INVOLVEMENT OF THE PRH P16/CDK4/CYCLIN D1 PATHWAY IN THE TUMORIGENESIS OF SPORADIC MALIGNANT MELANOMAS/

Biopsies from 61 sporadic metastatic malignant melanomas and five mela noma cell lines were examined for homozygous deletions and mutations i n the CDKN2 gene (p16). As the p16 protein is involved in a cell cycle regulatory pathway consisting of at least pRb, cdk4 and cyclin D1, th e tumours were also screened for amplifications of the last two genes. Moreover, the transcript levels of the genes were determined and the results compared with the immunohistochemically assessed expression of pRb. Altogether, homozygous deletions of CDKN2 were found in seven tu mours (11%) and two of five cell lines, whereas a mutation was detecte d in only one biopsy, indicating that in sporadic melanomas the former mechanism is predominant for inactivating this gene. Notably, in tota l 59% of the metastatic lesions lacked detectable expression of p16 mR NA, whereas all the biopsies were found to express pRb. In accordance with the postulated negative feedback loop between p16 and pRb, one me lanoma cell line showed overexpression of CDKN2 mRNA together with ver y low levels of the Rb protein. Amplification of the other two genes m ay not be important in the tumorigenesis of melanomas, as only one CDK 4 and no CCND1 amplification was observed. However, highly elevated CD K4 mRNA levels, compared with that seen in a panel of normal tissues, were observed in 76% of the rumours, accompanied in 71% of the cases b y high expression of the CCND1 cyclin activator. Although a low freque ncy of CDKN2 DNA aberrations was observed, the high number of tumours that lacked CDKN2 expression but showed overexpression of CDK4 and/or CCND1, suggest that functional inactivation of pRb through this pathwa y may be involved in the development or progression of sporadic human melanomas.