Despite initial positive responses with chemotherapy, many cancer patients experience relapse, continued tumor growth, and metastatic spread due to drug resistance. It is well documented that a rare population of phenotypically heterogeneous cells contributes to intratumour heterogeneity and drug resistance. To date, these rare populations are poorly characterized. To identify the potential role of these rare populations in drug resistance, here we have performed single-cell RNA sequencing of human oral squamous cell carcinomas lines presenting with sensitive, early, and late cisplatin-resistance patterns. The single-cell RNA-sequencing data identified two different transitional clusters within the three, sensitive, early, and late cisplatin-resistant major clusters. The differential gene expression profile and deregulated pathways analysis suggested Brain Abundant Membrane-Attached Signal Protein 1 (BASP1) as a major upregulated gene not only in major drug-resistant clusters but also in transitional clusters. Selective knockdown of BASP1 reverses epithelial to mesenchymal transition (EMT) phenotype in cisplatin-resistant cells and restores cisplatin-induced cell death. Mechanistically, BASP1 positively regulates LIN7A expression through phosphorylation of RAC-alpha serine/threonine-protein kinase as well as by supressing microRNA hsa-mir-501-3p, which in turn induces β-catenin-mediated EMT in chemoresistant cells. Overall, our study demonstrates that BASP1 acts as a key regulator of EMT in cisplatin-resistant oral squamous cell carcinoma and represents a promising therapeutic target to overcome drug resistance in advanced stages of the disease.

Docetaxel is the most common chemotherapy regimen for several neoplasms, including advanced OSCC (Oral Squamous Cell Carcinoma). Unfortunately, chemoresistance leads to relapse and adverse disease outcomes. We performed CRISPR-based kinome screening to identify potential players of Docetaxel resistance. Immunohistochemistry was performed to examine the expression profile of the target gene across tumour tissues. Global transcriptome analysis was performed to determine the molecular mechanism underlying Docetaxel resistance. NEK9 kinase assay was performed to identify a putative kinase inhibitor. Upon conducting CRISPR-based kinome screening, Never In Mitosis Gene-A Related Kinase-9 (NEK9) was identified as a major player of Docetaxel resistance in OSCC, prostate, and pancreatic cancer lines. NEK9 expression was found to be upregulated in chemotherapy non-responder OSCC patients as compared to responders. NEK9 ablation restores Docetaxel-induced cell death in chemoresistant cells. Mechanistically, we found that NEK9 deletion upregulates Transducin-like enhancer protein 3 (TLE3), which in turn represses Wnt signalling. Fostamatinib was identified as a potent NEK9 inhibitor that overcomes Docetaxel resistance. Our study demonstrated that NEK9 plays an important role in Docetaxel resistance. The novel combination of NEK9 inhibitor Fostamatinib and Docetaxel needs further clinical investigation in advanced OSCC.

Docetaxel alone or in combination with other drugs is the most common chemotherapy regimen for several neoplasms including advanced OSCC. Unfortunately, chemoresistance leads to relapse and continued tumor growth. It is therefore important to explore the causative factors for docetaxel resistance. In this study, we performed a CRISPR-based kinome screening that identified Never In Mitosis Gene-A Related Kinase-9 (NEK9) as a major player of docetaxel resistance in OSCC, prostate, and pancreatic cancer lines. NEK9 expression was upregulated in tumor samples of chemotherapy non-responders compared to responder OSCC patients. Our validation data suggests selectively knocking out NEK9 sensitizes cancer cells to docetaxel. Mechanistically, we found that ablation of NEK9 induces DNA damage, activating ERK(p-T202/Y204) that leads to Gasdermin-E mediated Cancer Cell pyroptosis. The in-vitro kinase activity assay identified fostamatinib as a potent inhibitor of NEK9. The xenograft data suggest that fostamatinib restores docetaxel sensitivity and facilitates a significant reduction of tumor burden. Overall, our data suggests a novel combination of fostamatinib and docetaxel needs further clinical investigation in advanced OSCC. ### Competing Interest Statement The authors have declared no competing interest.

Cisplatin, 5FU and docetaxel (TPF) are the most common chemotherapy regimen used for advanced OSCC. However, many cancer patients experience relapse, continued tumor growth, and spread due to drug resistance, which leads to treatment failure and metastatic disease. Here, using a CRISPR/Cas9 based kinome knockout screening, Misshapen-like kinase 1 (MINK1) is identified as an important mediator of 5FU resistance in OSCC. Analysis of clinical samples demonstrated significantly higher MINK1 expression in the tumor tissues of chemotherapy non-responders as compared to chemotherapy responders. The nude mice and zebrafish xenograft experiments indicate that knocking out MINK1 restores 5FU mediated cell death in chemoresistant OSCC. An antibody based phosphorylation array screen revealed MINK1 as a negative regulator of p53. Mechanistically, MINK1 modulates AKT phosphorylation at Ser473, which enables p-MDM2 (Ser 166) mediated degradation of p53. We also identified lestaurtinib as a potent inhibitor of MINK1 kinase activity. The patient derived TPF resistant cell based xenograft data suggest that lestaurtinib restores 5FU sensitivity and facilitates a significant reduction of tumor burden. Overall, our study suggests that MINK1 is a major driver of 5FU resistance in OSCC. The novel combination of MINK1 inhibitor lestaurtinib and 5FU needs further clinical investigation in advanced OSCC.

CMTM6, a type 3 transmembrane protein, is known to stabilize the expression of programmed cell death ligand 1 (PD-L1) and hence facilitates the immune evasion of tumor cells. Recently, we demonstrated that CMTM6 is a major driver of cisplatin resistance in oral squamous cell carcinomas (OSCC). However, the detailed mechanism of how CMTM6 rewires cisplatin resistance in OSCC is yet to be explored. RNA sequencing analysis of cisplatin-resistant OSCC lines stably expressing Nt shRNA and CMTM6 shRNA revealed that CMTM6 might be a potential regulator of the ribosome biogenesis network. Knocking down CMTM6 significantly inhibited transcription of 47S precursor rRNA and hindered the nucleolar structure, indicating reduced ribosome biogenesis. When CMTM6 was ectopically over-expressed in CMTM6KD cells, almost all ribosomal machinery components were rescued. Mechanistically, CMTM6 induced the expression of C-Myc, which promotes RNA polymerase I mediated rDNA transcription. In addition to this, CMTM6 was also found to regulate the AKT-mTORC1-dependent ribosome biogenesis and protein synthesis in cisplatin-resistant lines. The nude mice and zebrafish xenograft experiments indicate that blocking ribosome synthesis either by genetic inhibitor (CMTM6KD) or pharmacological inhibitor (CX-5461) significantly restores cisplatin-mediated cell death in chemoresistant OSCC. Overall, our study suggests that CMTM6 is a major regulator of the ribosome biogenesis network and targeting the ribosome biogenesis network is a viable target to overcome chemoresistance in OSCC. The novel combination of CX-5461 and cisplatin deserves further clinical investigation in advanced OSCC.

m6A RNA methylation, which serves as a critical regulator of transcript expression, has gathered tremendous scientific interest in recent years. From RNA processing to nuclear export, RNA translation to decay, m6A modification has been studied to affect various aspects of RNA metabolism, and it is now considered as one of the most abundant epitranscriptomic modification. RNA methyltransferases (writer), m6A-binding proteins (readers), and demethylases (erasers) proteins are frequently upregulated in several neoplasms, thereby regulating oncoprotein expression, augmenting tumor initiation, enhancing cancer cell proliferation, progression, and metastasis. Though the potential role of m6A methylation in growth and proliferation of cancer cells has been well documented, its potential role in development of therapy resistance in cancer is not clear. In this review, we focus on m6A-associated regulation, mechanisms, and functions in acquired chemoresistance, radioresistance, and resistance to immunotherapy in cancer.

BACKGROUND:Chemoresistance is one of the major factors for treatment failure in OSCC. Identifying key resistance triggering molecules will be useful strategy for developing novel treatment methods.METHODS:To identify the causative factors of chemoresistance, we performed RNA sequencing and global proteomic profiling of human OSCC lines presenting with sensitive, early and late cisplatin-resistance patterns.RESULTS:From the common set of dysregulated genes from both the analysis, RRBP1 was identified to be upregulated in both early and late cisplatin-resistant cells with respect to the sensitive counterpart. Analysis of OSCC patient sample indicates that RRBP1 expression is upregulated in chemotherapy-non-responder tumours as compared to chemotherapy-responder tumours. Genetic (knockout) or pharmacological (Radezolid, represses expression of RRBP1) inhibition of RRBP1 restores cisplatin-mediated cell death in chemo-resistant OSCC. Mechanistically, RRBP1 regulates Yes-associated protein1 (YAP1), a key protein in the Hippo pathway to induce chemoresistance. The PDC xenograft data suggests that knockout of RRBP1 induces cisplatin-mediated cell death and facilitates a significant reduction of tumour burden.CONCLUSION:Overall, our data suggests that (I) RRBP1 is a major driver of cisplatin-resistance in OSCC, (II) RRBP1 regulates YAP1 expression to mediate cisplatin-resistance, (III) Radezolid represses RRBP1 expression and (IV) targeting RRBP1 reverses cisplatin-induced chemoresistance in advanced OSCC.

Rewiring tumor cells to undergo drug-induced apoptosis is a promising way to overcome chemoresistance. Therefore, identifying causative factors for chemoresistance is of high importance. Unbiased global proteome profiling of sensitive, early, and late cisplatin-resistant oral squamous cell carcinoma (OSCC) lines identified CMTM6 as a top-ranked upregulated protein. Analyses of OSCC patient tumor samples demonstrated significantly higher CMTM6 expression in chemotherapy (CT) nonresponders as compared with CT responders. In addition, a significant association between higher CMTM6 expression and poorer relapse-free survival in esophageal squamous cell carcinoma, head and neck squamous cell carcinoma, and lung squamous cell carcinoma was observed from Kaplan-Meier plot analysis. Stable knockdown (KD) of CMTM6 restored cisplatin-mediated cell death in chemoresistant OSCC lines. Upon CMTM6 overexpression in CMTM6-KD lines, the cisplatin-resistant phenotype was rescued. The patient-derived cell xenograft model of chemoresistant OSCC displaying CMTM6 depletion restored the cisplatin-induced cell death and tumor burden substantially. The transcriptome analysis of CMTM6-KD and control chemoresistant cells depicted enrichment of the Wnt signaling pathway. We demonstrated that CMTM6 interaction with membrane-bound Enolase-1 stabilized its expression, leading to activation of Wnt signaling mediated by AKT-glycogen synthase kinase-3β. CMTM6 has been identified as a stabilizer of programmed cell death ligand 1. Therefore, as CMTM6 facilitates tumor cells for immune evasion and mediates cisplatin resistance, it could be a promising therapeutic target for treating therapy-resistant OSCC.

Despite of having the initial positive responses of chemotherapy, many cancer patients experience relapse with continued tumor growth and metastasis due to development of drug resistance. Chemoresistance is one of the main reasons for treatment failure in cancer patients. Understanding the molecular mechanism of drug resistance and rewiring of chemoresistant cells to undergo drug-induced apoptosis are viable approaches to overcome chemoresistance. The traditional two-dimensional (2D) monolayer culture system has limitations to study drug resistance, as it does not mimic the in vivo tumor microenvironment (TME). In contrast, three-dimensional (3D) culture is an ideal model to study chemoresistance as it accurately recapitulates in vivo TME, that is enrichment of stem cells, cell to cell or cell to extracellular matrix interaction, and tumor heterogeneity. Recently, tumor heterogeneity and microenvironment have drawn much attention for its impact on tumor development, drug resistance, and patient outcome. Therefore, it is possible to carry out cell-based assays like drug screening, and drug action using 3D culture model to explore agents those overcome chemoresistance. Henceforth, 3D culture model can be an appropriate platform to identify anticancer agents with potential of future clinical relevance.

Chemoresistance is one of the important factors for treatment failure in OSCC, which can culminate in progressive tumor growth and metastatic spread. Rewiring tumor cells to undergo drug-induced apoptosis is a promising way to overcome chemoresistance, which can be achieved by identifying the causative factors for acquired chemoresistance. In this study, to explore the key cisplatin resistance triggering factors, we performed global proteomic profiling of OSCC lines representing with sensitive, early and late cisplatin-resistant patterns. The top ranked up-regulated protein appeared to be CMTM6. We found CMTM6 to be elevated in both early and late cisplatin-resistant cells with respect to the sensitive counterpart. Analyses of OSCC patient samples indicate that CMTM6 expression is upregulated in chemotherapy-non-responder tumors as compared to chemotherapy-naïve tumors. Stable knockdown of CMTM6 restores cisplatin-mediated cell death in chemoresistant OSCC lines. Similarly, upon CMTM6 overexpression in CMTM6KD lines, the cisplatin resistant phenotype was efficiently rescued. Mechanistically, it was found that CMTM6 interacts with membrane bound Enolase-1 and stabilized its expression, which in turn activates the AKT-GSK3β mediated Wnt signaling. CMTM6 triggers the translocation of β-catenin into the nucleus, which elevates the Wnt target pro-survival genes like Cyclin D, c-Myc and CD44. Moreover, incubation with lithium chloride, a Wnt signaling activator, efficiently rescued the chemoresistant phenotype in CMTM6KD OSCC lines. In a patient-derived cell xenograft model of chemoresistant OSCC, knock-down of CMTM6 restores cisplatin induced cell death and results in significant reduction of tumor burden. CMTM6 has recently been identified as a stabilizer of PD-L1 and henceforth it facilitates immune evasion by tumor cells. Herewith for the first time, we uncovered another novel role of CMTM6 as one of the major driver of cisplatin resistance.