KRAS: The Critical Driver and Therapeutic Target for Pancreatic Cancer

Abstract

RAS genes (HRAS, KRAS, and NRAS) comprise the most frequently mutated oncogene family in human cancer. With the highest RAS mutation frequencies seen with the top three causes of cancer deaths in the United States (lung, colorectal, and pancreatic cancer), the development of anti-RAS therapies is a major priority for cancer research. Despite more than three decades of intense effort, no effective RAS inhibitors have yet to reach the cancer patient. With bitter lessons learned from past failures and with new ideas and strategies, there is renewed hope that undruggable RAS may finally be conquered. With the KRAS isoform mutated in 84% of all RAS-mutant cancers, we focus on KRAS. With a near 100% KRAS mutation frequency, pancreatic ductal adenocarcinoma (PDAC) is considered the most RAS-addicted of all cancers. We review the role of KRAS as a driver and therapeutic target in PDAC.

Figure 1.

RAS proteins. (A) Sequence identity of human RAS proteins. Amino acid sequence identity was determined by CLUSTALW multiple sequence alignment. (B) RAS mutation frequencies. Data were compiled from Catalogue of Somatic Mutations in Cancer (COSMIC) v80. (C) RAS domains. The amino-terminal amino acids (1–164) comprise the G domain involved in guanosine triphosphate (GTP) binding and hydrolysis and interaction with guanine nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs), and effectors. RAS protein structure changes in the Switch I (SI; amino acids 30–38) and II (SII; amino acids 60–76) regions during guanosine diphosphate (GDP)–GTP cycling, with the GTP-bound form having higher affinity for effectors. Circled cysteine amino acids indicate sites of covalent modification by addition of a palmitate fatty acid. The boxed serine residue is phosphorylated by protein kinase C. The underlined lysine residues promote membrane targeting. The italicized cysteines in the CAAX motif are sites of covalent modification by addition of a farnesyl isoprenoid and carboxylmethylation.

Figure 2.

KRAS mutations in pancreatic cancer. (A) Frequency of mutations in the four major genes in pancreatic cancer. (B) Genetic progression of pancreatic cancer. (C) KRAS amino acid substitutions in pancreatic cancer. (Compiled from data in Jones et al. 2008, Biankin et al. 2012, Sausen et al. 2015, Waddell et al. 2015, and Witkiewicz et al. 2015.)

Figure 3.

KRAS guanosine diphosphate (GDP)–guanosine triphosphate (GTP) regulation and effector signaling. RAS-selective guanine nucleotide exchange factors (GEFs) and GTP-activating proteins (GAPs) regulate GDP–GTP cycling. Cancer associated mutations, found primarily (>99%) at residues G12, G13, or Q61 disrupt GDP–GTP cycling by impairing intrinsic and GAP-stimulated GTP hydrolysis and/or accelerating GDP–GTP exchange. KRAS effectors are characterized by a RAS-binding domain (RBD) or RAS association (RA) domains. AKT1 phosphorylates and inactivates GSK3β, preventing MYC phosphorylation at T58 and FBW7-mediated polyubiquitination and degradation.

Figure 4.

Components of the three-tiered extracellular regulated kinase (ERK) mitogen-activated protein kinase (MAPK) cascade. Each level of the ERK MAPK cascade is comprised of highly related isoforms. BRAF missense or deletion mutants (shown is the deletion in the BxPC-1 cell line) are found in pancreatic ductal adenocarcinoma (PDAC). RAF isoforms are regulated by positive (green) and negative (red) regulatory phosphorylation activities. Shown are the phosphorylation sites that activate MEK1/2 or ERK1/2. Domain topology was determined in simple modular architecture research tool (SMART) (smart.embl-heidelberg.de). Total protein and kinase domain sequence identities are indicated (%/%) as determined by CLUSTALW multiple sequence alignment. *, point mutation; ▵, deletion mutation.

Figure 5.

Clinical candidate inhibitors of the RAF-MEK-extracellular regulated kinase (ERK) mitogen-activated protein kinase (MAPK) cascade. (Compiled from data in www.clinicaltrials.gov.)