Dimethylbenzanthracene carcinogenesis in Gadd45a-null mice is associated with decreased DNA repair and increased mutation frequency

Mice lacking the Gadd45a gene are susceptible to ionizing radiation-induced tumors. Increased levels of Gadd45a transcript and protein are seen after treatment of cells with ionizing radiation as well as many other agents and treatments that damage DNA, Because cells deficient in Gadd45a were shown to have a partial defect in the global genomic repair component of the nucl eotide excision repair pathway of UV-induced photoproducts, dimethylbenzant hracene (DMBA) carcinogenesis was investigated because this agent produces bulky adducts in DNA that are also repaired by nucleotide excision repair. Wild-type mice and mice deficient for Gadd45a were injected with a single i .p. dose of DMBA at 10-14 days of age. The latency for spontaneous deaths w as slightly decreased for Gadd45a-null mice compared with wild-type mice. A t 17 months, all surviving animals were killed, and similar percentages of each genotype were found to have tumors. However, nearly twice as many Gadd 45a-null than wild-type mice had multiple tumors, and three times as many h ad multiple malignant tumors, The predominant tumor types in wild-type mice were lymphoma and tumors of the intestines and liver. In Gadd45a-null mice , there was a dramatic increase in female ovarian tumors, male hepatocellul ar tumors, and in vascular tumors in both sexes. In wild-type mice, this do se of DMBA induced a >5-fold increase in Gadd45a transcript in the spleen a nd ovary, whereas the increase in liver was >20-fold. Nucleotide excision r epair, which repairs both UV- and DMBA-induced DNA lesions, was substantial ly reduced in Gadd45a-null lymphoblasts, Mutation frequency after DMBA trea tment was threefold higher in Gadd45a-null liver compared with mild-type li ver, Therefore, lack of basal and DMBA-induced Gadd45a may result in enhanc ed tumorigenesis because of decreased DNA repair and increased mutation fre quency. Genomic instability, decreased cell cycle checkpoints, and partial loss of normal growth control in cells from Gadd45a-null mice may also cont ribute to this process.