Mutation frequency is not elevated in the cerebellum of harlequin/Big Blue® mice but Class II deletions occur preferentially in young harlequin cerebellum

The harlequin (hq)/Big Blue® mouse is a model of premature aging with neurodegeneration and a transgenic mutation target (λ cII gene) for in vivo mutation analysis in individual tissues. The hq mutation results in downregulation of the apoptosis-inducing factor (Aif) gene with conflicting reports of elevated oxidative stress in neurons. Previously, the cerebellum of hq disease mice was reported to have a three-fold elevated frequency of −1 bp deletions at a monobasic run (G11), a common mutation signature of reactive oxygen species [1]. To examine a diversity of mutations types associated with oxidative DNA damage, the frequency and nature of spontaneous cII mutations were determined in the cerebellum of three- and seven-month-old hq disease and Aif-proficient Big Blue® mice. Unlike the previous report, no elevation in spontaneous mutation frequency was observed in the cerebellum of hq disease compared to Aif-proficient mice. The G11 sequence used in the previous study may be particularly susceptible to oxidative DNA damage leading to replication errors and the previous assay may detect transcriptional errors. We observed intrachromosomal deletions (10–21 bp) associated with oxidative DNA damage exclusively in the cerebellum of four out of five young hq disease mice (p = 0.0001). The pattern of mutation in the cerebellum of hq disease and Aif-proficient mice was similar. This mutation pattern is typical of previous reports of spontaneous mutations across multiple tissues and ages of wild type mice. Finally, an Aif-proficient mouse had the heaviest mutation shower yet reported with seven point mutations over 21 bp in a single mutant, consistent with transient hypermutability, a phenomenon relevant to disease and genome evolution.