Aberrant p16 expression is correlated with hemizygous deletions at the 9p21-22 chromosome region in non-small cell lung carcinomas

The p16 protein is encoded by the CDKN2 gene, and functions as an inhibitor of cyclin-dependent kinase 4 anal 6 (CDK4/6). Phosphorylation of the retin oblastoma protein pRb) by CDK4/6 represents a vital step in cell cycle prog ression. Alterations of p16(INK4A) are frequent events in human malignancie s. In non-small cell lung carcinoma (NSCLC) the data concerning the mechani sms of p16(INK4A) inactivation suggest that point mutations and aberrant me thylation of its promoter can only account for a proportion of the cases wi th abnormal p16 immunoexpression. The role of deletions in this procedure i s not yet clarified. In order to gain more insight into the role of deletio ns in p16(INK4A) deregulated expression, we investigated the state of the c hromosomal region 9p21-22 in a series of 57 NSCLCs, by performing a detaile d mapping analysis, using a tight cluster of highly polymorphic microsatell ite markers, and correlating the findings with p16 immunostaining. Abnormal p16 expression was observed in 46% of the NSCLCs examined. No relationship was observed between p16 abnormal staining and various clinicopathological parameters. Abnormal p16 protein staining was strongly associated with hem izygous deletions at the IFNA and D9S171 microsatellite loci, which demarca te the region encoding the p16(INK4A) gene (P=0.002). These findings sugges t that deregulated expression of p16 is involved in the multistage process of NSCL carcinogenesis and that deletions may represent a predominant mecha nism of p16(INK4A) inactivation. A significant percentage also of LOH was n oticed at the D9S162 (35%) and D9S126 (38%) loci which lie 6cM and 4cM, res pectively, far from the area which encodes p16(INK4A), implying that other tumor suppressor genes (TSGs) may reside in this region. Although the overa ll incidence of LOH at the examined region was high (58%), we did not obser ve any correlation with smoking habits, histology and lymph node status. An other noteworthy finding was the existence of microsatellite instability (M I) in 11% of the patients. MI provides a marker for replication error pheno type (RER+), a recently defined manifestation of genetic instability observ ed in a wide range of tumors. In conclusion, alterations (LOH+MI) at the 9p 21-22 chromosome region are frequent events in NSCLCs and may affect direct ly or indirectly the expression of p16.