Human Retinoblastoma Gene: Long-Range Mapping and Analysis of Its Deletion in a Breast Cancer Cell Line

Abstract

Mutational inactivation of the retinoblastoma (RB) gene is considered a fundamental event in the formation of several types of human cancer. A substantial proportion of RB gene mutations are partial or complete deletions that extend an unknown distance beyond one or both ends of the gene. To provide a framework for measuring the extent of these deletions, we have constructed a long-range restriction map of SfiI sites spanning 850 kilobases around the RB gene. This map was applied in a molecular analysis of RB gene deletion in breast cancer cell line MB468. A previous study of this cell line demonstrated deletion of the entire RB gene except for exons 1 and 2 (E. Y.-H. P. Lee, H. To, J.-Y. Shew, R. Bookstein, P. Scully, and W.-H. Lee, Science 241:218-221, 1988). Genomic clones containing the deletion junction were isolated from a library made from MB468 DNA. A probe obtained from the far side of the deletion junction was used to localize and clone the unknown 3′ endpoint, demonstrating that the chromosomal mutation in this case was a simple deletion spanning 200 kilobases. Sequence analysis of the deletion junction indicated a conservative deletion with no loss or gain of nucleotides. The deletion endpoints had no sequence homology to each other or to any repetitive sequence family, such as Alu, so the recombination event was illegitimate. Structural analysis of this and other RB gene deletions is important for understanding molecular mechanisms of recessive oncogenesis.