G C A T T A C G G C A T genes Review The Role of Wild-Type RAS in Oncogenic RAS Transformation Erin Sheffels and Robert L. Kortum * Department of Pharmacology and Molecular Therapeutics, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA; [email protected] * Correspondence: [email protected] Abstract: The RAS family of oncogenes (HRAS, NRAS, and KRAS) are among the most frequently mutated protein families in cancers. RAS-mutated tumors were originally thought to proliferate independently of upstream signaling inputs, but we now know that non-mutated wild-type (WT) RAS proteins play an important role in modulating downstream effector signaling and driving therapeutic resistance in RAS-mutated cancers. This modulation is complex as different WT RAS family members have opposing functions. The protein product of the WT RAS allele of the same isoform as mutated RAS is often tumor-suppressive and lost during tumor progression. In contrast, RTK-dependent activation of the WT RAS proteins from the two non-mutated WT RAS family members is tumor-promoting. Further, rebound activation of RTK–WT RAS signaling underlies therapeutic resistance to targeted therapeutics in RAS-mutated cancers. The contributions of WT RAS to proliferation and transformation in RAS-mutated cancer cells places renewed interest in upstream signaling molecules, including the phosphatase/adaptor SHP2 and the RasGEFs SOS1 and SOS2, as potential therapeutic targets in RAS-mutated cancers. Keywords: RAS; KRAS; HRAS; NRAS; SOS1; SOS2; RTK; SHP2; resistance Citation: Sheffels, E.; Kortum, R.L. The Role of Wild-Type RAS in Oncogenic RAS Transformation. 1. Introduction Genes 2021, 12, 662. https://doi.org/ The RTK/RAS pathway (Figure1A) is among the most commonly mutated pathways 10.3390/genes12050662 in cancer [1,2]. The three RAS genes, HRAS, NRAS, and KRAS, encode four highly homolo- gous protein isoforms (HRAS, NRAS, KRAS4A, and KRAS4B), driver mutations in RAS Academic Editor: genes occur in ~20% of human tumors (reviewed in [3]). KRAS is the most frequently mu- Esther Castellano-Sánchez tated RAS family member (75% of RAS mutations), including high incidence of mutations in lung [4], colon [5], and pancreatic cancers [6], three of the top four causes of cancer-related Received: 9 April 2021 death [2,7,8]. HRAS and NRAS mutations are common in other cancer types including Accepted: 27 April 2021 head and neck, skin, and hematopoietic cancers [9]. RAS-mutated cancers respond poorly Published: 28 April 2021 to standard chemotherapy [10–14], so targeted approaches are needed to treat patients with RAS-mutated tumors. While advances in targeting specific mutant RAS proteins have been Publisher’s Note: MDPI stays neutral made [15–18], the majority of RAS-mutated tumors remain resistant to currently available with regard to jurisdictional claims in treatments [4,12,14,19,20]. Novel strategies for targeting the RAS pathway are necessary published maps and institutional affil- iations. to provide effective therapeutics to the majority of patients with RAS-mutated cancers. Understanding the signaling context of mutant RAS is key to developing indirect targeting and combination therapy strategies to better manage these cancers. While typical models of oncogene activation assume that the mutated protein drives oncogenesis separately from the wild-type family members, the evidence that non-mutant Copyright: © 2021 by the authors. wild-type (WT) RAS proteins influence cancer initiation and growth in RAS-mutated Licensee MDPI, Basel, Switzerland. cancers is now well-established and several mechanisms for the effects have been proposed. This article is an open access article Here, we summarize the current understanding of the effects of WT RAS on RAS-mutated distributed under the terms and conditions of the Creative Commons cancers and the proposed mechanisms behind those effects. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Genes 2021, 12, 662. https://doi.org/10.3390/genes12050662 https://www.mdpi.com/journal/genes Genes 2021, 12, x FOR PEER REVIEW 2 of 18 Genes 2021, 12, 662 2 of 18 Figure 1. MutantFigure RAS 1. and Mutant WT RAS RAS cooperate and WT RAS to promote cooperate oncogenesis. to promote (A oncogenesis.) Schematic showing(A) Schematic mutant showing RAS and WT RAS signaling in RASmutant-mutated RAS cancer and WT cells. RAS (B )signaling RAS family in RAS members-mutated show cancer differential cells. (B activation) RAS family of downstream members show RAS effectors. HRAS activatesdifferential PI3K well activation but RAF relatively of downstream poorly; RAS KRAS effector activatess. HRAS RAF activates well but PI3K PI3K well poorly. but (RAFC) Schematic relatively showing proposed activationpoorly; of KRAS RAF/MEK/ERK activates RAF versus well PI3K/AKT but PI3K poorly. signaling (C) inSchematicHRAS- and showingKRAS -mutatedproposed canceractivation cells. of In HRAS- RAF/MEK/ERK versus PI3K/AKT signaling in HRAS- and KRAS-mutated cancer cells. In HRAS- mutated cells, mutant HRAS activates PI3K/AKT signaling, whereas RTK-WT N/KRAS activate RAF/MEK/ERK signaling. mutated cells, mutant HRAS activates PI3K/AKT signaling, whereas RTK-WT N/KRAS activate WT HRAS is tumor suppressive and inhibits mutant HRAS signaling. In KRAS-mutated cells, mutant KRAS activates RAF/MEK/ERK signaling. WT HRAS is tumor suppressive and inhibits mutant HRAS signaling. RAF/MEK/ERKIn signaling,KRAS-mutated whereas cells, RTK-WT mutant H/NRAS KRAS activates activate RAF/MEK/ERK PI3K/AKT signaling. signaling, WT whereas KRAS is RTK-WT tumor suppressive and inhibits mutantH/NRAS KRAS signaling. activate PI3K/AKT signaling. WT KRAS is tumor suppressive and inhibits mutant KRAS signaling. 2. Contributions of WT RAS to Mutant RAS-Driven Cancers 2. ContributionsNon-mutant of WT RAS WTto Mutant RAS proteins RAS-Driven play anCancers important role in modulating downstream Non-mutanteffector WT signaling RAS proteins and oncogenesis play an important in RAS-mutated role in modulating cancers. While downstream the contributions of effector signalingWT RAS and to RASoncogenesis-mutated in cancers RAS-mutated varies based cancers. on factors While suchthe contributions as the specific of RAS isoform WT RAS to thatRAS is-mutated mutated cancers and the varies cancer based type, on WT factor RASs proteinssuch as the can specific be broadly RAS categorized isoform into two that is mutatedgroups and withthe cancer opposing type, biologic WT RAS functions. proteins can The be protein broadly product categorized of the into WT twoRAS allele of groups withthe opposing same isoform biologic as func mutatedtions. TheRAS proteinis tumor-suppressive, product of the WT whereas RAS allele the WT of the RAS proteins same isoformfrom as themutated two non-mutated RAS is tumor-suppressive, WT RAS family wherea memberss the are WT tumor-promoting RAS proteins from (Figure 1B and the two non-mutatedreviewed inWT [21 RAS]). family members are tumor-promoting (Figure 1B and re- viewed in [21]). 2.1. The WT RAS Allele of the Same Isoform as Mutated RAS Inhibits Tumorigenesis 2.1. The WT RASSeveral Allele of studies the Same have Isoform found as Mutated that the WTRAS allele Inhibits corresponding Tumorigenesis to the specific mutated RAS gene (WT HRAS in HRAS-mutated cancers [22,23]; WT NRAS in NRAS-mutated Several studies have found that the WT allele corresponding to the specific mutated cancers [24,25]; WT KRAS in KRAS-mutated cancers [23,26–30]) suppresses tumorigenesis. RAS gene (WT HRAS in HRAS-mutated cancers [22,23]; WT NRAS in NRAS-mutated can- Further, many RAS-mutated cancers have loss of heterozygosity (LOH) at the mutated cers [24,25];gene, WT KRAS suggesting in KRAS that-mutated loss of the cancers wild-type [23,26–30]) allele confers suppresses a growth tumorigenesis. advantage. Evidence for Further, manyLOH RAS as-mutated a frequent cancers event in have cancer loss initiation of heterozygosity has been observed(LOH) at inthe model mutated systems for all gene, suggestingthree RAS that genesloss of (HRAS the wild-type[23,31–34 allele], NRAS confers[21,24 a,35 growth], KRAS advantage.[21,36–39]). Evidence A survey of human for LOH as tumora frequent samples, event cancer in cancer cell lines,initia andtion xenograftshas been observed of lung, pancreatic,in model systems and colorectal for cancers all three RASfound genes mutant (HRAS allele [23,31–34], specific NRAS imbalance [21,24,35], (MASI), KRAS where [21,36–39]). the mutant A allelesurvey makes of up more human tumorthan samples, half of cancer the gene cell copies, lines, and in 58% xenografts of KRAS of-mutated lung, pancreatic, samples; and over colorectal half of these imbal- cancers foundances mutant were allele due tospecific complete imbalance loss of (MASI), the wild-type where allele the mutant (uniparental allele makes disomy) up [40]. Other more than halfsurveys of the of gene both copies,KRAS-mutated in 58% of [41 KRAS,42] and-mutatedHRAS samples;-mutated over [43] half patient of these tumor samples imbalances havewere founddue to similar complete results. loss KRASof the MASIwild-type is associated allele (uniparental with worse disomy) prognosis [40]. in colorectal Other surveyscancer of both and KRAS pancreatic-mutated cancer [41,42] [44] and and loss HRAS of the-mutated WT KRAS [43]allele patient has tumor been foundsam- at a higher ples have foundrate in similar metastatic results.KRAS KRAS-mutated MASI lungis associated and pancreatic with worse cancers prognosis compared in colo- to the primary rectal cancertumors and pancreatic [36,37]. Severalcancer [44] mechanisms and loss of have the WT been KRAS proposed allele forhas inhibition been found of at mutant RAS a higher rateby in the metastatic corresponding KRAS-mutated wild-type lung RAS. and MASI pancreatic and concomitant cancers compared loss of the to wild-type the allele primary tumorswould [36,37]. increase Several the dosagemechanisms of the have mutant been allele, proposed increasing for inhibition the number of mutant of mutant RAS RAS by theproteins corresponding signaling wild-type in the cell, RAS.