Altered spectra of hypermutation in antibodies from mismatch repair-deficient mice

Somatic hypermutations are introduced into rearranged immunoglobulin variable (V) genes at a frequency of 10^-2 mutations per base pair by an unknown mechanism. DNA repair pathways may generate or remove mutations, which predicts that the frequency and pattern of mutation will be different in V genes from mice defective in repair. Therefore, hypermutation was studied in mice deficient for mismatch and nucleotide excision repair. There were similar frequencies of mutation in rearranged V_κ Ox1 genes from immunized wildtype, PMS2-mismatch repair-deficient, and XPA-nucleotide excision repair-deficient mice. The pattern of mutation was also similar in V genes from XPA-deficient and wildtype mice, but was different in V genes from PMS2-deficient mice, which had a significant increase in the number of adjacent base substitutions. A biochemical assay was used to measure the efficiency of repair of tandem mismatches by various cell lines defective for DNA repair genes. Tandem mispairs were repaired by wildtype and Xpa^-/- cells but not by Pms2^-/- cells. The analysis of hypermutation in mice deficient for DNA repair has revealed two new findings. First, neither mismatch repair nor nucleotide excision repair are required to generate hypermutation in V genes. Second, tandem base substitutions were frequently found in V genes from Pms2^-/- mice, suggesting that they are produced during the hypermutation process and then removed by a PMS2-dependent pathway. Biochemical evidence supports the existence of a novel pathway that processes tandem mispairs which requires PMS2.