPLUNC1 (p.G22E) showed reduced antibiofilm activity, increased meningococcal adhesion, and increased invasion of cells, compared with wild-type SPLUNC1. CONCLUSIONS A mutation in SPLUNC1 affecting mucosal attachment, biofilm formation, and invasion of mucosal epithelial cells is a new genetic cause of meningococcal disease