FHIT and TSG101 in thyroid tumours: aberrant transcripts reflect rare abnormal RNA processing events of uncertain pathogenetic or clinical significance

OBJECTIVE The chromosomal regions containing the two putative tumour suppre ssors, fragile histidine triad gene (FHIT) and tumour suppressor gene 101 ( TSG101), are deleted frequently in thyroid tumours. We therefore analysed F HIT and TSG101 transcripts in a group of advanced thyroid tumours to establ ish their role in thyroid tumorigenesis. DESIGN Retrospective analysis of FHIT and TSG101 mRNA transcripts and genom ic DNA from cryo-preserved thyroid tumours. TP53, previously shown at the g enomic level not to be mutated in this cohort of tumours, served as a contr ol. PATIENTS We analysed nine follicular thyroid carcinomas (FTC), six papillar y thyroid carcinomas and six follicular adenomas (FA) and histologically no rmal thyroid tissue from four of the FA patients. MEASUREMENTS Single stage and nested reverse transcription polymerase chain reaction (RT-PCR) products of FHIT, TSG101, and TP53 were analysed by agar ose or polyacrylamide gel electrophoresis and sequenced. Genomic DNA was al so analysed by polymerase chain reaction and sequencing (FHIT) or by Southe rn blotting (TSG101). Clinical data were correlated with the results of the mutation analysis. RESULTS Truncated FHIT transcripts were observed frequently alongside full length transcripts with nested RT-PCR, most often in FTC, while single stag e RT-PCR revealed only normal length transcripts in all tumours. Similar re sults were obtained for TP53, while abnormal TSG101 transcripts were detect able by single stage RT-PCR. Sequence analysis of the truncated FHIT and TS G101 transcripts revealed mainly exon skipping and alternate RNA processing events. Only a single point mutation (of TSG101) was found. Southern blott ing for the TSG101 gene, and PCR amplification and sequencing of the FHIT g ene showed no evidence of genomic abnormalities in either case, and there w as no evidence of splice site mutations in the FHIT gene, suggesting that t he truncated transcripts result from altered RNA processing. There was no r elationship between tumour stage, grade or survival and the presence of FHI T or TSG101 abnormalities. CONCLUSIONS Truncated FHIT and TSG101 transcripts in thyroid tumours reflec t alternate mRNA splicing events, rather than genomic deletions. Such abnor mal RNA processing seems to be common and widespread in thyroid neoplasms, as similar results were obtained by analysis of transcripts of TP53, which we had previously shown not to be mutated in these specimens. Although a pa thogenetic role for these aberrant transcripts remains possible, no correla tion was found with stage, histological grade or outcome in this small grou p of advanced thyroid malignancies. Relaxation of mRNA splice control appea rs to be a feature of follicular cell-derived thyroid neoplasms.