The biological function of m6A reader YTHDF2 and its role in human disease

As far as we know, there are over 160 post-transcriptional modifications of RNA, which are characterized among all living organisms and produce a functional diversity that allows four basic ribonucleotide residues to obtain multiple functions [1]. Accumulating researches discovered a variety of RNA modifications in eukaryotic mRNAs, such as pseudouridine (Ψ), 5-methylcytosine (m5C), N1-methyladenosine (m1A) and m6A modifications [2]. m6A modification was first identified in mRNA-enriched RNA fractions in 1974 [3]. It was the methylation of the N6 position of adenosine bases and the most common internal RNA modification in eukaryotic cells [4]. With the application of available methods for detecting m6A, insights into the underlying mechanisms have been disclosed in recent decades. m6A RNA methylation is enriched in 3′untranslated regions (3′UTRs) [5, 6], and functions to modify kinds of RNAs, such as microRNAs (miRNAs) [7, 8], long non-coding RNAs (lncRNAs) [9] and messenger RNAs (mRNAs) [10].

m6A modification was recognized as a dynamic and reversible post-transcriptional modification in mammalian cells and maintained by multi-components, such as methyltransferase (MTase) complex and demethylase [11]. MTase complex referred to “writers”, including methyltransferase-like 3/14/16 (METTL3/14/16), KIAA1429, wilms tumor 1-associated protein (WTAP), RBM15 and RBM15B, which appended m6A sites and significantly influenced specific physiopathological processes [12]. Demethylase referred to as “erasers”, including alkB homolog 5 (ALKBH5) and fat mass and obesity-associated protein (FTO), acted to changeover the methylation [13, 14]. Besides, another group of m6A binding proteins, including YT521-B homology (YTH) domain family, IGF2BP1/2/3 and HNRNPA2B1, were termed as "readers" and functioned to recognize m6A sites [15‐18].

Accumulating evidence showed that YTHDF2, as a member of m6A “readers”, played a significant role in multiple diseases, such as hematopathy and cancers [19‐21]. Besides, significant progress had been made in fully understanding the role of m6A modifications in mRNA decay [22], pre- ribosomal RNA (rRNA) processing [23], and so on. These mechanisms were associated with diverse physiological and pathological processes, such as regulating inflammatory response [24], inducing pluripotent stem cells [25], adipogenesis [19]. Therefore, YTHDF2 might have a far-reaching influence on the development of human diseases, especially cancers.

Herein, we concluded the biological functions and the role of YTHDF2 in diverse diseases, especially cancers (Fig. 1), and hoped to reveal the underlying mechanisms.

The structure of YTHDF2 provided great help in disclosing the specific molecular mechanisms of the recognition of m6A sites. Zhu et al. [26] discovered that YTHDF2 consisted of a C-terminal YTH domain, which targeted the m6A-containing RNA. There were five known YTH domain containing proteins in human genome, including YTHDF1-3 and YTHDC1-2, and YTH domain is highly conserved in these YTH domain-containing proteins [27]. YTH domain of YTHDF2 was a globular fold with four-stranded β-sheets (β1-β4), four α helices (α1-α4) and flanking regions on two sides. Besides, residues K416 and R527 of the above C-terminal YTH domain greatly participated in targeting RNA backbone and residues W432 and W486 within the hydrophobic pocket. All these specific bindings contributed to the specific recognition of m6A. However, Li et al. [28] declaimed in the same year that YTH domain of YTHDF2 showed a globular fold with a central core of eight β strands (β1-β8), three α helices (α1-α3) and two 3₁₀ helices. Additionally, the m6A mononucleotide was positioned in an aromatic cage, which consisted of residue Trp486 in β4-β5 loop, Trp432 in β2 strand and Trp491 in β4-β5 loop (Fig. 2).

Furthermore, YTHDF2 recruited the CCR4–NOT deadenylase complex. The modulation relied on the communication between YTHDF2 and the superfamily homology domain of the CNOT1 subunit, thus inducing the decay of m6A-containing mRNAs [29]. Besides, in vitro and in vivo assays suggested that m6A modification significantly enhanced the phase separation of YTHDF2 [30]. Liquid-like phase separation (LLPS) of YTHDF2 might depend on binding m6A mRNAs .

In the field of cancer research, YTHDF2 had been found to greatly participate in the development of various cancers, including bladder cancer, hepatocellular carcinoma (HCC), gastric cancer, breast cancer, osteosarcoma, cervical cancer, prostate cancer, pancreatic cancer, acute myeloid leukemia (AML) and so on. It was interesting that YTHDF2 was up-regulated or down-regulated in different cancers, and played an oncogenic role or acted as a tumor suppresser (Table 1). Next, we will further discuss the specific role of YTHDF2 in multiple cancers.

In this review, we discussed the specific structure of YTHDF2 and the underlying mechanisms. In addition, YTHDF2 was dysregulated in human cancers, including bladder cancer, HCC, gastric cancer, breast cancer, osteosarcoma, cervical cancer, prostate cancer, pancreatic cancer, acute myeloid leukemia (AML) and so on. Although surgical treatment, chemoradiotherapy, immunotherapy and targeted therapy have significantly improve the prognosis of cancer patients, the underlying mechanisms of cancers remain unclear. This review summarized the current researches and concluded that YTHDF2 played an oncogenic role or acted as a tumor suppresser by regulating tumor cell proliferation, survival, growth, apoptosis, cell cycle, migration, invasion, metastasis, cell viability and so on,. These multiple effects were achieved through diverse pathways, such as collaborating with miRNAs or circRNAs, communicating with METTL3, and recognizing and decaying corresponding mRNA. However, the relevant reasons why YTHDF2 played opposite roles in different cancers were still unclear. Further researches might focus on the upstream molecular regulation mechanisms to explore the causes of YTHDF2 disorders in human cancers. Besides, regarding that YTHDF2 influenced various signaling pathways through recognizing m6A-modified sites of mRNAs, we’d better pay close attention to the most advantageous signaling pathways and get rid of those interfering ones. It was also declared that YTHDF2 was dysregulated in human non-cancers. For example, YTHDF2 took a great part in hematopoietic stem cell self-renewal and differentiation, inducing pluripotent stem cells, regulating adipogenesis, mediating viral gene expression, modulating male and female fertility. Further exploration might also focus on the detailed mechanisms regarding human non-cancers.

In conclusion, YTHDF2 has great potential for clinical application by acting as a novel diagnostic/prognostic biomarker. However, further researches are still needed to expound the specific role of YTHDF2 in human cancers and non-cancers.