KRAS Allelic Imbalance: Strengths and Weaknesses in Numbers

KRAS Loss of Heterozygosity Promotes MAPK-Dependent Pancreatic Ductal Adenocarcinoma Initiation and Induces Therapeutic Sensitivity to MEK Inhibition

Pancreatic cancer is characterized by the prevalence of oncogenic mutations in KRAS. Previous studies have reported that altered KRAS gene dosage drives progression and metastasis in pancreatic cancer. Whereas the role of oncogenic KRAS mutations is well characterized, the relevance of the partnering wild-type (WT) KRAS allele in pancreatic cancer is less well understood and controversial. Using in vivo mouse modeling of pancreatic cancer, we demonstrated that WT KRAS restrains the oncogenic impact of mutant KRAS and dramatically impacts both KRAS-mediated tumorigenesis and therapeutic response. Mechanistically, deletion of WT Kras increased oncogenic KRAS signaling through the downstream MAPK effector pathway, driving pancreatic intraepithelial neoplasia initiation. In addition, in the KPC mouse model, a more aggressive model of pancreatic cancer, lack of WT KRAS led to accelerated initiation but delayed tumor progression. These tumors had altered stroma and an enrichment of immunogenic gene signatures. Importantly, loss of WT Kras sensitized Kras mutant tumors to MEK1/2 inhibition though tumors eventually became resistant and then rapidly progressed. This study demonstrates the repressive role of WT KRAS during pancreatic tumorigenesis and highlights the critical impact of the presence of WT KRAS in both tumor progression and therapeutic response in pancreatic cancer. Significance: KRAS allelic status impacts pancreatic cancer progression and has the potential to guide effective treatment in a substantial subset of patients.

Wild-Type KRAS Allele Effects on Druggable Targets in KRAS Mutant Lung Adenocarcinomas

KRAS mutations are one of the most common oncogenic drivers in non-small cell lung cancer (NSCLC) and in lung adenocarcinomas in particular. Development of therapeutics targeting KRAS has been incredibly challenging, prompting indirect inhibition of downstream targets such as MEK and ERK. Such inhibitors, unfortunately, come with limited clinical efficacy, and therefore the demand for developing novel therapeutic strategies remains an urgent need for these patients. Exploring the influence of wild-type (WT) KRAS on druggable targets can uncover new vulnerabilities for the treatment of KRAS mutant lung adenocarcinomas. Using commercially available KRAS mutant lung adenocarcinoma cell lines, we explored the influence of WT KRAS on signaling networks and druggable targets. Expression and/or activation of 183 signaling proteins, most of which are targets of FDA-approved drugs, were captured by reverse-phase protein microarray (RPPA). Selected findings were validated on a cohort of 23 surgical biospecimens using the RPPA. Kinase-driven signatures associated with the presence of the KRAS WT allele were detected along the MAPK and AKT/mTOR signaling pathway and alterations of cell cycle regulators. FoxM1 emerged as a potential vulnerability of tumors retaining the KRAS WT allele both in cell lines and in the clinical samples. Our findings suggest that loss of WT KRAS impacts on signaling events and druggable targets in KRAS mutant lung adenocarcinomas.

Two Distinct Structures of Membrane-Associated Homodimers of GTP- and GDP-Bound KRAS4B Revealed by Paramagnetic Relaxation Enhancement

KRAS homo-dimerization has been implicated in the activation of RAF kinases, however, the mechanism and structural basis remain elusive. We developed a system to study KRAS dimerization on nanodiscs using paramagnetic relaxation enhancement (PRE) NMR spectroscopy, and determined distinct structures of membrane-anchored KRAS dimers in the active GTP- and inactive GDP-loaded states. Both dimerize through an α4-α5 interface, but the relative orientation of the protomers and their contacts differ substantially. Dimerization of KRAS-GTP, stabilized by electrostatic interactions between R135 and E168, favors an orientation on the membrane that promotes accessibility of the effector-binding site. Remarkably, "cross"-dimerization between GTP- and GDP-bound KRAS molecules is unfavorable. These models provide a platform to elucidate the structural basis of RAF activation by RAS and to develop inhibitors that can disrupt the KRAS dimerization. The methodology is applicable to many other farnesylated small GTPases.

The Function of RAS Mutation in Cancer and Advances in its Drug Research

RAS (H-ras, K-ras, and N-ras), as the second largest mutated gene driver in various human cancers, has long been a vital research target for cancer. Its function is to transform the extracellular environment into a cascade of intracellular signal transduction. RAS mutant protein regulates tumor cell proliferation, apoptosis, metabolism and angiogenesis through downstream MAPK, PI3K and other signaling pathways. In KRAS or other RAS-driven cancers, current treatments include direct inhibitors and upstream/downstream signaling pathway inhibitors. However, the research on these inhibitors has been largely restricted due to their escape inhibition and off-target toxicity. In this paper, we started with the role of normal and mutant RAS genes in cancer, elucidated the relevant RAS regulating pathways, and highlighted the important research advancements in RAS inhibitor research. We concluded that for the crosstalk between RAS pathways, the effect of single regulation may be limited, and the multi-target drug combined compensation mechanism is becoming a research hotspot.

Cancer Treatment in the Genomic Era

The complexity of human cancer underlies its devastating clinical consequences. Drugs designed to target the genetic alterations that drive cancer have improved the outcome for many patients, but not the majority of them. Here, we review the genomic landscape of cancer, how genomic data can provide much more than a sum of its parts, and the approaches developed to identify and validate genomic alterations with potential therapeutic value. We highlight notable successes and pitfalls in predicting the value of potential therapeutic targets and discuss the use of multi-omic data to better understand cancer dependencies and drug sensitivity. We discuss how integrated approaches to collecting, curating, and sharing these large data sets might improve the identification and prioritization of cancer vulnerabilities as well as patient stratification within clinical trials. Finally, we outline how future approaches might improve the efficiency and speed of translating genomic data into clinically effective therapies and how the use of unbiased genome-wide information can identify novel predictive biomarkers that can be either simple or complex.

Oncogenic Ras Isoforms Signaling Specificity at the Membrane

How do Ras isoforms attain oncogenic specificity at the membrane? Oncogenic KRas, HRas, and NRas (K-Ras, H-Ras, and N-Ras) differentially populate distinct cancers. How they selectively activate effectors and why is KRas4B the most prevalent are highly significant questions. Here, we consider determinants that may bias isoform-specific effector activation and signaling at the membrane. We merge functional data with a conformational view to provide mechanistic insight. Cell-specific expression levels, pathway cross-talk, and distinct interactions are the key, but conformational trends can modulate selectivity. There are two major pathways in oncogenic Ras-driven proliferation: MAPK (Raf/MEK/ERK) and PI3Kα/Akt/mTOR. All membrane-anchored, proximally located, oncogenic Ras isoforms can promote Raf dimerization and fully activate MAPK signaling. So why the differential statistics of oncogenic isoforms in distinct cancers and what makes KRas so highly oncogenic? Many cell-specific factors may be at play, including higher KRAS mRNA levels. As a key factor, we suggest that because only KRas4B binds calmodulin, only KRas can fully activate PI3Kα/Akt signaling. We propose that full activation of both MAPK and PI3Kα/Akt proliferative pathways by oncogenic KRas4B-but not by HRas or NRas-may help explain why the KRas4B isoform is especially highly populated in certain cancers. We further discuss pharmacologic implications. Cancer Res; 78(3); 593-602. ©2017 AACR.

Oncogenic Ras Isoforms Signaling Specificity at the Membrane

How do Ras isoforms attain oncogenic specificity at the membrane? Oncogenic KRas, HRas, and NRas (K-Ras, H-Ras, and N-Ras) differentially populate distinct cancers. How they selectively activate effectors and why is KRas4B the most prevalent are highly significant questions. Here, we consider determinants that may bias isoform-specific effector activation and signaling at the membrane. We merge functional data with a conformational view to provide mechanistic insight. Cell-specific expression levels, pathway cross-talk, and distinct interactions are the key, but conformational trends can modulate selectivity. There are two major pathways in oncogenic Ras-driven proliferation: MAPK (Raf/MEK/ERK) and PI3Kα/Akt/mTOR. All membrane-anchored, proximally located, oncogenic Ras isoforms can promote Raf dimerization and fully activate MAPK signaling. So why the differential statistics of oncogenic isoforms in distinct cancers and what makes KRas so highly oncogenic? Many cell-specific factors may be at play, including higher KRAS mRNA levels. As a key factor, we suggest that because only KRas4B binds calmodulin, only KRas can fully activate PI3Kα/Akt signaling. We propose that full activation of both MAPK and PI3Kα/Akt proliferative pathways by oncogenic KRas4B-but not by HRas or NRas-may help explain why the KRas4B isoform is especially highly populated in certain cancers. We further discuss pharmacologic implications. Cancer Res; 78(3); 593-602. ©2017 AACR.