Breast cancer is the most common form of malignancy in American women. Apart from age, a strong family history of breast cancer confers the highest known risk for neoplastic development by the various etiologic factors identified to date. Four genes have been identified (p53, BRCA1, BRCA2, ATM) that appear to confer substantial predispositions to human breast cancer. Gene targeting techniques have been used to create mice with specific defects in these genes. This review describes the status of these mice as models for breast cancer susceptibility and suggests future research directions which may increase our understanding of breast carcinogenesis.

Genetic and molecular studies have implicated the region of the alpha-interferon gene cluster on mouse chromosome 4 as the location of a putative tumor suppressor gene. A region of homology on human chromosome 9p21-22 that is frequently deleted in multiple human cancers has recently been found to contain a candidate tumor suppressor gene called multiple tumor suppressor-1 (MTS1); which was previously shown to encode an inhibitor of cyclin-dependent kinase 4. We performed loss of heterozygosity and deletion analyses to map the most commonly deleted region on chromosome 4 in F1 hybrid mouse lung tumors. Ten simple sequence length polymorphism markers were analyzed with focus on the alpha-interferon region. Allelic losses were detected in 29 of 61 (48%) of the lung adenocarcinomas but in only 1 of 38 (3%) of the lung adenomas examined. In most cases, the losses appeared to occur by nondisjunction. However, in three carcinomas, we detected homozygous deletions that overlapped at simple sequence length polymorphism marker D4MIT77. These data suggest a critical region of about 2 cM immediately distal to the alpha-interferon locus as the likely domain of a novel tumor suppressor gene on mouse chromosome 4, the loss of which appears to be involved in the progression of mouse lung tumorigenesis.

Fibroblast cultures were derived from mouse embryos containing either one (p53+/-) or two (p53-/-) inactivated p53 alleles and compared to normal embryo fibroblasts for a number of growth parameters. Early passage p53-deficient embryo fibroblasts (p53-/-) divided faster than normal embryo fibroblasts, achieved higher confluent densities, and had a higher fraction of division-competent cells under conditions of low cell density. Flow cytometry studies of early passage embryo fibroblasts showed that the percent of p53-deficient cells in G0/G1 was lower than in normal cells, consistent with the argument that p53 mediates a G1 block. When p53-deficient and normal cells were passaged for long periods of time, the homozygote (p53-/-) fibroblasts grew at a high rate for over 50 passages and never entered a non-growing senescent phase characteristic of the heterozygote (p53+/-) and normal (p53+/+) cells. The p53-deficient fibroblasts were genetically unstable during passaging, with the p53-/- cells showing a high degree of aneuploidy and the p53+/- cells displaying a moderate level of chromosomal abnormalities by passage 25. Surprisingly, the heterozygote cells lost their single wild type allele very early during culturing and in spite of this loss most heterozygote lines entered into senescence. We conclude that the loss of p53 by itself is insufficient to confer immortality on a cell, but does confer a growth advantage. Taken together, the findings confirm that the absence of p53 promotes genomic instability, which in turn may result in genetic alterations which directly produce immortality.

Carcinogen-induced point mutations resulting in activation of ras oncogenes have been demonstrated in various experimental systems such as skin carcinogenesis, mammary, and liver carcinogenesis. In many cases, the data support the conclusion that these point mutations are critical changes in the initiation of these tumors. The Syrian hamster embryo (SHE) cell transformation model system has been widely used to study the multistep process of chemically induced neoplastic transformation. Recent data suggest that activation of the Ha-ras gene via point mutation is one of the crucial events in the transformation of these cells. We have now cloned the c-Ha-ras proto-oncogene from SHE cDNA-libraries, and we have performed polymerase chain reaction and direct sequencing to analyze tumor cell lines induced by different chemical carcinogens for the presence of point mutations. No changes were detectable at codons 12, 13, 59, 61, and 117 or adjacent regions in tumor cell lines induced by diethylstilbestrol, asbestos, benzo(a)pyrene, trenbolone, or aflatoxin B1. Thus, it is not known whether point mutations in the Ha-ras proto-oncogene are essential for the acquisition of the neoplastic phenotype of SHE cells. Activation of other oncogenes or inactivation of tumor suppressor genes may be responsible for the neoplastic progression of these cells. However, in SHE cells neoplastically transformed by diethylstilbestrol or trenbolone, a significant elevation of the c-Ha-ras expression was observed. Enhanced expression of c-myc was detected in SHE cells transformed by benzo(a)pyrene or trenbolone.