IDENTIFICATION OF A NOVEL BREAST-CANCER-ANTI-ESTROGEN-RESISTANCE (BCAR2) LOCUS BY CELL-FUSION-MEDIATED GENE-TRANSFER IN HUMAN BREAST-CANCERCELLS

Development of anti-estrogen resistance limits the benefit of endocrin e therapy of breast cancer. The mechanistic basis for resistance to th e anti-estrogen tamoxifen may involve (epi)genetic alterations within tumor cells. We have initiated a random search for genes allowing estr ogen-dependent ZR-75-1 human breast-cancer cells to proliferate in the presence of tamoxifen. The strategy was based on insertion mutagenesi s of ZR-75-1 cells using defective retrovirus and subsequent identific ation of common integration sites. As an alternative approach to ident ify integration loci involved in anti estrogen resistance, we have app lied cell fusion. Integration regions from lethally irradiated, tamoxi fen-resistant cells were transferred to hygromycin B-resistant ZR-75-1 cells. Somatic cell hybrids were established by selection for resista nce to G418 (encoded by the integrated virus) and hygromycin B. Indivi dual integration loci were thus separated among different cell hybrids and tested for their role in anti-estrogen resistance. Analysis of a panel of 29 somatic-cell hybrids revealed that tamoxifen resistance co -segregated with only I of the 2 integration loci present in the tamox ifen-resistant donor cell line. This locus was further identified as a common integration site in our panel of tamoxifen-resistant cell clon es. Our results designate this integration site as the second breast-c ancer-anti-estrogen-resistance locus (BCAR2), which most likely contai ns a gene responsible for the anti-estrogen-resistant phenotype in clo se proximity to the integrated virus. Our data also imply that individ ual genes can alter the estrogen dependency of human breast-cancer cel ls in a dominant manner in vitro. (C) 1997 Wiley-Liss, Inc.