Cyclin D1 (PRAD1) alternative transcript b: full-length cDNA cloning and expression in breast cancers

The cyclin D1/PRAD1 protooncogene is a key regulator of the G1 phase of the cell cycle and has been incriminated in the pathogenesis of a variety of primary human tumors. Recently, part of a novel alternatively spliced cyclin D1 transcript, called transcript b, has been identified. This variant transcript showed a failure of splicing at the 3′ end of exon 4 and as a result, the expected protein product is altered at its C-terminus. Because of similar transcript sizes, previous Northern analyses would not have been expected to distinguish the two variants, and the relative levels of the two cyclin D1 transcripts in human tumors is unknown. To elucidate the role of cyclin D1 transcript b, we have isolated cDNA clones of this variant transcript from human breast cancer cell lines and report the sequence of the entire coding region of the cDNA. The protein predicted from the cDNA sequence consists of 274 amino acid residues and lacks a PEST sequence in its C-terminus. Examination of the levels of the two alternative cyclin D1 transcripts in primary breast cancers and breast cancer cell lines by Northern blot analysis and reverse transcription-polymerase chain reaction (RT-PCR) assays showed that the variant transcript b is indeed expressed in primary breast cancers and breast cancer cell lines, but the level of transcript b is dramatically lower than that of the originally reported transcript a of the cyclin D1 gene. In breast cancers, oncogenic overexpression of cyclin D1 mRNA appears to consist overwhelmingly of transcript a, and the role of transcript b, if any, in oncogenesis remains to be established.