PROBING IMMUNOGLOBULIN GENE HYPERMUTATION WITH MICROSATELLITES SUGGESTS A NONREPLICATIVE SHORT PATCH DNA-SYNTHESIS PROCESS

As the rate of Ig gene hypermutation approximates the level of nucleot ide discrimination of DNA polymerases (10(-3) to 10(-4)), a local inhi bition of proofreading and mismatch repair during semiconservative rep lication could generate the mutations introduced by the process. To ad dress this question, we have constructed transgenic mice that carry a hypermutation substrate containing a ''polymerase slippage trap'': an Ig gene with a mono or dinucleotide tract inserted in its V region. Th e low amount of slippage events as compared to the number of mutations , the absence of transient misalignment mutations at the border of the repeats, and the dissociation between the amount of frameshifts and m utations when the transgene is put on mismatch repair-deficient geneti c backgrounds, suggest that Ig gene hypermutation occurs by an error-p rone short patch DNA synthesis taking place outside global DNA replica tion.