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Mutations and altered expression of p16INK4 in human cancer
A Okamoto1, D J Demetrick, E A Spillare
1Laboratory of Human Carcinogenesis, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892.
Abstract:
Cell cycle arrest at the G1 checkpoint allows completion of critical macromolecular events prior to S phase. Regulators of the G1 checkpoint include an inhibitor of cyclin-dependent kinase, p16INK4; two tumor-suppressor proteins, p53 and RB (the product of the retinoblastoma-susceptibility gene); and cyclin D1. Neither p16INK4 nor the RB protein was detected in 28 of 29 tumor cell lines from human lung, esophagus, liver, colon, and pancreas. The presence of p16INK4 protein is inversely correlated with detectable RB or cyclin D1 proteins and is not correlated with p53 mutations. Homozygous deletions of p16INK4 were detected in several cell lines, but intragenic mutations of this gene were unusual in either cell lines or primary tumors. Transfection of the p16INK4 cDNA expression vector into carcinoma cells inhibits their colony-forming efficiency and the p16INK4 expressing cells are selected against with continued passage in vitro. These results are consistent with the hypothesis that p16INK4 is a tumor-suppressor protein and that genetic and epigenetic abnormalities in genes controlling the G1 checkpoint can lead to both escape from senescence and cancer formation.
Insights
The p16INK4 protein, a cell cycle regulator, was absent in most tested human cancer cell lines. Loss of p16INK4 function is linked to cancer development, suggesting its role as a tumor suppressor.
Area of Science:
- Oncology
- Molecular Biology
- Cell Biology
Background:
- The G1 checkpoint in the cell cycle is crucial for preventing DNA replication errors.
- Key regulators of the G1 checkpoint include p16INK4, p53, retinoblastoma (RB) protein, and cyclin D1.
Purpose of the Study:
- To investigate the role of p16INK4 in human cancers.
- To determine the frequency of p16INK4 alterations in various tumor types.
Main Methods:
- Analysis of p16INK4, RB, and cyclin D1 protein expression in 29 human tumor cell lines.
- Detection of homozygous deletions and intragenic mutations in the p16INK4 gene.
- Functional assessment of p16INK4 by cDNA transfection into carcinoma cells.
Main Results:
- p16INK4 and RB proteins were undetectable in 28 of 29 tumor cell lines.
- p16INK4 expression inversely correlated with RB or cyclin D1 presence.
- Homozygous deletions of p16INK4 were observed, but intragenic mutations were rare.
- p16INK4 transfection inhibited colony formation and led to in vitro selection against expressing cells.
Conclusions:
- p16INK4 functions as a tumor suppressor protein.
- Abnormalities in G1 checkpoint regulators contribute to cancer formation by enabling cells to bypass senescence.