1. METTL16 is differentially expressed in pan-cancer and is correlated with patient prognosis
In order to investigate the expression of METTL16 in pan-cancer, we conducted a search for METTL16 associated diseases on OpenTarget. The bubble plots indicated that METTL16 was associated with gastric carcinoma, osteosarcoma, Ewing sarcoma, and pancreatic carcinoma (Fig. 1A). Then, we investigated whether there were variations in the mRNA expression levels of METTL16 between 33 tumors and their respective normal tissues. The mRNA level of METTL16 was up-regulated in CHOL, COAD, ESCA, GBM, HNSC, KIRP, LIHC, and STAD, while it was down-regulated in BLCA, BRCA, CESC, KICH, LUAD, LUSC, PRAD, THCA, and UCEC (Fig. 1B). Due to the absence of normal tissue control in certain tumors, we utilized GEPIA2.0 for supplementary analysis in order to determine that METTL16 mRNA was highly expressed in DLBC and LGG but decreased in OV (Fig. 1C). Additionally, the expression level of METTL16 was negatively correlated with the pathological stage of STAD, OV, and PAAD (Fig. 1D). Besides, the protein level of METTL16 was up-regulated in GBM, LIHC, BRCA, KIRC, and HNSC, in contrast to UCEC (Fig. 1E). This finding was in accordance with the orientation of the METTL16 mRNA changes described above, with the exception of BRCA. We obtained immunohistochemical images from the HPA database to more visually evaluate the protein expression changes of METTL16 in tumors. These images demonstrated that the protein expression of METTL16 was significantly higher than that of normal tissues in breast, cervical, colorectal, endometrial, renal, liver, lung, skin, testis cancers, and lymphoma (Fig. S1A-J). Conversely, it was observed to be lower in ovarian, pancreatic, prostate, stomach, and thyroid cancer (Fig. S1K-O). This implies that METTL16 is differentially expressed at both the transcriptional and translational levels in pan-cancer.
Subsequently, we employed forest plots (Fig. S2) and Kaplan-Meier curves to investigate whether the expression level of METTL16 influences the prognosis of pan-cancer patients. The findings indicated that patients with BRCA, LIHC, and SARC endured a low OS and a low DSS, while those with BRCA, LUSC, and UVM suffered a low DSS. However, patients with ESCA, KIRC, PAAD, READ, and UCEC had a longer OS, while patients with KIRC, PAAD, READ, and THYM received a prolonged DSS. Additionally, patients with KIRC and PAAD experienced a prolonged PFI (Fig. 2). Therefore, the clinical significance of the differential expression of METTL16 in tumors is certain. There are two aspects to this gene: in BRCA, it may function as a pro-oncogene, whereas in PAAD, it serves as a protective factor.
2. Involvement of METTL16 in methylation modification
The GO functional enrichment analysis demonstrated that METTL16 possesses a variety of methylation modification activities (Fig. 3A). METTL16 expression resulted in an increase in the levels of tumor methyltransferase, with a particular emphasis on COAD, ESCA, HNSC, LGG, LUSC, OV, STAD, THCA, and UVM (Fig. 3B). A hallmark of tumorigenesis progression is abnormal methylation [47]. The methylation of gene promoters occurs in the early phases of tumorigenesis and has significant potential as a biomarker for cancer prognosis and diagnosis [48]. Therefore, we investigated whether there was a correlation between METTL16 promoter methylation and CTL and prognosis. We selected the top 10 cancer types with high CTL correlation coefficients (Fig. 3C), which include BRCA, SKCM, CHOL, KIRC, SARC, and partial subtypes. All of these cancers exhibited positive correlations between METTL16 promoter methylation and CTL infiltration. Finally, the correlation scatterplot is depicted in Fig. 3D. The survival of patients was extended in BRCA, SKCM, and their subtypes as a consequence of methylation of the METTL16 promoter (Fig. 3E).
Further, we discovered experimentally verified proteins that can interact with METTL16 using the String web tool. Figure 3F illustrates 10 of these proteins (WTAP, CBLL1, METTL14, RMB15, METTL5, RMB15B, ZC3H13, ZCCHC4, VIRMA, METTL3). Additionally, GEPIA2.0's analysis demonstrated that the mentioned proteins exhibited altered expression in a diverse array of tumors (Fig. 3G). In pan-cancer, METTL16 was positively co-expressed with WTAP, CBLL1, METTL14, RBM15, METTL5, RBM15B, ZC3H13, ZCCHC4, and METTL3 (Fig. 3H). Likewise, the methylation function of all of these genes can contribute to the development of cancer [49–53].
3. METTL16 has an influence on DNA repair and stemness in tumor cells
As a further study of the function of METTL16 participated in pan-cancer, we initially compared the expression of METTL16 in altered and unaltered pathways with UALCAN, and found that altered of mTOR pathway status was accompanied by the upregulation of METTL16 in 7 tumors (BRCA, COAD, KIRC, HNSC, GBM, LGG, PRCA), altered chromatin modifier status was accompanied by up-regulation of METTL16 in 7 tumors (BRCA, COAD, OV, KIRC, UCEC, HNSC, LGG), altered MYC/MYCN pathway status was accompanied by upregulation of METTL16 in 6 tumors (OV, LUAD, HNSC, GBM, LGG, PRCA), altered WNT pathway status was accompanied by upregulation of METTL16 in 5 tumors (COAD, KIRC, HNSC, GBM, LGG), altered SWI/SNF complex status was accompanied by upregulation of METTL16 in 5 tumors (BRCA, KIRC, HNSC, GBM, LGG), and altered HIPPO complex status was accompanied by upregulation of METTL16 in 7 tumors (COAD, KIRC, LUAD PAAD, HNSC, GBM, PRCA) (Fig. 4A). As the aforementioned results were restricted to a subset of pan-cancer, we performed a thorough analysis of all tumors using GEPIA2.0 to investigate the expression correlation between METTL16 and these pathway-related signatures. We discovered that the expression of METTL16 was positively correlated with all of these signatures (Fig. 4B). Since these pathways are associated with DDR [18, 54, 55], we believed that METTL16 plays a role in it.
We focused on the correlation between METTL16 and HRD and MMR-related genes (MLH1, MSH2, MSH6, PMS2, EPCAM). We observed that METTL16 was positively correlated with HRD in KICH, HNSC, LGG, GBM, and LIHC, and negatively correlated with HRD in THYM, PAAD, BRCA, LAML, UCEC, PRAD, TGCT, CESC, and STAD (Fig. 4C). METTL16 has a positive association with a number of MMR genes in the bulk of cancers, including BLCA, BRCA, CESC, COAD, ESCA, GBM, HNSC, KICH, KIRC, KIRP, LAML, LGG, LIHC, LUAD, LUSC, OV, PAAD, PCPG, PRAD, READ, SARC, SKCM, STAD, TGCT, THCA, UCEC, and UVM (Fig. 4D).
Variations in the expression of DNA repair genes can also influence the self-renewal capacity, tumorigenicity, and invasiveness of cancer stem cells [56]. Additionally, a growing body of evidence has linked the poor prognosis of tumors to the stemness properties of cancer cells [57]. The HIPPO pathway is also known to be associated with DNA damage repair and to maintain the stemness of tumor stem cells [58]. The expression of METTL16 is correlated with the alteration of HIPPO complex status in pan-cancer (Fig. 4A, B). That is why we subsequently conducted an investigation into the potential impact of METTL16 on the stemness of cancer. Figure 4E indicates that METTL16 is positively correlated with stemness in GBM, HNSC, LUSC, and UVM, and negatively correlated with it in PRAD and THYM. It is intriguing that the same trend was observed when HRR signatures were conducted on the aforementioned cancers (Fig. 4F). We therefore hypothesize that METTL16 may affect the stemness of cancer that is mediated by DNA repair.
4. METTL16 can cause genome instability in pan-cancer
Effective DNA damage repair is essential for the preservation of genomic stability and the prevention of carcinogenesis. Mutations or recombination that disrupt DNA damage repair can activate proto-oncogenes and deactivate oncogenes [19]. Thus, we conducted an extended investigation of the potential impact of METTL16 on genomic stability. The cBioPortal online tool was used to analyze the CNA levels of METTL16 in pan-cancers, which included 32 studies and 10,967 samples. The results indicated that DLBC, ACC, and LIHC exhibited high levels of deep deletion of METTL16, while DLBC, UCEC, SKCM, and COAD exhibited high levels of mutation of METTL16 (Fig. 5A). The most frequently altered location is R200Q, with 59 missense variants, 7 truncation mutations, 1 splice mutation, and 7 fusion mutations within the 562 amino acids of METTL16 (Fig. 5B). In all but LAML, KIRP, THCA, and UVM, shallow deletion of METTL16 was the most prevalent among the 32 cancers (Fig. 5C). This suggests that METTL16 expression can impact genome stability in pan-cancer. The survival of patients with LGG, UCEC, and PAAD improved when METTL16 CNA levels were elevated, while the survival of patients with DLBC was decreased (Fig. 5D). It reveals that changes in the METTL16 genome can influence the prognosis of cancer.
Since TMB and MSI are indicators of the frequency of genomic mutations, we then examined the correlation between METTL16 and these two in pan-cancer. METTL16 was positively correlated with TMB in COAD, LGG, MESO, OV, and UCEC, and negatively correlated with TMB in BLCA, BRCA, ESCA, PAAD, PRAD, SKCM, and THCA. In addition, METTL16 was positively correlated with MSI in COAD, DLB, HNSC, LUAD, and LUSC, while negatively correlated with MSI in PRAD and THCA (Fig. 5E). Given that genetic mutations cause tumor cells to create proteins called neoantigens, which differ in amino acid sequence, we also explored the relationship between METTL16 and tumor neoantigens. Figure 5F showed a positive correlation between the expression of neoantigen and METTL16 in COAD, KICH, and UCEC. In addition to simple deoxyribonucleic acid sequence alterations, genomic instability encompasses chromosomal aberrations [59]. METTL16 was positively correlated with MESO, OV, LIHC, and LUSC in ploidy analysis, and negatively correlated with THYM, DLBC, PRAD, and UCEC (Fig. 5G). The results presented above demonstrate that METTL16 can result in genome instability, which is particularly evident in COAD and UCEC.
5. Differential expression of METTL16 AS events affect cancer prognosis
Maintaining genomic instability requires AS [59]. By conducting an analysis of the NCBI database, we discovered that METTL16 contains numerous variable splices (Fig. 6A). We conducted PSI analysis using OncoSplicing to identify variations in the expression of METTL16 AS events in pan-cancer. The results showed that PSI was increased in CESC and KICH compared to normal tissues, and decreased in BLCA, COAD, and PAAD (Fig. 6B). Similarly, Fig. 6C indicates that the PSI of cancers is significantly different from that of normal or GTEx tissues, as evidenced by the PSI of KICH, SKCM, BRCA, LUSC, and LUAD. Furthermore, PanCox (PanOS or PanPFI) plots demonstrate that the PSI distribution of METTL16 AS events was negatively correlated with OS in ACC, KIRC, and STAD, and positively correlated with OS in GBM, KIRP, SARC, UCS, and LGG. It was also negatively correlated with PFI in ACC, LIHC, THYM, TGCT, and STAD, and positively correlated with PFI in UCS, DLBC, LUAD, and LGG (Fig. 6D, E). Kaplan-Meier curves were employed to determine whether METTL16 AS events influence cancer prognosis. Elevated PSI levels were associated with shorter OS in CHOL, GBM, KIRP, LGG, SARC, and UCS (Fig. 6F). In contrast, the OS of STAD, KIRC, and ACC patients and the PFI of TGCT and THYM patients were extended by a high level of PSI (Fig. 6F, G). As a result, we predicted that METTL16 AS events are crucial for cancer prognosis.
6. METTL16 is involved in tumor immune infiltration and remodeling of TME
As DNA repair defects and AS have been implicated in the regulation of the TME, we examined the immune function of METTL16 in the TME. In BLCA, BRCA, GBM, KIRP, LAML, LGG, MESO, SARC, and THCA, METTL16 was negatively correlated with ESTIMATEScore, ImmuneScore, and StromalScore. Conversely, in COAD, PAAD, and STAD, METTL16 was positively correlated with ESTIMATEScore, ImmuneScore, and StromalScore. The negative correlation between METTL16 and ImmuneScore was observed in LAML and THCA, as well as with several immune checkpoint genes (Fig. 7A, B). In addition, immune checkpoint genes can also be positively correlated with METTL16 in a diverse array of cancers. For instance, in ACC, BLCA, KIRP, LGG, LIHC, OV, SARC, SKCM, TGCT, and UVM, METTL16 expression was positively correlated with CD80, a mark for M1 macrophages. In ACC, BLCA, GBM, KIRP, LGG, LIHC, MESO, OV, SARC, SKCM, and TGCT, the expression of METTL16 was also positively correlated with CD86, another marker of M1 macrophages. Up to 28 immune check genes were positively correlated with METTL16 in SARC (Fig. 7B). The results presented above imply that METTL16 participates in the effects of immune checkpoints.
Following this, we conducted an investigation using TISIDB and discovered that METTL16 expression varied among cancer immune subtypes, with the highest levels of significance observed in BRCA, LUAD, PAAD, and UCEC (Fig. 7C). Tumor immune subtypes have been demonstrated to be correlated with patient prognosis in prior research. The OS of patients with the C3 immune subtype is significantly higher than that of the other immune subtypes. Conversely, the prognosis of patients with C1 and C2 is markedly worse, and that of patients with C4 and C6 is the worst [42]. Figure 7D illustrates the METTL16 expression of each immune subtype in the top four cancers with the highest correlation factor. The expression of METTL16 was higher in the C3 immune subtype of LUAD and PAAD and lower in the C4 and C6 immune subtypes. This indicates that METTL16 may have a tumor-suppressive effect in LUAD and PAAD. Significant differences in METTL16 expression were also observed in the immune subtypes of BLCA, ESCA, HNSC, KIRC, LGG, LIHC, LUSC, OV, PRAD, SARC, STAD, STAD, and TGCT in the other 26 cancers (Fig. S3).
Previously, we discovered that the methylation of METTL16 could influence the prognosis of pan-cancer, and that m6A modification could influence the recruitment, activation, and polarization of immune cells, thereby remodeling the TME [60]. Therefore, in order to determine whether the methylation of METTL16 could play a role in the TME, we examined it in relation to chemokines, chemokine receptors, and immunomodulatory factors. As illustrated in Fig. 7E, the methylation of METTL16 is positively correlated with a variety of chemokines, receptors, MHCs, and immunostimulators, and it is particularly significant in OV and TGCT. Moreover, we conducted a comparison of the expression levels of METTL16 in tumor cell lines prior to and following cytokine treatment. Increased expression of METTL16 was observed in both B16 and LLC cell lines following IFN-β treatment (Fig. 7F). Hence, the expression of METTL16 is a critical factor in reprogramming the TME and can serve as an indicator of the efficacy of tumor immunotherapy.
7. METTL16 is a potential marker of M1 macrophages infiltration
In order to further investigate the impact of METTL16 on TME, we carried an analysis of its expression in immune cells. We identified 22 immune cells that were associated with METTL16 by employing the CIBERSORT algorithm. Our findings indicated that METTL16 expression were predominantly negatively correlated with immunosuppressive cells in pan-cancer. For instance, METTL16 was negatively correlated with M2 macrophages in CESC, CHOL, KICH, KIRP, LAML, LGG, LIHC, PRAD, TGCH, and THCA; METTL16 was negatively correlated with regulatory T cells (Tregs) in BRCA, KIRC, LUAD, PAAD, SKCM, STAD, and THCA; and METTL16 was negatively correlated with Dendritic cells in BLCA, BRCA, CESC, DLBC, LUAD, LUSC, PAAD, UCEC, and UCS (Fig. 8A).
Macrophages are capable of participating in coordinated immunosuppression due to their status as the most prevalent cell type in the TME [61]. So, we concentrated on macrophages and discovered that METTL16 expression was positively correlated with M1 macrophage infiltration in COAD, ESCA, KIRC, SKCM, STAD, and TGCT (Fig. 8B). The METTL16 expression in M1 and M2 macrophages was investigated using the cancer single cell database from TISCH. In contrast to M2 macrophages, METTL16 was predominantly expressed in M1 macrophages, which included CLL, CRC, Glioma, NET, NHL, PAAD, PBMC, SKCM, and UVM (Fig. 8C). The expression of METTL16 in various immune cells in the above tumors is illustrated in Fig. 8D. It follows that METTL16 has the potential to serve as a marker for M1 macrophage infiltration in pan-cancer.
8. METTL16 expression correlates with therapeutic response and its target drugs
In addition, we assessed the impact of METTL16 expression on the efficacy of routine chemotherapeutic agents and immune checkpoint inhibitors in a clinical cancer cohort. We found that in colorectal cancer, responders of Irinotecan had lower levels of METTL16 expression, whereas in Melanoma or pan-cancer, patients who responded to Pembrolizumab had higher levels of METTL16 expression (Fig. 9A). Also, we conducted a search on RNAactDrug to identify drugs that are linked to the expression of METTL16 mRNA. In pan-cancer, Tanespimycin was the only drug with a positive correlation with METTL16 expression among the top 28 drugs with FDR < 0.05. The remaining drugs were negatively correlated with METTL16 expression, and Irinotecan and Topotecan were more sensitive in targeting METTL16 (Fig. 9B). The GI50 of pan-cancer cell lines treated with Irinotecan and Topotecan was compared using the COMPARE tool. The mean -log10(GI50) for Irinotecan was − 4.94, while the mean -log10(GI50) for Topotecan was − 7.51 (Fig. 9C). In order to further investigate the potential of these drugs to bind to the METTL16 protein, we performed molecular docking studies. Figure 9D illustrates the three-dimensional structures of the docking pockets of Irinotecan and Topotecan with the METTL16 protein, which have binding affinities of -10.6 kal/mol and − 8.7 kal/mol, respectively.