Therapies targeting the RAF–MEK–ERK pathway are generally considered to have limited efficacy in KRAS-mutant cancers. However, specific KRAS mutants exhibit distinct behaviors. Notably, KRASG12R pancreatic ductal adenocarcinoma (PDAC) tumors have shown sensitivity to MEK inhibitors (MEKi) in combination with autophagy inhibitors, but a better understanding of the underlying mechanisms is needed to optimize this treatment strategy. Using a systems-level approach, we uncovered a mechanistic explanation for this phenomenon. Due to distinct biophysical properties, KRASG12R had an impaired ability to activate wild-type HRAS and NRAS (WT-RAS) compared with other KRAS mutants, such as KRASG12D. This reduced activation stemmed from the weaker interaction between KRASG12R and guanine exchange factors (SOS), as well as the tumor suppressor neurofibromin (NF1), crucial in regulating WT-RAS activity. The impaired ability to activate WT-RAS led to weaker holistic MAPK signaling in KRASG12R-driven tumors, which conferred increased sensitivity to MEKi. To substantiate the preclinical findings, the utility of MEKi in combination with the autophagy inhibitor hydroxychloroquine was analyzed in patients with KRASG12R-mutated metastatic PDAC. Five of the eight (62.5%) patients treated in first- or second-line settings had a progression-free survival exceeding 6 months. Three patients had impressive disease control: two had stable disease of 11 and 22.7 months, and one achieved a partial response with an 83% decrease in tumor size that lasted for 8.9 months. Overall, this work highlights how systems-based approaches in precision medicine can uncover mechanistic insights to guide the identification of patients with PDAC most likely to benefit from tailored therapeutic strategies. Significance: The unique sensitivity of KRASG12R-mutant cancers to MEK inhibitors offers a critical advancement in understanding MAPK signaling and paving the way for precision-targeted therapies in previously untreatable contexts. See related commentary by Tiriac and Engle, p. 1817 See related article by Burge et al., p. 1854 See related article by Burge et al., p. 1868

Pancreatic ductal adenocarcinoma (PDAC) is defined by the near universal occurrence of KRAS mutations. The KRASG12R mutation is detected in approximately 15% to 20% of patients with PDAC and rare in other KRAS-mutant cancers. KRASG12R is unable to activate the lipid kinase PIK3CA, suggesting that alternative mechanisms might be employed to activate KRASG12R-independent PI3K signaling in PDAC. In this study, we detected elevated expression of all four PI3K isoforms in PDAC cell lines, with the PIK3CG isoform showing higher overall expression in KRASG12R-mutant PDAC. All four PI3K isoforms contributed to global PI3K signaling, and inhibition of any single isoform was insufficient to limit PDAC proliferation. The combined inhibition of all PI3K isoforms was required to limit proliferation, providing a potential explanation for the limited efficacy of PI3K inhibitors in the clinic. Additionally, PTEN, a negative regulator of PI3K signaling, was inactivated in PDAC by the formation of an intramolecular disulfide, which elevated overall PI3K signaling and reduced the dependency of PI3K signaling on KRAS. Oxidation of PTEN was independent of KRAS mutation status. Finally, nutrient-limiting conditions mimicking the PDAC tumor microenvironment further elevated PTEN oxidation and significantly increased macropinocytosis. Thus, this study uncovered a mechanism that supports elevated PI3K signaling in PDAC, thereby reducing the need for KRAS to directly activate the PI3K pathway. Significance: PTEN inactivation by intramolecular disulfide bond formation and elevated expression of PI3K isoforms in pancreatic cancer leads to unchecked KRAS-independent PI3K signaling, highlighting the need for therapeutic approaches targeting constitutive PI3K signaling. See related commentary by Tiriac and Engle, p. 1817 See related article by Burge et al., p. 1868 See related article by Kamgar et al., p. 2042

Patients with pancreatic ductal adenocarcinoma (PDAC) harboring KRASG12R mutations have increased overall survival relative to patients with KRASG12D/V mutations. To investigate the mechanisms underlying this differential outcome, we developed a genetically engineered mouse model (GEMM) harboring KrasG12R and Trp53R172H mutations (KrasLSL-G12R/+;Trp53LSL-R172H/+;p48Cre-ERTM). Unlike KrasG12D models, KrasG12R GEMMs exhibited limited tumorigenesis, with only 10% developing pancreatic tumors after 1 year. Additionally, mice harboring whole-body expression of KrasG12R remained healthy for over 1 year, whereas KrasG12D mice developed rapid multifocal disease. Comparison of KRAS mutant-selective transcription and signaling in murine and human PDAC cell lines, GEMMs, and patient-derived xenograft (PDX) mouse models revealed that direct KRAS-mediated PI3K activation is necessary for robust tumor initiation in GEMMs. Unexpectedly, KRAS was not the primary driver of PI3K activity in human PDAC cell lines and PDX models, regardless of KRAS mutation. KRASG12R and KRASG12D activated a similar pancreas-specific transcriptional network, but KRASG12R promoted these pathways less robustly due to limited ERK/MAPK nuclear translocation. Finally, KRASG12R human pancreatic tumors had an altered tumor microenvironment (TME) with reduced collagen deposition and metastatic liver invasion. Together, this study demonstrated that KRASG12R is capable of driving tumorigenesis despite the reduced ERK/MAPK nuclear translocation and transcriptional output. Although human KRASG12D- and KRASG12R-mutant tumors display unexpected similarities in PI3K activity, the differential ERK/MAPK signaling activity and the extrinsic consequences on the TME provide support for using KRASG12R mutation status as a prognostic biomarker for therapeutic strategies. Significance: KRASG12R-mutant pancreatic cancer is characterized by lower ERK/MAPK nuclear translocation and transcriptional output than KRASG12D-mutant tumors, offering a potential window for patients with KRASG12R mutations to derive additional benefit from neoadjuvant therapy. See related commentary by Tiriac and Engle, p. 1817 See related article by Burge et al., p. 1854 See related article by Kamgar et al., p. 2042

The RAS family of small GTPases is among the most frequently mutated gene families in human cancer. In pancreatic ductal adenocarcinoma (PDAC), ∼95% of cases harbor an activating KRAS mutation, primarily at codon 12, 13, or 61, with G12D being the most common overall (40%). In contrast, the KRASQ61L mutation, though constitutively active, is virtually absent in tumors of patients with PDAC. This suggests that KRASQ61L may engage in distinct, allele-specific signaling that limits its ability to drive tumorigenesis. Determining the mechanisms that limit the occurrence of this mutation will aid in our understanding of the critical KRAS effectors and pathways that drive tumorigenesis. To investigate these mechanisms, we utilized a tightly controlled doxycycline-inducible KRAS expression system in an isogenic, immortalized pancreatic cell line, enabling direct comparison of KRASQ61L with the common PDAC mutant KRASG12D. Using TurboID proximity labeling alongside RNA sequencing, we mapped early effector interactions and transcriptional responses, revealing that KRASQ61L induces greater hyperactivation of the ERK/MAPK pathway, resulting in increased nuclear translocation of ERK1/2. Finally, pancreatic cells are highly tolerant to overexpression of KRASG12D, but KRASQ61L overexpression leads to impaired proliferation and increased apoptosis. These findings provide experimental support for the long-standing “Goldilocks” model of oncogenic signaling, in which too much ERK/MAPK pathway activation is detrimental to tumorigenesis. Our work offers a mechanistic explanation for the relative absence of KRASQ61L in PDAC and contributes to our understanding of KRAS allele–specific vulnerabilities, which can inform future therapeutic strategies targeting KRAS-driven pancreatic cancer. Significance: This study demonstrates that KRASQ61L drives hyperactivation of ERK/MAPK signaling and triggers apoptosis, which limits the proliferation of pancreatic cells. These findings support a “Goldilocks” model of RAS signaling and suggest that strong hyperactivation of the ERK/MAPK pathway contributes to the selective absence of KRASQ61L in pancreatic tumorigenesis.

KRASQ61L hyperactivates ERK1/2 while limiting proliferation in pancreatic cancer cells. A, Immunoblot of KRAS mutants expressed in human telomerase reverse transcriptase–HPNE E6/E7 cell lines for 48 hours with DOX-induced KRAS expression (n = 3). B, Quantification of pERK1/2 in HPNE KRAS-mutant cell lines from three independent experiments. C, Representative images of KRASWT/G12D/Q61L and EV expressed in human telomerase reverse transcriptase–HPNE E6/E7 cell lines (N = 3). D, Box and whisker plot quantifying the proliferation of HPNE cells expressing KRASG12D/Q61L from C. KRAS mutants were normalized to HPNE EV. P values were calculated using the Student t test. Error bars represent mean ± SEM. **, P < 0.002; *, P < 0.05. All statistical tests were run using R 4.2.2.

Figure S4. KRAS mutants after 3-day doxycycline treatment.