CDKN2A GENE DELETIONS AND LOSS OF P16 EXPRESSION OCCUR IN OSTEOSARCOMAS THAT LACK RB ALTERATIONS

Osteosarcomas often suffer mutations of the RB (retinoblastoma) gene, with resultant inactivation of the pRb protein. pRb is one component i n a cell-cycle control pathway that includes the p16 (encoded by the C DKN2A gene) and cyclin-dependent kinase 4 (cdk4, encoded by the CDK4 g ene) proteins. We therefore sought to determine whether the CDKN2A and CDK4 genes were altered in those osteosarcomas that lacked RE inactiv ation Twenty-one osteosarcomas (2 low-grade and 19 high-grade) were ev aluated for homozygous deletion of the CDKN2A gene, CDK4 amplification , and allelic loss of the RE gene, as well as for expression of p16 an d pRb proteins. Five high-grade osteosarcomas showed loss of p16 expre ssion; four of these had homozygous CDKN2A deletions, and the fifth ha d a probable deletion obscured by numerous nonneoplastic, p16-immunopo sitive multinucleated giant cells. Thus, p16 immunohistochemistry may provide a sensitive means for assessing CDKN2A status. Twelve tumors ( including the two low-grade osteosarcomas) were immunopositive for pRb , and nine tumors were immunonegative for pRb. Of the five cases with CDKN2A/p16 alterations, none had allelic loss of the RE gene and all e xpressed pRb, suggesting that each of these tumors had an intact RB ge ne. None of the tumors showed CDK4 amplification. No alterations were detected in the two low-grade osteosarcomas, This study suggests that CDKN2A is a tumor suppressor inactivated in osteosarcomas that lack RB mutations and that the p16-pRb cell-cycle control pathway is deregula ted in a large number of high-grade osteosarcomas.