High susceptibility of transgenic rats carrying the human c-Ha-ras proto-oncogene to chemically-induced mammary carcinogenesis

A rat line carrying three copies of the human c-Ha-ras proto-oncogenes, inc luding its own promoter region, was established and designated as Hras128. Expression of the transgene was detected in all organs by Northern blot ana lysis. To examine its influence on susceptibility to mammary carcinogenesis , female rats were treated with N-methyl-N-nitrosourea (MNU) or 7,12-dimeth ylbenz[a]anthracene (DMBA) at 50 days of age, With MNU, all the transgenic rats rapidly developed multiple mammary carcinomas within as short as 8 wee ks 114.1 tumors/rat), in contrast to 0.46 tumors/rat in non-transgenic rats . PCR-RFLP analysis and direct sequencing for the transgene indicated that the large majority of carcinomas (38/44, 86.4%) contained cells with mutati ons at codon 12 in exon 1. However, comparison of the signal densities of t he mutated band to dilution scale bands revealed that the cells with the mu tated transgene were not in the majority. By PCR-SSCP analysis for codons 1 2 and 61 of the rat endogenous c-Ha-ras gene, no mutations were detected. S imilarly, with DMBA, almost all (13/14, 92.9%) the transgenic rats develope d multiple mammary carcinomas (9.39 tumors/rat) within 16 weeks, and 4 out of 12 (33.3%) non-transgenic rats had only small tumors (0.83 tumors/rat). A lower incidence of mutation of the transgene was found in codon 12 (5/25, 25%) than in MNU-induced tumors, but mutations were detected in codon 61 ( 7/20, 35%, No mutations were detected in the rat endogenous gene. No mutati on was found in the rat endogenous c-Ha-ras gene in non-transgenic rats. As observed in both the MNU- and DMBA-induced tumor cases, the population of cells with the mutated transgene were in the minority. The results thus ind icate that rats carrying the transduced human c-Ha-ras proto-oncogene are h ighly susceptible to MNU- and DMBA-induced mammary carcinogenesis and that this is not primarily due to mutations of the transgene or endogenous c-Ha- ras gene. Furthermore, irrespective of the mechanism of enhanced susceptibi lity, the Hras128 transgenic rats can be utilized for the screening of mamm ary carcinogens. (C) 2001 Elsevier Science B.V. All rights reserved.