Coamplification of the CDK4 Gene with MDM2 and GLI in Human Sarcomas

The 34-kilodalton cyclin-dependent kinase, p34cdk4, is a major catalytic subunit of mammalian D-type cyclins, which act during the G1 phase of the cell cycle to enforce the decision of cells to enter S phase. A murine complementary DNA clone was used to clone the cognate human CDK4 gene, which was localized to human chromosome 12, band q13, by fluorescence in situ hybridization. Because this chromosomal band contains the GLI and MDM2 genes, which are frequently amplified in human sarcomas, we analyzed CDK4 copy number and expression in a panel of sarcoma cell lines. An osteosarcoma cell line, OsACL, manifested a 25-fold increased copy number of CDK4, amplified concordantly with both GLI and MDM2, whereas a rhabdomyosarcoma cell line, SJRH30, was found to have an amplicon that included CDK4 and GLI but not MDM2. CDK4 mRNA and protein were overexpressed in both cell lines, and nucleotide sequencing analysis indicated that the gene had not sustained mutations. These observations provide the first evidence for amplification of a gene encoding a cell division cycle protein kinase, complement recent data indicating that genes encoding D-type cyclins are targets of chromosomal rearrangement and gene amplification in tumor cells, and suggest that CDK4 amplification might contribute to oncogenesis.