Infrequent mutation of the p16/MTS1 gene and overexpression of cyclin-dependent kinase 4 in human primary soft-tissue sarcoma

The pl6INK4a/MTS1 (p16) gene encodes a specific inhibitor of cyclin-dependent kinase (CDK)4 and CDK6. The p16 gene is frequently mutated or deleted in many types of cancer cell lines as well as in certain types of primary tumors. p16 knockout mice are viable but predisposed to sarcoma and B-cell lymphoma. To investigate the role of p16 in human soft-tissue sarcoma tumor progression, we examined the p16 gene by Southern blot analysis and PCR sequencing in 30 pairs of primary soft-tissue sarcomas and autologous normal tissue. Only one tumor sample showed possible rearrangement of the p16 gene. In contrast, Western blot analysis of the p16 protein in 20 pairs of samples showed decreased p16 expression in only 20% of the tumors but elevated p16 expression in 40% of the tumors when compared with the autologous normal controls. Overexpression of p16 was not concomitant with loss of the RB protein as is found in several other types of cancers, because more than one-half of the tumors with increased p16 expression also had high levels of RB protein. On the other hand, the p16 target protein CDK4 was overexpressed in at least 60% of the tumors. In the majority of cases, CDK4 overexpression accompanied elevated p16 and/or RB levels. Our results suggest that: (a) alteration of the p16 gene is infrequent in primary soft-tissue sarcoma; (b) Cdk4 may act as an oncogene in soft-tissue sarcoma; and (c) elevated p16 and RB levels might be the result of compensatory up-regulation of these proteins to counteract CDK4 overexpression in these tumors. Our results also suggest that it is more informative to examine aberrations in the "p16-CDK4/cyclin D-RB" pathway than to selectively examine individual components in this pathway when investigating genetic changes involved in human malignancy.