and multiple endocrine neoplasia type 2 (MEN2) Ret tyrosine kinase and multiple endocrine neoplasia type 2 (MEN2)

Ret is a receptor tyrosine kinase  The ret gene encodes a transmembrane protein tyrosine kinase. It has an extracellular ligand-binding domain, a cysteine-rich domain, and intracellular tyrosine kinase domains.  Associated with the cadherin superfamily  Chromosomal locus 10q11.2  Expressed in cells of neural crest origin - in rodent embryonic and adult tissue, expressed in peripheral enteric, sympathetic and sensory neurons, the excretory system (mesonephric duct and branching ureteric bud during embryogenesis)

Review: Receptor Tyrosine Kinases (RTKs)  Receptor tyrosine kinases are involved in signaling in cell growth, differentiation, survival, and apoptosis.  In response to binding of extracellular ligands, RTKs generally form homodimers or heterodimers. This is usually followed by autophosphorylation and signal transduction through the pathway.

Ret Activation and Promotion of Signaling Pathways  Ligands are glial cell line-derived neurotrophic factor (GDNF) family members  Ligands bind glycosyl-phosphatidylinositol-anchored coreceptors (GFR) 1-4  Ret dimerizes as a result of activation by ligand/coreceptor binding, autophosphorylates itself, and continues the phosphorylation cascade.  Among others, the GDNF/GFR1/RET complex initiates both the RAS and PI3K pathways - Pathways are activated through Tyr1062, which is a binding site for SHC - SHC further associates with GRB2/SOS and GAB1/2 complexes in the Ras and PI3K pathways, respectively

Ret activation initiates PI3 Kinase and Ras pathways Ligand = GDNF Co-receptor = GFR-1

Wild-type Ras has multiple functions  Development of the enteric nervous system (ENS) is primarily dependent on GDNF/GFR1/RET - loss of enteric ganglia if ret has a loss-of-function mutation (Moore, M., et. al. (1996) Nature 382) - c-ret homozygous mice develop an aganglionic phenotype and die because of a lack of ganglia posterior to the stomach (Taraviras, S, et.al. (1999) Development 126)  Activation of the PI3K pathway by GDNF/GFR1/RET blocks neuroectodermal apoptosis (Mograbi, B., et.al, (2001) J. Biol. Chem. 276(48))  Renal organogenesis - GDNF/GFR1/RET null mice show renal agenesis and hypoplastic kidneys due to lack of ureteric bud growth (Baloh, R.H, et.al. (2001) Curr. Opin. Neurobiol. 10)

(Mograbi, B., et.al, (2001) J. Biol. Chem. 276(48)) PI3K and Ras pathways interact in neural crest cells to promote growth, differentiation, and survival (Mograbi, B., et.al, (2001) J. Biol. Chem. 276(48))

Effects of Ret Mutations Hirschsprung Disease (HSCR):  A congenital absence of enteric innervation which results in intestinal obstruction  The mutations are varied and scattered throughout the Ret coding sequence, which include deletions and a variety of point mutations  This is the result of a loss-of-function mutation Papillary Thyroid Carcinoma (PTC)  RET/PTC oncoproteins in thyroid follicular cells, are frequently found in radiation-induced papillary thyroid carcinomas Multiple Endocrine Neoplasia (MEN) Type 2  A group of cancer syndromes characterized by medullary thyroid carcinoma  This condition is the result of a gain-of-function mutation, causing proliferation of thyroid cells

Multiple Endocrine Neoplasia Type 2 (MEN2)  Rare familial cancer syndrome  Usually germline mutations  Autosomal dominant mode of inheritance  Three types: FMTC (familial medullary thyroid carcinoma), MEN2A, and MEN2B  Affected cells are the C cells of the thyroid (these are derived from neural crest cells)  Medullary thyroid carcinomas are indicative of each type of MEN2 - 75% of all MTCs are sporadic; the remainder are hereditary - initially present as a mass on the neck or metastatic disease

FMTC  patient presents with bilateral medullary thyroid carcinoma (MTC)  Approximately 85% of families with FMTC have an identifiable RET mutation - Mutations occur at one of the five cysteine residues (codons 609, 611, 618, 620, and 634) with mutations of codons 618, 620, and 634 each accounting for 25 to 35% of mutations.  Presents at 20-40 years of age  Believed to be more benign than MEN2A or B and prognosis is good

MEN2A  patient presents with MTC, pheochromocytomas (~50%) and/or hyperparathyroidism (~15-30%)  Approximately 95% of families with MEN 2A have a RET mutation in exon 10 or 11 - Mutations of codon 634 Cys occur in about 85% of families; mutation of cysteine residues at codons 609, 611, 618, and 620 together account for the remainder of identifiable mutations in exons 10 and 11  50% of individuals with mutations in the Ret gene develop the disease by age 50, and 70% by age 70  Most common form of MEN2, accounting for ~90% of all cases

MEN2B  patient not only presents with MTC and bilateral pheochromocytomas, but also with diffuse ganglioneuromas of the intestinal tract, mucosal neuromas (on lips or tongue), and skeletal abnormalities  Approximately 95% of individuals with the MEN 2B phenotype have a single point mutation in the tyrosine kinase domain of the RET gene at codon 918 in exon 16, which substitutes a threonine for methionine  Accounts for ~5% of all MEN2 cases  Age of onset is about 10 years earlier than of MEN2A, but pheochromocytomas are sometimes detected in childhood

Ret mutations in MEN2  Codon 634 mutations render the RTK constitutively active.  This is a result of the dimerization of the Ret monomers due to the mutated cysteine. The mutation leaves an unpaired residue, and each mutant Ret monomer forms a disulfide bond with its unpaired counterpart from another mutant Ret.  The same mechanism applies to other mutations in the cysteine-rich region of the protein.

Detection and Treatment Options  Ret testing, elevated calcitonin levels (produced in C cells), elevated blood pressure if pheochromocytoma is present  Prophylactic thyroidectomy by age of 6 if mutation is detected (by age of 3 if MEN2B is detected)  Complete thyroidectomy after detection of MTC