Combined immunodeficiency caused by a novel homozygous NFKB1 mutation

Background

Genetic faults in several components of the nuclear factor-κB pathway cause immunodeficiency. Most defects lead to combined immunodeficiency with a range of severity. Heterozygous mutations in NFKB1 were associated with common variable immunodeficiency, however, homozygous mutations have not been described.

Objective

We studied the molecular basis of combined immunodeficiency in a patient who presented with failure to thrive, persistent EBV viremia and hepatitis, pneumocystis jirovecii pneumonitis, and generalized lymphadenopathy.

Methods

Whole genome and exome sequencing followed by Sanger confirmation were performed to identify the genetic defect. Molecular and cellular techniques were used to assess the variant impact on the nuclear factor-κB pathway and lymphocyte function.

Results

Genetic analysis revealed a novel homozygous mutation in NFKB1, c.2878G>A, p.Gly960Arg (G960R). This affected p105 phosphorylation and p50 formation on antigen and cytokine stimulation, as well as attenuating nuclear signal transmission. As a result, both T- and B-cell maturation and function were perturbed. The number of memory CD4⁺ T cells were reduced, while CD8⁺ T cells consisted predominately of expanded differentiated populations. The function of T cells were diminished as shown by reduced responses to mitogens as well as diminished cytokine secretion. B-cell maturation was also affected, with decreased IgD⁺CD27⁺ memory B cells while transitional B cells were increased, likely contributing to the reduced ability to produce specific antibodies.

Conclusion

Homozygous G960R mutation in NFKB1 leads to a severe clinical presentation of combined immunodeficiency. This was associated with blockade of nuclear factor-κB pathway signaling, resulting in aberrations in T- and B-cell maturation and function.

Introduction

The nuclear factor-κB (NF-κB) signaling pathway plays a major role in mediating multiple cellular events including immune and inflammatory responses, lymphocyte development, cell growth, and programmed death.¹ The NF-κB system consists of 5 structurally related proteins that function in the classical or alternative NF-κB pathways and sense different receptors, but share significant cross-talking.2, 3, 4 The pathway members consist of NFKB1 (p50 and its precursor, p105), NFKB2 (p52 and its precursor, p100), transcription factor p65, REL-associated protein A ([RelA] p65), RelB, and c-Rel, all bound to each other in a dimerized form and sequestered in the cytoplasm by the inhibitor of NF-κB. On activation of the classical pathway by Toll-like receptors, B- or T-cell receptors, the inhibitor of NF-κB kinase enzyme complex is recruited, initiating phosphorylation and degradation events that ultimately lead to formation of the transcription factor NFKB1 heterodimer (p50/RelA) that can readily migrate into the nucleus.^5,6 Proteolytic processing of p105 to p50 is a key step in this cascade and is regulated by the ubiquitin-proteasome system.⁷

A variety of immunodeficiencies have been shown to arise from mutations in the classical as well as alternative pathway components.8, 9, 10, 11, 12, 13

Recently, heterozygous mutations in NFKB1 resulting in haploinsufficiency have been identified in a relatively large proportion of patients with common variable immunodeficiency.¹⁴

We report here the first case of homozygous NFKB1 deficiency in a patient with severe life-threatening infections and combined immunodeficiency.