Myc/p53 interactions in transgenic mouse mammary development, tumorigenesis and chromosomal instability

Abstract

We have examined defects in mammary development and tumorigenesis in a transgenic model expressing the c-myc gene under the MMTV ± LTR promoter. The stochastic tumors which arise from hyperplastic ductal and lobular lesions in this model are characterized by high rates both of apoptosis and of chromosomal instability. Since the p53 gene product is thought to be central in the maintenance of genomic integrity, in part due to its ability to induce apoptosis in cells harboring DNA damage, we examined its expression and possible mutation. Initially, we observed that unmutated p53 is strongly expressed in premalignant mammary glands and in mammary tumors derived from the MMTV-c-myc strain. We then mated the MMTV-myc strain to a p53-de®cient strain as a means of examining the eect of this lesion on mammary development and tumorigenesis in the context of c-myc overexpression. A lack of both p53 alleles in the presence of c-myc overexpression resulted in a dramatic hyerplastic alteration in mammary gland development. Speci®cally, in female bitransgenic MMTV-c-myc/p53 null mice (MMTV-myc/p53 7/7 ), lobular hyperplasias were observed at almost every ductal end bud as early as 32 days of age. In contrast, only mild ductal and lobular hyperplasias were seen in MMTV-myc mice that contained both p53 alleles (MMTV-myc/p53 +/+ ); an intermediate phenotype occurred in mice with a single intact (MMTV-myc/p53 +/7 ) p53 allele. Mammary carcinomas arose with a high frequency in MMTV-myc/p53 +/7 mice; the tumors were comparable in frequency, histology and apoptotic index to the tumors in MMTV-myc/p53 +/+ mice. Also, as previously observed (Elson et al., 1995), lymphomas arose with extremely short latency in MMTV-myc/ p53 7/7 mice, precluding study of the fate of their hyperplastic mammary lesions in situ. The frequency of p53 mutations in MMTV-myc/p53 +/+ and MMTV-myc/ p53 +/7 mammary tumors and in cell lines derived from these tumors was examined by direct sequencing. No point mutations or deletions in p53 were observed in mammary tumors or cell lines from either genotype. Finally, a detailed chromosomal analysis using multicolor spectral karyotyping (SKY) revealed that there were multiple chromosomal alterations in the c-mycoverexpressing cells that contained either one or two unmutated p53 alleles. Variable ploidy changes, a common translocation of chromosome 11, and other chromosomal aberrations were observed. Our data thus support an interaction between c-Myc and p53 in mammary development, but suggest that loss of p53 is required neither for c-myc-dependent tumorigenesis nor for c-myc-dependent chromosomal instability.

Use of MMTV-Wnt-1 transgenic mice for studying the genetic basis of breast cancer

“…p53 de®ciency (p53+/7, p537/7) enhances cell proliferation in the Wnt-1 transgene-derived tumors, but the modestly ongoing apoptosis that accompanies Wnt-1 overexpression does not seem to be attenuated (Jones et al, 1997). Similarly, absence of one allele of p53 does not a ect the apoptotic index in mammary tumors induced by an MMTV-c-myc transgene (McCormack et al, 1998).…”

“…This frequent occurrence of LOH contrasts with the very rare loss of the wild-type p53 allele in mammary tumors from p53 heterozygotes carrying an MMTV-cmyc transgene (Elson et al, 1995;McCormack et al, 1998). It is notable that inactivation of p53 collaborates with MMTV-c-myc, MMTV-H-ras, and MMTV-neu transgenes to produce lymphomas and salivary tumors, but rarely mammary tumors (C-X Deng, personal communication, Elson et al, 1995;Hundley et al, 1997).…”

“…Translocations, trisomy, and aneuploidy have been detected in cells cultured from some of these tumors (Z Weaver and WP Hively, unpublished). Karyotype instability in mammary tumors has been reported in mammaryspeci®c Brca1 knockout mice (Xu et al, 1999) and other transgenic models including MMTV-c-myc (McCormack et al, 1998;Weaver et al, 1999).…”

Transforming growth factor alpha- and c-myc-induced mammary carcinogenesis in transgenic mice

“…Mice that overexpress c-myc in the mammary gland are predisposed to develop mammary neoplasia (Stewart et al, 1984;Leder et al, 1986;Andres et al, 1988;Oncogene (2000) 19, 1092 ± 1096 ã 2000 Macmillan Publishers Ltd All rights reserved 0950 ± 9232/00 $15.00 www.nature.com/onc *Correspondence: EP Sandgren Schoenenberger et al, 1988;Sandgren et al, 1995). Furthermore, as for TGFa, co-expression of c-myc with growth factors or other oncogenes increases the incidence of tumors and shortens their latency (Sinn et al, 1987;Andres et al, 1988;Sandgren et al, 1995;Amundadottir et al, 1995Amundadottir et al, , 1996Jager et al, 1997;McCormack et al, 1998).…”

Centrosome abnormalities, recurring deletions of chromosome 4, and genomic amplification of HER2/neu define mouse mammary gland adenocarcinomas induced by mutant HER2/neu

“…Tumor bearing animals (FVB 25%, CD1 25% and BALB/c 50%) were sacri®ced and cell lines prepared from tumor tissue as previously described (Amundadottir et al, 1995;McCormack et al, 1998). Metaphase chromosomes were prepared following exposure to colcemid arrest (3 ± 4 h, ®nal concentration 100 mg/ml) and standard hypotonic treatment and ®xation in methanol/acetic acid.…”