Correlation between DNA alterations and p53 and p16 protein expression in cancer cell lines

To investigate the interaction between DNA abnormalities, p53 and p16 gene mutations, and methylation and protein expression, 20 cancer cell lines were examined by Western blotting. A clear relation was found to exist between p53 accumulation and mutation status. Of 20 cell lines examined, 14 demonstrated p53 homozygous mutations in exons 3-10, including 12 missense mutations, one nonsense mutation, and one frameshift mutation. Overexpression of p53 was always linked to missense mutations in exons 6-8. Intermediate expression of p53 was noted in cells with missense mutations or polymorphism to proline at codon 72 in exons 4-5, whereas there was slight or no visible expression in wild type cells and in cells with nonsense and frameshift mutations. DNA aberration in the p16 promoter gene correlated significantly with protein expression of the p16 suppressor gene. Overexpression was noted in six cell lines, intermediate expression in two, and slight or no visible expression in 12. Methylation-caused disappearance of p16 protein was noted in 40% (8/20) of the cell lines. Of six cell lines overexpressing p16 protein, two could be amplified with primers for both unmethylated and methylated forms in a methylation-specific RCP analysis. One cell line with no visible expression could also be amplified with both primers. Overexpression or disappearance of p16 protein may readily occur when one of two alleles has been methylated.