RET and MEN2 Flashcards
Why Study RET (of all RTKs)?
- Human diseases are associated with both gain and loss of function mutations.
- RET provides insight into mechanisms in cancer and developmental biology.
- Identifying the RET mutations allows us to begin appropriate treatment earlier
What does RET stand for?
rearranged during transformation
THE RET SYSTEM- Receptors
- RET (rearranged during transformation):
Receptor Tyrosine Kinase
- GFRα(1-4) (GDNF family receptor-a, 1-4):
Accessory receptors that increase the affinity of the ligands for RET.
Glycosylphosphatidylinositol-linked (NOT transmembrane)
Structure of RET
- Draw RET—cysteine rich domain & cadherin domain.
- There are 3 isoforms of RET generated by alternate mRNA splicing:
- RET51, RET43, RET9. They have different length tails AFTER the kinase domain
RET LIGANDS
GDNF family of ligands (GFLs)
- GDNF (glial cell-derived neurotrophic factor)
- NRTRN (Neurturin)
- PRSP (Persephin)
- ARTN (Artemin)
Different GFLs prefer different GFRαs
Show RET and its Accessory Receptors
RET signal transduction
Germline Mutations in RET
- 98% of the cases of inherited MEN2 (multiple endocrine neoplasia Type 2) are associated with mutations in RET.
- MEN2 is rare (1:30,000)
MEN2 has 2 Main Subtypes:
- MEN2A (FMTC is now seen as a subset)
- MEN2B
(About 25% of MTC (medullary thyroid carcinoma) is inherited and 75% is sporadic; Mutations in RET account for 40-50% of the sporadic)
Characteristics of MEN2 Subtypes: MEN2A
- Medullary thyroid cancer
- Pheochromocytoma
- Hyperparathyroidism
- MEN2A with Hirshsprung’s disease MEN2A with cutaneous lichen
- amyloidosis
- FMTC
Characteristics of MEN2 Subtypes: MEN2A
- Medullary thyroid cancer
- Pheochromocytoma
- Marfanoid habitus
- Intestinal neuromas
- mucosal ganglioneuromas
Survival Rates for MEN2a and MEN2b
10 year survival rate:
- 75% of MEN2B patients with medullary thyroid carcinoma (MTC).
- 97% of MEN2A patients with MTC.
MEN2B is more aggressive since it is usually diagnosed later (denovo germline mutations with no family history)
Medullary Thyroid Carcinoma (MTC)
MTC is multi-centric, bilateral, and characterised by whitish nodules
Genotype to Phenotype
Figure 1. RET proto-oncogene is the cause of MEN2. (A) The proto-oncogene RET is composed of 21 exons located on chromosome
- 10 (10q11.2) and encodes for a transmembrane receptor tyrosine kinase. (B) The RET protein is composed of 3 functional
- domains,including an extracellular ligand-binding domain, a transmembrane domain, and a cytoplasmic tyrosine kinase domain.
- The extracellular domain contains a signal peptide that is cleaved, 4 cadherin-like repeats, and a cysteine-rich region critical for
- disulfide bond formation needed in dimerization. (C) The locations of known RET mutations are highlighted according to American
- Thyroid Association risk classification (blue 5 rating A; green 5 rating B; orange 5 rating C; red 5 rating D).
- (D) Mutations are grouped by the MEN (multiple endocrine neoplasia) subtype they cause.
Location of Common and Rare Mutations in RET
Cysteine Residues are Mutated in MEN2a
- Point mutations in one of six cysteine codons in the extracellular domain of RET are responsible for FMTC and MEN2A
- C634 is mutated in 90% of MEN2A (C634R in >50%) (Frank-Raue estimates only 54% of MEN2A—may depend on the design of the genetic test)
- C634, C618, C620 each accounts for about 30% of FMTC
The Mutated Cysteines are in the Extracellular Domain of RET
Mechanism of the Cysteine Mutations
- Substitutions of cysteines activate RET as a dominant transforming gene by causing ligand-independent dimerization.
- The substitution of a cysteine with another amino acid disrupts its normal intramolecular disulphide bond and leaves the partner cysteine free to form an intermolecular bond between RET molecules.
Forming Disulphide Bonds
Structure of RET C634R Homodimers
(Receptor is autophosphorylated in the absence of ligand; dimers are held together by intermolecular disulphide bonds)
MEN2B
- medullary thyroid carcinoma (MTC; 100% of patients)
- pheochromocytoma (tumours of the adrenal medulla; 50% of patients)
- ganglioneuromatosis (tumours of the ganglion nerve cells of the gastrointestinal tract and mucosa; 40% of patients)
TRUE or FALSE: MEN2B is the most aggressive version of MEN2
TRUE
Ganglioneuromatosis
Neuromas in the mucosa in MEN2B
MEN2B Is Caused by An Activating Mutation in the Cytoplasmic Domain of RET
- The point mutation M918T is found in 95% of the
- patients with MEN2B
- M918T is ligand-independent—the mutant signals in the absence of GDNF.
- M918 is in the substrate recognition pocket of RET’s tyrosine kinase domain.
- M918T results in increased kinase activity and an expanded range of substrates (it acts on proteins that are normally a target of the cytoplasmic tyrosine kinase Src).
Homology Model of RET kinase domain and M918
Hierarchical clustering of differentially expressed genes in tumours with MEN2A and MEN2B
- Hierarchical clustering of differentially expressed genes identified by whole-genome gene chip analysis on NIH-RET(MEN2A) (C609Y, C634R), NIH-RET(MEN2B) (A883F, M918T) and NIH-RET(FMTC) (Y791F)-specific tumors.
- Genes are colored according to the normalized expression values.
- The colors in the heat map represent expression levels (red, overexpression; yellow, average expression; blue, underexpression/no expression).
- The tumor samples are ordered in columns and the corresponding gene clusters in rows.
Mechanisms Underlying the Activity of Oncogenic RET Mutations
