A high-risk lesion for invasive breast cancer, ductal carcinoma in situ, exhibits frequent overexpression of retinoid X receptor.

ABSTRACT

The development of prevention strategies for breast cancer will require a molecular map of carcinogenesis. We have investigated gene expression patterns in premalignant and early carcinomatous human breast lesions that confer to the patient varying risks for developing invasive breast cancer. The relative expression levels of one of the retinoid receptors, retinoid X receptor (RXR), was determined by in situ hybridization to 58 biopsy specimens; RXR mRNA grain density over each lesion was compared to that over the normal ductal/lobular units in each section. Overexpression of RXR mRNA was observed in 66% of noncomedo ductal carcinoma in situ (DCIS), which confer a >8-fold increase in breast cancer risk, and 88% of comedo DCIS lesions, which are associated with a yet higher risk. In contrast, only 8% of lesions that confer little or no increase in breast cancer risk overexpressed RXR mRNA (P = 0.0008). Limited in situ hybridization data using retinoic acid receptor (RAR) riboprobes showed overexpression of RAR alpha, but not RAR beta or -gamma, in only a modest percentage (36%) of cases, suggesting that all members of the retinoid receptor superfamily are not similarly regulated. Immunohistochemistry performed on 52 DCIS specimens for alpha, beta, and gamma isoforms of RXR confirmed its overexpression at the protein level and implicate RXR alpha as the predominant overexpressed form. The data indicate that RXR overexpression is associated with an increased risk for the development of invasive breast cancer in human breast lesions and suggest the hypothesis that it is causally involved in breast oncogenesis. The implications for retinoid chemoprevention are discussed.