Mutation analysis of 8p genes POLB and PPP2CB in bladder cancer

The DNA polymerase beta gene (POLB), which encodes a DNA polymerase believed to be involved in short gap-filling DNA synthesis, has been mapped to the proximal region of 8p (8p12-p11), a region commonly deleted in bladder carcinoma and a wide variety of other neoplasms. Also mapped to this region (8p12-p11.2) is the gene encoding the beta isoform of the catalytic subunit of protein phosphatase 2A (PPP2CB), a major serine/threonine phosphatase thought to play a regulatory role in many cellular pathways. The known functions of these proteins make them good candidates for 8p tumor suppressor genes. To test this hypothesis, we assessed a series of bladder tumors and bladder tumor cell lines for sequence variation in POLB and PPP2CB. Single strand conformation polymorphism (SSCP) analysis and direct sequencing of POLB cDNA derived from cell lines and tumors, many with known deletions of proximal 8p, revealed one sequence variant that was shown to represent a normal sequence polymorphism. No tumor-specific sequence variants were identified. The promotor sequence in genomic DNA from tumors with 8p LOH was also screened by SSCP. Four polymorphisms were identified but no tumor-specific mutations were found. PPP2CB was analyzed by SSCP analysis of all 7 coding exons in genomic DNA of bladder tumors and cell lines. Polymorphisms were detected in exons 4 and 5 but no tumor-specific mutations were found. We conclude that these genes are unlikely to be the suppressor genes for bladder cancer targeted by deletions of chromosome arm 8p.