Background: Several estrogen receptor (ER) variant messenger RNAs (mRNAs) have been identified previously in human breast cancer biopsy samples and cell lines. The relative levels of certain ER variant mRNAs have been observed to increase with breast tumor progression. In vitro assays of the function of polypeptides encoded by some of these variant mRNAs have led to speculation that ER variants may be involved in the progression from hormone dependence to independence in breast cancer.
Purpose: We set out to establish if ER variant mRNAs are present in normal human breast tissues and, if so, to compare levels of these variants between normal and neoplastic human breast tissues.
Methods: Four human breast tissue samples from reduction mammoplasties and five samples from tissue adjacent to breast tumors were analyzed. The tissue samples were confirmed to be normal (i.e., not malignant) by histopathologic analysis. RNA was extracted immediately from adjacent frozen sections. Human breast tumor specimens originally resected from 19 patients were acquired from a tumor bank and processed in the same way as the normal tissue samples. The RNAs were then reverse transcribed and subsequently amplified with the use of the polymerase chain reaction (PCR). PCR primer sets were designed to detect several different exon-deleted ER variants and a truncated ER variant (i.e., clone 4). A semiquantitative PCR-based method was used to determine the relative expression of exon 5- and exon 7-deleted variants to wild-type ER mRNAs in the nine normal tissues and in 19 ER-positive breast tumor tissues. The Mann-Whitney rank sum test (two-sided) was used to determine P values.
Results: ER variant mRNAs corresponding to the clone 4 ER truncated variant and to variants deleted in either exon 2, exon 3, exons 2-3, exon 5, or exon 7 were detected in all normal samples. The results were confirmed by restriction enzyme analyses and sequencing of the PCR products. The expression of exon 5-deleted ER variant relative to the wild-type ER mRNA was significantly lower (P< .001) in normal tissue than in tumor tissue. A similar trend was noted for expression of the exon 7-deleted ER variant mRNA; however, the difference did not achieve statistical significance (P= .476).
Conclusion: Several ER variant mRNAs are present in normal human breast tissue, but the level of expression of some of these variants may be lower in normal tissue than in tumor tissue.
Implication: These data suggest that the mechanisms generating ER variant mRNAs exist in normal breast tissue and may be deregulated in breast cancer tissues. Further investigation of the role of variant ER expression in development and progression of human breast cancer appears warranted.