AN EXON-5 DELETION VARIANT OF THE ESTROGEN-RECEPTOR FREQUENTLY COEXPRESSED WITH WILD-TYPE ESTROGEN-RECEPTOR IN HUMAN BREAST-CANCER

Recent evidence suggests that the expression of estrogen receptor (ER) variants in breast cancer may interfere with wild-type (wt) ER function and be related to tumor progression and resistance to hormone treatment. One of these variants, ERDELTAE5, lacking that part of the hormone-binding domain encoded by exon 5, has previously been identified in breast tumors with the unusual estrogen receptor negative (ER-) and progesterone receptor positive (PgR+) phenotype and found to possess constitutive and hormone-independent transcriptional activity. Using a ribonuclease protection assay, we analyzed 27 breast tumors and 4 breast cell lines for the presence of this variant. We found the ERDELTAE5 variant to be expressed, not only in all of three ER-/PgR+ tumors but also in 19 of 20 ER+/PgR+ or ER+/PgR- tumors. Moreover, the variant was always coexpressed with and often in excess of wtER. ERDELTAE5 was also found in three breast cancer cell lines (MCF7, T47D, and ZR75-1), although to a lesser extent than wtER. A complete absence of both ERDELTAE5 and wtER was noted in four ER-/PgR- tumors and one normal breast cell line (HBL-100). Thus, our data suggest that the occurrence of ERDELTAE5 in breast cancer may represent a critical stage in tumor progression to autonomy.