The responsible gene for MEN 2 (OMIM 171400, 162300, 155240) was localized to centromeric chromosome 10 (10q11.2) by genetic linkage analysis in 1987. Activating germline point mutations of the RET proto- oncogene were identified in 1993. Analysis of RET in families with MEN 2 revealed that only affected family members had germline missense mutations in eight closely located exons (5, 8, 10, 11, and 13– 16). Genetic testing detects nearly 100% of mutation carriers and is considered the standard of care for all first- degree relatives of patients with newly diagnosed MTC.
The RET gene has 21 exons and encodes a receptor tyrosine kinase that appears to transduce growth and differentiation signals in several developing tissues including those derived from the neural crest. It is expressed in cells such as C cells, the precursors of MTC, and in adrenal medullary cells, the precursors of pheochromocytomas, and parasympathetic and sympathetic and colonic ganglia cells, precursors of the ganglioneuromas. Unregulated RET activation leads to various cancers, including hereditary MTC, while humans lacking RET expression due to inactivating germline mutations develop Hirschsprung’s disease.
The RET protein consists of an extracellular segment with a ligand- binding domain, a cadherin (Ca2+- dependent cell adhesion)- like domain, and a cysteine- rich domain that is positioned near the cell membrane. RET protein has a single transmembrane domain and an intracellular segment with two tyrosine kinase subdomains, TK1 and TK2. It is activated by ligand- induced dimerization.
Hereditary MTC is caused by autosomal dominant gain- of- function mutations in the RET proto- oncogene. Mutation of the extracellular cysteine at exon 11 codon 634 causes ligand- independent dimerization of receptor molecules, enhanced phosphorylation of intracellular substrates, and cell transformation. Mutation of the intracellular tyrosine kinase (codon 918) has no effect on receptor dimerization but causes constitutive activation of intracellular signalling pathways and also results in cellular trans formation. There is a significant age- related progression from C- cell hyperplasia to MTC, which correlates with the transforming capacity of the respective RET mutations.
At present, mutation analysis has identified over 100 different missense mutations associated with the development of MEN 2 (Figure 1.). Although some overlap exists between RET mutations and the resulting clinical subtype of MEN 2, most of patients with MEN 2A have a mutation of codon 634 (exon 11), followed by mutations of codons 609, 611, 618, and 620 (exon 10). In FMTC, germline mutations are distributed throughout the RET gene with an accumulation in exon 13 (codons 768, 790, and 791), exon 14 (co dons 804 and 844), and rarely exon 10 (codons 618 and 620); MEN 2B patients have mutations in codon 918 (exon 16), but mutations are rarely identified at codon 883 exon 15. Phaeochromocytomas are associated with codon 634 and 918 mutations in approximately 40– 50% of patients, and are associated with mutations in exon 10 (codon 609, 611, 618, 620) in about 30% of patients and rarely in exon 15 (codon 791, 804). Hyperparathyroidism in MEN 2A is most commonly associated with codon 634 mutations, and in particular with the C634R mutation. The association between disease phenotype and RET mutation genotype has important implications for the clinical management of MEN 2 patients and their families. There is a correlation between the specific germline RET mutation and the age of onset and aggressiveness of MTC development and the presence of nodal metastases.
Fig1. Germline mutations of the RET proto- oncogene associated with MEN 2 and FMTC. Numbers indicated mutated codons of the RET gene
This information is used to stratify RET mutations into three risk levels namely moderate, high, and highest risk concerning the age of onset and aggressiveness of MTC: MEN2- patients with ATA (American Thyroid Association)- moderate risk have mutations at exon 10 (codons 609, 611, 618, 620), exon 13– 15 (codon 768, 790, 791, 804, and 891), with ATA higher risk at exon 11 (codon 634) and at exon 15 (codons 883) and with ATA highest risk at exon 16 (codon 918). This risk classification is important in presymptomatic RET mutation carriers because prophylactic thyroidectomy must be performed before MTC development or while the tumour is confined to the thyroid gland. This strategy for preventing familial MTC should be tailored according to the specific risk of the mutation carried by each patient.
Approximately 23– 60% of sporadic MTC have a somatic (present in tumour only) mutation at codon 918 identical to the germline mutation found in MEN 2B. Some reports suggest that patients with sporadic MTC with codon 918 somatic mutations have more aggressive tumour growth and a poorer prognosis.
