Deletion mapping at 12p12-13 in metastatic prostate cancer

The identification of homozygous deletions in malignant tissue is a powerful tool for the localization of tumor suppressor genes. Representational difference analysis (RDA) uses selective hybridization and the polymerase chain reaction (PCR) to isolate regions of chromosomal loss and has facilitated the identification of tumor suppressor genes, such as BRCA2 and PTEN. We have recently identified a 1-5-cM homozygous deletion on 12p12-13 in a prostate cancer xenograft and found that 47% of patients who died of prostate carcinoma demonstrate focal loss of heterozygosity (LOH) in this region in metastatic deposits. We have now characterized the region of interest by assembling a yeast artificial chromosome (YAC) contig spanning the homozygous deletion and identifying which known genes and expressed sequence tags (EST) lie within the homozygous deletion. A rib metastasis was harvested at autopsy and placed subcutaneously in a male SCID mouse. Genomic DNA from this xenograft and from the patient's normal renal tissue was extracted. Multiplex PCR, with the xenograft and normal DNA used as template, was performed using primers for loci on the Whitehead contig 12.1 believed to be near our region of interest. We found that our deletion lay in a 1-2-Mb interval between WI-664 and D12S358. We then used the same primers to construct a YAC contig across the homozygous deletion. PCR amplification of YAC DNA, using primers for the genomic sequences of known genes and ESTs reported to lie on 12p12-13, was used to identify candidate genes that lay within the deletion. Duplex PCR, with control primers known not to be deleted in the xenograft, was used to confirm that both the CDKN1B and ETV6 genes were homozygously deleted in the xenograft. Mutations in either or both of these genes may play an important role in metastatic prostate carcinoma.