Characterization of Chromosome 9 in Human Ovarian Neoplasia Identifies Frequent Genetic Imbalance on 9q and Rare Alterations Involving 9p, Including CDKN2

We have examined 41 forms of ovarian cancer for genetic alterations on chromosome 9 using a combination of five RFLP DNA probes and 15 simple tandem repeat polymorphisms. Genetic imbalance (i.e., loss of heterozygosity, microsatellite instability, amplification) for 1 or more informative markers on chromosome 9 was observed in 66% (27 of 41) of our tumor panel. Genetic imbalance was observed on 9q in 59% (24 of 41) of tumors informative for at least one locus. In contrast, only 13% (5 of 40) of informative tumors demonstrated a genetic alteration involving 9p. Furthermore, allelic loss on 9q was more common in late stage tumors (63%, 17 of 27) and poorly differentiated tumors (75%, 15 of 20) as compared to benign and early stage tumors (30%, 3 of 10). Evaluation of 15 tumors showing limited regions of genetic imbalance has identified 2 candidate tumor suppressor regions on 9q and 1 on 9p. Interestingly, the regions defined to 9p21-p24,9q31, and 9q32-q34 all overlap with several known disease loci. In this aspect, the potential role of the CDKN2 gene at 9p21-p22 in ovarian carcinogenesis was assessed in an extended panel of ovarian tumors, 11 human ovarian carcinoma cell lines, and 1 cervical tumor cell line. With the use of comparative multiplex PCR, homozygous deletions were detected in 16 of 115 (14%) fresh tumors and 3 of 12 cancer cell lines. For those tumors demonstrating allelic loss for markers on 9p no somatic mutations were observed in the retained allele of CDKN2, as determined by single-strand conformation polymorphism analysis, but a mutation was observed in an additional cell line. Furthermore, CDKN2 mRNA levels were similar in the 9 cancer cell lines that retain CDKN2, as compared to normal human ovarian surface epithelial cell lines. Overall, our results suggest the potential involvement of a gene or genes on chromosome 9q and de-emphasize a significant role for the CDKN2 gene on 9p in the initiation and progression of ovarian cancer.