Quantification of random genomic mutations

Cancer cells contain numerous clonal mutations. It has been theorized that malignant cells sustain an elevated mutation rate and, as a consequence, harbor yet larger numbers of random point mutations. Testing this hypothesis has been precluded by lack of an assay to measure random mutations-that is, mutations that occur in only one or a few cells of a population. We have established a method that has permitted us to detect and identify rare random mutations in human cells, at a frequency of 1 per 108 base pairs. The assay is based on gene capture, by hybridization with a uracil-containing probe, followed by magnetic separation. Mutations that render the mutational target sequence non-cleavable by a restriction enzyme are quantified by dilution to single molecules and real-time quantitative PCR amplification. The assay can be extended to quantify mutation in any DNA-based organism, at different sites in the genome, in introns and exons, in unselected and selected genes, and in proliferating and quiescent cells.