Roles of N6‐methyladenosine (m6A) RNA modifications in urological cancers

Abstract Epigenetics has long been a hot topic in the field of scientific research. The scope of epigenetics usually includes chromatin remodelling, DNA methylation, histone modifications, non‐coding RNAs and RNA modifications. In recent years, RNA modifications have emerged as important regulators in a variety of physiological processes and in disease progression, especially in human cancers. Among the various RNA modifications, m6A is the most common. The function of m6A modifications is mainly regulated by 3 types of proteins: m6A methyltransferases (writers), m6A demethylases (erasers) and m6A‐binding proteins (readers). In this review, we focus on RNA m6A modification and its relationship with urological cancers, particularly focusing on its roles and potential clinical applications.


| ROLE S OF RNA m 6 A MOD IFI C ATI ON IN UROLOG IC AL C AN CER S
Emerging evidence shows that RNA m 6 A methylation is closely associated with the progression of urological cancers, including their carcinogenesis, proliferation and metastasis. Herein, we briefly review recent studies of m 6 A methylation in urological cancers (Table 1).

| Prostate cancer
Prostate cancer is one of the most commonly diagnosed cancers worldwide and is especially prevalent in developed countries. 45 In our previous study, we found that the expression of YTHDF2 in prostate cancer was up-regulated, and the increased expression of YTHDF2 was related to prostate cancer proliferation and metastasis. 29 The up-regulation of YTHDF2 in prostate cancer was possibly contributed to the regulation of miR-493-3p, which increased the m 6

| Bladder cancer
Bladder cancer is one of most commonly diagnosed cancers worldwide, especially in men. 46 Both the incidence and mortality of bladder cancer have increased rapidly in recent years. 47 The m 6 A methyltransferase METTL3 seems to play important roles in the car- our previous study, we also found that the expression of METTL3 and YTHDF2 was up-regulated in bladder cancer and showed that METTL3 inhibited the expression of SETD7 and KLF4 in an m 6 A-YTHDF2-dependent manner to further promote the proliferation and metastasis of bladder cancer. 32 In addition, a nine-gene panel that included METTL3 was identified as a prognostic indicator for the recurrence of muscle invasive bladder cancer. 33  regulates pri-miR221/222 processing in an m 6 A-dependent manner to promote carcinogenesis. 36 In addition, a high expression pattern of METTL3 is demonstrated in bladder cancer via bioinformatic analysis. 37 Therefore, METTL3 serves as an oncogene in the carcinogenesis of bladder cancer ( Figure 2).

| Renal cell carcinoma
The incidence of renal cell carcinoma is still increasing rapidly worldwide. 51 Although mortality is decreasing in developed countries, it Interestingly, the down-regulated expression of ALKBH5 and FTO was found to be related to poor overall survival and cancer-specific survival in renal cell carcinoma, which implied that ALKBH5 and FTO could serve as potential prognostic biomarkers. 43 In another study, the expression of FTO was also found to be down-regulated in renal cell carcinoma, and FTO suppressed carcinogenesis of renal cell carcinoma via the FTO-PGC-1α signalling axis. 44 To determine the potential downstream targets of m 6

| m 6 A modifications as therapeutic targets
The crucial roles of m 6  However, in our ongoing research, we found that the m 6 A methyltransferase WTAP and the m 6 A demethylase ALKBH5 act as oncogenes in renal cell carcinoma. Therefore, due to the pathological diversity of renal cell carcinoma, the actual role of m 6 A regulators in it and its subtypes needs further elucidation.
PD-1/PD-L1-related immunotherapy has proven to be effective in many types of tumours. 64 Recent studies illustrated that PD-1/PD-L1-related immunotherapy was effective in urological cancers. [65][66][67][68] FTO is demonstrated to promote carcinogenesis and anti-PD-1 resistance in melanoma, suggesting that FTO could be a potential therapeutic target in immunotherapy, 69  cancer, indicating the potential role of METTL3 in chemo-and radiotherapy resistance. 72 In addition, FTO was found to enhance chemoradiotherapy resistance in cervical squamous cell carcinoma via β-catenin. 73 As we mentioned before, both METTL3 and FTO play crucial roles in carcinogenesis in both bladder cancer and renal cell carcinoma. Taken together, these findings suggest that m 6 A regulators could be potential therapeutic targets for patients receiving chemoand radiotherapy.

| miRNAs and m 6 A modifications
In addition to the functions of m 6 A modifications in mRNA, recent studies have shown that m 6 A modifications also have roles in regulating non-coding RNAs, especially microRNAs (miRNAs). 24,[48][49][50] miRNAs are mainly processed by the microprocessor complex, which includes RNA-binding protein DGCR8 and ribonuclease type III DROSHA. 74 Alarcón et al 48 found that METTL3 methylated pri-miRNAs, facilitating their recognition and processing by DGCR8, indicating that the m 6 A regulator could possibly be a key factor in the initiation of miRNA biogenesis. They further identified that HNRNPA2B1 bound to m 6 A-modified sites in a group of pri-miRNAs, interacted with DGCR8, and promoted the maturation of pri-miRNAs. 24 In addition, METTL14 was identified to interact with DGCR8 and regulate the pri-miR-126 mature process in an m 6 A-dependent manner in hepatocellular carcinoma. 75 Pri-miR-25 in pancreatic cancer could be matured by smoking via enhanced m 6 A modification, which was catalysed by METTL3. 76 A similar mechanism was found in bladder cancer. METTL3 interacted with DGCR8 and regulated the pri-miR-221/222 maturation process. 36 Taken together, considering the crucial roles of m 6 A regulators and miRNAs in urological cancers and the potential regulatory mechanisms between m 6 A regulators and miRNAs, miRNAs and microprocessor proteins (such as DGCR8) could be potential therapeutic targets in urological cancers. However, future studies are needed to further clarify the underlying mechanisms.

| Alternative splicing and m 6 A modifications
Alternative splicing is the process of generating numerous mRNA variants from a single gene transcript, leading to proteome complexity and diversification. 77 Alternative splicing exists in almost 95% of human genes and exerts functions in many biological aspects, including chromatin modification, signal transduction and carcinogenesis. 78 In addition to cis-regulatory elements, trans-acting splicing factors including m 6 A regulators play critical roles in the alternative splicing process. 9,79 Previous studies indicated that dysregulation of m 6 A regulators drastically affects the process of alternative splicing.
METTL3 regulated MyD88 alternative splicing on the lipopolysaccharide-induced inflammatory response. 80

| Brief summary
The regulatory functions of m 6  Additionally, some m 6 A modification-based enzyme inhibitors have demonstrated potential effects on cancer; however, there is still a long way to go before m 6 A-based cancer therapy can be applied in the clinic. 88

| CON CLUS IONS
Epigenetics has long been a hot topic in the field of scientific research. 89 RNA m 6 A modifications are of great importance in urological cancers and serve as diagnostic and prognostic biomarkers that regulate carcinogenesis and metastasis, indicating their potential as therapeutic targets. However, further studies are still necessary to elucidate the underlying mechanisms in urological cancers so that these findings can be translated from bench to bedside in the future.

CO N FLI C T O F I NTE R E S T
The authors confirm that there are no conflicts of interest.