1. Medical Research Council Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK
2. Immunology and Molecular Genetics, Kawasaki Medical School, 577 Matsushima, Kurashiki, Okayama 701-0192, Japan
3. CREST Research Project, Radiation Genetics, Faculty of Medicine, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan

Correspondence to: Julian E. Sale¹ Correspondence should be addressed to J.E.S. (e-mail: Email: jes@mrc-lmb.cam.ac.uk) or M.S.N. (e-mail: Email: msn@mrc-lmb.cam.ac.uk).

Abstract

After gene rearrangement, immunoglobulin V genes are further diversified by either somatic hypermutation or gene conversion¹. Hypermutation (in man and mouse) occurs by the fixation of individual, non-templated nucleotide substitutions. Gene conversion (in chicken) is templated by a set of upstream V pseudogenes. Here we show that if the RAD51 paralogues² XRCC2, XRCC3 or RAD51B are ablated the pattern of diversification of the immunoglobulin V gene in the chicken DT40 B-cell lymphoma line³ exhibits a marked shift from one of gene conversion to one of somatic hypermutation. Non-templated, single-nucleotide substitutions are incorporated at high frequency specifically into the V domain, largely at G/C and with a marked hotspot preference. These mutant DT40 cell lines provide a tractable model for the genetic dissection of immunoglobulin hypermutation and the results support the idea that gene conversion and somatic hypermutation constitute distinct pathways for processing a common lesion in the immunoglobulin V gene. The marked induction of somatic hypermutation that is achieved by ablating the RAD51 paralogues is probably a consequence of modifying the recombination-mediated repair of such initiating lesions.