Bi-allelic mutation of CARD11 resulting in fatal immune dysregulation with dramatic B cell expansion

Abstract Caspase recruitment domain family member 11 (CARD11) functions as a scaffold to bridge antigen receptors with downstream immune signaling pathways that enable antigen-dependent lymphocyte activation. Given its pleiotropic role in regulating adaptive immunity, different genetic variants in CARD11 can result in diverse phenotypic consequences in humans. Here we describe a patient harboring a novel bi-allelic CARD11 mutation that results in potentially fatal immune dysregulation with massive B cell expansion. He presented with splenomegaly, amenia, leukocytosis and thrombocytopenia within the first year of life. A sibling showed similar symptoms before passing away in infancy. Due to parental consanguinity, an autosomal recessive inborn error of immunity was suspected; whole exome sequencing revealed a homozygous missense mutation in the coiled-coil (CC) domain of CARD11 (p.Arg331Pro, R331P). Functional testing revealed the R331P mutation caused a mild gain-of-function effect relative to wild-type CARD11 in driving constitutive NF-kB activation, suggesting that R331P may disrupt interactions needed to maintain CARD11 autoinhibition in resting lymphocytes. Dramatic B cell lymphocytosis noted in the patient was not present in his heterozygous parents, indicating that 2 mutant alleles are required for this phenotype. In contrast, the patient’s T cells responded poorly to TCR stimulation with severely reduced proliferation. Collectively, these results indicate that the novel R331P CARD11 mutation manifests as the first known recessive form of B cell Expansion with NF-kB and T cell Anergy (BENTA disease), and illustrate how rare monogenic inborn errors of immunity provide a unique discovery platform for basic immunology.