Transgenic mutation assays utilizing bacterial target genes display a high frequency of spontaneous mutation at CpG sequences. This is believed to result from the fact that: (1) the prokaryotic genes currently being used as transgenic mutation targets have a high CpG content and (2) these sequences are methylated by mammalian cells to produce 5-methylcytosine (5MC), a known promutagenic base. To study the effect of CpG content on the frequency and type of spontaneous mutation, we have synthesized an analogue of the bacterial lacI target gene (mrkII) that contains a reduced number of CpG sequences. This gene was inserted into a lambda vector and used to construct transgenic mice that undergo vector rescue from genomic DNA upon in vitro packaging. Results on spontaneous mutation frequency and spectrum have been collected and compared to those observed at the lacI gene in Big Blue transgenic mice. Spontaneous mutations at the mrkII gene occurred at a frequency in the mid-10-5 range and were predominantly base pair substitutions, similar to results seen in Big Blue. However, mrkII mutations were distributed toward the carboxyl end of the gene instead of the bias toward the amino terminus seen in lacI. Unexpectedly, 23% of the spontaneous mrkII mutations were GC-->AT transitions at CpG sequences (compared to 32% in lacI), despite the reduction in CpG number from 95 in lacI to only 13 in mrkII. Nine of the CpG bases undergoing transition mutations in mrkII have not been recorded previously as spontaneous sites in Big Blue. Therefore, substantial reduction of the number of CpG sequences in the lacI transgene did not significantly reduce the rate of spontaneous mutation or alter the contribution of CpG-related events. This suggests that other factors are also operating to establish frequency and composition of spontaneous mutations in transgenic targets.
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