Integrated Proteogenomic Characterization across Major Histological Types of Pediatric Brain Cancer.

We report a comprehensive proteogenomics analysis, including whole-genome sequencing, RNA sequencing, and proteomics and phosphoproteomics profiling, of 218 tumors across 7 histological types of childhood brain cancer: low-grade glioma (n = 93), ependymoma (32), high-grade glioma (25), medulloblastoma (22), ganglioglioma (18), craniopharyngioma (16), and atypical teratoid rhabdoid tumor (12). Proteomics data identify common biological themes that span histological boundaries, suggesting that treatments used for one histological type may be applied effectively to other tumors sharing similar proteomics features. Immune landscape characterization reveals diverse tumor microenvironments across and within diagnoses. Proteomics data further reveal functional effects of somatic mutations and copy number variations (CNVs) not evident in transcriptomics data. Kinase-substrate association and co-expression network analysis identify important biological mechanisms of tumorigenesis. This is the first large-scale proteogenomics analysis across traditional histological boundaries to uncover foundational pediatric brain tumor biology and inform rational treatment selection.

A Vulnerability of a Subset of Colon Cancers with Potential Clinical Utility.

BRAF(V600E) mutant colon cancers (CCs) have a characteristic gene expression signature that is also found in some tumors lacking this mutation. Collectively, they are referred to as "BRAF-like" tumors and represent some 20% of CCs. We used a shRNA-based genetic screen focused on genes upregulated in BRAF(V600E) CCs to identify vulnerabilities of this tumor subtype that might be exploited therapeutically. Here, we identify RANBP2 (also known as NUP358) as essential for survival of BRAF-like, but not for non-BRAF-like, CC cells. Suppression of RANBP2 results in mitotic defects only in BRAF-like CC cells, leading to cell death. Mechanistically, RANBP2 silencing reduces microtubule outgrowth from the kinetochores, thereby inducing spindle perturbations, providing an explanation for the observed mitotic defects. We find that BRAF-like CCs display far greater sensitivity to the microtubule poison vinorelbine both in vitro and in vivo, suggesting that vinorelbine is a potential tailored treatment for BRAF-like CCs.

RASopathy mutations provide functional insight into the BRAF cysteine-rich domain and reveal the importance of autoinhibition in BRAF regulation.

BRAF is frequently mutated in human cancer and the RASopathy syndromes, with RASopathy mutations often observed in the cysteine-rich domain (CRD). Although the CRD participates in phosphatidylserine (PS) binding, the RAS-RAF interaction, and RAF autoinhibition, the impact of these activities on RAF function in normal and disease states is not well characterized. Here, we analyze a panel of CRD mutations and show that they increase BRAF activity by relieving autoinhibition and/or enhancing PS binding, with relief of autoinhibition being the major factor determining mutation severity. Further, we show that CRD-mediated autoinhibition prevents the constitutive plasma membrane localization of BRAF that causes increased RAS-dependent and RAS-independent function. Comparison of the BRAF- and CRAF-CRDs also indicates that the BRAF-CRD is a stronger mediator of autoinhibition and PS binding, and given the increased catalytic activity of BRAF, our studies reveal a more critical role for CRD-mediated autoinhibition in BRAF regulation.

Lyso-PAF, a biologically inactive phospholipid, contributes to RAF1 activation.

Phospholipase A2, group VII (PLA2G7) is widely recognized as a secreted, lipoprotein-associated PLA2 in plasma that converts phospholipid platelet-activating factor (PAF) to a biologically inactive product Lyso-PAF during inflammatory response. We report that intracellular PLA2G7 is selectively important for cell proliferation and tumor growth potential of melanoma cells expressing mutant NRAS, but not cells expressing BRAF V600E. Mechanistically, PLA2G7 signals through its product Lyso-PAF to contribute to RAF1 activation by mutant NRAS, which is bypassed by BRAF V600E. Intracellular Lyso-PAF promotes p21-activated kinase 2 (PAK2) activation by binding to its catalytic domain and altering ATP kinetics, while PAK2 significantly contributes to S338-phosphorylation of RAF1 in addition to PAK1. Furthermore, the PLA2G7-PAK2 axis is also required for full activation of RAF1 in cells stimulated by epidermal growth factor (EGF) or cancer cells expressing mutant KRAS. Thus, PLA2G7 and Lyso-PAF exhibit intracellular signaling functions as key elements of RAS-RAF1 signaling.

The Power of Human Cancer Genetics as Revealed by Low-Grade Gliomas.

The human brain contains a vast number of cells and shows extraordinary cellular diversity to facilitate the many cognitive and automatic commands governing our bodily functions. This complexity arises partly from large-scale structural variations in the genome, evolutionary processes to increase brain size, function, and cognition. Not surprisingly given recent technical advances, low-grade gliomas (LGGs), which arise from the glia (the most abundant cell type in the brain), have undergone a recent revolution in their classification and therapy, especially in the pediatric setting. Next-generation sequencing has uncovered previously unappreciated diverse LGG entities, unraveling genetic subgroups and multiple molecular alterations and altered pathways, including many amenable to therapeutic targeting. In this article we review these novel entities, in which oncogenic processes show striking age-related neuroanatomical specificity (highlighting their close interplay with development); the opportunities they provide for targeted therapies, some of which are already practiced at the bedside; and the challenges of implementing molecular pathology in the clinic.

IKKα Kinase Regulates the DNA Damage Response and Drives Chemo-resistance in Cancer.

Phosphorylated IKKα(p45) is a nuclear active form of the IKKα kinase that is induced by the MAP kinases BRAF and TAK1 and promotes tumor growth independent of canonical NF-κB signaling. Insights into the sources of IKKα(p45) activation and its downstream substrates in the nucleus remain to be defined. Here, we discover that IKKα(p45) is rapidly activated by DNA damage independent of ATM-ATR, but dependent on BRAF-TAK1-p38-MAPK, and is required for robust ATM activation and efficient DNA repair. Abolishing BRAF or IKKα activity attenuates ATM, Chk1, MDC1, Kap1, and 53BP1 phosphorylation, compromises 53BP1 and RIF1 co-recruitment to sites of DNA lesions, and inhibits 53BP1-dependent fusion of dysfunctional telomeres. Furthermore, IKKα or BRAF inhibition synergistically enhances the therapeutic potential of 5-FU and irinotecan to eradicate chemotherapy-resistant metastatic human tumors in vivo. Our results implicate BRAF and IKKα kinases in the DDR and reveal a combination strategy for cancer treatment.

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.

The Dietary Supplement Chondroitin-4-Sulfate Exhibits Oncogene-Specific Pro-tumor Effects on BRAF V600E Melanoma Cells.

Dietary supplements such as vitamins and minerals are widely used in the hope of improving health but may have unidentified risks and side effects. In particular, a pathogenic link between dietary supplements and specific oncogenes remains unknown. Here we report that chondroitin-4-sulfate (CHSA), a natural glycosaminoglycan approved as a dietary supplement used for osteoarthritis, selectively promotes the tumor growth potential of BRAF V600E-expressing human melanoma cells in patient- and cell line-derived xenograft mice and confers resistance to BRAF inhibitors. Mechanistically, chondroitin sulfate glucuronyltransferase (CSGlcA-T) signals through its product CHSA to enhance casein kinase 2 (CK2)-PTEN binding and consequent phosphorylation and inhibition of PTEN, which requires CHSA chains and is essential to sustain AKT activation in BRAF V600E-expressing melanoma cells. However, this CHSA-dependent PTEN inhibition is dispensable in cancer cells expressing mutant NRAS or PI3KCA, which directly activate the PI3K-AKT pathway. These results suggest that dietary supplements may exhibit oncogene-dependent pro-tumor effects.

Transcriptional features of low-grade neuroepithelial tumors with the BRAF V600E mutation associated with epileptogenicity.

Low-grade neuroepithelial tumors are major causes of drug-resistant focal epilepsy. Clinically, these tumors are defined as low-grade epilepsy-associated neuroepithelial tumors (LEATs). The BRAF V600E mutation is frequently observed in LEAT and linked to poor seizure outcomes. However, its molecular role in epileptogenicity remains elusive. To understand the molecular mechanism underlying the epileptogenicity in LEAT with the BRAF V600E genetic mutation (BRAF V600E-LEAT), we conducted RNA sequencing (RNA-seq) analysis using surgical specimens of BRAF V600E-LEAT obtained and stored at a single institute. We obtained 21 BRAF V600E-LEAT specimens and 4 control specimens, including 24 from Japanese patients and 1 from a patient of Central Asian origin, along with comprehensive clinical data. We submitted the transcriptome dataset of 21 BRAF V600E-LEAT plus 4 controls, as well as detailed clinical information, to a public database. Preliminary bioinformatics analysis using this dataset identified 2134 differentially expressed genes between BRAF V600E-LEAT and control. Additionally, gene set enrichment analysis provided novel insights into the association between estrogen response-related pathways and the epileptogenicity of BRAF V600E-LEAT patients. Our datasets and findings will contribute toward the understanding of the pathology of epilepsy caused by LEAT and the identification of new therapeutic targets.

Comprehensive clinicopathological, molecular, and methylation analysis of mesenchymal tumors with NTRK and other kinase gene aberrations.

Alterations in kinase genes such as NTRK1/2/3, RET, and BRAF underlie infantile fibrosarcoma (IFS), the emerging entity 'NTRK-rearranged spindle cell neoplasms' included in the latest WHO classification, and a growing set of tumors with overlapping clinical and pathological features. In this study, we conducted a comprehensive clinicopathological and molecular analysis of 22 cases of IFS and other kinase gene-altered spindle cell neoplasms affecting both pediatric and adult patients. Follow-up periods for 16 patients ranged in length from 10 to 130 months (mean 38 months). Six patients were treated with targeted therapy, achieving a partial or complete response in five cases. Overall, three cases recurred and one metastasized. Eight patients were free of disease, five were alive with disease, and two patients died. All cases showed previously reported morphological patterns. Based on the cellularity and level of atypia, cases were divided into three morphological grade groups. S100 protein and CD34 were at least focally positive in 12/22 and 14/22 cases, respectively. Novel PWWP2A::RET, NUMA1::RET, ITSN1::RAF1, and CAPZA2::MET fusions, which we report herein in mesenchymal tumors for the first time, were detected by RNA sequencing. Additionally, the first uterine case with BRAF and EGFR mutations and CD34 and S100 co-expression is described. DNA sequencing performed in 13 cases uncovered very rare additional genetic aberrations. The CNV profiles showed that high-grade tumors demonstrate a significantly higher percentage of copy number gains and losses across the genome compared with low- and intermediate-grade tumors. Unsupervised clustering of the tumors' methylation profiles revealed that in 8/9 cases, the methylation profiles clustered with the IFS methylation class, irrespective of their clinicopathological or molecular features. © 2024 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

BRAFV600E promotes anchorage-independent growth but inhibits anchorage-dependent growth in hTERT/Cdk4-Immortalized normal human bronchial epithelial cells.

Certain oncogenes, including mutant RAS and BRAF, induce a type of senescence known as oncogene-induced senescence (OIS) in normal cells in a cell-type-specific manner. OIS serves as a barrier to transformation by activated oncogenes. Our previous studies showed that mutant KRASV12 did not efficiently induce OIS in an hTERT/Cdk4-immortalized normal human bronchial epithelial cell line (HBEC3), but it did enhance both anchorage-dependent and anchorage-independent growth. In this study, we investigated whether mutant BRAF, a well-known inducer of OIS, could trigger OIS in HBEC3 cells. We also assessed the impact of mutant BRAF on the growth of HBEC3 cells, as no previous studies have examined this using a normal bronchial epithelial cell line model. We established an HBEC3 cell line, designated as HBEC3-BIN, that expresses mutant BRAFV600E in a doxycycline-regulated manner. Unlike our previous finding that KRASV12 upregulated both pERK and pAKT, mutant BRAFV600E upregulated pERK but not pAKT in HBEC3-BIN cells. Similar to KRASV12, BRAFV600E did not efficiently induce OIS. Interestingly, while BRAFV600E inhibited colony formation in anchorage-dependent conditions, it dramatically enhanced colony formation in anchorage-independent conditions in HBEC3-BIN. In HBEC3 cells without BRAFV600E or KRASV12 expression, p21 was only detected in the cytoplasm, and its localization was not altered by the expression of BRAFV600E or KRASV12. Next-generation sequencing analysis revealed an enrichment of gene sets known to be involved in carcinogenesis, including IL3/JAK/STAT3, IL2, STAT5, and the EMT pathway. Our results indicate that, unlike KRASV12, which promoted both, BRAFV600E enhances anchorage-independent growth but inhibits anchorage-dependent growth of HBEC3. This contrast may result from differences in activation signaling in the downstream pathways. Furthermore, HBEC3 cells appear to be inherently resistant to OIS, which may be partly due to the fact that p21 remains localized in the cytoplasm upon expression of BRAFV600E or KRASV12.

Development and in-depth characterization of BRAFi-resistant melanoma cell lines in vitro and in vivo.

Regardless of the clinical response and improved patient survival observed following treatment with BRAFi like Vemurafenib (Vem), rapid development of resistance still remains as a major obstacle in melanoma therapy. In this context, we developed and characterized two acquired Vem-resistant melanoma cell lines, A375V and SK-MEL-28V, and an intrinsically Vem-resistant cell line, RPMI-7951. Altered morphology and growth rate of the resistant cell lines displayed spindle-shaped cells with filopodia formation and enhanced proliferation rate as compared to parental cells. Further in vitro characterization in 2D models confirmed the emergence of a resistant phenotype in melanoma cells. To mimic the in vivo tumor microenvironment, spheroids were developed for both parental and resistant cell lines to recognize materialization of invadopodia structures demonstrating elevated invasiveness and proliferation of resistant cells-based spheroids, especially A375V. Importantly, we validated A375V cell line in vivo to prove its tumorigenicity and drug resistance in tumor xenograft model. Taken together, our established clinically relevant Vem-resistant tumor model could be beneficial to elucidate drug resistance mechanisms, screen and identify novel anticancer therapies to overcome BRAFi resistance in melanoma.

Targeting NG2 relieves the resistance of BRAF-mutant thyroid cancer cells to BRAF inhibitors.

BRAFV600E represents a constitutively active onco-kinase and stands as the most prevalent genetic alteration in thyroid cancer. However, the clinical efficacy of small-molecule inhibitors targeting BRAFV600E is often limited by acquired resistance. Here, we find that nerve/glial antigen 2 (NG2), also known as chondroitin sulfate proteoglycan 4 (CSPG4), is up-regulated in thyroid cancers, and its expression is increased with tumor progression in a BRAFV600E-driven thyroid cancer mouse model. Functional studies show that NG2 knockout almost does not affect tumor growth, but significantly improves the response of BRAF-mutant thyroid cancer cells to BRAF inhibitor PLX4720. Mechanistically, the blockade of ERK-dependent feedback by BRAF inhibitor can activate receptor tyrosine kinase (RTK) signaling, causing the resistance to this inhibitor. NG2 knockout attenuates the PLX4720-mediated feedback activation of several RTKs, improving the sensitivity of BRAF-mutant thyroid cancer cells to this inhibitor. Based on this finding, we propose and demonstrate an alternative strategy for targeting NG2 to effectively treat BRAF-mutant thyroid cancers by combining multiple kinase inhibitor (MKI) Sorafenib or Lenvatinib with PLX4720. Thus, this study uncovers a new mechanism in which NG2 contributes to the resistance of BRAF-mutant thyroid cancer cells to BRAF inhibitor, and provides a promising therapeutic option for BRAF-mutant thyroid cancers.

Reporter cell lines to screen for inhibitors or regulators of the KRAS-RAF-MEK1/2-ERK1/2 pathway.

The RAS-regulated RAF-MEK1/2-ERK1/2 signalling pathway is activated in cancer due to mutations in RAS proteins (especially KRAS), BRAF, CRAF, MEK1 and MEK2. Whilst inhibitors of KRASG12C (lung adenocarcinoma) and BRAF and MEK1/2 (melanoma and colorectal cancer) are clinically approved, acquired resistance remains a problem. Consequently, the search for new inhibitors (especially of RAS proteins), new inhibitor modalities and regulators of this pathway, which may be new drug targets, continues and increasingly involves cell-based screens with small molecules or genetic screens such as RNAi, CRISPR or protein interference. Here we describe cell lines that exhibit doxycycline-dependent expression KRASG12V or BRAFV600E and harbour a stably integrated EGR1:EmGFP reporter gene that can be detected by flow cytometry, high-content microscopy or immunoblotting. KRASG12V or BRAFV600E-driven EmGFP expression is inhibited by MEK1/2 or ERK1/2 inhibitors (MEKi and ERKi). BRAFi inhibit BRAFV600E-driven EmGFP expression but enhance the response to KRASG12V, recapitulating paradoxical activation of wild type RAF proteins. In addition to small molecules, expression of iDab6, encoding a RAS-specific antibody fragment inhibited KRASG12V- but not BRAFV600E-driven EmGFP expression. Finally, substitution of EmGFP for a bacterial nitroreductase gene allowed KRASG12V or BRAFV600E to drive cell death in the presence of a pro-drug, which may allow selection of pathway inhibitors that promote survival. These cell lines should prove useful for cell-based screens to identify new regulators of KRAS- or BRAF-dependent ERK1/2 signalling (drug target discovery) as well as screening or triaging 'hits' from drug discovery screens.

LINEAGE: Label-free identification of endogenous informative single-cell mitochondrial RNA mutation for lineage analysis.

Single-cell RNA-sequencing (scRNA-seq) has become a powerful tool for biomedical research by providing a variety of valuable information with the advancement of computational tools. Lineage analysis based on scRNA-seq provides key insights into the fate of individual cells in various systems. However, such analysis is limited by several technical challenges. On top of the considerable computational expertise and resources, these analyses also require specific types of matching data such as exogenous barcode information or bulk assay for transposase-accessible chromatin with high throughput sequencing (ATAC-seq) data. To overcome these technical challenges, we developed a user-friendly computational algorithm called "LINEAGE" (label-free identification of endogenous informative single-cell mitochondrial RNA mutation for lineage analysis). Aiming to screen out endogenous markers of lineage located on mitochondrial reads from label-free scRNA-seq data to conduct lineage inference, LINEAGE integrates a marker selection strategy by feature subspace separation and de novo "low cross-entropy subspaces" identification. In this process, the mutation type and subspace-subspace "cross-entropy" of features were both taken into consideration. LINEAGE outperformed three other methods, which were designed for similar tasks as testified with two standard datasets in terms of biological accuracy and computational efficiency. Applied on a label-free scRNA-seq dataset of BRAF-mutated cancer cells, LINEAGE also revealed genes that contribute to BRAF inhibitor resistance. LINEAGE removes most of the technical hurdles of lineage analysis, which will remarkably accelerate the discovery of the important genes or cell-lineage clusters from scRNA-seq data.

BRaf signaling principles unveiled by large-scale human mutation analysis with a rapid lentivirus-based gene replacement method.

Rapid advances in genetics are linking mutations on genes to diseases at an exponential rate, yet characterizing the gene-mutation-cell-behavior relationships essential for precision medicine remains a daunting task. More than 350 mutations on small GTPase BRaf are associated with various tumors, and ∼40 mutations are associated with the neurodevelopmental disorder cardio-facio-cutaneous syndrome (CFC). We developed a fast cost-effective lentivirus-based rapid gene replacement method to interrogate the physiopathology of BRaf and ∼50 disease-linked BRaf mutants, including all CFC-linked mutants. Analysis of simultaneous multiple patch-clamp recordings from 6068 pairs of rat neurons with validation in additional mouse and human neurons and multiple learning tests from 1486 rats identified BRaf as the key missing signaling effector in the common synaptic NMDA-R-CaMKII-SynGap-Ras-BRaf-MEK-ERK transduction cascade. Moreover, the analysis creates the original big data unveiling three general features of BRaf signaling. This study establishes the first efficient procedure that permits large-scale functional analysis of human disease-linked mutations essential for precision medicine.

Identification of Rare EIF3E::RSPO2 Fusion in Recurrent and Aggressive Urachal Adenocarcinoma.

INTRODUCTION: Urachal cancer (UC) is a rare genitourinary malignancy arising from the urachus, an embryonic remnant of the placental allantois. Its diagnosis remains ambiguous with late-stage cancer detection and represents a highly aggressive disease. Due to its rarity, there is no clear consensus on molecular signatures and appropriate clinical management of UC. CASE REPORT: We report a 45-year-old man with recurrent urachal adenocarcinoma (UA) treated with cystectomies, chemotherapy, and radiotherapy. The patient initially presented with hematuria and abdominal pain. Imaging revealed a nodular mass arising from the superior wall of the urinary bladder and extending to the urachus. Biopsy results suggested moderately differentiated UA with muscle layer involvement. The tumor recurred after 20 months, following which, another partial cystectomy was performed. Repeat progression was noted indicating highly aggressive disease. Targeted next-generation sequencing revealed the presence of EIF3E::RSPO2 fusion, along with BRAF and TP53 mutations, and EGFR gene amplification. This is the first case reporting the presence of this fusion in UA. Palliative medication and radiotherapy were administered to manage the disease. CONCLUSION: Current treatment modality of surgery may be effective in the early stages of recurrent UA; however, a standard chemotherapy and radiotherapy regimen is yet to be determined for advanced stages. The detection of the rare EIF3E::RSPO2 fusion warrants further studies on the significance of this variant as a possible therapeutic target for improved clinical management.

USP25 Promotes the Antimycobacterial Response of Macrophages Through Stabilizing B-Raf and C-Raf.

Ubiquitin-specific peptidase 25 (USP25) is one of the best-characterized deubiquitinating enzymes and plays a vital regulatory role in various biological processes, especially in cancer development and immune regulation. However, the exact role of USP25 and its underlying mechanisms in macrophage activation and immunogenicity during Mycobacterium tuberculosis infection remain unclear. In this study, we found that M tuberculosis infection induced USP25 expression in human and mouse macrophages. In particular, USP25 expression is elevated in multiple cell types, especially monocytes, in patients with tuberculosis. Additionally, USP25 deficiency in macrophages and mice resulted in compromised immunity against M tuberculosis infection, accompanied by reduced expressions of various proinflammatory cytokines and chemokines. Mechanistically, USP25 in macrophages promoted the activation of the ERK signaling pathway through deubiquitination and stabilization of B-Raf and C-Raf. These findings collectively suggest the critical roles of USP25 in M tuberculosis infection and its potential as a therapeutic target.

Targeted Mass Spectrometry Analyses of Somatic Mutations in Colorectal Cancer Specimens Using Differential Ion Mobility.

Identification of K-Ras and B-Raf mutations in colorectal cancer (CRC) is essential to predict patients' response to anti-EGFR therapy and formulate appropriate therapeutic strategies to improve prognosis and survival. Here, we combined parallel reaction monitoring (PRM) with high-field asymmetric waveform ion mobility (FAIMS) to enhance mass spectrometry sensitivity and improve the identification of low-abundance K-Ras and B-Raf mutations in biological samples without immunoaffinity enrichment. In targeted LC-MS/MS analyses, FAIMS reduced the occurrence of interfering ions and enhanced precursor ion purity, resulting in a 3-fold improvement in the detection limit for K-Ras and B-Raf mutated peptides. In addition, the ion mobility separation of isomeric peptides using FAIMS facilitated the unambiguous identification of K-Ras G12D and G13D peptides. The application of targeted LC-MS/MS analyses using FAIMS is demonstrated for the detection and quantitation of B-Raf V600E, K-Ras G12D, G13D, and G12V in CRC cell lines and primary specimens.

BRAF V600E-induced distinct DNA damage response defines the therapeutic potential of p53 activation for TP53 wild-type colorectal cancer.

BRAF V600E, one of the most frequent mutations in the MAPK pathway, confers poor prognosis to colorectal cancers (CRCs), partly because of chemotherapeutic resistance. Oncogene-induced DNA damage responses (DDRs) that primarily activate p53 are important mechanistic barriers to the malignant transformation of cells; however, the mechanism underlying this impairment in cancer remains unknown. Here, we evaluated the responses of BRAFV600E-induced DDRs in two CRC cell lines, SW48 and LIM1215, both of which harbor wild-type TP53, KRAS, and BRAF. BRAFV600E transduction exhibited distinct phenotypes in these cells: SW48 cell proliferation markedly decreased, whereas that of LIM1215 increased. BRAFV600E expression induced the activation of oncogene-induced DDR signaling in SW48 cells, but not in LIM1215 cells, whereas chemotherapeutic agents similarly activated DDRs in both cell lines. Knockdown experiments revealed that these responses in SW48 cells were mediated by p53-p21 pathway activation. Comet assay (both alkaline and neutral) revealed that BRAFV600E increased single-strand breaks to the same extent in both cell lines; however, in case of LIM1215 cells, it only facilitated double-strand breaks. Furthermore, the proliferation of LIM1215 cells, wherein no oncogene-induced DDRs occurred, was synergistically inhibited upon MDM2 inhibitor-mediated p53 activation combined with MEK inhibition. Taken together, these distinct DDR signaling responses highlight the novel characteristics of BRAFV600E-mutated CRC cells and define the therapeutic potential of p53 activation combined with MAPK inhibition against TP53 wild-type CRC harboring a BRAFV600E mutation.