Variable expression of p16 protein in patients with acute myeloid leukemia without gross rearrangements at the DNA level

The p16 protein is an inhibitor of cyclin-dependent kinases (cdk) 4 and 6. Both cdk4 and cdk6 phosphorylate the retinoblastoma protein (pRb) and are thought to be required for cell proliferation. Mutations and homozygous deletions in the p16 gene have been found in a number of cell lines but the incidence of abnormalities in primary tumours is controversial. We have studied the p16 gene in 76 cases of acute myeloid leukemia (AML) and 18 hematologically normal controls by quantitative Southern blotting. No deletions or rearrangements were detected. Twenty-five cases also showed no abnormal band patterns by RT-PCR-SSCP analysis of the coding sequence. We analyzed 60 AML samples for p16 protein expression by Western blot analysis. A reduced level of p16 was observed in six cases, however, none of them showed methylation of the 5'-CpG island. Over-expression of p16 protein was detected in six cases. Studies in cell lines have suggested a feedback loop between p16 and pRb with a futile overproduction of p16 protein in cells containing abnormal pRb. In accordance with these findings, four of the AML cases with high levels of p16 had abnormal pRb (two alteration in band size, two absent). From our data we suggest that gross abnormalities in the p16 gene are rare in AML, but that p16 levels are variable and high levels are associated with pRb abnormalities in a subset of cases.