EVIDENCE OF GENE DELETION OF P21 (WAF1 CIP1), A CYCLIN-DEPENDENT PROTEIN-KINASE INHIBITOR, IN THYROID CARCINOMAS/

Eukaryotic cell cycle progression is controlled by a host of cyclin/cy clin-dependent kinases (Cdks), that are themselves regulated by multip le factors, including a group of small cyclin-Cdk inhibitor proteins ( p15, p16, p21 and p27). The involvement of Cdk inhibitors in carcinoge nesis has been demonstrated by the studies of p16, p53 is frequently m utated in thyroid carcinomas and p21/Waf1 is a downstream effector of p53. It is conceivable that genetic defects of genes downstream in the p53 pathway could also be oncogenic. We, therefore, examined a series of 57 thyroid tumour specimens (eight follicular adenomas and 49 carc inomas) for deletion and point mutation of the p21/Waf1 gene. Three di fferent kinds of deletions ranging from 349 to 450 bp were detected in five papillary carcinoma specimens by reverse transcription-polymeras e chain reaction (RT-PCR). All the deletions were involved in the seco nd exon of the p21/Waf1 gene. RT-PCR single strand conformational poly morphism (SSCP) analysis of remaining samples failed to reveal any poi nt mutations in the coding region of the gene, except for a polymorphi sm at codon 31 (Ser to Arg). Genomic Southern blot analysis did not de monstrate any gene deletion or rearrangement in these samples, indicat ing abnormal RNA splicing may be involved. Analysis of intron- exon bo undary and the ending region of the second exon did not reveal any mut ation except for a point mutation (C to G) located 16 bp downstream fr om the splice donor site of the second intron in three out of five sam ples with p21/Waf1 deletions. Whether the mutation plays any role in a berrant RNA splicing remains to be determined. Among the five samples with p21/Waf1 gene deletions, none of them simultaneously carried a p5 3 or retinoblastoma (Rb) gene mutation. No p21/Waf1 abnormality was fo und in the benign adenomas. Thus, 12.5% (5/40) of thyroid papillary ca rcinoma specimens harboured p21/Waf1 gene deletions. Our data suggest that p21/Waf1 gene deletion is involved in thyroid carcinogenesis and may play an important role in thyroid cell transformation.