Identification of a novel somatic mutation in the RET proto-oncogene in a patient with sporadic medullary thyroid carcinoma.

Abstract

Germline mutations in exons 10, 11, and 13 of the RET proto-oncogene occur in the vast majority of patients suffering from multiple endocrine neoplasia (MEN) type IIA and familial medually thyroid carcinoma (Donis-Keller et al., 1993; Mulligan et al., 1993), whereas patients with MEN IIB usually carry germline mutations in codon 918 of exon 16 (Eng et al., 1994). In contrast, somatic mutations in the RET proto-oncogene are detected in a subset of sporadic medullary thyroid carcinomas (SMTC). These mutations are located in exon 16 and, more rarely, in exons 13 and 15. The detection of a point mutation in exons 10 and 11 in either the DNA of the tumour or of peripheral blood is suggestive of a hereditary disease. In one case of SMTC, Dou and Donis-Keller detected a six base-pair somatic deletion that removed a cysteine residue at codon 630 of the exon 11 (Donis-Keller, 1993). Recently, Romei et al. (1996) identified three somatic mutations in codon 634 of exon 11. In the present report, we describe a novel somatic point mutation in codon 630 of exon 11 of the RET proto-oncogene in a SMTC, demonstrating that somatic mutations may occur in such location. A 62-yr-old woman with no previous family history of thyroid cancer was found to have a thyroid nodule. After thyroidectomy, a 1.6 cm medullary thyroid carcinoma was detected. The tumor showed all the pathologic features of a sporadic tumour; it was unifocal and was not associated with C-cell hyperplasia. Genomic DNA from two areas of the tumour and two areas of normal thyroid tissue was extracted from formalin-fixed, paraffin-embedded tissue blocks with a precise microdissection under microscopic control. PCR-based, single-strand conformation polymorphism (SSCP) analysis using published oligonucleotide primers of exon 11 demonstrated the presence of additional bands with aberrant migration in tumour DNA. These bands were not detected in normal thyroid tissue or in constitutional DNA. SSCP analysis of exon 10, and restriction analysis of exon 16 did not show germline or somatic mutations. Sequence analysis demonstrated a RET missense somatic point mutation (TGC → CGC, Cys → Arg) at codon 630 of exon 11, which was identified in tumour DNA but not in DNA extracted from peripheral blood. Several authors including ourselves have demonstrated the presence of exon 16 somatic mutations in 23–86% of SMTC and the absence of exon 10 and 11 somatic point mutations in the vast majority of the reported cases, which account for almost 150. However, the results of the present study confirm that somatic mutations may occur also in exon 11, a location that was thought to be exclusive for familial tumours. Similarly to the germline missense point mutations described in familial medullary thyroid carcinoma and MEN IIA patients, the somatic mutation removed one of the cysteine residues found in the extracellular domain of the RET protein, the same cysteine residue as in the case noted by Donis-Keller et al. (1993). It seems that codon 630 may be an additional locus for sporadic medullary thyroid carcinoma, since no germline mutations involving such codon have been reported.