Using single-strand conformational polymorphism (SSCP) analysis, we have examined the coding region of the PRAD1 gene in 30 primary breast cancers and 25 parathyroid adenomas.
Cyclin D1 (PRAD1) protein expression in breast cancer: approximately one-third of infiltrating mammary carcinomas show overexpression of the cyclin D1 oncogene.
PRAD1 (previously D11S287) is a putative proto-oncogene at 11q13, activated by overexpression through gene rearrangement or gene amplification in several types of human tumors including parathyroid adenomas, centrocytic lymphomas and other B-cell tumors with t(11;14), and breast cancers.
Four antibodies were reactive with both PRAD1 fusion products and cell lysates of B-cell tumor cell lines with t(11;14)(q13;q32) and a breast cancer cell line with 11q13 amplification, on immunoblotting.
These results suggest that the frequency of overexpression is much higher than previously concluded from DNA-based analyses and that more than one-third of human breast cancers may contain excessive levels of cyclin D1.
In spite of the accumulating genetic evidence, however, there are no data regarding abundance and properties of the cyclin D1 protein in breast cancer.
We conclude that overexpression of cyclin D1 deregulates cell proliferation and can induce tumorigenic changes in mammary tissues, suggesting that cyclin D1 indeed plays an important oncogenic role in breast cancer.
These alterations in the cyclin D1 gene and mRNA suggest that altered expression of cyclin D1 may be important in the malignant transformation of this cell line, and support the identification of cyclin D1 as a dominant oncogene at 11q13 in human breast cancer.
These data implicate dysregulated expression of several cyclin genes, particularly cyclin D1, as a potential factor in the pathogenesis of breast cancer.
We have previously identified two genes (EMS1 and PRAD1/cyclin D1) in the chromosome 11q13 region that are frequently coamplified and overexpressed in human breast cancer and in squamous cell carcinomas of the head and neck (E. Schuuring, E. Verhoeven, W.J.Mooi, and R.J.A.M.Michalides, Oncogene 7:355-361, 1992).
Elevated levels of expression of cyclin D1 protein have been found in a variety of cancers, including breast cancer, head and neck cancer, non-small-cell lung cancer, and mantle cell lymphomas.
Direct cooperation of these distinct abnormalities to cell-cycle, deregulation in NHL cells was suggested by G1 acceleration upon inducible overexpression of cyclin D1 in a control breast cancer cell line lacking p16CDKN2, an effect which could be prevented by ectopic expression of p16CDKN2.
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.
Cyclin D1's oncogenic properties have been suggested by its cooperation with ras or adenovirus E1a to transform cultured cells, as well its overexpression in transgenic mice that leads to breast cancer.