Malignant melanomas that arise from the iris, ciliary body, and choroid layers of the eye-collectively referred to as uveal melanomas—represent the most common primary cancer of the eye and the second most common form of melanoma. Until recently, the identification of effective therapies for metastatic uveal melanoma has been hampered by a lack of known driver mutations. This situation has changed in recent years with the discovery of several common driver mutations, which has opened the door to rational targeted therapies.
Mutually exclusive mutations in the G protein-coupled receptor (GPCR) alpha subunits GNAQ and GNA11 (encoding Gq and G11 proteins, respectively) are present in ∼85% of uveal melanocytic tumors, including benign nevi, primary melanomas of all stages, and metastatic lesions. This spectrum suggests that GNAQ/11 mutations occur early and may even represent initiating events in tumorigenesis. These mutations occur as single amino acid substitutions at residues Q209 or R183, and they abrogate the intrinsic GTPase activity that normally serves to inactivate the subunit. As such, these inactivating mutations result in constitutive activation of oncogenic Gq/11 subunits. The recessive nature of these mutations at the molecular level, despite their dominant action at the cellular level, has posed a major challenge for direct pharmacologic inhibition. Instead, most efforts have focused on inhibiting downstream targets of activated Gq/11. The best understood target of Gq/11 is phospholipase C beta (PLCβ), which cleaves phosphatidylinositol (4,5)-bisphosphate (PIP2) to yield diacylglycerol (DAG) and inositol triphosphate (IP3). Both products promote stimulation of protein kinase C (PKC), which leads to activation of the mitogen-activated protein kinase (MAPK or MEK) pathway and cell proliferation. MEK and PKC inhibitors inhibit the proliferation of Gq/11 mutant uveal melanoma cell lines in vitro. Yet, clinical trials so far have shown little or no activity of such agents in patients with metastatic uveal melanoma, raising the question of whether there may be other targets that are critical for therapeutic inhibition in cancers harboring oncogenic forms of Gq/11.
One such target may be the Hippo tumor suppressor pathway, which controls tissue growth and cell fate through the regulation of cell proliferation and apoptosis . Key effectors of the pathway include the homologous oncoproteins YAP and TAZ, which promote tissue growth by regulating the activity of transcription factors such as TEADs and SMADs. In most proliferating cells, YAP is localized in the nucleus in its active form. Hippo pathway signaling leads to phosphorylation of YAP by the serine/threonine-protein kinases LATS1/2, resulting in YAP inactivation and retention in the cytoplasm and degradation via the proteasome.
Feng et al. and Yu et al. publicized studies showing that Gq/11 mutants found in uveal melanoma promote tumorigenesis by activating YAP. Mutant Gq/11, but not wild-type Gq/11, was found to trigger dephosphorylation and nuclear localization of YAP, associated with YAP-dependent transcription. Importantly, this activity of mutant Gq/11 is independent of PLCβ. In uveal melanoma cell lines and human tumor samples, there was a strong correlation between the presence of Gq/11 mutations and activated YAP, as indicated by its nuclear localization and increased levels of unphosphorylated YAP.
The question then arises as to whether this YAP activation by mutant Gq/11 is mediated solely through inhibition of LATS1/2. In their current article and in a recent publication by the same group show that activation of YAP by mutant Gq requires the guanine nucleotide exchange factor, Trio, and downstream small GTPases RhoA and Rac1. Activation of RhoA and Rac1 induces actin polymerization of G-actin to F-actin, triggering dissociation of the cytoskeletal-associated protein angiomotin (AMOT) from YAP, thereby allowing YAP to translocate from the cytoplasm to the nucleus to activate YAP-dependent transcription. Thus, mutant Gq/11 may activate YAP not only by inhibiting LATS1/2, but also by promoting actin polymerization independently of the canonical Hippo pathway.
Although these findings are promising, it is unlikely that inhibition of mutant Gq/11 signaling alone will be sufficient for treating metastatic uveal melanoma. Mutant Gq and G11 are relatively weak oncoproteins that are only able to transform immortalized melanocytes that have been genetically altered to be deficient in the p53 and p16/CDK4/RB pathways. Nevertheless, these findings will play an important role in the ongoing quest for effective therapy against metastatic uveal melanoma.