Genome-wide mutational signatures of immunological diversification in normal lymphocytes

A lymphocyte suffers many threats to its genome, including programmed mutation during differentiation, antigen-driven proliferation and residency in diverse microenvironments. After developing protocols for single-cell lymphocyte expansions, we sequenced whole genomes from 717 normal naive and memory B and T lymphocytes and hematopoietic stem cells. Lymphocytes carried more point mutations and structural variation than stem cells, accruing at higher rates in T than B cells, attributable to both exogenous and endogenous mutational processes. Ultraviolet light exposure and other sporadic mutational processes generated hundreds to thousands of mutations in some memory lymphocytes. Memory B cells acquired, on average, 18 off-target mutations genome-wide for every one on-target IGV mutation during the germinal center reaction. Structural variation was 16-fold higher in lymphocytes than stem cells, with ~15% of deletions being attributable to off-target RAG activity. One Sentence Summary: The mutational landscape of normal lymphocytes chronicles the off-target effects of programmed genome engineering during immunological diversification and the consequences of differentiation, proliferation and residency in diverse microenvironments. ### Competing Interest Statement P.J.C. is a founder, consultant and director for Mu Genomics Ltd. G.G. receives research funds from Pharmacyclics and IBM. G.G. is an inventor on multiple patents related to bioinformatics methods (MuTect, MutSig, ABSOLUTE, MSMutSig, MSMuTect, POLYSOLVER and TensorQTL). G.G. is a founder, consultant and holds privately held equity in Scorpion Therapeutics.