Ret

The human RET (REarranged during Transfection) gene maps on chromosome 10 q11.2 and codes for a single-pass transmembrane protein. Its extracellular portion contains four cadherin-like repeats, a calcium-binding site and a cysteine-rich domain. The intracellular portion features a typical tyrosine kinase domain. Two major protein products (RET-9 and RET-51) are generated through an alternative splicing mechanism. RET is the functional receptor for ligands of the glial cell-line derived neurotrophic factor (GDNF) family. Four GDNF family ligands (GFL) have been isolated: GDNF, neurturin (NRTN), artemin (ARTN) and persephin (PSPN). GFLs bind RET in conjunction with glycosylphosphatidylinositol (GPI)-anchored co-receptors designated "GDNF family receptor-α" (GFR-α). Each of the four GFLs uses one of the four GFR-α (GFR-α 1-4) as preferential receptor. GFLs form a high-affinity complex with the corresponding GFR-α homodimer, and the complex brings together two RET molecules thereby triggering autophosphorylation and intracellular signaling. RET contains at least 12 autophosphorylation sites. A complex network of intracellular signaling pathways follows the autophosphorylation of some of them. RET tyrosines 905, 981, 1015 and 1062 are docking sites for Grb7/10 proteins, c-Src, phospholipase Cγ and Grb2, respectively. Tyrosine 1062 recruits many proteins including Shc/N-Shc, FRS2, IRS1/2, DOK1/4/5 and enigma. Thus, RET appears to trigger a complex network of intracellular signaling pathways, with Tyrosine1062 being central to most of them.

RET is expressed primarily in peripheral enteric, sympathetic and sensory neurons, and in central motor, dopamine and noradrenaline neurons. It is also expressed in branching ureteric bud during embryogenesis and in differentiating spermatogonia. RET is essential for the development of the sympathetic, parasympathetic and enteric nervous systems (ENS) and the kidney. In humans, RET loss-of-function mutations cause about 50% of familial megacolon (Hirschprung's disease, HSCR) and 15-35% of sporadic HSCR cases. Mutations identified in the RET extracellular domain markedly impair cell surface expression of the protein, probably because of incorrect folding. Mutations in the kinase domain virtually abolish RET enzymatic activity. Finally, a few mutations in the RET carboxyl-terminal tail impair binding to adaptors, i.e., Shc, FRS2, IRS1 and ShcC (Rai/N-Shc).

Activating germline point mutations in RET cause three related dominantly-inherited cancer syndromes: multiple endocrine neoplasia type 2A (MEN 2A), 2B (MEN 2B) and familial medullary thyroid carcinoma (FMTC). MEN 2 patients are affected by medullary thyroid carcinoma (MTC), a malignant tumor arising from calcitonin-secreting C cells of the thyroid gland. Additional features can be present in MEN 2A and MEN 2B, but not FMTC patients. FMTC patients usually develop MTC at a later stage in life. Most MEN 2B patients carry the M918T mutation in the RET kinase domain. Most MEN 2A and FMTC mutations affect one cysteine in the extracellular cysteine-rich domain of RET. MEN 2A is associated most frequently with mutations of codon 634 (85%), whereas FMTC mutations are evenly distributed among the various cysteines.

MEN 2-associated RET mutations have a gain-of-function effect, i.e., they promote activation of the kinase and oncogenic conversion. Indeed, MEN 2 was the first example of an inherited cancer syndrome caused by the germline transmission of a dominantly activated oncogene. Mechanisms leading to RET oncogenic conversion in MEN 2 depend on the location of the amino-acid change. Extracellular cysteine mutants display constitutive kinase activity consequent to disulfide-bonds stabilized homodimerization. These cysteine residues are thought to be normally involved in the formation of intramolecular disulfide bonds. Thus, when a cysteine is mutated, a partner cysteine may become free and form an aberrant intermolecular bond between two mutated RET monomers.

The first evidence of RET involvement in human cancer was obtained in thyroid gland papillary carcinomas. Chromosomal inversions or translocations of 10q11.2 occur in 2.5% to 40% of papillary thyroid carcinomas (PTC) and cause the recombination of the kinase-encoding domain of RET with heterologous genes thereby generating the RET/PTC oncogenes. RET/PTC1 (H4-RET fusion) and 3 (RFG-RET fusion) are the most prevalent variants. RET/PTC rearrangement cause a ligand-independent activation of RET oncogenic signaling. A body of evidence supports the notion that RET/PTC oncogenes can be causative in thyroid tumorigenesis and also an early genetic change in PTC development.

RET is a suitable target for the design of novel therapeutic approaches for human cancer. Accordingly, several small-molecule ATP-mimetics, including PP1, PP2, ZD6474, CEP-701, CEP-751, RPI-1, have been demonstrated to efficiently (nM range) obstruct RET enzymatic function.

 

The Table below contains accepted modulators and additional information.

 

Family Members RET
Other Names CDHF12, D-ret, Dret, Hirschsprung disease: HSCR1, MEN2A, MEN2B, MTC1, multiple endocrine neoplasia and medullary thyroid carcinoma 1, PTC, RET51, RET9
Molecular Weight
(kDa)
123 kDa
Structural Data 1114 aa (RET-51, human)
1072 aa (RET-9, human)
Isoforms Not Known
Species Human, mouse, Xenopus, rat, dog, Drosophila, chicken
Domain Organization Cadherin domain
N-terminal extracellular domain
Cysteine-rich extracellular domain
C-terminal cytoplasmic tyrosine kinase domain
Phosphorylation Sites Tyr806
Tyr809
Tyr900
Tyr905
Tyr981
Tyr1015
Tyr1062
Tyr1090
Tyr1096
Tissue Distribution Peripheral and central nervous system, thyroid C-cells, adrenal medulla developing ureteric bud, spermatogonia
Subcellular Localization Not Known
Binding Partners/Associated Proteins DOK4
DOK5
PLC-γ
DOK1
DOK3
DOK6
DOK2
FRS2
GRB2
Grb10
Shc
N-Shc
c-Src
IRS 1/2
Gab 1/2
Upstream Activators GDNF
PAX3
SOX10
NRTN
ARTN
PSPN
Downstream Activation Raf/MAP kinase
PI3K/Akt
PLCγ
c-Src
STAT3
Rac/c-Jun-terminal kinase
Shc
Braf
PDK1
Inhibitors PP1 (P0040)
PP2 (P0042)
ZD6474
CEP-701 (C7869)
CEP-751
RPI-1 (R8907)
Activators Not Known
Selective Activators Not Known
Physiological Function Neuronal cells survival and differentiation, kidney morphogenesis spermatogonia differentiation
Disease Relevance Hirschsprung's disease (congenital magacolon) (HSCR), multiple endocrine neoplasia type 2 syndromes (MEN2A, MEN2B, FMTC), sporadic medullary thyroid carcinomas (MTC), thyroid papillary carcinomas, congenital hypoventilation syndrome

 

Abbreviations

ARTN: Artemin
NRTN: Neurturin
PSPN: Persephin

 

References