Up-regulation of cyclin-dependent kinase 4/Cyclin d2 expression but down-regulation of cyclin-dependent kinase 2/cyclin E in testicular germ cell tumors

Testicular germ cell tumors (GCT) characteristically display two chromosome 12 abnormalities: the isochromosome i(12p) and concomitant deletions of th e long arm. Some genes important in the control of the G(1)-S cell cycle ch eckpoint G(1)-S, i.e., cyclin-dependent kinases 2 and 4, cyclin D2 are loca ted on this chromosomal region. Therefore, testicular GCTs mere analyzed as to the expression of CDK2, CDK4, CDK6, and the expression of their catalyt ic partners cyclins D1, D2 and E by semiquantitative reverse transcription- PCR. Cyclin D2, located on 12p, was overexpressed in 69% (31 of 45) of the tumors by a mean factor of 8, including all histological subtypes. In addit ion, the cyclin D2 partner CDK4 was increased in 41% (21 of 51) of all tumo rs by a factor of 6, most strongly in embryonal carcinomas. Sixty-four perc ent of the seminomas and 23% of the non-seminomas had decreased expression of CDK6 by a mean factor of 5 (P = 0.009). Statistical analysis using confi gural frequency analysis and regression analysis revealed that cyclin D2 an d CDK4 expression were strongly correlated (r(2) = 0.682; P = 0.000052), wh ereas expression of CDK6 did not correlate with either of them (r(2) = 0.38 2; P = 0.00085). CDK2 and its catalytic partner cyclin E were down-regulate d in 40% (19 of 47) and 42% (19 of 45) of the tumors, respectively, by a fa ctor of 7 each. Western blots and immunohistochemical experiments confirmed cyclin D2 and CDK4 overrepresentation and reduced expression of cyclin E a nd CDK2 tumors in the few tumors under protein study. Despite its localizat ion on 12q13, a hot spot for loss of heterozygosity in testicular GCTs (> 4 0%), Southern blotting revealed no gross DNA alteration of the CDK2 gene. B ecause up-regulation of the cyclin D2/CDK4 complex and downregulation of cy clin E/CDK2 complex were found in seminomas as well as in non-seminomas and in all tumor stages, these findings seem to be early events during tumorig enesis of testicular GCTs. Together with previous findings that retinoblast oma mRNA and protein expression is strongly decreased in these tumors, thes e data suggest an unusual deregulated G(1)-S checkpoint as a decisive event for germ cell tumors.