A patient with MEN 2 and multiple mutations of RET in the germline

Molecular pathogenesis of MEN2-associated tumors

Although the gene responsible for multiple endocrine neoplasia type 2 (MEN2) was discovered many years ago, the exact mechanisms of tumor development in patients affected with RET germline mutations remain unknown. In vitro studies have certain pitfalls, one of which is the use of cell culture systems such as the NIH3T3 cells, in which RET usually is not expressed in contrast to the in vivo situation. Recent data suggest that an overrepresentation of mutant RET as a 'second hit' event might trigger tumorigenesis. However, alterations in other genes might contribute to this overrepresentation of RET or impact on MEN 2-related tumor development through completely different mechanisms and pathways. The final goal of further elucidating the natural history and pathogenesis of MEN2-related tumors should be the chance to offer patients with RET germline mutations an optimal cancer prevention (e.g. codon specific recommendations for prophylactic thyroidectomy) and treatment program, especially for metastatic medullary thyroid carcinoma for which presently no effective therapy other than surgery exists.

RET polymorphisms in codons 769 and 836 are not associated with predisposition to medullary thyroid carcinoma

The study was undertaken to verify whether the RET gene polymorphisms are associated with MTC in patients negative for germline mutations. Two hundred five patients with apparent sporadic MTC were subjected to genetic analysis of RET exons 10, 11, 13, 14, 16 and 22 RET germline mutation carriers were identified with 10.7% frequency. The frequency among 26 patients not older than 30 was 27%. In patients excluded for known mutations we analyzed two polymorphic sites: RET codon 769 and 836. As control group, 90 healthy subjects were investigated. In young patients the observed allelic frequencies were 32% for variant L769/CTG and 5% for variant S836/AGT. Although these values were higher than in older MTC patients (22 and 3%, respectively), as well as in the control group (27 and 2%) the difference was insignificant. We conclude that in Polish patients polymorphisms at RET codons 769 and 836 are not associated with medullary thyroid carcinoma.

Prophylactic thyroidectomy in multiple endocrine neoplasia: the impact of molecular mechanisms of RET proto-oncogene

BACKGROUND

Multiple endocrine neoplasia (MEN) type 2, a cancer syndrome inherited in the dominant fashion, is defined by the occurrence of medullary thyroid carcinoma (MTC), either as a singular lesion (familial medullary thyroid carcinoma, FMTC) or with the variable expression of pheochromocytoma, hyperparathyroidism (MEN 2A), ganglioneuromas, buccal neuromas and Marfanoid-like phenotype (MEN 2B).

DISCUSSION

Germline mutations of the RET proto-oncogene, localized on chromosome 10q11.2, have been identified as the underlying genetic cause of the disorder. In the majority of patients with MEN 2A/FMTC missense mutations at exon 10 or exon 11 are identifiable. Cysteine to arginine exchange at codon 634 is the mutation most frequently found. In MEN 2B approximately 95% of patients present with a mutation at codon 918 (exon 16). Additionally, less frequent mutations in other codons have been found in both syndromes. The DNA-based genotype analysis enables the identification of gene carriers at risk of developing MTC and offer them prophylactic thyroidectomy prior to development of any thyroid pathologies. Prophylactic surgery is generally recommended for MEN 2A/FMTC gene carriers at the age of 4-6 years. Due to the aggressiveness of the MEN 2B syndrome gene carriers should be operated by the age of 1 year. Presumably some less virulent mutations allow postponement of the prophylactic treatment to the second to fourth decade of life.

CONCLUSIONS

Compared to standard presymptomatic biochemical screening, genetic testing and consecutive prophylactic treatment contribute to better outcome of individuals at risk for MTC.

New insights into the genetics of familial chromaffin cell tumors

We review genetic aspects and recent advances in our understanding of the molecular pathogenesis of familial chromaffin cell tumors (pheochromocytoma, paraganglioma). About 10 percent of pheochromocytomas are familial and occur as part of multiple endocrine neoplasia type 2 (MEN 2), von Hippel-Lindau (VHL) disease, and neurofibromatosis type 1 (NF 1). A subset of paragangliomas, tumors that can also produce and secrete catecholamines, are also familial and occur in patients with germline mutations in genes that encode subunits of the mitochondrial complex II. The precise molecular mechanisms underlying the pathogenesis of chromaffin cell tumors remain widely unknown, although recent studies in hereditary tumors help elucidate their development. In MEN 2, overrepresentation of mutant RET in selected adrenomedullary cells may be an important mechanism in initiating the formation of a pheochromocytoma. In VHL disease, pheochromocytoma development appears to occur according to Knudson's two-hit model, a VHL germline mutation and wildtype allelic deletion. Tumorigenesis of NF1-associated pheochromocytomas remains unknown, as does tumor formation (i.e., carotid body tumor) in patients with germline mutations in SDHB, SDHC, and SDHD, genes that encode subunits of the mitochondrial complex II, the smallest complex in the respiratory chain. Many genetic alterations have been found in sporadic chromaffin cell tumors. However, at present such genetic changes are difficult to place into context with regard to tumor formation and progression.