Variant estrogen receptor mRNA species detected in human breast cancer biopsy samples

Thirty to 40% of estrogen receptor (ER) positive human breast tumors are resistant to endocrine therapy. To investigate the possibility that abnormal ER proteins may be associated with this endocrine resistant phenotype we have looked for the presence of abnormally sized ER mRNA in human breast tumor biopsies. Poly(A+) enriched RNA was isolated from 46 human breast tumor biopsies and analyzed by Northern blotting and hybridization with the human estrogen receptor OR-8 cDNA. Seventy percent of the tumors contained detectable 6.5 kilobase (kb) ER mRNA. Some tumors also contained smaller sized ER mRNA of approximately 3.8 and 2.4 kb. These variant sized transcripts were only detected in biopsies where the normal 6.5 kb mRNA was present. In some cases the abundance of the variant mRNAs was equal to or greater than the normal ER mRNA. The variant ER mRNAs were not due to nonspecific RNA degradation nor was there any evidence of gross alteration of the ER gene in tumors where variant mRNAs were detected. ER cDNA fragments representing only the E/F domain of the receptor showed little or no hybridization with the variant sized ER mRNAs, while a ER cDNA fragment covering the A/B, C and D domains hybridized to both the variant and normal ER mRNAs. This suggested that the smaller variant ER mRNAs may be missing some or all of the E/F domain which is thought to contain the steroid hormone binding domain of the receptor. It is hypothesized that if these intermediates were translated into viable proteins they may interfere with the normal ER function.