Mechanisms of G₁ checkpoint loss in resected early stage non-small cell lung cancer

Loss of the G₁ checkpoint appears to be extremely common among virtually all neoplasms. A variety of genetic and epigenetic mechanisms have been demonstrated to play significant roles in this process. In a consecutive series of early stage non-small cell lung cancer (NSCLC), we have established the loss of expression of the G₁ Cdk inhibitors p15^INK4b and p16^INK4a by DNA methylation is very common (37%), and methylation of p16^INK4a is tightly correlated with loss of expression of p16^INK4a protein (P=0.0018). Furthermore, methylation of p15^INK4b and p16^INK4a appear inversely correlated, although methylation of p15^INK4b is an infrequent event in this cohort (4%). Methylation was detected in all stages of NSCLC equally, and did not correlate with survival in these patients. Evidence for methylation was more frequent in squamous cell carcinomas in comparison to other tumor histologies (P=0.0156). In addition, over-expression of cyclin D1 was found to be tightly restricted (P=0.0032) to those tumors that had retained wild-type expression of pRB, and did not correlate with methylation or expression of p16^INK4a gene product. Although loss of p16^INK4a function remains tightly correlated with pRB expression, loss of other regulatory elements in NSCLC such as p53 mutation and cyclin D1 over-expression appear independent of loss of the p16^INK4a gene product.