Although alterations in the p53 tumor suppressor gene are detected frequently in human breast cancers, mammary tumors are observed infrequently in p53 null mice. This has led to the suggestion that absence of p53 alone is not sucient for induction of mammary tumors. However, early death of p53 null mice from thymic lymphomas may obscure tumor phenotypes that would develop later. Therefore, p53null mammary epithelium was transplanted into cleared mammary fat pads of wild type p53 BALB/c hosts to allow long-term analysis of mammary tumor phenotypes. Five treatments were compared for their eects on tumor incidence in hosts bearing transplants of p53 null vs p53 wt mammary transplants for each treatment group were 62% vs 0%, 100% vs 0%, 68% vs 0%, 60% vs 4% and 91% vs 14%, respectively. The mammary tumors that developed in the p53 null mammary epithelium were all adenocarcinomas and were frequently aneuploid. These data demonstrate that the absence of p53 is sucient to cause development of mammary tumors and that hormonal stimulation enhances the tumorigenicity of p53 null mammary epithelium to a greater extent than DMBA exposure alone. This model provides an in situ approach to examine the molecular basis for the role of p53 in the regulation of mammary tumorigenesis.
Breast cancer is the most frequent tumor type among women in the United States and in individuals with Li-Fraumeni syndrome. The p53 tumor suppressor gene is altered in a large proportion of both spontaneous breast malignancies and Li-Fraumeni breast cancers. This suggests that loss of p53 can accelerate breast tumorigenesis, yet p53-deficient mice rarely develop mammary tumors. To evaluate the effect of p53 loss on mammary tumor formation, the p53(null) allele was back-crossed onto the BALB/c genetic background. Median survival was 15.4 weeks for BALB/c-p53(-/-) mice compared to 54 weeks for BALB/c-p53(+/-) mice. Sarcomas and lymphomas were the most frequent tumor types in BALB/c-p53(-/-) mice, whereas 55% of the female BALB/c-p53(+/-) mice developed mammary carcinomas. The mammary tumors were highly aneuploid, frequently lost the remaining wild-type p53 allele, but rarely lost BRCA1. Although mammary tumors were rarely detected in BALB/c-p53(-/-) female mice, when glands from BALB/c-p53(-/-) mice were transplanted into wild-type BALB/c hosts, 75% developed mammary tumors. The high rate of mammary tumor development in the BALB/c background, but not C57Bl/6 or 129/Sv, suggests a genetic predisposition toward mammary tumorigenesis. Therefore, the BALB/c-p53(+/-) mice provide a unique model for the study of breast cancer in Li-Fraumeni syndrome. These results demonstrate the critical role that the p53 tumor suppressor gene plays in preventing tumorigenesis in the mammary gland.
Most types of invasive breast cancer are thought to evolve over long periods from specific preexisting benign lesions. Of the many types of benign entities found in the human breast, only a few have clinically significant premalignant potential. Currently, the best-characterized premalignant lesions are atypical ductal hyperplasia, atypical lobular hyperplasia, and lobular carcinoma in situ. Ductal carcinoma in situ is considered to be a preinvasive malignant lesion. Two additional lesions, unfolded lobules and usual ductal hyperplasia, are sometimes considered to be very early premalignant epithelial abnormalities. Premalignant lesions are currently defined by their histologic features, and not all necessarily progress to invasive cancer. This suggests that although lesions within specific categories look alike, they must possess underlying genetic differences that cause some to remain stable and others to advance. The development of modern molecular genetic techniques has allowed breast cancer researchers to clarify the multistep model of breast carcinogenesis. Recent studies indicate that cancer evolves by highly diverse genetic mechanisms, and research into these altered pathways may identify specific early defects that might be targeted to prevent progression of premalignant lesions to invasive cancer. Current clinical management is heterogeneous and depends on histologic examination and individual patient factors. Options for breast cancer risk reduction and prevention are available.
Introduction-Extracellular matrix changes occur in many heart valve pathologies. For example, myxomatous mitral valves are reported to contain excess proteoglycans (PGs) and hyaluronan (HA). However, it is unknown which specific PGs are altered in myxomatous valves. Because PGs perform varied functions in connective tissues, this study was designed to identify and localize three matrixassociated PGs as well as HA and the HA receptor for endocytosis (HARE) within myxomatous and normal mitral valves.Methods-Human mitral posterior leaflets (control n=6−9, myxomatous n=14−21, mean age 61 for all groups) were histochemically stained for PG core proteins, HA, and HARE. Stain intensity was semi-quantitatively graded to determine differences in marker abundance betweennormal and myxomatous valves. The PGs were localized to different regions of the leaflet by correspondence to parallel Movat stained sections Results-The PGs decorin, biglycan and versican were more abundant in myxomatous valves than in normal controls (p<0.03). There was a gender effect on PG presence but no age related trends were observed. HA and HARE were distributed throughout all valves. There was no significant difference in HA between groups, but HARE expression was reduced in myxomatous valves compared to normal controls (p<0.002).Conclusion-Excess decorin, biglycan and versican may be associated with the remodeling of other matrix components in myxomatous mitral valves. Decreased expression of HARE in myxomatous valves suggests that HA metabolism could be altered in myxomatous mitral valve disease. These finding contribute towards elucidating the pathogenesis of myxomatous mitral valve disease and developing potential new therapies.
Missense mutations in the p53 tumor suppressor occur frequently in human breast cancer and in¯uence both the prognosis and response to chemotherapy. Amino acid 175 (equivalent to murine 172) is the second most common site of missense mutations in p53 in human breast cancer. Over 95% of these mutations are arginine-to-histidine (R-H) substitutions resulting in a gain-of-function, and not merely a dominant-negative phenotype. Transgenic mice expressing a p53 172 R ± H construct targeted to the mammary gland by means of a whey acidic protein (WAP) promoter were characterized as a model system in order to determine the speci®c e ects of this mutation on mammary tumorigenesis. Although transgene expression alone had no apparent e ect on normal mammary development, transgenic mice treated with the chemical carcinogen dimethylbenz(a)anthracene developed tumors with much shorter latency than did control littermates and had a greater tumor burden. Tumors arising in transgenic mice did not exhibit either decreased apoptosis or increased cell proliferation relative to tumors arising in nontransgenic littermates, but did display increased genomic instability. Large pleiomorphic nuclei were visible in many tumors from transgenic mice, and DNĀ ow analysis con®rmed the presence of signi®cant aneuploid cell populations. Since these transgenic mice develop very few spontaneous tumors, while accelerating carcinogen-and oncogene-mediated tumorigenesis, this mouse model will, therefore, be useful in the investigation of early events in mammary tumorigenesis. It may also be used as a preclinical model to test newly developed chemotherapeutic strategies.
Mammary tumorigenesis was analysed in transgenic mice which overexpress des(1-3)hIGF-I (WAP-DES) and/or a mutant form of p53 (p53 172R-H ). Nonlactating, multiparous WAP-DES mice exhibited hyperplastic lesions termed mammary interepithelial neoplasia (MIN) which constitutively expressed WAP-DES. By 23 months of age, 53% of the WAP-DES mice developed mammary adenocarcinomas. A 75% reduction in both apoptosis and proliferation was observed in the normal mammary glands of WAP-DES mice. Mammary tumor incidence in WAP-DES/p53 bitransgenic mice was similar to that of WAP-DES and 2 ± 3-fold greater than that of nontransgenic and p53 172R-H females. Tumor latency, however, was reduced by 8 months in bitransgenic mice as compared to mice of the other three genotypes. Aneuploidy was frequently observed in tumors from bitransgenic and p53 172R-H mice, but not from mice expressing only the WAP-DES transgene. Expression of IGFBP3 was elevated in tumors from WAP-DES, but not bitransgenic mice, indicating an alteration in the p53/IGF-I axis. These studies indicate that overexpression of des(1-3)hIGF-I increases the frequency of MIN and stochastic mammary tumors and that the appearance of tumors displaying genomic instability is accelerated by mutant p53 172R-H . Oncogene (2000) 19, 889 ± 898.
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