Nonconservative Amino Acid Substitution Variants Exist at Polymorphic Frequency in DNA Repair Genes in Healthy Humans1

The removal or repair of DNA damage has a key role in protecting the genome of the cell from the insults of cancer-causing agents. This was originally demonstrated in individuals with the rare genetic disease xeroderma pigmentosum, the paradigm of cancer genes, and subsequently in the relationship between mismatch repair and colon cancer. Recent re ports suggest that individuals with less dramatic reductions in the capacity to repair DNA damage are observed at polymorphic frequency in the population; these individuals have an increased susceptibility to breast, lung, and skin cancer. We report initial results from a study to estimate the extent of DNA sequence variation among individuals in genes encoding proteins of the DNA repair pathways. Nine different amino acid substitution variants have been identified in resequencing of the exons of three nucleotide excision repair genes (ERCC1, XPD, and XPF), a gene involved in double-strand break repair/recombination genes (XRCC3), and a gene functioning in base excision repair and the repair of radiation-induced damage (XRCC1). The frequencies for the nine different variant alÃ-eles range from 0.04 to 0.45 in a group of 12 healthy individuals; the average alÃ-elefrequency is 0.17. The potential that this variation, and especially the six nonconservative amino acid substitutions occurring at residues that are identical in human and mouse, may cause reduc tions in DNA repair capacity or the fidelity of DNA repair is intriguing; the role of the variants as cancer risk factors or susceptibility alÃ-eles remains to be addressed.