Detection of wild type and exon 5-deleted splice variant oestrogen receptor (ER) mRNA in ER-positive and -negative breast cancer cell lines by reverse transcription/polymerase chain reaction.

Using reverse transcription (RT)/PCR we have shown that four breast cancer cell lines expressed oestrogen receptor (ER) mRNA, irrespective of whether they were assessed as ER-positive (MCF-7 and BT-474) or ER-negative (MDA-MB-231 and BT-20) by enzyme immunoassay (EIA). In addition to the wild type (WT) form, they were all found to express the exon 5-deleted variant (V) form of ER mRNA by RT/PCR; this is thought to code for a truncated constitutively active protein. By Northern blot analysis only the ER-positive cell lines (MCF-7 and BT-474) were found to express detectable levels of ER mRNA. Oestradiol-induced growth was found only in the ER-positive (by EIA) cell lines. These results confirm that the differences between ER-positive and ER-negative cell lines are quantitative rather than qualitative. As low levels of ER mRNA could be detected by RT/PCR, this may reflect the greater sensitivity of this approach. The presence of exon 5-deleted V form ER mRNA in addition to the WT form in all four breast cancer cell lines may allow these lines to be used to assess differential regulation of transcription and the impact of this on their oestrogen dependence.