The 5′ boundary of somatic hypermutation in a Vχ gene is in the leader intron

Abstract

The maturation of the immune response involves the hypermutation of antibody genes and the selection of B cells expressing receptors with improved antigen binding properties. Somatic hypermutation of antibody genes is targeted to a small region ∼1 kb surrounding the rearranged V gene. The precise definition of the 5′ limit is not yet clear since the data base of somatic mutations upstream of the V region is very restricted. The available data suggest that it lies close to the promoter region and this has been used to implicate transcription in the mechanism leading to hypermutation. Here we present an extensive analysis of mutations in the 5′ region of a single χ light chain gene. A large data base from highly mutated sequences was obtained from anti‐oxazolone hybridomas expressing the V_χOx1‐J_χ5 light chain and from polymerase chain reaction‐derived clones from splenic and Peyer's patches of transgenic mice expressing the same V_χOx1‐J_χ5 gene combination. Although mutations were found in the 5′‐flanking segment, the rate of mutation in the V‐J segment was about 20‐fold higher. A sharp decline between those two mutation rates is evident but the boundary was found in the leader intron of the V_χOx1 gene, about 150 bases downstream of the initiation of transcription site.