Whole-Exome Sequencing Links CARD11 Inactivation with SCID

Abstract 258

Primary immunodeficiencies represent model diseases for the mechanistic understanding of the human innate and the adaptive immune response and are per se clinically highly relevant, because in SCID patients infections by opportunistic pathogens are typically life-threatening early in life. We identified an infant of consanguineous parents suffering from a novel form of SCID, who presented with a life-threatening Pneumocystis jirovecii pneumonia. This entity was characterized by agammaglobulinemia and profoundly deficient T-cell function despite quantitatively normal T- and B-lymphocytes. Lymphocyte proliferation was strongly inhibited after stimulation of PBMCs with T-cell mitogens such as PHA, Con A, or anti-CD3 monoclonal antibody. The expression of several T-cell response associated cytokines upon stimulation with PMA/ionomycin was dramatically reduced in comparison to normal controls. By contrast, proliferation induced by the classical B-cell mitogen PWM was almost comparable to healthy controls. Immunophenotyping revealed a predominantly naïve phenotype (CD45RA⁺ CCR7⁺) in CD4⁺ and CD8⁺ T-lymphocytes, whereas central memory T-lymphocytes (CD45RA⁻ CCR7⁺) were nearly absent. B-lymphocytes from peripheral blood were mainly naïve B-cells (CD27⁻) with a uniformly immature transitional B-lymphocyte phenotype (CD24⁺⁺, CD38⁺⁺). Patient B-lymphocytes retained the ability to proliferate and differentiate in response to BCR-independent stimuli, while their response to BCR activation was defective. Our findings thus revealed a combined defect of TCR-mediated T-lymphocyte functions and BCR-mediated B-lymphocyte functions but did not enable us to link the immunological phenotype with one of the known molecularly defined categories of SCID. Diagnostic whole-exome sequencing and systematic variant categorization revealed a single pathogenic homozygous nonsense mutation of the caspase recruitment domain 11 (CARD11) gene. CARD11 is a scaffold protein that is known to be required for the assembly and activation of the NF-kB complex. In reconstitution assays we demonstrated that the patient derived truncated CARD11 protein is defective in antigen receptor signaling and NF-kB activation. Several lines of evidence substantiate the involvement of the identified CARD11 mutation in the new form of SCID that we report here. First, PCR and Sanger re-sequencing validated the truncating CARD11 mutation to be homozygous in the patient and heterozygous in the parents, in agreement with the recessive transmission of the mutation through the healthy consanguineous parents. Second, CARD11 is a scaffold protein required for TCR- and BCR-induced NF-kB activation as well as lymphocyte activation and proliferation, which is specifically expressed in hematopoietic cells, consistent with a causative role of CARD11 mutations in the context of an immune disorder. Third, the GUK domain of CARD11, which is missing in the mutated form of CARD11 due to truncation, was previously reported to be necessary for NF-kB activation by PMA/ionomycin treatment, further supporting the presumed damaging nature of the homozygous CARD11 mutation observed in the female patient reported here. Finally, the immunological findings in this patient are compatible with the phenotype of a previously described Card11 −/− k.o. mouse, which shows a selective defect in NF-κB activation leading to diminished antigen receptor or PKC mediated proliferation and defective cytokine production in T-cells and B-cells. Thus, we have identified an inactivating CARD11 mutation linking defective NF-kB signaling with a novel cause of autosomal recessive SCID, which must be considered in the diagnostic assessment of patients with suspected SCID but with quantitatively normal T-cells.