A DNA repair abnormality specific for rearranged immunoglobulin variable genes in germinal center B cells.

The somatic hypermutation mechanism produces high-rate mutagenesis specifically targeted to rearranged immunoglobulin (Ig) variable (V) gene segments during the germinal center (GC) stage of B lymphocyte differentiation. The mechanism of this process remains uncertain, partly due to the lack of a direct assay for hypermutation activity. In this study, a gene-specific DNA repair assay was used to compare the rate and quality of DNA repair in the mantle zone (MZ) and GC B cells at rearranged and unrearranged Ig V genes. GC B cells were distinguished from MZ B cells by a retarded repair rate specific for rearranged Ig V genes. In addition, a unique feature of GC cells after DNA repair was the appearance of predominant mutations in rearranged Ig VH5 gene PCR products. These predominant mutations also occurred in natural mutants of VH5 genes. However, repair-associated mutations reflected, at least in part, "template-jumping" during amplification of the residually damaged genomic template. Overall, these findings reflect a repair abnormality associated with the hypermutation process by the criteria of sequence- and B cell stage-specificity. We conclude that locus-specific retardation of DNA repair is a component of the hypermutation mechanism. RFLP or SSCP analysis provides a simple assay to monitor this repair abnormality as a surrogate biochemical marker for hypermutation during B cell differentiation.