Human thyroid cancer cells as a source of iso-genic, iso-phenotypic cell lines with or without functional p53

Differentiated thyroid carcinomas tin contrast to the rarer anaplastic form ) are unusual among human cancers in displaying a remarkably low frequency of p53 mutation and appear to retain wild-type (wt) p53 function as assesse d by the response of derived cell lines to DNA damage. Using one such cell line, K1, we have tested the effect of experimental abrogation of p53 funct ion by generating matched subclones stably expressing either a neo control gene, a dominant-negative mutant p53 (143(ala)) or human papilloma virus pr otein HPV16 E6. Loss of p53 function in the latter two groups was confirmed by abolition of p53-dependent 'stress' responses including induction of th e cyclin/CDK inhibitor p21WAF1 and G1/S arrest following DNA-damage. In con trast, no change was detected in the phenotype of 'unstressed' clones, with respect to any of the following parameters: proliferation rate in monolaye r, serum-dependence for proliferation or survival, tumorigenicity, cellular morphology, or tissue-specific differentiation markers. The K1 line theref ore represents a 'neutral' background with respect to p53 function, permitt ing the derivation of functionally p53 + or - clones which are not only iso -genic but also iso-phenotypic. Such a panel should be an ideal tool with w hich to test the p53-dependence of cellular stress responses, particularly the sensitivity to potential therapeutic agents, free from the confounding additional phenotypic differences which usually accompany loss of p53 funct ion. The results also further support the hypothesis that p53 mutation alon e is not sufficient to drive progression of thyroid cancer to the aggressiv e anaplastic form.