RET signaling pathway and RET inhibitors in human cancer

Abstract

Rearranged during transfection (RET) receptor tyrosine kinase was first identified over thirty years ago as a novel transforming gene. Since its discovery and subsequent pathway characterization, RET alterations have been identified in numerous cancer types and are most prevalent in thyroid carcinomas and non-small cell lung cancer (NSCLC). In other tumor types such as breast cancer and salivary gland carcinomas, RET alterations can be found at lower frequencies. Aberrant RET activity is associated with poor prognosis of thyroid and lung carcinoma patients, and is strongly correlated with increased risk of distant metastases. RET aberrations encompass a variety of genomic or proteomic alterations, most of which confer constitutive activation of RET. Activating RET alterations, such as point mutations or gene fusions, enhance activity of signaling pathways downstream of RET, namely PI3K/AKT, RAS/RAF, MAPK, and PLCγ pathways, to promote cell proliferation, growth, and survival. Given the important role that mutant RET plays in metastatic cancers, significant efforts have been made in developing inhibitors against RET kinase activity. These efforts have led to FDA approval of Selpercatinib and Pralsetinib for NSCLC, as well as, additional selective RET inhibitors in preclinical and clinical testing. This review covers the current biological understanding of RET signaling, the impact of RET hyperactivity on tumor progression in multiple tumor types, and RET inhibitors with promising preclinical and clinical efficacy.

Keywords: RET; cancer; lung cancer; therapeutics; thyroid cancer.

Figure 1

Canonical RET signaling. RET activation occurs upon fulfillment of multiple steps. Binding of GDNF-family ligands (GFLs) such as Artemin, Neurturin, Persephin, (ARTN, NRTN, PSPN, respectively), and GDNF to co-receptor GFRα1-4, concurrently with binding of calcium ions to the calcium binding domain, induces recruitment of RET, forming RET-GFRα complex. Formation of RET-GFRα complex brings two RET monomers in close proximity to induce homodimerization and crossphosphorylation of key RET tyrosine residues that recruit adaptor proteins important for propagation of RET signaling, such as PI3K/AKT, MAPK, PLCγ, and RAS/RAF/ERK. Thus, activation of RET signaling ultimately promotes cell proliferation, growth, and survival through activation of multiple downstream signaling cascades. CRD, cysteine-rich domain; TMD, transmembrane domain; TK, tyrosine kinase domain.

Figure 2

Altered RET and their mechanisms of activation. (A) Schematic representation of the fusion between the tyrosine kinase domain of wild-type (WT) RET receptor, and the kinesin and coiled-coil (CC) domain of a fusion partner. Dashed lines represent fusion sites. (B) Cysteine mutations in RET cysteine-rich domain (CRD) promote formation of intermolecular disulfide bridges, leading to constitutive dimerization and activation of RET that is GDNF-independent. Mutations in RET tyrosine kinase domains (TKD) can elicit steric conformations that regulate access to the ATP binding pocket of RET (E768X, V804X), alter hinge or inter-lobe flexibility (L790X, Y791X), promote activation of RET monomers (S891X, M918T), or destabilize the inactive form of RET (A883X), all of which confer constitutive RET activation albeit with varying activities. (C) Activation of either WT RET or oncogenic RET fusions can promote activation of downstream pathways, PI3K/AKT, RAS/RAF/MEK/ERK, JAK2/STAT3, and PLCγ resulting in enhanced proliferation, migration, cell survival and differentiation, ultimately promoting neoplastic growth and tumorigenesis.

Figure 3

RET/PTC rearrangements in papillary thyroid carcinomas. Wild-type RET and RET rearrangements in papillary thyroid carcinoma (PTC). Coiled-Coil domain containing 6 (CCDC6)-RET, Protein Kinase CAMP-Dependent Type I Regulatory Subunit Alpha (PRKAR1A)-RET, and nuclear receptor coactivator 4 (NCOA4)-RET are frequently found in PTC cases. Oncogenic RET fusions lack the transmembrane domain (TMD) and result in chimeric, cytosolic proteins that are constitutively activated.