The Rate of CpG Mutation in AluRepetitive Elements within the p53 Tumor Suppressor Gene in the Primate Germline

Abstract

Cytosine to thymine transition mutations at the CpG dinucleotide are the most common point mutations in cancer and genetic disease. We calculated the in vivorate of CpG mutation in the primate germline by deriving a primordial consensus sequence for an Alurepetitive element which inserted into intron 6 of the primate p53 gene 35 to 55 million years ago. Comparison of this primordial sequence to the Alusequence in intron 6 of present-day primates was used to determine the nature and rate of mutations which occurred during evolution. We estimate the half-life of a CpG nucleotide to be 24 to 60 million years, and the rate constant for mutation at this dinucleotide to be 1.2 × 10 −8to 2.9 × 10 −8years −1. These results were confirmed by the analysis of a second Alusequence in intron 10 of the p53 gene. The in vivomutation rate is at least 1250-fold slower than the in vitrochemical rate of 5-methylcytosine deamination in double-stranded DNA, showing that current estimates of CpG mutation repair have been significantly underestimated. Furthermore, the mutability of the CpG dinucleotide has led to the depletion of this dinucleotide from the vertebrate genome, and calculations in this study suggest that current levels of the CpG dinucleotide in the primate genome are very close to a steady state equilibrium in which the rate of CpG mutation is equal to the rate of CpG formation by random mutation.