A Detrimental NFKB2 Missense Variant is Associated with Hypogammaglobulinemia.

NFKB2 encodes the precursor p100 which undergoes processing to generate the mature NF-κB2 transcription factor subunit p52. Most of the known pathogenic NFKB2 variants render p100 un-processable and are typically linked to immunodeficiency disorders with antibody deficiency, susceptibility to infections and often autoimmunity. We describe a heterozygous germline NFKB2 missense variant (c.781C>T; R261W) associated with antibody deficiency and recurrent respiratory tract infections in a German family with incomplete penetrance. Patient-derived cells had reduced p52 levels, indicating protein insufficiency as the primary defect. Characterization of the R261W variant in vitro employing overexpression of EGFP-fused wildtype and mutant p100/p52 in HEK293T cells revealed restricted levels of mutant p100, suggesting unsustainable protein expression while maintenance of mutant p52 was almost precluded, confirming a detrimental protein defect. Enforced p100-processing driven by constitutively active NF-κB inducing kinase (NIK) caused subnuclear aggregation of mutant EGFP-p52 while DNA-binding activity was undetectable. Although ectopically over-expressed EGFP-p52-R261W also formed intra-nuclear aggregates - a previously established diagnostic indicator of protein-decaying NFKB1 mutations - its DNA-binding ability was not abolished. In summary, the single amino acid substitution R261W in the N-terminal Rel-homology domain of p100/p52 causes protein loss, particularly of the mature subunit p52. This genetic condition is associated with an inheritable form of hypogammaglobulinemia with mild clinical symptoms. We therefore recommend genetic screening for NFKB2 loss-of-expression variants in mildly affected patients with recurrent respiratory tract infections.