Abstract 2157: Is CDK6 the target of amplification at 7q21 in esophageal adenocarcinoma

Abstract

Abstract Introduction: The cyclin-dependent kinase 6 (CDK6) is a positive regulator of the cell cycle. It binds to D-type cyclins and phosphorylates the pRb tumor suppressor proteins allowing progression through the G1-phase of the cell cycle. Amplification of the 7q21 chromosomal region has been reported in esophageal adenocarcinoma (EAC) and it has been proposed that CDK6 is the key gene targeted by this event. However, overexpression of CDK6 in vitro suppresses proliferation of breast cancer cells and causes nuclear accumulation of pRb2 tumor suppressor protein. Thus, it remains unclear whether amplification and overexpression of CDK6 is truly driving the 7q21 amplicon in EAC. Methods: For DNA copy number analysis we analyzed genomic DNA from 83 EAC tumors using Affymetrix 6.0 SNP arrays. Protein expression was analyzed in tissue microarrays comprised of 116 independent EAC samples and pre-EAC lesions (15 high grade dysplasia (HGD), 18 low grade dysplasia (LGD), and 283 cases of columnar cell change (CCM)) using a monoclonal antibody against CDK6. CDK6 was overexpressed by stable transfection of OE19, OE33 and Flo1A esophageal cancer cells using CDK6 cDNA under the control of the pCMV promoter. Results: The microarray DNA copy number analysis showed amplification of the 7q21 region in at least 25% of cases. The minimal amplicon contained CDK6 along with 4 other genes. Immunohistochemistry showed both cytoplasmic and nuclear CDK6 staining in EAC (36, 31.3%), HGD (6, 40.0%), LGD (5, 27.7%), and the CCM (29, 10.2%). Overexpression of CDK6 in EAC cell lines caused either suppression of proliferation and anchorage independence (OE19 cells) or had no effect on proliferation (OE33 and Flo1A cells). The suppression of proliferation in OE19/CDK6 cells is dependent on the kinase activity of CDK6 as OE19 cells overexpressing a kinase-deficient form of CDK6 proliferate normally. As previously reported in breast cancer cells, pRb2 (p130) tumor suppressor protein is upregulated in OE19/CDK6 cells. The upregulation of pRb2, however, does not seem to account for the hypo-proliferative effect of OE19/CDK6 cells as it is also upregulated in normally proliferative cells that overexpress the kinase-negative form of CDK6. Conclusion: Although CDK6 is amplified and overexpressed in EAC and its preneoplastic lesions, it does not seem to account for the tumorigenic importance of the 7q21 amplicon. CDK6 overexpression in vitro does not show any mitogenic phenotype in EAC tumor cells. The presence of other genes within the minimal amplicon raises the possibility that one or more of them might drive such a potential. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2157.