Advance and Briefs Mechanisms of Inactivation of p14, p15, and p16 Genes in Human Esophageal Squamous Cell Carcinoma

The 9p21 gene cluster, harboring growth suppressive genes p14, p15, and p16, is one of the major aberration hotspots in human cancers. It was shown that p14 and p16 play active roles in the p53 and Rb tumor suppressive pathways, respectively, and p15 is a mediator of the extracellular growth inhibition signals. To elucidate specific targets and aberrations affecting this subchromosomal region, we constructed a detailed alteration map of the 9p21 gene cluster by analyzing homozygous deletion, hypermethylation, and mutation of the p14, p15, and p16 genes individually in 40 esophageal squamous cell carcinomas (ESCCs) and compared the genetic alterations with mRNA expression in 18 of these samples. We detected aberrant promoter methylation of the p16 gene in 16 (40%), of p14 in 6 (15%), and of p15 in 5 (12.5%) tumor samples. Most p16 methylations were exclusive, whereas all but one of the p14/ p15 methylations were accompanied by concomitant p16 methylation. We detected homozygous deletion of p16 in 7 (17.5%), of p14-E1b in 13 (33%), and of p15 in 16 (40%) tumor samples. Most deletions occurred exclusively on the E1b-p15 loci. Two samples contained p14 deletion but with p16 and p15 intact. No mutation was detected in the p14 and p16 genes. Comparative RT-PCR showed good concordance between suppressed mRNA expression and genetic alteration for p15 and p16 genes in the 18 frozen samples, whereas 5 of the 13 cases with suppressed p14 mRNA expression contained no detectable E1b alteration but aberrations in the p16 locus. Our results show that in human ESCCs, p14 is a primary target of homozygous deletion along with p15, whereas p16 is the hotspot of hypermethylation of the 9p21 gene cluster. The frequent inactivation of the p14 and p16 genes may be an important mechanism for the dysfunction of both the Rb and p53 growth regulation pathways during ESCC development.