FREQUENT INACTIVATION OF THE RETINOBLASTOMA GENE IN HUMAN THYROID CARCINOMAS

The retinoblastoma-susceptibility gene, Rb, is a tumor suppressor gene whose inactivation by deletions or mutations is the basis for retinob lastoma. somatic inactivation of Rb has also been described in a varie ty of human tumors not related to retinoblastoma. Using single-strand conformation polymorphism analysis of cDNA fragments amplified by reve rse transcription polymerase chain reaction (RT-PCR-SSCP), we analysed 57 thyroid tumor specimens (eight follicular adenomas and 49 carcinom as) for structural abnormalities of the gene. Since mutations which af fect Rb protein function has been mapped to the genomic sequences enco ding exon 13-22, we examined these exons in the cDNAs for potential Rb mutations. Mutant Rb allele was found in 55% (27/49) of thyroid carci nomas, but in none of the eight benign thyroid tumors: Mutations were observed in three out of five (60%) anaplastic carcinomas and 24 out o f 44 (54.5%) differentiated thyroid carcinomas. All the mutations foun d were in the region containing exon 21. Most (26/27) were exon 21 del etion, resulting in the fusion of exon 20 to 22. The deletion was, how ever, not present in the genomic sequence of Rb gene, indicating that the deletion was the result of abnormal Rb mRNA splicing. Another patt ern of deletion was noted in one papillary carcinoma sample where most of exon 19 and 21, and entire exon 20 were deleted. The deletion was in-frame and the same sample also contained exon 21 deletion on the re maining allele. A point mutation was also observed at codon 712 of exo n 21 (TCC to CCC), changing Cys to Arg in a follicular cancer sample. Six samples contained both Rb and p53 mutations. Three were found in a dvanced carcinoma samples. Rb/p53 double mutations did not bear a rela tionship to tumor histology. The presence of both Rb and p53 gene muta tions appears to be more frequent in the advanced disease stage. Rb in activation may, therefore, be involved in the pathogenesis and/or prog ression of thyroid carcinomas.