Association of RET codon 691 polymorphism in radiation-induced human thyroid tumours with C-cell hyperplasia in peritumoural tissue

Abstract

The RET proto-oncogene encodes a protein structurally related to transmembrane receptors with an intracellular tyrosine kinase domain. In human thyroid gland, the RET proto-oncogene is normally expressed in parafollicular C-cells. Thyroid C-cell hyperplasia is associated with inherited medullary thyroid carcinomas and is considered as a pre-neoplastic stage of C-cells disease. It has also been observed in thyroid tissues adjacent to follicular and papillary carcinomas. In order to study the relationship between a misfunctioning of the RET proto-oncogene and the presence of C-cell hyperplasia, we compared a series of thyroid glands presenting sporadic or radiation-associated tumours, as well as samples of unrelated normal thyroid tissues, for alteration in exons 10 and 11 of the gene and for the presence or absence of C-cell hyperplasia. Here we report a significantly higher frequency of C-cell hyperplasia present in peritumoural thyroid tissues of radiation-induced epithelial thyroid tumours, than in peritumoural of sporadic thyroid tumours or in control normal thyroid tissues (P=0.001). A G691S RET polymorphism was present with a higher frequency in radiation-induced epithelial thyroid tumours (55%) than in sporadic tumours (20%) and in control normal thyroid tissues (15%). Interestingly, this polymorphism was associated in the majority (88%) of radiation-induced tumours with a C-cell hyperplasia in the peritumoural tissues. Several explanations for this association are discussed.

Figure 1

(A) Presence of C-cell hyperplasia in the peritumoural tissue of a radiation-induced thyroid tumour (T) of patient Ti226 (×100 magnification). The arrows indicate calcitonin positive C-cells. (B) The same C-cell hyperplasia seen with ×250 magnification. The calcitonin C-cells was detected by immunohistochemistry using a polyclonal anti-calcitonin antibody.

Figure 3

Example of G691 SNP in patient Ti226. (A) Direct sequence of exon 11 amplified DNA: (1) radiation-induced thyroid tumour showing mutated allele A2; (2) lymphocytic DNA showing heterozygote form (A1+A2); (3) normal thyroid sample showing wild type allele A1. (B) BanI restriction enzyme digestion of exon 11 amplified DNA: (1) non-digested; (2) radiation associated tumour (one band: allele A2); (3) lymphocytic DNA (three bands: alleles A1+A2) and (4) normal thyroid sample (two bands: allele A1). In A and B the material studied after PCR is the same. In B M: marker Ø×174/HaeIII digested DNA; 2% agarose gell stained with ethidium bromide. In A the arrow indicates the location of the transition G to A.

Figure 2

Frequency of radiation-induced (A) and sporadic (B) thyroid tumours (n=29) presenting a wild type allele A1, or mutated allele A2, or both (A1+A2) in the presence or absence of C-cell hyperplasia in peritumoural tissue. C-cell hyperplasia was associated with 55% (16/29) of the tumours in (A) and with only 7% (2/29) of the tumours in (B).