Comparative expression of endogenous retrotransposons in adult mouse mammary gland during normal development and differentiation

Mutations involving the endogenous ecotropic murine leukemia virus in primary mammary carcinomas of BALB/c mice

Endogenous murine leukemia virus-related elements (MLVEs) are often overexpressed in primary mammary carcinomas of BALB/c mice. We therefore searched for mutations associated with MLVEs and found amplified sequences of the ecotropic MLVE in hormonally and chemically induced mammary neoplasms. Restriction fragment length polymorphism (RFLP) analysis revealed DNA rearrangements consistent with 1-10 or more new copies of the ecotropic MLVE in the genome of these tumors. This is the first evidence of mutations involving an endogenous retrovirus other than mouse mammary tumor virus in mouse mammary carcinomas.

Tumor viruses and endogenous retrotransposons in mammary tumorigenesis

Much of our knowledge about the intricate pathways and molecular mechanisms involved in the conversion of a normal mammary epithelial cell to malignancy derives from studies on mammary tumorigenesis induced by the retrovirus mouse mammary tumor virus. In addition, three DNA tumor viruses, simian virus 40, polyomavirus, and human papillomavirus, have been instrumental in dissecting the series of steps comprising neoplastic progression of mammary epithelium, particularly with cultured human breast cells. Endogenous transposons are analogous bioagents receiving increased attention recently. At least 10% of the cell genome consists of transposable elements, a growing number of which have been implicated in mutagenizing DNA in a variety of human tissues and disorders. Research efforts have therefore intensified to determine if endogenous elements such as retrotransposons participate in the development of breast cancer in animals and humans.