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A truncated cyclin D1 gene encodes a stable mRNA in a human breast cancer cell line
D E Lebwohl1, R Muise-Helmericks, L Sepp-Lorenzino
1Department of Medicine, Memorial Sloan-Kettering Cancer Center, New York, New York 10021.
Abstract:
The G1 cyclin D1 is amplified in approximately 20% of human breast cancers and is frequently overexpressed as part of an amplicon in these tumors, suggesting a potential role for this gene in the pathogenesis of breast cancer. Although amplification of cyclin D1 occurs in human breast cancer, it is possible that another gene in the amplicon is the relevant oncogene in these cancers. We now report a truncation of the cyclin D1 gene in a human breast cancer cell line, associated with overexpression of a short cyclin D1 mRNA. In a survey of breast cancer cell lines and tumors by Southern blot hybridization, using a 1.2 kb human cyclin D1 cDNA, we observed that genomic DNA derived from the MDA MB-453 cell line contains an extra band in the Bg1II and BamHI digests, suggesting that one allele of gene is altered. Moreover, the altered allele is amplified three-fold relative to the normal allele, and contains a 3' deletion. On Northern analysis, the MDA MB-453 line has a marked increase in 1.1 to 1.3 kb transcripts, which are truncated at the 3' end, in contrast to the normally predominant 4.2 kb transcript. The 1.1-1.3 kb cyclin D1 mRNA has a longer half-life than the 4.2 kb mRNA, indicating that the 3' truncation may contribute an increased stability and therefore an elevated steady-state level of the short mRNA. 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.
Insights
Cyclin D1 gene alterations, including a 3' deletion, were found in breast cancer cells, leading to a shorter, more stable mRNA. This suggests cyclin D1 acts as an oncogene in breast cancer development.
Area of Science:
- Molecular Biology
- Oncology
- Genetics
Background:
- Cyclin D1 amplification is observed in ~20% of human breast cancers.
- Overexpression of cyclin D1 suggests its role in breast cancer pathogenesis.
- The specific oncogene within amplified regions in breast cancer remains under investigation.
Purpose of the Study:
- To investigate alterations in the cyclin D1 gene in human breast cancer.
- To determine the functional consequences of observed cyclin D1 gene modifications.
- To confirm cyclin D1 as a dominant oncogene in breast cancer.
Main Methods:
- Southern blot hybridization using human cyclin D1 cDNA.
- Analysis of genomic DNA from breast cancer cell lines and tumors.
- Northern blot analysis to assess mRNA transcripts.
Main Results:
- A 3' deletion and amplification of one cyclin D1 allele were identified in the MDA MB-453 cell line.
- MDA MB-453 cells exhibit increased expression of truncated, 3' end-deleted cyclin D1 mRNA (1.1-1.3 kb).
- The truncated cyclin D1 mRNA demonstrates enhanced stability compared to the normal 4.2 kb transcript.
Conclusions:
- Altered expression of cyclin D1 due to gene truncation and increased mRNA stability is implicated in breast cancer cell transformation.
- These findings support cyclin D1 as a dominant oncogene at chromosome 11q13 in human breast cancer.