Evidence for mutation showers.

Mutants in the Big Blue transgenic mouse system show spontaneous clustered multiple mutations with unexpectedly high frequency, consistent with chronocoordinate events. We tested the prediction that the multiple mutations seen within the lacI mutation target sometimes occur in the context of chronocoordinate multiple mutations spanning multiple kilobases (mutation showers). Additional sequencing of mutants was performed in regions immediately flanking the lacI region (total of 10.7 kb). Nineteen additional mutations were found outside the lacI region ("ectomutations") from 10 mutants containing two or more lacI mutations, whereas only one ectomutation was found in 130 mutants with a single mutation (P < 0.0001). The mutation showers had an average of approximately one mutation per 3 kb. Four mutants showed closely spaced double mutations in the new sequence, and analysis of the spacing between these mutations revealed significant clustering (P = 0.0098). To determine the extent of the mutation showers, regions (8.5 kb total) remote from the lacI region (approximately 16-17 kb away) were sequenced. Only two additional ectomutations were found in these remote regions, consistent with mutation showers that generally do not extend more than approximately 30 kb. We conclude that mutation showers exist and that they constitute at least 0.2% and possibly 1% or more of mutational events observed in this system. The existence of mutation showers has implications for oncogenesis and evolution, raising the possibilities of "cancer in an instant" and "introns as sponges to reduce the deleterious impact of mutation showers."