An Integrative Model of Cellular States, Plasticity, and Genetics for Glioblastoma.

Diverse genetic, epigenetic, and developmental programs drive glioblastoma, an incurable and poorly understood tumor, but their precise characterization remains challenging. Here, we use an integrative approach spanning single-cell RNA-sequencing of 28 tumors, bulk genetic and expression analysis of 401 specimens from the The Cancer Genome Atlas (TCGA), functional approaches, and single-cell lineage tracing to derive a unified model of cellular states and genetic diversity in glioblastoma. We find that malignant cells in glioblastoma exist in four main cellular states that recapitulate distinct neural cell types, are influenced by the tumor microenvironment, and exhibit plasticity. The relative frequency of cells in each state varies between glioblastoma samples and is influenced by copy number amplifications of the CDK4, EGFR, and PDGFRA loci and by mutations in the NF1 locus, which each favor a defined state. Our work provides a blueprint for glioblastoma, integrating the malignant cell programs, their plasticity, and their modulation by genetic drivers.

RAS Proteins and Their Regulators in Human Disease.

RAS proteins are binary switches, cycling between ON and OFF states during signal transduction. These switches are normally tightly controlled, but in RAS-related diseases, such as cancer, RASopathies, and many psychiatric disorders, mutations in the RAS genes or their regulators render RAS proteins persistently active. The structural basis of the switch and many of the pathways that RAS controls are well known, but the precise mechanisms by which RAS proteins function are less clear. All RAS biology occurs in membranes: a precise understanding of RAS' interaction with membranes is essential to understand RAS action and to intervene in RAS-driven diseases.

ARAF protein kinase activates RAS by antagonizing its binding to RASGAP NF1.

RAF protein kinases are effectors of the GTP-bound form of small guanosine triphosphatase RAS and function by phosphorylating MEK. We showed here that the expression of ARAF activated RAS in a kinase-independent manner. Binding of ARAF to RAS displaced the GTPase-activating protein NF1 and antagonized NF1-mediated inhibition of RAS. This reduced ERK-dependent inhibition of RAS and increased RAS-GTP. By this mechanism, ARAF regulated the duration and consequences of RTK-induced RAS activation and supported the RAS output of RTK-dependent tumor cells. In human lung cancers with EGFR mutation, amplification of ARAF was associated with acquired resistance to EGFR inhibitors, which was overcome by combining EGFR inhibitors with an inhibitor of the protein tyrosine phosphatase SHP2 to enhance inhibition of nucleotide exchange and RAS activation.

K128 ubiquitination constrains RAS activity by expanding its binding interface with GAP proteins.

The RAS pathway is among the most frequently activated signaling nodes in cancer. However, the mechanisms that alter RAS activity in human pathologies are not entirely understood. The most prevalent post-translational modification within the GTPase core domain of NRAS and KRAS is ubiquitination at lysine 128 (K128), which is significantly decreased in cancer samples compared to normal tissue. Here, we found that K128 ubiquitination creates an additional binding interface for RAS GTPase-activating proteins (GAPs), NF1 and RASA1, thus increasing RAS binding to GAP proteins and promoting GAP-mediated GTP hydrolysis. Stimulation of cultured cancer cells with growth factors or cytokines transiently induces K128 ubiquitination and restricts the extent of wild-type RAS activation in a GAP-dependent manner. In KRAS mutant cells, K128 ubiquitination limits tumor growth by restricting RAL/ TBK1 signaling and negatively regulating the autocrine circuit induced by mutant KRAS. Reduction of K128 ubiquitination activates both wild-type and mutant RAS signaling and elicits a senescence-associated secretory phenotype, promoting RAS-driven pancreatic tumorigenesis.

SPRED proteins and their roles in signal transduction, development, and malignancy.

The roles of SPRED proteins in signaling, development, and cancer are becoming increasingly recognized. SPRED proteins comprise an N-terminal EVH-1 domain, a central c-Kit-binding domain, and C-terminal SROUTY domain. They negatively regulate signaling from tyrosine kinases to the Ras-MAPK pathway. SPRED1 binds directly to both c-KIT and to the RasGAP, neurofibromin, whose function is completely dependent on this interaction. Loss-of-function mutations in SPRED1 occur in human cancers and cause the developmental disorder, Legius syndrome. Genetic ablation of SPRED genes in mice leads to behavioral problems, dwarfism, and multiple other phenotypes including increased risk of leukemia. In this review, we summarize and discuss biochemical, structural, and biological functions of these proteins including their roles in normal cell growth and differentiation and in human disease.

Chemical genetic screens reveal defective lysosomal trafficking as synthetic lethal with NF1 loss.

Neurofibromatosis type 1, a genetic disorder caused by germline mutations in NF1, predisposes patients to the development of tumors, including cutaneous and plexiform neurofibromas (CNs and PNs), optic gliomas, astrocytomas, juvenile myelomonocytic leukemia, high-grade gliomas, and malignant peripheral nerve sheath tumors (MPNSTs), which are chemotherapy- and radiation-resistant sarcomas with poor survival. Loss of NF1 also occurs in sporadic tumors such as glioblastoma (GBM), melanoma, breast, ovarian, and lung cancers. We performed a high-throughput screen for compounds that were synthetic lethal with NF1 loss, which identified several leads, including the small molecule Y102. Treatment of cells with Y102 perturbed autophagy, mitophagy, and lysosome positioning in NF1-deficient cells. A dual proteomics approach identified the BORC complex, which is required for lysosome positioning and trafficking, as a potential target of Y102. Knockdown of a BORC complex subunit using siRNA recapitulated the phenotypes observed with Y102 treatment. Our findings demonstrate that the BORC complex may be a promising therapeutic target for NF1-deficient tumors.

Distinct hyperactive RAS/MAPK alleles converge on common GABAergic interneuron core programs.

RAS/MAPK gene dysfunction underlies various cancers and neurocognitive disorders. Although the roles of RAS/MAPK genes have been well studied in cancer, less is known about their function during neurodevelopment. There are many genes that work in concert to regulate RAS/MAPK signaling, suggesting that if common brain phenotypes could be discovered they could have a broad impact on the many other disorders caused by distinct RAS/MAPK genes. We assessed the cellular and molecular consequences of hyperactivating the RAS/MAPK pathway using two distinct genes in a cell type previously implicated in RAS/MAPK-mediated cognitive changes, cortical GABAergic interneurons. We uncovered some GABAergic core programs that are commonly altered in each of the mutants. Notably, hyperactive RAS/MAPK mutants bias developing cortical interneurons towards those that are somatostatin positive. The increase in somatostatin-positive interneurons could also be prevented by pharmacological inhibition of the core RAS/MAPK signaling pathway. Overall, these findings present new insights into how different RAS/MAPK mutations can converge on GABAergic interneurons, which may be important for other RAS/MAPK genes and related disorders.

Mosaicism in Tumor Suppressor Gene Syndromes: Prevalence, Diagnostic Strategies, and Transmission Risk.

A mosaic state arises when pathogenic variants are acquired in certain cell lineages during postzygotic development, and mosaic individuals may present with a generalized or localized phenotype. Here, we review the current state of knowledge regarding mosaicism for eight common tumor suppressor genes-NF1, NF2, TSC1, TSC2, PTEN, VHL, RB1, and TP53-and their related genetic syndromes/entities. We compare and discuss approaches for comprehensive diagnostic genetic testing, the spectrum of variant allele frequency, and disease severity. We also review affected individuals who have no mutation identified after conventional genetic analysis, as well as genotype-phenotype correlations and transmission risk for each tumor suppressor gene in full heterozygous and mosaic patients. This review provides new insight into similarities as well as marked differences regarding the appreciation of mosaicism in these tumor suppressor syndromes.

Molecular and clinicopathological features of KIT/PDGFRA wild-type gastrointestinal stromal tumors.

Approximately 10% of gastrointestinal stromal tumors (GISTs) harbor reportedly no KIT and PDGFRA mutations (wild-type GISTs). The clinicopathological features and oncologic outcomes of wild-type GISTs based on molecular profiles are unknown. We recruited 35 wild-type GIST patients from the two registry studies of high-risk GISTs between 2012 and 2015 and primary GISTs between 2003 and 2014. Molecular profiling of wild-type GISTs was performed by targeted next-generation sequencing (NGS) using formalin-fixed paraffin-embedded tumor samples. Among 35 wild-type GISTs, targeted NGS analysis detected NF1, SDH, or BRAF mutation: 16 NF1-GISTs with various NF1 mutations, 12 SDH-GISTs (4 with SDHA mutations, 4 with SDHB mutations, and 4 with SDHB-negative staining), and 5 BRAF-GISTs with the V600E mutation. Two GISTs showed no mutations based on our targeted NGS analysis. Additional gene mutations were infrequent in primary wild-type GISTs and found in TP53, CREBBP, CDKN2A, and CHEK2. Most NF1-GISTs were located in the small intestine (N = 12; 75%) and showed spindle cell features (N = 15; 94%) and multiple tumors (N = 6, 38%) with modest proliferation activities. In contrast, SDH-GISTs were predominantly found in the stomach (N = 11; 92%), exhibiting epithelioid cell (N = 6; 50%) and multiple (N = 6, 50%) features. The overall survival of patients with SDH-GISTs appeared to be better than that of BRAF-GISTs (p = 0.0107) or NF1-GISTs (p = 0.0754), respectively. In conclusion, major molecular changes in wild-type GISTs include NF1, SDH, and BRAF. NF1-GISTs involved multifocal spindle cell tumors in the small intestine. SDH-GISTs occurred in young patients and were multifocal in the stomach and clinically indolent.

Primary results and characterization of patients with exceptional outcomes in a phase 1b study combining PARP and MEK inhibition, with or without anti-PD-L1, for BRCA wild-type, platinum-sensitive, recurrent ovarian cancer.

BACKGROUND: This phase 1b study (ClinicalTrials.gov identifier NCT03695380) evaluated regimens combining PARP and MEK inhibition, with or without PD-L1 inhibition, for BRCA wild-type, platinum-sensitive, recurrent ovarian cancer (PSROC). METHODS: Patients with PSROC who had received one or two prior treatment lines were treated with 28-day cycles of cobimetinib 60 mg daily (days 1-21) plus niraparib 200 mg daily (days 1-28) with or without atezolizumab 840 mg (days 1 and 15). Stage 1 assessed safety before expansion to stage 2, which randomized patients who had BRCA wild-type PSROC to receive either doublet or triplet therapy, stratified by genome-wide loss of heterozygosity status (<16% vs. ≥16%; FoundationOne CDx assay) and platinum-free interval (≥6 to <12 vs. ≥12 months). Coprimary end points were safety and the investigator-determined objective response rate (ORR) according to Response Evaluation Criteria in Solid Tumors (RECIST). Potential associations between genetic parameters and efficacy were explored, and biomarker profiles of super-responders (complete response or those with progression-free survival [PFS] >15 months) and progressors (disease progression as the best response) were characterized. RESULTS: The ORR in patients who had BRCA wild-type PSROC was 35% (95% confidence interval, 20%-53%) with the doublet regimen (n = 37) and 27% (95% confidence interval, 14%-44%) with the triplet regimen (n = 37), and the median PFS was 6.0 and 7.4 months, respectively. Post-hoc analyses indicated more favorable ORR and PFS in the homologous recombination-deficiency-signature (HRDsig)-positive subgroup than in the HRDsig-negative subgroup. Tolerability was consistent with the known profiles of individual agents. NF1 and MKNK1 mutations were associated with sustained benefit from the doublet and triplet regimens, respectively. CONCLUSIONS: Chemotherapy-free doublet and triplet therapy demonstrated encouraging activity, including among patients who had BRCA wild-type, HRDsig-positive or HRDsig-negative PSROC harboring NF1 or MKNK1 mutations.

Expression analysis of NF1-mutated alleles in a rare compound heterozygous spinal NF1 patient by digital PCR.

BACKGROUD: Neurofibromatosis type 1 (NF1) is a heterogeneous neurocutaneous disorder. Spinal neurofibromatosis (SNF) is a distinct clinical entity of NF1, characterized by bilateral neurofibromas involving all spinal nerve roots. Although both forms are caused by intragenic heterozygous variants of NF1, missense variants have been associated with SNF, according to a dominant inheritance model causing haploinsufficiency. Most patients carry pathogenic variants in one of the NF1 alleles; nevertheless, patients with both NF1-mutated copies have been described. Interestingly, all NF1 variants carried by the known SNF compound heterozygotes were missense/splicing variants or in-frame insertion-deletions. AIMS: To investigate whether there is a differential expression of NF1 variant alleles in an NF1 compound heterozygous SNF patient possibly contributing to clinical phenotype. MATERIALS & METHODS: We performed an allele-specific expression study, by chip-based digital PCR, in an SNF family carrying two NF1 missense variants. We evaluated the expression levels of the two NF1-mutated alleles both carried by the compound heterozygous SNF patient and his relatives. RESULTS: Both alleles were expressed at comparable levels in the patient and hyper-expressed compared to the wild-type alleles of healthy controls. DISCUSSION: Here we provide new insights into expression studies of NF1-mutated transcripts suggesting that a novel pathogenetic mechanism, caused by gain-of-function variants, could be associated with SNF. CONCLUSIONS: Further studies should be performed in larger cohorts, opening new perspectives in the NF1 pathogenesis comprehension.

Biallelic inactivation of the NF1 tumour suppressor gene in juvenile myelomonocytic leukaemia: Genetic evidence of driver function and implications for diagnostic workup.

Juvenile myelomonocytic leukaemia (JMML) is characterized by gene variants that deregulate the RAS signalling pathway. Children with neurofibromatosis type 1 (NF-1) carry a defective NF1 allele in the germline and are predisposed to JMML, which presumably requires somatic inactivation of the NF1 wild-type allele. Here we examined the two-hit concept in leukaemic cells of 25 patients with JMML and NF-1. Ten patients with JMML/NF-1 exhibited a NF1 loss-of-function variant in combination with uniparental disomy of the 17q arm. Five had NF1 microdeletions combined with a pathogenic NF1 variant and nine carried two compound-heterozygous NF1 variants. We also examined 16 patients without clinical signs of NF-1 and no variation in the JMML-associated driver genes PTPN11, KRAS, NRAS or CBL (JMML-5neg) and identified eight patients with NF1 variants. Three patients had microdeletions combined with hemizygous NF1 variants, three had compound-heterozygous NF1 variants and two had heterozygous NF1 variants. In addition, we found a high incidence of secondary ASXL1 and/or SETBP1 variants in both groups. We conclude that the clinical diagnosis of JMML/NF-1 reliably indicates a NF1-driven JMML subtype, and that careful NF1 analysis should be included in the genetic workup of JMML even in the absence of clinical evidence of NF-1.

Inflammatory Rhabdomyoblastic Tumor: Clinicopathologic and Molecular Analysis of 13 Cases.

Inflammatory rhabdomyoblastic tumors (IRMTs) are newly recognized skeletal muscle tumors with uncertain malignant potential. We investigated 13 IRMTs using clinicopathologic, genetic, and epigenetic methods. The cohort included 7 men and 6 women, aged 23 to 80 years (median, 50 years), of whom 2 had neurofibromatosis type 1. Most tumors occurred in the deep soft tissues of the lower limbs, head/neck, trunk wall, and retroperitoneum/pelvis. Two tumors involved the hypopharyngeal submucosa as polypoid masses. Eight tumors showed conventional histology of predominantly spindled cells with nuclear atypia, low mitotic activity, and massive inflammatory infiltrates. Three tumors showed atypical histology, including uniform epithelioid or plump cells and mitotically active histiocytes. The remaining 2 tumors demonstrated malignant progression to rhabdomyosarcoma; one had additional IRMT histology and the other was a pure sarcoma. All 11 IRMTs without malignant progression exhibited indolent behavior at a median follow-up of 43 months. One of the 2 patients with IRMTs with malignant progression died of lung metastases. All IRMTs were positive for desmin and PAX7, whereas myogenin and MyoD1 were expressed in a subset of cases. Targeted next-generation sequencing identified pathogenic mutations in NF1 (5/8) and TP53 (4/8). All TP53 mutations co-occurred with NF1 mutations. TP53 variant allele frequency was much lower than that of NF1 in 2 cases. These tumors showed geographic (subclonal) strong p53 immunoreactivity, suggesting the secondary emergence of a TP53-mutant clone. DNA methylation-based copy number analysis conducted in 11 tumors revealed characteristic flat patterns with relative gains, including chromosomes 5, 18, 20, 21, and/or 22 in most cases. Widespread loss of heterozygosity with retained biparental copies of these chromosomes was confirmed in 4 tumors analyzed via allele-specific profiling. Based on unsupervised DNA methylation analysis, none of the 11 tumors tested clustered with existing reference entities but formed a coherent group, although its specificity warrants further study.

Whole-exome sequencing has revealed novel genetic characteristics in intracranial germ cell tumours in the Chinese.

AIMS: Intracranial germ cell tumour (IGCT) is a type of rare central nervous system tumour that mainly occurs in children and adolescents, with great variation in its incidence rate and molecular characteristics in patients from different populations. The genetic alterations of IGCT in the Chinese population are still unknown. METHODS AND RESULTS: In this study, 47 patients were enrolled and their tumour specimens were analysed by whole-exome sequencing (WES). We found that KIT was the most significantly mutated gene (15/47, 32%), which mainly occurred in the germinoma group (13/20, 65%), and less frequently in NGGCT (2/27, 7%). Copy number variations (CNVs) of FGF6 and TFE3 only appeared in NGGCT patients (P = 0.003 and 0.032, respectively), while CNVs of CXCR4, RAC2, PDGFA, and FEV only appeared in germinoma patients (P = 0.004 of CXCR4 and P = 0.027 for the last three genes). Compared with a previous Japanese cohort, the somatic mutation rates of RELN and SYNE1 were higher in the Chinese. Prognostic analysis showed that the NF1 mutation was associated with shorter overall survival and progression-free survival in IGCT patients. Clonal evolution analysis revealed an early branched evolutionary pattern in two IGCT patients who underwent changes in the histological subtype or degree of differentiation during disease surveillance. CONCLUSION: This study indicated that Chinese IGCT patients may have distinct genetic characteristics and identified several possible genetic alterations that have the potential to become prognostic biomarkers of NGGCT patients.

Neurofibromatosis-Noonan syndrome and growth deficiency in an Iranian girl due to a pathogenic variant in NF1 gene.

BACKGROUND: Mutations in NF1 gene could cause allelic disorders with clinical spectrum of Neurofibromatosis type 1 to Noonan syndrome. Here, a 7-year-old Iranian girl is described with Neurofibromatosis-Noonan syndrome due to a pathogenic variant in NF1 gene. METHODS: Clinical evaluations were performed along with genetic testing using whole exome sequencing (WES). The variant analysis including pathogenicity prediction was also done using bioinformatics tools. RESULTS: The chief compliant of the patient was short stature and lack of proper weight gain. Other symptoms were developmental delay, learning disability, inadequate speech skill, broad forehead, hypertelorism, and epicanthal folds, low set ears and webbed neck. A small deletion, c.4375-4377delGAA, was found in NF1 gene using WES. This variant was classified as pathogenic according to ACMG. CONCLUSIONS: NF1 variants may show variable phenotypes among the patients; identifying such variants is helpful in therapeutic management of the disease. WES is considered as an appropriate test to diagnose Neurofibromatosis-Noonan syndrome.

Genetically confirmed coexistence of neurofibromatosis type 1 and Cherubism in a pediatric patient.

BACKGROUND: Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder typified by various combination of numerous Café-au-lait macules, cutaneous and plexiform neurofibromas, freckling of inguinal or axillary region, optic glioma, Lisch nodules and osseous lesions. Cherubism is a rare genetic syndrome described by progressive swelling of the lower and/or upper jaw due to replacement of bone by fibrous connective tissue. Patients are reported in the literature with NF1 and cherubism-like phenotype due to the NF1 osseous lesions in the jaws. The purpose of this case report is the description of a young male genetically diagnosed with both NF1 and cherubism. METHODS AND RESULTS: A 9 years and six month old patient with clinical findings of NF1 and cherubism in whom both diseases were genetically confirmed, is presented. The patient was evaluated by a pediatrician, a pediatric endocrinologist, an ophthalmologist, and an oral and maxillofacial surgeon. A laboratory and hormonal screening, a histological examination, a chest X-ray, a magnetic resonance imaging (MRI) of the orbit and a digital panoramic radiography were performed. Genetic testing applying Whole Exome Sequencing was conducted. CONCLUSIONS: A novel and an already reported pathogenic variants were detected in NF1 and SH3BP2 genes, respectively. This is the first described patient with coexistence of NF1 and cherubism. The contribution of Next Generation Sequencing (NGS) in gene variant identification as well as the importance of close collaboration between laboratory scientists and clinicians, is highlighted. Both are essential for optimizing the diagnostic approach of patients with a complex phenotype.

Surgical revascularization as a procedure to prevent neurological complications in children with moyamoya syndrome associated with neurofibromatosis I: a single institution case series.

BACKGROUND: The optimal timing and surgical approach for surgical revascularization in patients with moyamoya syndrome (MMS) associated with neurofibromatosis type I (NF1) remain so far elusive. We aimed to compare the long-term clinical, radiological, and cognitive effects of different revascularization procedures in a pediatric cohort of NF1-associated MMS. METHODS: We reviewed the clinical, radiological, and surgical data of 26 patients with NF1-associated MMS diagnosed at our institution between 2012 and 2022, at the clinical onset and last follow-up. RESULTS: Indirect bypasses were performed in 12/26 patients (57.1%), while combined direct and indirect procedures in 9/26 subjects (42.9%); 5 patients did not undergo surgery. Through logistic regression analysis, pathological Wechsler Intelligence Scale for Children (WISC) at onset was found to be associated with symptom improvement at 1-year follow up (p = 0.006). No significant differences were found in long-term neurocognitive outcome and stroke rate in patients receiving combined or indirect bypass (p > 0.05). CONCLUSIONS: Currently, whether combined or indirect bypass should be considered the treatment of choice in pediatric patients with NF1-associated MMS remains unclear, as well as the optimal time approach. In our series, no significant differences were found in long-term neurocognitive outcome and stroke rate between patients treated with either of these two approaches. Clinical evidence supports the crucial role of early diagnosis and surgical revascularization in subjects with MMS-associated NF1, even in case of mildly symptomatic vasculopathy. This allows to achieve a good long-term outcome with improved intellectual function and prevention of stroke and seizure in these patients.

Case of embryonal tumor multilayered rosettes in a patient with neurofibromatosis type 1.

BACKGROUND: ETMR is a unique and highly malignant brain tumor mostly occurring in infants. This report provides a comprehensive overview of the clinical presentation, histological aspects, radiological features, and therapeutic options of ETMR. Being the first report on the co-occurrence of NF1 with ETMR, it highlight the challenges of managing a patient with complex medical conditions. CASE REPORT: We present a case of a 3 and 1/2-year-old girl with neurofibromatosis type 1 (NF1), later diagnosed with a supratentorial brain tumor reported as an embryonal tumor with multilayered rosettes (ETMR), along with possible co-occurrence of constitutional mismatch repair deficiency (CMMRD) on immunohistochemistry (IHC); however, germline testing was not performed. Even though NF1 can be associated with tumors such as gliomas, the literature has no previous case reports of ETMR coexisting with NF1. CONCLUSION: Exploring the link between NF1 and ETMR with CMMRD is crucial to improving and establishing more treatment protocols. Therefore, reporting each case's unique features would be essential in developing appropriate treatment protocols.

Human Genetics of Ventricular Septal Defect.

Ventricular septal defects (VSDs) are recognized as one of the commonest congenital heart diseases (CHD), accounting for up to 40% of all cardiac malformations, and occur as isolated CHDs as well as together with other cardiac and extracardiac congenital malformations in individual patients and families. The genetic etiology of VSD is complex and extraordinarily heterogeneous. Chromosomal abnormalities such as aneuploidy and structural variations as well as rare point mutations in various genes have been reported to be associated with this cardiac defect. This includes both well-defined syndromes with known genetic cause (e.g., DiGeorge syndrome and Holt-Oram syndrome) and so far undefined syndromic forms characterized by unspecific symptoms. Mutations in genes encoding cardiac transcription factors (e.g., NKX2-5 and GATA4) and signaling molecules (e.g., CFC1) have been most frequently found in VSD cases. Moreover, new high-resolution methods such as comparative genomic hybridization enabled the discovery of a high number of different copy number variations, leading to gain or loss of chromosomal regions often containing multiple genes, in patients with VSD. In this chapter, we will describe the broad genetic heterogeneity observed in VSD patients considering recent advances in this field.

Oncogenic KRAS engages an RSK1/NF1 pathway to inhibit wild-type RAS signaling in pancreatic cancer.

Pancreatic ductal adenocarcinoma (PDAC) is a lethal malignancy with limited treatment options. Although activating mutations of the KRAS GTPase are the predominant dependency present in >90% of PDAC patients, targeting KRAS mutants directly has been challenging in PDAC. Similarly, strategies targeting known KRAS downstream effectors have had limited clinical success due to feedback mechanisms, alternate pathways, and dose-limiting toxicities in normal tissues. Therefore, identifying additional functionally relevant KRAS interactions in PDAC may allow for a better understanding of feedback mechanisms and unveil potential therapeutic targets. Here, we used proximity labeling to identify protein interactors of active KRAS in PDAC cells. We expressed fusions of wild-type (WT) (BirA-KRAS4B), mutant (BirA-KRAS4BG12D), and nontransforming cytosolic double mutant (BirA-KRAS4BG12D/C185S) KRAS with the BirA biotin ligase in murine PDAC cells. Mass spectrometry analysis revealed that RSK1 selectively interacts with membrane-bound KRASG12D, and we demonstrate that this interaction requires NF1 and SPRED2. We find that membrane RSK1 mediates negative feedback on WT RAS signaling and impedes the proliferation of pancreatic cancer cells upon the ablation of mutant KRAS. Our findings link NF1 to the membrane-localized functions of RSK1 and highlight a role for WT RAS signaling in promoting adaptive resistance to mutant KRAS-specific inhibitors in PDAC.