KRAS: the Achilles' heel of pancreas cancer biology

Abstract

The genetic landscape of pancreatic ductal adenocarcinoma (PDAC) is well-established and dominated by four key genetic driver mutations. Mutational activation of the KRAS oncogene is the initiating genetic event, followed by genetic loss of function of the CDKN2A, TP53, and SMAD4 tumor suppressor genes. Disappointingly, this information has not been leveraged to develop clinically effective targeted therapies for PDAC treatment, where current standards of care remain cocktails of conventional cytotoxic drugs. Nearly all (~95%) PDAC harbors KRAS mutations, and experimental studies have validated the essential role of KRAS mutation in PDAC tumorigenic and metastatic growth. Identified in 1982 as the first gene shown to be aberrantly activated in human cancer, KRAS has been the focus of intensive drug discovery efforts. Widely considered "undruggable," KRAS has been the elephant in the room for PDAC treatment. This perception was shattered recently with the approval of two KRAS inhibitors for the treatment of KRASG12C-mutant lung and colorectal cancer, fueling hope that KRAS inhibitors will lead to a breakthrough in PDAC therapy. In this Review, we summarize the key role of aberrant KRAS signaling in the biology of pancreatic cancer; provide an overview of past, current, and emerging anti-KRAS treatment strategies; and discuss current challenges that limit the clinical efficacy of directly targeting KRAS for pancreatic cancer treatment.

Figure 1. KRAS mutations in PDAC.

(A) Schematic illustrating pancreatic ductal adenocarcinoma (PDAC) pathogenesis and progression (adapted from ref. with permission from Springer Nature Limited, which retains the rights to the reference image). Mutations in KRAS oncogene are the initiating step in PDAC development, and they induce transformation of normal pancreas epithelium to low-grade pancreatic intraepithelial neoplasia (PanIN). Progression from low-grade PanINs to high-grade PanINs and eventually invasive PDAC is caused by loss-of-function mutations in CDKN2A, TP53, and SMAD4 tumor suppressor genes. The severity of disease is also associated with increased KRAS^mut copy numbers. (B) KRAS mutation frequencies in PDAC. Data were compiled from the cBioPortal GENIE Cohort v17.0 database (48) from 7,407 patients with PDAC. Of the three RAS isoforms, KRAS is the predominantly mutated isoform, with NRAS and HRAS mutations accounting for <1% of PDAC cases. Of the three mutational hot spots, G12X mutations are most common in PDAC, with G12D, G12V, and G12R representing the predominant amino acid mutations at this position. G13X mutations are rare in PDAC and comprise less than 1% of KRAS mutations. Q61X mutations are also uncommon, accounting for 7% of KRAS point mutations, with Q61H representing the predominant mutation. The authors would like to acknowledge the American Association for Cancer Research and its financial and material support in the development of the AACR Project GENIE registry, as well as members of the consortium for their commitment to data sharing. Interpretations are the responsibility of the authors.

Figure 2. KRAS GTPase regulation and signaling.

(A) KRAS encodes a small GTPase comprising the G domain and hypervariable region (HVR). Alternative splicing of exon four results in two KRAS isoforms (KRAS4A/KRAS4B, denoted as 4A/B), which differ in their carboxyl-terminal 151–188/189 amino acids. The G domain is involved in guanosine triphosphate (GTP) and guanosine diphosphate (GDP) binding and interactions with guanine nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs), and effectors. HVR contains the CAAX tetrapeptide motif that acts as a signal for posttranslational modifications that promote KRAS plasma membrane association essential for KRAS oncogenic function. Switch I and II regions (denoted as SI and SII) are highlighted, and mutational hot spots at G12, G13, and Q61 positions are indicated with red asterisks. (B) KRAS cycles between active GTP-bound and inactive GDP-bound states. Receptor tyrosine kinase (RTK) signaling promotes GEF-mediated GTP loading and activation of KRAS, which then engages downstream effector signaling (i.e., the RAF/MEK/ERK MAPK cascade). GAPs accelerate intrinsic KRAS GTPase activity and GTP hydrolysis to return KRAS to the inactive GDP-bound state. Amino acid substitutions at G12, G13, and Q61 hot spot positions accelerate GDP to GTP exchange rates and/or impair intrinsic or GAP-induced GTP hydrolysis, resulting in constitutively active KRAS. (C) KRAS undergoes three posttranslational modifications at the carboxyl-terminal CAAX motif (where C denotes cysteine, A denotes aliphatic, and X denotes terminal residues), which is required for association with membranes. Farnesyltransferase (FTase) adds a 15-carbon farnesyl group to the cysteine amino acid at the CAAX motif, RAS-converting enzyme (RCE1) removes -AAX residues, and isoprenylcysteine carboxylmethyltransferase (ICMT) catalyzes carboxylmethylation of farnesylated cysteine. Inhibition of FTase (FTIs) leads to alternative prenylation of KRAS by geranylgeranyltransferase-I (GGTase-I), which adds a 20-carbon geranylgeranyl group and facilitates KRAS associate with membranes. C, cysteine; Ome, carboxyl methylation.

Figure 3. Direct KRAS inhibitors.

(A) The current landscape of direct KRAS inhibitors and their status in preclinical and clinical stages. Blue indicates approved drugs, green indicates clinical trials that are recruiting, gray indicates active clinical trials that are not recruiting, red indicates terminated clinical trials, and purple indicates trials with unknown status. (B) The mechanisms of action of KRAS inhibitors are diverse. Mutant-selective inhibitors can be off-state, on-state, or off- and on-state inhibitors. Some inhibitors covalently modify mutant KRAS, others do not. There are multi-mutant or pan-KRAS and pan-RAS inhibitors that target WT and mutant KRAS/RAS proteins. Tri-complex inhibitors utilize cytosolic cyclophilin A (CypA) scaffold and KRAS degraders utilize ubiquitin-mediated proteasomal degradation of KRAS protein.

Figure 4. Resistance mechanisms to KRAS^G12C inhibitors and combination strategies.

(A) Sequencing of circulating tumor DNA from patients who relapsed on adagrasib, sotorasib, divarasib, or LY3537982 treatment demonstrated that genetic alterations occurred at the level of RAS or in the upstream and downstream components of RAS signaling. RAS-level alterations included mutations and/or amplifications in KRAS and NRAS and mutations in NF1. Upstream signaling alterations included mutations, amplifications, and fusions in RTKs. Downstream signaling alterations included mutational activation of downstream ERK MAPK and PI3K effector signaling components, amplification of MYC, etc. No genetic mutations were found in 50% of patients who relapsed on KRAS^G12C treatment. (B) Most combination strategies with KRAS inhibitors are based on resistance mechanisms that have been identified in relapsed patients and in preclinical studies that include signal transduction and kinase inhibitors, among others (Tables 3 and 4). (C) Nongenetic mechanisms driving resistance to KRAS inhibitors may include transcriptional reprogramming, changes in cellular states (epithelial to mesenchymal [EMT], adeno-to-squamous carcinoma, or adenocarcinoma to mucinous differentiation), and/or changes in molecular subtypes. MET,mesenchymal to epithelial transition; RASi, RAS inhibitor.