RET is an abbreviation for "rearranged during transfection", as the DNA sequence of this gene was originally found to be rearranged within a 3T3 fibroblast cell line following its transfection with DNA taken from human lymphoma cells.[8]
The human geneRET is localized to chromosome 10 (10q11.2) and contains 21 exons.[9]
The natural alternative splicing of the RETgene results in the production of 3 different isoforms of the protein RET. RET51, RET43 and RET9 contain 51, 43 and 9 amino acids in their C-terminal tail respectively.[10] The biological roles of isoforms RET51 and RET9 are the most well studied in-vivo as these are the most common isoforms in which RET occurs.
In order to activate RET, GFLs first need to form a complex with a glycosylphosphatidylinositol (GPI)-anchored co-receptor. The co-receptors themselves are classified as members of the GDNF receptor-α (GFRα) protein family. Different members of the GFRα family (GFRα1, GFRα2, GFRα3, GFRα4) exhibit a specific binding activity for a specific GFLs.[14]
Upon GFL-GFRα complex formation, the complex then brings together two molecules of RET, triggering trans-autophosphorylation of specific tyrosine residues within the tyrosine kinase domain of each RET molecule. Tyr900 and Tyr905 within the activation loop (A-loop) of the kinase domain have been shown to be autophosphorylation sites by mass spectrometry.[15]Phosphorylation of Tyr905 stabilizes the active conformation of the kinase, which, in turn, results in the autophosphorylation of other tyrosine residues mainly located in the C-terminal tail region of the molecule.[11]
The structure shown to the left was taken from the protein data bank code 2IVT.[5]
The structure is that of a dimer formed between two protein molecules each spanning amino acids 703-1012 of the RET molecule, covering RETs intracellular tyrosine kinase domain. One protein molecule, molecule A is shown in yellow and the other, molecule B in grey. The activation loop is coloured purple and selected tyrosine residues in green. Part of the activation loop from molecule B is absent.
Phosphorylation of Tyr981 and the additional tyrosines Tyr1015, Tyr1062 and Tyr1096, not covered by the above structure, have been shown to be important to the initiation of intracellular signal transduction processes.
Role of RET signalling during development
Mice deficient in GDNF, GFRα1 or the RET protein itself exhibit severe defects in kidney and enteric nervous system development. This implicates RET signal transduction as key to the development of normal kidneys and the enteric nervous system.[11]
At least 26 disease-causing mutations in this gene have been discovered.[16] Activating point mutations in RET can give rise to the hereditary cancer syndrome known as multiple endocrine neoplasia type 2 (MEN 2).[17] There are three subtypes based on clinical presentation: MEN 2A, MEN 2B, and familial medullary thyroid carcinoma (FMTC).[18] There is a high degree of correlation between the position of the point mutation and the phenotype of the disease.
Chromosomal rearrangements that generate a fusion gene, resulting in the juxtaposition of the C-terminal region of the RET protein with an N-terminal portion of another protein, can also lead to constitutive activation of the RET kinase. These types of rearrangements are primarily associated with papillary thyroid carcinoma (PTC) where they represent 10-20% of cases, and non-small cell lung cancer (NSCLC) where they represent 2% of cases. Several fusion partners have been described in the literature, and the most common ones across both cancer types include KIF5B, CCDC6 and NCOA4.
While older multikinase inhibitors such as cabozantinib or vandetanib showed modest efficacy in targeting RET-driven malignancies, newer selective inhibitors (such as selpercatinib and pralsetinib) have shown significant activity in both mutations and fusions. The results of the LIBRETTO-001 trial studying selpercatinib showed a progression-free survival of 17.5 months in previously treated RET-positive NSCLC, and 22 months for RET-positive thyroid cancers, which prompted an FDA approval for both these indications in May 2020. Several other selective RET inhibitors are under development, including TPX-0046, a macrocyclic inhibitor of RET and Src intended to inhibit mutations providing resistance to current inhibitors.
^Veiga-Fernandes H, Pachnis V (February 2017). "Neuroimmune regulation during intestinal development and homeostasis". Nature Immunology. 18 (2): 116–122. doi:10.1038/ni.3634. PMID28092371. S2CID5519816.
^Bahrami A, Joodi M, Moetamani-Ahmadi M, Maftouh M, Hassanian SM, Ferns GA, Avan A (January 2018). "Genetic Background of Hirschsprung Disease: A Bridge Between Basic Science and Clinical Application". Journal of Cellular Biochemistry. 119 (1): 28–33. doi:10.1002/jcb.26149. PMID28543993. S2CID12086686.
^Ceccherini I, Bocciardi R, Luo Y, Pasini B, Hofstra R, Takahashi M, Romeo G (November 1993). "Exon structure and flanking intronic sequences of the human RET proto-oncogene". Biochemical and Biophysical Research Communications. 196 (3): 1288–1295. doi:10.1006/bbrc.1993.2392. PMID7902707.
^Myers SM, Eng C, Ponder BA, Mulligan LM (November 1995). "Characterization of RET proto-oncogene 3' splicing variants and polyadenylation sites: a novel C-terminus for RET". Oncogene. 11 (10): 2039–2045. PMID7478523.
^ abcArighi E, Borrello MG, Sariola H (2005). "RET tyrosine kinase signaling in development and cancer". Cytokine & Growth Factor Reviews. 16 (4–5): 441–467. doi:10.1016/j.cytogfr.2005.05.010. PMID15982921.
^Baloh RH, Enomoto H, Johnson EM, Milbrandt J (February 2000). "The GDNF family ligands and receptors - implications for neural development". Current Opinion in Neurobiology. 10 (1): 103–110. doi:10.1016/S0959-4388(99)00048-3. PMID10679429. S2CID32315320.
^Airaksinen MS, Titievsky A, Saarma M (May 1999). "GDNF family neurotrophic factor signaling: four masters, one servant?". Molecular and Cellular Neurosciences. 13 (5): 313–325. doi:10.1006/mcne.1999.0754. PMID10356294. S2CID46427535.
^ abBorrello MG, Pelicci G, Arighi E, De Filippis L, Greco A, Bongarzone I, et al. (June 1994). "The oncogenic versions of the Ret and Trk tyrosine kinases bind Shc and Grb2 adaptor proteins". Oncogene. 9 (6): 1661–1668. PMID8183561.
^Hwang JH, Kim DW, Suh JM, Kim H, Song JH, Hwang ES, et al. (June 2003). "Activation of signal transducer and activator of transcription 3 by oncogenic RET/PTC (rearranged in transformation/papillary thyroid carcinoma) tyrosine kinase: roles in specific gene regulation and cellular transformation". Molecular Endocrinology. 17 (6): 1155–1166. doi:10.1210/me.2002-0401. PMID12637586.
Hofstra RM, Osinga J, Buys CH (1998). "Mutations in Hirschsprung disease: when does a mutation contribute to the phenotype". European Journal of Human Genetics. 5 (4): 180–185. doi:10.1159/000484760. PMID9359036.
Santoro M, Carlomagno F, Melillo RM, Fusco A (December 2004). "Dysfunction of the RET receptor in human cancer". Cellular and Molecular Life Sciences. 61 (23): 2954–2964. doi:10.1007/s00018-004-4276-8. PMID15583857. S2CID36766644.
Niccoli-Sire P, Conte-Devolx B (June 2005). "[RET mutations and preventive treatment of medullary thyroid cancer]". Annales d'Endocrinologie. 66 (3): 168–175. doi:10.1016/s0003-4266(05)81748-2. PMID15988377.
