The RNA m6A writer WTAP in diseases: structure, roles, and mechanisms

Abstract

N6-methyladenosine (m⁶A) is a widely investigated RNA modification in studies on the "epigenetic regulation" of mRNAs that is ubiquitously present in eukaryotes. Abnormal changes in m⁶A levels are closely related to the regulation of RNA metabolism, heat shock stress, tumor occurrence, and development. m⁶A modifications are catalyzed by the m⁶A writer complex, which contains RNA methyltransferase-like 3 (METTL3), methyltransferase-like 14 (METTL14), Wilms tumor 1-associated protein (WTAP), and other proteins with methyltransferase (MTase) capability, such as RNA-binding motif protein 15 (RBM15), KIAA1429 and zinc finger CCCH-type containing 13 (ZC3H13). Although METTL3 is the main catalytic subunit, WTAP is a regulatory subunit whose function is to recruit the m⁶A methyltransferase complex to the target mRNA. Specifically, WTAP is required for the accumulation of METTL3 and METTL14 in nuclear speckles. In this paper, we briefly introduce the molecular mechanism of m⁶A modification. Then, we focus on WTAP, a component of the m⁶A methyltransferase complex, and introduce its structure, localization, and physiological functions. Finally, we describe its roles and mechanisms in cancer.

Fig. 1. Mechanism of m6A and fuctional domais in m⁶A methyltransferase.

A The dynamic molecular mechanism of m⁶A modification. m⁶A is installed by “writers” (METTL3/14, WTAP, RBM15/15B, VIRMA, and ZC3H13), removed by “erasers” (FTO, ALKBH5, and ALKBH3), and recognized by “readers” (YTHDC1/2, YTHDF1/2/3, IGF2BP1/2/3, HNRNP, and eIF3). B Functional domains in m⁶A writer, eraser, and reader proteins.

Fig. 2. The function of WTAP in cell cycle transition.

In keratinocytes and renal cell carcinoma cells, WTAP enhances the stability of the CDK2 mRNA by directly binding to its 3’-UTR. In human umbilical vein endothelial cells (HUVECs), WTAP stabilizes cyclin-A2 mRNA by binding to its AUUUA motif ACAAAUUAU, which corresponds to the 3ʹ UTR (1526–1534). These findings indicated that WTAP promotes the G1/S transition and the G2/M transition.

Fig. 3

A Model of the mechanism through which WTAP regulates SMC proliferation. The balance between WTAP and WT1 influences the state of SMCs. When the expression of WTAP is reduced, WT1-mediated transcriptional events proceed. Amphiregulin is a direct transcriptional target of WT1 that drives SMC proliferation by upregulating the EGF pathway. Thus, SMCs switch to a proliferative state. When the balance of WTAP and WT1 is reversed, WT1-mediated transcription may be blocked, and the transcription of Bcl-2, which is suppressed by WT1, is activated. SMC apoptosis is increased, and the cells switch to a nonproliferative state. B WTAP in the antiviral immune response. WTAP is degraded in virus-infected cells. After viral infection, degradation of WTAP leads to a decrease in the m⁶A level of IRF3 mRNA and IFNAR1 mRNA, which leads to IRF3 mRNA translation blockade and accelerated IFNAR1 mRNA degradation. This biological process restricts the antiviral immune response and maintains homeostasis.

Fig. 4. The function of WTAP in biological process.

Immunohistochemistry has been performed in many studies. Strong staining for WTAP was observed in grade IV gliomas, renal cell carcinoma, hepatocellular carcinoma, colorectal cancer, and high-grade ovarian carcinoma, with low staining in adjacent normal tissues.

Fig. 5. WTAP serves as a methyltransferase in cancers.

WTAP plays a significant role in RNA methylation by recruiting METTL3/METTL14 to form a complex that binds to target RNAs. In this process, WTAP regulates the differential expression of oncogenes and tumor suppressor genes in an m⁶A-dependent manner. It enhances the stability of the HK2 and DUSP6 mRNAs, inducing drug resistance in hepatocellular carcinoma, gastric cancer, and NKTCL. Additionally, WTAP induces the degradation of the ETS1, HMBOX1, and c-Myc mRNAs in an m⁶A-dependent manner, enhancing HCC proliferation and suppressing the invasion and metastasis of osteosarcoma and acute myeloid leukemia.

Fig. 6. Other functions of WTAP in cancers.

WTAP regulates the differential expression of oncogenes and tumor suppressor genes at the non-posttranscriptional level. WTAP induces the expression of Muc1, which regulates EGFR activity in cholangiocarcinoma. Hsp90 forms a complex with WTAP and stabilizes its protein level to promote chemoresistance in AML. In DLBCL, Hsp90 also stabilizes the WTAP protein, which forms a complex with BCL6. In colorectal cancer, CA4 interacts with WTAP and promotes its degradation in a polyubiquitination-dependent manner so that WT1 is released from the WT1-WTAP complex, resulting in the induction of transducin β-like protein 1 (TBL1) and the degradation of β-catenin, which blocks the Wnt pathway. WTAP was found to facilitate the nuclear translocation of β-catenin and enhance the phosphorylation of GSK3b at Ser9, which induced chemoresistance to cisplatin in endometrial carcinoma by activating the Wnt/β-catenin pathway. Additionally, WTAP was found to regulate the expression of the EMT-related proteins E-cadherin and vimentin. Furthermore, WTAP is involved in the activation of the AKT and MAPK pathways. Overall, WTAP contributes to cell proliferation, apoptosis, invasion, metastasis, and chemo- or radioresistance in different cancers.