CARCINOGEN-INDUCED LOSS OF HETEROZYGOSITY AT THE APRT LOCUS IN SOMATIC-CELLS OF THE MOUSE

Genetic events leading to the loss of heterozygosity (LOH) have been s hown to play a crucial role in the development of cancer. However, LOH events do not occur only in genetically unstable cancer cells but als o have been detected in normal somatic cells of mouse and man. Mice, i n which one of the alleles for adenine phosphoribosyltransferase (Aprt ) has been disrupted by gene targeting, were used to investigate the p otency of carcinogens to induce LOH in vivo. After 7,12-dimethyl-1,2-b enz[a]anthracene (DMBA) exposure, a 5-fold stronger mutagenic response was detected at the autosomal Aprt gene than at the X chromosomal hyp oxantine-guanine phosphoribosyltransferase (Hprt) gene in splenic T-ly mphocytes. Allele-specific PCR analysis shelled that the normal, nonta rgeted Aprt allele was lost in 70% of the DMBA-induced Aprt mutants. F luorescence in situ hybridization analysis demonstrated that the targe ted allele had become duplicated in almost all DMBA-induced mutants th at displayed LOH at Aprt. These results indicate that the main mechani sms by which DMBA caused LOH were mitotic recombination or chromosome loss and duplication but not deletion. However, after treatment with t he alkylating agent N-ethyl-N-nitrosourea,Aprt had a similar mutagenic response to Hprt while the majority (90%) of N-ethyl-N-nitrosourea-in duced Aprt mutants had retained both alleles. Unexpectedly, irradiatio n with x-rays, which induce primarily large deletions, resulted in a s ignificant increase of the mutant frequency at Hprt but not at Aprt. T his in vivo study clearly indicates that, in normal somatic cells, car cinogen exposure can result in the induction of LOH events that are co mpatible with cell survival and may represent an initiating event in t umorigenesis.