Regulation and perturbations of the p21 cyclin dependent kinase inhibitor in human cancer

grantor: University of Torontop21/WAF1/CIP1/SDI1 is a putative tumour suppressor gene which may be targeted during carcinogenesis as a mechanism of evading the G1/S checkpoint. To test the hypothesis that primary human tumours have mutations in the p21 gene, which propagate the carcinogenic process, primary breast and sarcoma tumour specimens were examined for alterations in the p21 gene. Unique or acquired somatic mutations were not observed, indicating that they are not selected for during the carcinogenic process; however, two p21 sequence variants were detected. The variants occur together in apparently unaffected individuals at a frequency of 10.7%. The frequency of the variant allele (4.1%) was found to be significantly lower in DNA from people that have tumours with p53 mutations (p = 0.006) in comparison to tumours without p53 mutations (20.4%). These data suggest that the occurrence of the variants may have a direct effect on tumour development and may, in some cases, be incompatible with p53 mutations. 'In vitro' experiments from many laboratories have demonstrated that down regulation of p21 gene expression leads to the loss of normal growth suppression, differentiation, and senescence in neoplastic cells. We investigated the issue of p53-independent regulation and deregulation of p21 mRNA levels with particular emphasis on the putative role of the two variants. To accomplish this, we examined the regulation of p21 mRNA and protein expression under various conditions both 'in vivo' and 'in vitro'. This 'in vitro' system was then used to test the effects of the two variants on the post-transcriptional regulation of p21 expression. We observed that the two different p21 alleles were expressed at the same rate after induction and decayed at the same rate after transcription has stopped. The genetic differences observed between the two different alleles of p21 are not caused by changes in p21 mRNA stability, however effects on protein stability could not be ruled out. Identification of the mechanism by which a variant form of the p21 gene could modify the risk of cancer will. have wide reaching impact on understanding and management of human cancer.Ph.D