The RETproto-oncogene encodes a receptor tyrosine kinase for members of the glial cell line-derived neurotrophic factor (GDNF) family of extracellular signalling molecules. [5] RET loss of function mutations are associated with the development of Hirschsprung's disease, [6] [7] while gain of function mutations are associated with the development of various types of human cancer, including medullary thyroid carcinoma, multiple endocrine neoplasias type 2A and 2B, pheochromocytoma and parathyroid hyperplasia.[ citation needed ]
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 gene RET is localized to chromosome 10 (10q11.2) and contains 21 exons. [9]
The natural alternative splicing of the RET gene 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.
Common to each isoform is a domain structure. Each protein is divided into three domains: an N-terminal extracellular domain with four cadherin-like repeats and a cysteine-rich region, a hydrophobic transmembrane domain and a cytoplasmic tyrosine kinase domain, which is split by an insertion of 27 amino acids. Within the cytoplasmic tyrosine kinase domain, there are 16 tyrosines (Tyrs) in RET9 and 18 in RET51. Tyr1090 and Tyr1096 are present only in the RET51 isoform. [11]
The extracellular domain of RET contains nine N-glycosylation sites. The fully glycosylated RET protein is reported to have a molecular weight of 170 kDa although it is not clear to which isoform this molecular weight relates. [12]
RET is the receptor for GDNF-family ligands (GFLs). [13]
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.
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.
The RET gene variant database at the University of Utah, identifies (as of November 2014) 166 mutations that are implicated in MEN2.
RET proto-oncogene has been shown to interact with:
The insulin receptor (IR) is a transmembrane receptor that is activated by insulin, IGF-I, IGF-II and belongs to the large class of receptor tyrosine kinase. Metabolically, the insulin receptor plays a key role in the regulation of glucose homeostasis; a functional process that under degenerate conditions may result in a range of clinical manifestations including diabetes and cancer. Insulin signalling controls access to blood glucose in body cells. When insulin falls, especially in those with high insulin sensitivity, body cells begin only to have access to lipids that do not require transport across the membrane. So, in this way, insulin is the key regulator of fat metabolism as well. Biochemically, the insulin receptor is encoded by a single gene INSR, from which alternate splicing during transcription results in either IR-A or IR-B isoforms. Downstream post-translational events of either isoform result in the formation of a proteolytically cleaved α and β subunit, which upon combination are ultimately capable of homo or hetero-dimerisation to produce the ≈320 kDa disulfide-linked transmembrane insulin receptor.
Growth factor receptor-bound protein 7, also known as GRB7, is a protein that in humans is encoded by the GRB7 gene.
Glial cell line-derived neurotrophic factor (GDNF) is a protein that, in humans, is encoded by the GDNF gene. GDNF is a small protein that potently promotes the survival of many types of neurons. It signals through GFRα receptors, particularly GFRα1. It is also responsible for the determination of spermatogonia into primary spermatocytes, i.e. it is received by RET proto-oncogene (RET) and by forming gradient with SCF it divides the spermatogonia into two cells. As the result there is retention of spermatogonia and formation of spermatocyte.
Tropomyosin receptor kinase A (TrkA), also known as high affinity nerve growth factor receptor, neurotrophic tyrosine kinase receptor type 1, or TRK1-transforming tyrosine kinase protein is a protein that in humans is encoded by the NTRK1 gene.
Receptor tyrosine kinases (RTKs) are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Of the 90 unique tyrosine kinase genes identified in the human genome, 58 encode receptor tyrosine kinase proteins. Receptor tyrosine kinases have been shown not only to be key regulators of normal cellular processes but also to have a critical role in the development and progression of many types of cancer. Mutations in receptor tyrosine kinases lead to activation of a series of signalling cascades which have numerous effects on protein expression. Receptor tyrosine kinases are part of the larger family of protein tyrosine kinases, encompassing the receptor tyrosine kinase proteins which contain a transmembrane domain, as well as the non-receptor tyrosine kinases which do not possess transmembrane domains.
Growth factor receptor-bound protein 2, also known as Grb2, is an adaptor protein involved in signal transduction/cell communication. In humans, the GRB2 protein is encoded by the GRB2 gene.
Tyrosine-protein phosphatase non-receptor type 11 (PTPN11) also known as protein-tyrosine phosphatase 1D (PTP-1D), Src homology region 2 domain-containing phosphatase-2 (SHP-2), or protein-tyrosine phosphatase 2C (PTP-2C) is an enzyme that in humans is encoded by the PTPN11 gene. PTPN11 is a protein tyrosine phosphatase (PTP) Shp2.
Proto-oncogene c-KIT is the gene encoding the receptor tyrosine kinase protein known as tyrosine-protein kinase KIT, CD117 or mast/stem cell growth factor receptor (SCFR). Multiple transcript variants encoding different isoforms have been found for this gene. KIT was first described by the German biochemist Axel Ullrich in 1987 as the cellular homolog of the feline sarcoma viral oncogene v-kit.
Growth factor receptor-bound protein 10 also known as insulin receptor-binding protein Grb-IR is a protein that in humans is encoded by the GRB10 gene.
Janus kinase 2 is a non-receptor tyrosine kinase. It is a member of the Janus kinase family and has been implicated in signaling by members of the type II cytokine receptor family, the GM-CSF receptor family, the gp130 receptor family, and the single chain receptors.
Phosphatidylinositol 3-kinase regulatory subunit alpha is an enzyme that in humans is encoded by the PIK3R1 gene.
Cbl is a mammalian gene encoding the protein CBL which is an E3 ubiquitin-protein ligase involved in cell signalling and protein ubiquitination. Mutations to this gene have been implicated in a number of human cancers, particularly acute myeloid leukaemia.
Tyrosine-protein kinase HCK is an enzyme that in humans is encoded by the HCK gene.
GRB2-associated-binding protein 1 is a protein that in humans is encoded by the GAB1 gene.
Phosphatidylinositol 3-kinase regulatory subunit beta is an enzyme that in humans is encoded by the PIK3R2 gene.
GDNF family receptor alpha-1 (GFRα1), also known as the GDNF receptor, is a protein that in humans is encoded by the GFRA1 gene.
Discoidin domain receptor family, member 1, also known as DDR1 or CD167a, is a human gene.
GDNF family receptor alpha-3 (GFRα3), also known as the artemin receptor, is a protein that in humans is encoded by the GFRA3 gene.
GDNF family receptor alpha-2 (GFRα2), also known as the neurturin receptor, is a protein that in humans is encoded by the GFRA2 gene.
Fms-related tyrosine kinase 4, also known as FLT4, is a protein which in humans is encoded by the FLT4 gene.