INTRODUCTION OF P53 INDUCES CELL-CYCLE ARREST IN P53-DEFICIENT HUMAN MEDULLARY-THYROID-CARCINOMA CELLS

The structural integrity of the p53 gene in a human thyroid-medullary- carcinoma-derived cell line has been studied. Analysis of high-molecul ar-weight DNA showed that the p53 locus is severely rearranged. PCR an d single-strand conformation polymorphism analysis revealed that a lar ge portion of the 5' end of the p53 gene is lost, while a region encom passing exons 8 and 9 is rearranged. As a consequence, no virtual expr ession of a p53-specific transcript is detected in mRNA from the medul lary-carcinoma cell line. The absence of a p53 protein prompted us to analyze the biological effect of exogenous expression of this tumor-su ppressor gene on cell growth and viability, introducing retroviral con structs carrying full-length human wild-type p53 cDNA. Contrary to wha t has been described for other cell types, including most thyroid-carc inoma cell lines of follicular origin, these experiments allowed us to establish clonal-cell populations which constitutively express p53. C ytometric analysis revealed G(1)-specific cell-cycle arrest, responsib le for growth retardation in the transfected clones when compared with the parental cell line, However, medullary-thyroid-cavcinoma cells ex pressing p53 are able to partially overcome the G(1) block and progres s through the cell cycle. In the search of the mechanism(s) involved i n these processes, we describe the interaction of p53 with specific p2 1(WAFI/Cipl) promoter sequences by gel-retardation assays. (C) 1997 Wi ley-Liss, Inc.