Novel Point Mutations and Allele Loss at the RET Locus in Sporadic Medullary Thyroid Carcinomas

Abstract

Germline mutations in the RET proto-oncogene have been shown to be the underlying cause of multiple endocrine neoplasia type 2 (MEN 2A and 2B) and familial medullary thyroid carcinoma (FMTC). Some cases of sporadic medullary thyroid carcinoma (sporadic MTC) are reported to have specific codon 918, 883 and 768 mutations of the RET gene in tumor tissues. We examined RET gene mutations in 40 Japanese cases who had previously undergone surgery for sporadic MTC. DNA extracted from formalin-fixed tumor tissues and corresponding normal thyroid tissues or peripheral blood leukocytes was analyzed for mutations of exon 10, 11, 13, 14 and 16 of the RET gene by DNA sequencing and by mutation-specific restriction enzyme analysis. Germline RET point mutations were found in six of 40 cases (15%), cysteine residues at codon 618 in two, codon 634 in three and valine residue at codon 804 in one, and were newly identified as heritable MTC. Of the remaining 34 sporadic MTC cases, four (12%) had tumor-specific RET point mutations. Two were found in exon 16; one case showed an ATG to ACG (Met to Thr) mutation at codon 918, and the other showed two point mutations, ATG to ACG (Met to Thr) at codon 918 and GCA to GTA (Ala to Val) at codon 919 with loss of the wild-type allele, suggesting that both alleles at the RET locus were altered. The other two were found in exon 13; one case showed a CCG to TCG (Pro to Ser) mutation at codon 766 and the other showed a silent mutation, GTC to GTT (Val) at codon 778 with loss of the wild-type allele. There was no association of sporadic mutations with recurrence or prognosis in patients with sporadic MTCs. The low rate of somatic RET mutation at codon 918 in our sporadic MTC suggests that as yet unknown factors may be involved. Genetic alterations in both alleles may have an important role in a small fraction of sporadic MTCs.Key words: Sporadic medullary thyroid carcinoma -RET gene -Point mutation -Allele lossMultiple endocrine neoplasia Specific germline mutations in the RET proto-oncogene on chromosome 10q11.2 have been shown to be the underlying cause of multiple endocrine neoplasia (MEN) 2A, 2B and familial medullary thyroid carcinoma (FMTC).1, 2) Mutations in MEN 2A and FMTC occur in a cysteine-rich extracellular region and are concentrated on cysteine residues at codons 609, 611, 618, 620 or 634 of exon 10 or 11. 3,4) In MEN 2B, mutations are found in the intracellular tyrosine kinase domain at codon 918 of exon 16.5-7) In addition, mutations at codon 768 of exon 13 and codon 804 of exon 14 were reported in some FMTC families. 8,9) More than half of all MTC cases are sporadic MTC. Clinically, sporadic MTC is characterized by negative family history in patients who are usually in their fifties or sixties when MTC is diagnosed. Occasionally, patients who undergo thyroidectomy for benign thyroid disease are diagnosed as MTC postoperatively. Before the susceptibility gene for MEN 2 was discovered, patients without a family history of MTC or pheochromocytoma were likely to be regarded as having t...

Somatic Mutations in RET Exons 12 and 15 in Sporadic Medullary Thyroid Carcinomas: Different Spectrum of Mutations in Sporadic Type from Hereditary Type

“…Together with the results from 34 previous cases and 5 additional patients, 35) somatic alterations in exons 10-16 were found in 10 out of 39 (25.6%) sporadic MTCs. Somatic mutations were distributed throughout exons 11 to 16, and were found in both the cysteine-rich and tyrosine kinase domains (Table I).…”

“…25,33,34) In contrast, we detected the somatic M918T mutation in only 6% (2/34) of sporadic MTCs, and correlation of this mutation with poor prognosis could not be confirmed. 35,36) This low incidence of somatic mutations in codon 918 led us to examine RET mutations in exons 12 and 15. Here we report novel somatic point mutations in RET exons 12 and 15 in sporadic MTCs, and a different spectrum of mutations in sporadic MTCs compared with hereditary MTCs.…”

Correlation of RET somatic mutations with clinicopathological features in sporadic medullary thyroid carcinomas

“…Met918Thr RET mutation is the most common somatic mutation in sporadic forms of MTC, and its detection rate varies greatly (5 -66%) in the published literature (Zedenius et al, 1994;Marsh et al, 1996;Romei et al, 1996;Wohllk et al, 1996;Bugalho et al, 1997;Scurini et al, 1998;Uchino et al, 1998Uchino et al, , 1999Dvorakova et al, 2008;Elisei et al, 2008). However, in some of these studies, the authors have screened sporadic MTC for only a few specific mutations, mostly in codon 918 (Hofstra et al, 1994;Shan et al, 1998;Bockhorn et al, 1999;Marsh et al, 2003).…”

“…Therefore, the number of exons screened, as well as the sizes of the analysed series, may explain some of the reported differences in the prevalence of RET mutations in sporadic MTC. In addition, ethnic or environmental factors, differences in detection or in sampling methods may also account for the reported differences (Uchino et al, 1998;Dvorakova et al, 2008). In some cohorts, besides the Met918Thr mutation, other somatic mutations were also detected, at a lower frequency in exons 10, 11, 12, 13 and 15 (Bugalho et al, 1997;Scurini et al, 1998;Uchino et al, 1999).…”

“…The screening of RET mutations has been carried out in different series of sporadic MTC, however the observed frequencies are variable (12 -100%) (Hofstra et al, 1994;Zedenius et al, 1994;Jhiang et al, 1996;Marsh et al, 1996Marsh et al, , 2003Romei et al, 1996;Wohllk et al, 1996;Bugalho et al, 1997;Scurini et al, 1998;Shan et al, 1998;Uchino et al, 1998Uchino et al, , 1999Bockhorn et al, 1999;Dvorakova et al, 2008;Elisei et al, 2008). Met918Thr RET mutation is the most common somatic mutation in sporadic forms of MTC, and its detection rate varies greatly (5 -66%) in the published literature (Zedenius et al, 1994;Marsh et al, 1996;Romei et al, 1996;Wohllk et al, 1996;Bugalho et al, 1997;Scurini et al, 1998;Uchino et al, 1998Uchino et al, , 1999Dvorakova et al, 2008;Elisei et al, 2008).…”