Identifying Possible Candidate Factors Influencing the Penetrance of Heterozygous NFKB1 Loss of Function Mutations By Whole Exome Sequencing

Introduction

The NFκB signaling pathway is a master regulator of immune and inflammatory responses. Recently we and other groups reported heterozygous NFKB1 loss-of-function mutations in patients with combined variable immunodeficiency (CVID) characterized by recurrent infections, autoimmunity and immunoglobulin deficiency. Pedigree analysis revealed incomplete penetrance of the disease causing mutation in 5 of the 6 analyzed families. While patients showed a severe phenotype including hypogammaglobulinemia, chronic infections and cytopenias, other carriers of the same mutation were unaffected except for slightly perturbed immunoglobulin levels indicating the existence of other factors influencing the penetrance of these mutations.

Methods

To identify genetic factors associated with complete penetrance of dominant NFKB1 mutations, whole exome sequencing was carried out using DNA extracted from blood samples derived from two patients and their families. Sequencing data of two patients and X unaffected carriers of the same NFKB1 mutations (p.R157X and p.I47fsX2) were then screened in silico for single nucleotide variations, small insertions and deletions present in modulators of immune responses in general and the NFκB pathway in particular, employing lists generated based on publicly available data on gene interactions (including e.g. data of the KEGG, and STRING databases).

Results

We detected no deleterious mutations in known modifier genes such as IL10, IL1B, IL6, CCR5, CCL5, RANTES, TGFB1 and others. But strikingly both patients harbored two polymorphisms (g.797C>A, Gly54Asp, Gly57Glu) in the Mannose Binding Lectin 2 (MBL2) gene that were previously reported as disease causing mutations in patients with primary immunodeficiency. These polymorphisms lead to reduced MBL2 expression and are linked with high susceptibility to infections. We hypothesize that low MBL2 expression in an NFKB1 haploinsufficient background may promote disease penetrance or increase the predisposition to infections.

Conclusion

Our combined next-generation sequencing and bioinformatics analyses approach identified MBL2 as an interesting candidate factor whose deficient expression may influence the penetrance of NFKB1 loss-of-function mutations. Further analysis of greater cohorts is needed to reinforce the role of MBL2 in the pathogenesis of NFKB1 haploinsufficiency.