Mechanisms of G1 checkpoint loss in resected early stage non-small cell lung cancer
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J X Zhou1, G A Niehans, A Shar
1Department of Research Service, Minneapolis Veterans Affairs Medical Center, University of Minnesota Medical School, Minneapolis, MN 55417, USA.
Abstract:
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 p15INK4b) and p16INK4a by DNA methylation is very common (37%), and methylation of p16INK4a is tightly correlated with loss of expression of p16INK4a protein (P = 0.0018). Furthermore, methylation of p15INK4b and p16INK4a appear inversely correlated, although methylation of p15INK4b 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 p16INK4a gene product. Although loss of p16INK4a 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 p16INK4a gene product.
Insights
Loss of G1 checkpoint regulators p15INK4b and p16INK4a via DNA methylation is common in non-small cell lung cancer (NSCLC). Methylation of p16INK4a correlates with protein loss, particularly in squamous cell carcinomas.
Area of Science:
- Oncology
- Molecular Biology
- Epigenetics
Background:
- The G1 cell cycle checkpoint is frequently lost in neoplasms.
- Genetic and epigenetic alterations drive this loss.
- Understanding these mechanisms is crucial for cancer research.
Purpose of the Study:
- To investigate the frequency and correlation of DNA methylation of G1 cyclin-dependent kinase inhibitors (p15INK4b and p16INK4a) in early-stage non-small cell lung cancer (NSCLC).
- To determine the relationship between methylation, protein expression, and clinicopathological features, including tumor histology and patient survival.
Main Methods:
- Analysis of DNA methylation status of p15INK4b and p16INK4a in a series of early-stage NSCLC.
- Assessment of p16INK4a protein expression.
- Correlation of methylation and expression data with tumor histology (squamous cell carcinoma vs. others) and patient survival.
- Evaluation of cyclin D1 and pRB expression in relation to p16INK4a status.
Main Results:
- DNA methylation of p15INK4b and p16INK4a was observed in 37% of NSCLC cases.
- Methylation of p16INK4a strongly correlated with loss of its protein expression (P = 0.0018).
- Methylation was more frequent in squamous cell carcinomas (P = 0.0156) and occurred equally across all NSCLC stages, without impacting survival.
- Cyclin D1 overexpression was restricted to tumors with wild-type pRB (P = 0.0032) and was independent of p16INK4a status.
Conclusions:
- Epigenetic silencing of p15INK4b and p16INK4a by DNA methylation is a common event in NSCLC, particularly affecting p16INK4a expression.
- These alterations are frequent in squamous cell carcinomas but do not influence patient survival in early-stage disease.
- While p16INK4a loss correlates with pRB status, other regulatory pathways like p53 mutation and cyclin D1 overexpression operate independently in NSCLC.
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