Chromosome 9p Allelic Loss and pl6/CDKN2 in Breast Cancer and Evidence of pl6 Inactivation in Immortal Breast Epithelial Cells

To define the extent of involvement of chromosome 9p in breast carcinogenesis, we performed microsatellite length polymorphism analysis of markers spanning this region. Of 24 primary breast carcinomas analyzed, we observed a high frequency (58%) of loss of heterozygosity or allelic imbalance affecting subregion 9p21–22. Mutational analysis of CDKN2 (pi6) was performed to determine whether this gene was the target of such alterations. Of 21 tumors analyzed, only 1 showed a mutation of probable consequence, suggesting that CDKN2 appears not to be the target of loss of heterozygosity and indicating the possible existence of another tumor suppressor gene within this region. Additionally, since it has been suggested that some CDKN2 deletions and mutations could be due to an in vitro phenomenon, four immortal breast cell lines derived from normal epithelium, MCF10F, MCF12F, 184A1, and 184B5, were examined for loss or mutation of CDKN2. Two lines (MCF10F and MCF12F) showed homozygous deletions of CDKN2, and one (184A1) revealed a hemizygous deletion and a nonsense mutation in the remaining allele. This could imply an important role of CDKN2 in the control of immortalization or in vitro adaptation and is the first evidence of such in nontumor-derived cell lines. Additionally, this is the first report of frequent loss of heterozygosity in the 9p21–22 chromosome subregion of uncultured primary breast tumors.