Truncating PREX2 mutations activate its GEF activity and alter gene expression regulation in NRAS-mutant melanoma

Abstract

PREX2 (phosphatidylinositol-3,4,5-triphosphate-dependent Rac-exchange factor 2) is a PTEN (phosphatase and tensin homolog deleted on chromosome 10) binding protein that is significantly mutated in cutaneous melanoma and pancreatic ductal adenocarcinoma. Here, genetic and biochemical analyses were conducted to elucidate the nature and mechanistic basis of PREX2 mutation in melanoma development. By generating an inducible transgenic mouse model we showed an oncogenic role for a truncating PREX2 mutation (PREX2E824*) in vivo in the context of mutant NRAS. Using integrative cross-species gene expression analysis, we identified deregulated cell cycle and cytoskeleton organization as significantly perturbed biological pathways in PREX2 mutant tumors. Mechanistically, truncation of PREX2 activated its Rac1 guanine nucleotide exchange factor activity, abolished binding to PTEN and activated the PI3K (phosphatidyl inositol 3 kinase)/Akt signaling pathway. We further showed that PREX2 truncating mutations or PTEN deletion induces down-regulation of the tumor suppressor and cell cycle regulator CDKN1C (also known as p57KIP2). This down-regulation occurs, at least partially, through DNA hypomethylation of a differentially methylated region in chromosome 11 that is a known regulatory region for expression of the CDKN1C gene. Together, these findings identify PREX2 as a mediator of NRAS-mutant melanoma development that acts through the PI3K/PTEN/Akt pathway to regulate gene expression of a cell cycle regulator.

ERK/MAPK Signaling Drives Overexpression of the Rac-GEF, PREX1, in BRAF- and NRAS-Mutant Melanoma

“…Finally, the PREX1-related RhoGEF PREX2 is activated by missense mutations in 25% of metastatic melanomas, especially by truncating mutations mutations (11,12), and mutational activation of the PREX1/2 target RAC1 has been observed in ~11% of melanomas (15,16). The rarity of PREX1 truncating mutations and lack of apparent hotspots among the few missense mutations argue that this is not a significant mechanism of PREX1 activation in melanoma.…”

“…Instead, missense mutations in PREX2 have been identified in 25% of malignant melanomas (11). Although no clear mutational hotspots have been seen in melanomas, experimental studies support a gain-of-function consequence of these mutations (11,12). PREX2 missense mutations have also been found in 38% of cutaneous squamous cell carcinomas (13) and 17% of stomach adenocarcinomas (14).…”

The Rac GTPase in Cancer: From Old Concepts to New Paradigms

“…Whereas P-Rex1 mutations are infrequent, a high incidence of P-Rex2 point mutations has been described in melanoma (14%, third in prevalence after BRaf and NRas) (38). Although the biological consequences of the multiple P-Rex2 mutations, observed in melanoma and pancreatic cancer, have yet to be fully determined, a recent study has demonstrated enhanced Rac1-GEF activity and a pro-tumorigenic function of a truncated P-Rex2 mutant (39). …”

PREX1 Protein Function Is Negatively Regulated Downstream of Receptor Tyrosine Kinase Activation by p21-activated Kinases (PAKs)

“…PREX2 is also overexpressed in numerous cancer types and is frequently mutated in cancer, with especially high mutation rates in melanoma (25)(26)(27). PREX2 mutations found in cancer activate its GEF activity and promote tumorigenesis in a melanocyte xenograft mouse model (27,28). Furthermore, certain mutations can also evade PTEN-mediated inhibition of PREX2-driven breast cancer cell invasion (29).…”