The molecular biological characteristics of childhood thyroid carcinoma

We have used molecular biology to study mutation and expression of key oncogenes in childhood thyroid carcinomas from Belarus and Ukraine. All cases were histologically verified by two or more pathologists including at least one from the CIS and one from the EU. We chose to study six genes which have been shown to be involved in thyroid carcinogenesis in adults: ret. Ha, Ki and N ras genes, p53 and the TSH receptor. Expression of the ret oncogene, which has been shown to be activated by translocation in a proportion of papillary carcinomas has been studied by two independent methods. The first, used by the Cambridge group uses RT-PCR to identify the expression of the tyrosine kinase domain of the gene; as the gene is normally silent in follicular cells, this approach allows demonstration of activation of ret, but does not identify the particular translocation involved. The second approach, used by the Naples group, also uses RT-PCR, but amplifies across the breakpoint of each of the three translocations already identified to provide information on the proportion of tumors which express the individual translocations of this gene. Mutations in the TSH receptor, a key modulator of thyroid follicular growth have been sought by the Brussels group using SSCP and direct sequencing. The Munich group have analyzed the samples for presence of mutation in p53, which is believed to play a role in genetic instability which is a features of carcinomas derived from may different tissues. Mutations in the common sites of the ras oncogenes have been studied by the Cambridge group. Analysis of 26 papillary carcinomas so far studied has shown that mutations in the TSH receptor and in p53 do not play a significant role in the genesis of the tumours studied. The proportion of tumours showing ret expression does not differ significantly from that found in a control non exposed population from the UK. However, the pathological study shows that nearly all the increased number of thyroid carcinomas found in children exposed to fallout from Chernobyl are of the papillary type, and although the proportion showing ret activation does not increase, the numbers of tumors in which ret activation plays a role in carcinogenesis has very greatly increased. We therefore consider that radiation leading to ret translocation is a major feature in the increased number of childhood thyroid carcinomas in the population exposed to fallout from Chernobyl