Multi-omics approach to map a clinically diverse large cohort with homozygous founder RAG1 p.C176F variant among Old Order Mennonites

Partially preserved function of recombination-activating genes (RAG) 1 and 2 can be linked to divergent and hard-to-predict clinical and immunological phenotypes, even among close relatives. It is unclear how intrinsic RAG hypomorphism interacts with extrinsic environmental pressures to shape clinical trajectory. We dissect this phenomenon in a unique cohort of 18 Old Order Mennonite individuals with a novel missense founder RAG1 variant (p.C176F) with 25.8% recombinase activity of wild-type protein. After studying a myriad of markers, we demonstrate varying severity and progression of clinical disease (predominantly in females) that paralleled decline in CMV-specific immune competence, break in tolerance with hallmark presence of antibodies to IFNα, a TH1/IFNγ-driven inflammatory milieu with elevated serum markers (CXCL9/10), expansion of follicular helper T cells and clonal B cells enriched for autoreactive-prone VH4.34 B cell receptor. BCR and TCR repertoires carry the fingerprints of both extrinsic and RAG-driven intrinsic changes. Through this experiment of nature, our cohort demonstrated that partial RAG deficiency (pRD) is not a monolithic disease but the result of compounded RAG intrinsic and extrinsic events. Overall, this study uses multi-omics approaches to dissect the contribution of a hypomorphic RAG variant to variable severity of clinical disease and immunological dysregulation at cellular and molecular level. Emerging biomarkers of a multi-omic roadmap should allow monitoring of progression of clinical disease and guide who and when to treat with hematopoietic stem cell transplant in pRD.