Expression and Promoter Methylation of P16INK4A During Estrogen-Induced Mammary Carcinogenesis in the ACI Rat

摘要

Breast cancer is one of the leading causes of death for women in the United States and estrogen exposure has been implicated in the development of this cancer. Our lab is studying the ACI rat an estrogen-induced breast cancer animal model to begin to elucidate the role of estrogen in breast cancer. The ACI rat develops mammary cancer after prolonged exposure to 17p-estradiol while the BN and genetically related COP rats do not. We have mapped a OTL conferring susceptibility to estrogen-induced mammary cancer on rat chromosome 5 for which the p16INK4A gene is a positional candidate. We found no differences in Cdkn2a gene expression between the COP, ACI and BN strains; however gene expression was significantly elevated in the tumors relative to normal ACI mammary tissue. Methylation status of the promoter region was examined and no significant differences were found between tumor and normal tissue suggesting that an alternative mechanism to loss of methylation accounts for upregulation of Cdkn2a gene expression in ACI mammary tumors. Sequencing of the p16INK4A gene using spleen cDNA revealed no polymorphisms in untreated ACI COP or BN rats. In contrast both tumors and hyperplastic mammary tissue from ACI rats treated with estrogen for 28 weeks revealed a number of independently arising mutations and polymorphisms. We will sequence genomic DNA isolated from the same tumors and mammary tissue to confirm whether the intratumoral heterogeneity is at the genomic DNA or RNA level.