Department of Endocrinology and Metabolism University of Pisa, Pisa, Italy
April 2008
The human RET gene maps on 10q11.2 and is composed by 21 exons with an estimated size of about 55 kb (Pasini et al., 1995). The RET gene encodes for a tyrosine kinase transmembrane receptor (Takahashi et al., 1988) characterized by 3 different domains: a) the extracellular domain, which contains the signal peptide, the cadherin-like region and the cysteine-rich region; b) the transmembrane domain; c) the intracellular portion containing the tyrosine kinase domain. RET is expressed in a variety of neuronal cell lineages including thyroid C cells and adrenal medulla. Although still debated, recently it has been reported that RET gene expression may also occur in follicular thyroid cells (Fludge et al., 2001). The physiological ligands of RET belong to the glial derived neurotrophic factors (GDNFs) family. Four members of this family, neurturin, persephin, artemin and GDNF, have a specific trophic effect on RET (Robertson and Mason, 1997). The activation of RET is mediated by the interaction of the ligands with 4 co-receptors, GFRα1, 2, 3 and 4 (Jing et al., 1996; Treanor et al., 1996; Trupp et al., 1996; Klein et al., 1997; Buj-Bello et al., 1997; Milbrandt et al., 1998; Baloh et al., 1998). RET dimerization is the natural consequence of the formation of the ligand-coreceptor-receptor complex and is responsible for the activation of the kinase catalytic domain and of the signal transduction which induces cells proliferation through a complex network of second messengers (Marshall, 1995). The oncogenic transformation of RET gene determines a constitutive activation of the receptor itself with the consequent induction of an uncontrolled cell proliferation and tumoral development (Fig. 1).
MEN 2 is an autosomal dominant inherited disease characterized by the presence of MTC plus other endocrine tumors: pheochromocytoma (PHEO) and/or parathyroid adenomas (hyperPTH) in MEN 2A, PHEO and mucosal neurinomas in MEN 2B. MTC alone, lacking the association with other endocrine tumors, may also be inherited (FMTC) (Keiser et al., 1973; Cunliffe et al., 1970; Farndon et al. 1986). In these syndromes, MTC develops in nearly 100% of the affected individuals, while PHEO and hyperPTH develop in around 50% and 20% respectively. In 1993 two independent groups discovered that germline point mutations of the RET proto-oncogene are causative events in MEN 2A (Mulligan et al., 1993) and in FMTC (Donis-Keller et al., 1993). One year later, also MEN 2B was associated with germline point mutations of the RET proto-oncogene (Eng et al., 1994; Carlson et al., 1994; Hofstra et al., 1994). Since then, a large number of publications have addressed the relationship between RET mutations and the clinical phenotype of MEN 2 patients and the clinical implication of screening MEN 2 family members for RET gene mutations. The most comprehensive study correlating the genotype to the phenotype of MEN 2 patients was published in 1996 by the International RET Mutation Consortium (Eng et al., 1996), which reported the results of a large cooperative survey including 477 pedigrees screened for the presence of germline RET proto-oncogene mutations. Germline RET point mutations were found in 92% of the whole group, including 95% of 79 families with MEN 2B, 98.0% of 203 families with MEN 2A and 88% of 34 families with FMTC. A specific mutation in exon 16, at codon 918 (ATG to ACG) was invariably associated with MEN 2B. In MEN 2A, several different cystein codons in exon 10 and 11 were affected, but codon 634 mutations were by far the most common, accounting for 85% of the cases. This mutation (mainly TGC to CGC) was also found to correlate significantly with the presence of PHEO and hyperPTH. In FMTC, the mutations were almost evenly distributed among the 5 cysteine codons 609, 611, 618, 620 and 634. Interestingly, mutations in the cystein rich domain (codons 609, 611, 618 and 620) are not only found in families with MEN 2A/FMTC but also in patients with Hirschprung’s disease, a congenital malformation characterized by an absence of enteric galglia cells in the distal part of the colon, or patients having a combination of MEN 2 and Hirschprung’s disease (Arighi et al., 2004; Takahashi et al., 1999; Sijmons et al., 1998). The number and type of RET mutations have been grown over the last 10 years, especially after the introduction of RET genetic screening in the work up of all patients with MTC, both hereditary and apparently sporadic. As consequence of this more careful research, RET mutations have been found to be widely distributed not only among the 5 cysteine codons but also in other non-cysteine codons, such as codon 804 in exon 14, codon 883 and codon 891 in exon 15 and others (Jimenez et al., 2004; Niccoli-Sire et al., 2001). These widely spread mutations are mainly associated with FMTC phenotype (Niccoli-Sire et al., 2001; Elisei et al., 2007). Large series of MTC hereditary cases have shown a significant correspondence between specific RET mutations and both phenotype and the level of aggressiveness of the MTC tumor (Elisei et al., 2007; Machens et al., 2003; Machens et al., 2007; Frank-Raue et al., 2008; Machens et al., 2003) (Fig 2).
Since 1993, year in which RET oncogene was demonstrated to be the causative event for MTC, several mutations have been described in MEN2 series and new mutations still continue to be discovered. A strict correlation between the type of mutation and the disease phenotype has been largely demonstrated in several studies during the last years and the RET genetic screening has been revealed as a very important diagnostic procedure for hereditary MTC. Finally RET somatic mutations have been shown to be an important bad prognostic indicator for sporadic MTC. For these reasons it appears evident that RET genetic screening is of great clinical relevance for its well established diagnostic and prognostic role. The possibility to employ new targeted therapy directed against RET mutated protein is the challenge of the near future and several tyrosine kinase inhibitors are under investigation in clinical trials (Schlumberger et al., 2008).