Altered expression of exon 6 deleted progesterone receptor variant mRNA between normal human breast and breast tumour tissues.

The progesterone receptor (PR) is an important prognostic marker in breast cancer as well as a marker of responsiveness to endocrine therapies. The presence of several exon-deleted PR variant mRNAs in both normal and neoplastic breast samples has recently been reported. Amongst them, a variant mRNA deleted in exon 6 (D6-PR mRNA) that if translated would encode a truncated PR-like protein missing the hormone binding domain and one of the transactivating domains of the wild-type PR protein. In order to determine whether changes in D6-PR variant expression could occur during tumorigenesis, its expression was investigated by reverse transcription and polymerase chain reaction in ten normal reduction mammoplasty samples, nine breast tumours with high PR levels (> 100 fmol mg(-1) protein) and eight breast tumours with low PR levels (< 15 fmol mg(-1) protein), as determined by ligand binding assay. The relative expression of D6-PR to wild-type PR mRNA was lower (P< 0.01 ) in normal than in all tumour breast samples. Moreover, a trend to lower (P < 0.1) relative D6-PR expression was observed in high PR tumours, compared to low PR tumours. These data suggest that increased expression of D6-PR occurs during tumorigenesis.