N7-methylguanosine (m7G) modification in breast cancer: clinical significances and molecular mechanisms

Abstract

The therapeutic strategies for advanced breast cancer (BC) continue to present significant challenges. Consequently, the implementation of precise diagnostic biomarkers and prognostic targets is essential for effective BC management. Recently, N7-methylguanosine (m7G) modification has garnered considerable attention in the context of various cancer types. In this study, we conducted a comprehensive literature review to explore the potential role of m7G in the tumorigenesis of BC. Analysis of thirteen relevant studies revealed that m7G methyltransferases were usually aberrantly expressed in BC, including TNBC and breast invasive carcinoma. m7G modifications in mRNA, tRNA, and rRNA can ultimately affect the expression of target genes (i.e., m7G regulators [e.g., METTL1/WDR4], m7G-associated genes [e.g. P27 and AGO2], m7G-related lncRNAs [e.g., LINC01871 and LINC00115], and m7G-related miRNAs [e.g. miR-7 and miR-139]) and regulate BC-related biological functions. These novel insights indicate that m7G modification and its regulators hold significant potential for future clinical applications in the diagnosis and treatment of BC. In the future, how to apply m7G modifications to identify the implementation of clinically personalized BC treatment needs to be further explored.

Introduction

Breast cancer (BC) is an important health issue in women. Based on the findings of the Cancer Statistics 2023, BC is the predominant form of malignancy among women, constituting 31% of newly diagnosed female cancers [1]. Estimates suggest breast cancer incidence is projected to reach 297,790 new cases, which would mark a significant increase in lung and bronchus cancer cases (120,790) by more than double [1]. Additionally, BC ranks second in terms of mortality rates among women, accounting for approximately 15% of estimated cancer-related deaths (43,170 cases) [1]. Furthermore, it is anticipated that the annual number of new BC cases will exceed 3 million and the deaths will exceed 1 million by the year 2040 [2]. As reported, estrogen receptor (ER), prolactin (PR), and HER2 (ERBB2) are the key factors in the determination of the BC subtype. HER2 positive, Lumina-A, Lumina-B, and triple-negative BC (TNBC) are the common primary BC subtypes [3]. Currently, treatments for BC mainly depend on surgery, chemotherapy, radiotherapy, endocrine-therapy, and immunological therapy. The choice of these management can mainly be attributed to the tumor types, tumor stage, and the overall health status of the sufferers [4]. The survival rate of BC patients is greatly increased by these treatments, while a high mortality rate persists, especially in those with TNBC, which has a poor prognosis when it progresses into a more invasive form (TNBC has the poorest prognosis than any other types of BC) [5]. The World Health Organization (WHO) has highlighted that early detection is critical for enhancing patient outcomes and survival in BC [6]. Consequently, the integration of suitable diagnostic biomarkers and prognostic indicators into BC management—particularly for TNBC—is of significant importance.

Cancer cell development and progression are deeply intertwined with alterations in both genetic and epigenetic factors. There has been a demonstration of a vital association between RNA modification and cancer development [7]. Dynamic RNA modification pathways have been found to be dysregulated in multiple malignancies [8]. The common types of RNA modifications are methylation, cytosine modification, uridine isomerization, and ribose modification [9]. N6-methyladenosine (m6A), a common RNA modification found in eukaryotic cells, has been detected to participate in the tumorigenesis, progression, metastasis, and drug resistance of various tumors [10]. Besides, 5-methylcytosine (m5C) and N1 methyladenosine (m1A) are also common types of RNA modifications, playing key roles in the development of multiple cancers [11]. The recent development of bioinformatics technologies (i.e., high-throughput sequencing and RNA methylation profiling) have uncovered that N7-methylguanosine (m7G) modification partially accounts for the RNA modifications [12]. Recent research has brought attention to the substantial role played by m7G in multiple aspects of the cellular process, i.e., RNA transcription, RNA processing, degradation of RNA, and RNA translation [13]. In order to achieve optimal gene expression in eukaryotes, it is imperative to attach a m7G cap at the 5'end of mRNA. This cap is composed of an inverted m7G moiety associated with the initial nucleotide of RNA polymerase (Pol II) transcripts. The m7G modification pattern is a prevalent epigenetic alteration observed in transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs) [14]. tRNA modification is considered to be the predominant form of m7G modification, which plays a crucial role in cancer development and progression by modulating the metabolism of diverse RNA molecules. Besides, tRNA can also modulate the expression level of both oncogenes and anti-oncogenes [15]. In mammals, the complex combined with WD repeat domain 4 (WDR4) and methyltransferase-like 1 (METTL1) is a key mediator in the regulatory process of m7G modification [16]. This complex exerts its influence on RNA function and contributes to the manifestation of human cancers through various molecular mechanisms. It has been found that the METTL1/WDR4 pathway promotes cancer progression by modifying tRNA [17].

In recent studies, growing evidence highlights that m7G RNA modification, along with its regulatory factors and related genes, are pivotal in BC carcinogenesis and tumor progression. This review aims to systematically summarize and clarify the functional roles of m7G in the development of BC, providing a comprehensive overview of its involvement in this disease context.

Data sources and search strategy

Conventional databases were properly accessed to find the relevant studies related to this topic. The search encompassed the period from the establishment of these databases to April 15, 2025, and was limited to studies published in English. The search terms employed in the MEDLINE database were ‘‘Breast Neoplasms’’ [Mesh], “Breast Cancer”, “N7-methylguanosine”, and “m7G”. A manual examination of the reference lists was conducted to identify additional eligible studies. The characteristics of the relevant studies are presented in Table 1.

Table 1.

Role of m7G and its associated genes in the development of breast cancer

Study/References	m7G regulators or associated genes	Expression	Molecular axis	Function	Main findings of the included study
Eto et al. [64]	P27	NA	P27/cell cycle genes	Promoting cell proliferation	Anti-cancer agents inhibited the proliferation of the breast cancer cells by up-regulating the expression of p27, thus inhibiting G1-to-S phase transition of the cancer cells
Cao et al. [50]	m7G-related LncRNAs	Abnormal expression of 7 LncRNAs	NA	Prediction of the survival	Seven m7G associated LncRNAs (EGOT, TFAP2A-AS1, Z68871.1, LINC01871, AC245297.3, SEMA3B-AS1, and AP003469.4) were found to be independently associated with the overall survival of BC patients
Huang et al. [51]	m7G-related LncRNAs	Abnormal expression of 8 LncRNAs	NA	Diagnostic biomarkers	Eight m7G associated LncRNAs (NDUFA6-DT, TFAP2AAS1, FARP1-AS1, RERE-AS1, LINC00115, MIR302CHG, BAIAP2-DT, and COL4A2-AS1) were found to be applied as the diagnostic biomarkers of BC patients
Huang et al. [70]	m7G regulator genes	Highly expressed	NA	Oncogenes	Based on the available data from the TCGA database, m7G was found to serve as a novel innovative biomarker in predicting clinical outcomes and immunotherapeutic efficacy of BC (prediction of the prognosis of BC patients with an AUC of 0.81)
Xu et al. [35]	METTL1/WDR4 and m7G-related microRNAs	Abnormal expression	METTL1/WDR4/m7G-related miRNAs/targeted genes	Prediction of the survival and the expression of immune checkpoints	Six m7G-related miRNA signatures could significantly predict the overall survival and immune landscape of triple-negative BC
Zhang et al. [58]	m7G-related microRNAs and RNMT/FAM103A1	Differentially expressed	m7G-related miRNAs/RNMT/FAM103A1	Prediction of the prognosis and immune infiltration	A total of 12 m7G methyltransferase-related miRNAs were used to establish the prognostic model which showed a good accuracy for predicting the survival rates of BC patients (the AUC were almost > 0.7)
Huang et al. [72]	AGO2, EIF4E, DPCS, and EIF4E3	Differentially expressed	NA	Prediction of the prognosis, TMB, and the response to chemotherapy	Four m7G related genes (AGO2, EIF4E, DPCS, and EIF4E3) significant associated with the TIME of BC. The four prognostic biomarkers could be used for predicting the prognosis and the candidate drugs related to BC
Li TJ et al. [37]	METTL1 and WDR4	Overexpression	METTL1/WDR4	Prediction of BC prognosis and treatment responses	High levels of METTL1 and WDR4 were associated with poor prognosis and immunotherapeutic responses in BC patient. A positive association was found between m7G modification tumor microenvironment
Li JR et al. [39]	NCBP1 and WDR4	Differentially expressed	NA	Prediction of the OS of BC patients	Five differentially expressed m7G-related genes (CYFIP1, EIF4E, EIF4E3, NCBP1, and WDR4) were associated with OS. NCBP1 was an important target of m7G. Knockdown of NCBP1 suppressed BC malignancy
Li AY et al. [60]	m7G-related miRNAs	Differentially expressed	NA	Prognostic validity	Four m7G-related miRNAs (miR-7, miR-139, miR-10b, and miR-4728) were most prominent genes in the prognostic signature of BC. Among BRCA genes, CD52 showed the greatest positive correlation with immune cells and functions
Zhang XQ et al. [69]	TCP1 and ANKRD36	Differentially expressed	RBM15B/TCP1/ANKRD36	Prognosis prediction	RMW_Score might be a key biomarker for predicting the survival and therapeutic benefits of BC patients. TCP1 could promote the proliferation and migration of BC alisertib-resistant cells
Dai et al. 2024	m7G-related miRNAs	Differentially expressed	NA	Prognostic validity	Fifteen m7G-related miRNAs were independently associated with OS in BC patients. Time-dependent ROC analysis yielded AUC values over 0.7 for predicting the survival rates. The risk score based on m7G-related miRNAs can serve as a significant independent prognostic factor for BC
Luo et al. [42]	WDR4	Overexpression	WDR4/m7G modification/mTORC1	Oncogenes	The inhibition of WDR4 led to a significant impairment in breast cancer progression both in vitro and in vivo. Additionally, WDR4 played a crucial role in regulating the cell cycle and the mTORC1 signaling pathway, primarily through the modification of m7G
Du et al. [43]	METTL1	Down-regulation	METTL1-mediated tRNA m7G/GADD45A and RB1	Prognostic factor; Tumor suppressor	METTL1/WDR4 were down-regulated in BC tissues; METTL1 functionally suppressed BC cell proliferation and cell cycle progression. It elevated the m7G modifications of 19 tRNAs, influencing the translation of growth arrest and DNA damage 45 alpha (GADD45A) and retinoblastoma protein 1 (RB1) in a codon-dependent fashion linked to m7G. METTL1 overexpression potentiated the anti-tumor effects of Abemaciclib

TIME Tumor immune microenvironment, TMB tumor mutational burden, RMW_Score RNA methylation modification “writers” and related genes_risk score, RNMT RNA guanine-7 methyltransferase

Overview of m7G modification and its cross-talk with RNAs

m7G, a prevalent and evolutionarily conserved RNA modification initially identified at the 5’ cap of mRNAs, is among the most frequently occurring base modifications in post-transcriptional regulation. This modification exerts regulatory control over the transcription of mRNA, tRNA stability, the biosynthesis of microRNAs, and rRNA maturation [18]. Furthermore, m7G methylation is reported to be closely linked to a diverse range of biological processes [19]. In conjunction with its associated regulatory factors, the m7G modification is pivotal to maintaining the equilibrium of human physiological activities. According to comprehensive analyses, the m7G modification is prevalent in mRNAs, ranging from 0.02% to 0.05% of all guanosines present in mRNAs across multiple cell lines of human [16]. Thus far, numerous experimental studies have identified over 44,000 m7G methylation sites within the internal regions of mRNA molecules in mammalian transcriptomes [20]. Furthermore, there is growing evidence suggesting that upwards of 1,200 disease-related genetic variants could mediate their impacts via altering m7G methylation patterns [21]. m7GHub is a comprehensive online platform designed to analyze the location, regulation, and pathogenic roles of internal m7G modifications in mRNA [20]. It comprises four main components, such as m7GDB, m7GDiseaseDB, m7GFinder, and m7GSNPer. This platform facilitates the exploration of m7G modification distribution, function, and its association with diseases, advancing the field of epitranscriptomics [22]. It is freely accessible at: www.rnamd.org/m7GHub2.

tRNA, an important molecule, has been found to facilitate in deciphering the genetic code during mRNA translation. Conventional wisdom purports that tRNAs are stably expressed in all cells. However, later evidence suggests that gene expression of tRNAs is tissue-specific and cell-type-specific [23, 24]. In mammals, the METTL1/WDR4 complex is an important factor for modulating m7G tRNA modification, which is essentially designed to be expressed correctly under normal growth conditions. Inhibition of METTLl results in a declination in the abundance of m7G-modified tRNAs, alteration of the cell cycle, and repression of oncogenesis. In contrast, upregulated METTLl expression contributes to neoplastic cell transformation and promotes the expression of oncogenic processes [25]. Mutations in the WDR4 gene result in impaired tRNA m7G modification, reducing the expression of multiple genes associated to biological functions and leading to various diseases. In addition, m7G methylation is also prevalent in rRNA in a conserved form [26]. WBSCR22 and TRMT112 have been found to be participated in the m7G modification of rRNA [27]. m7G modification is also processed and developd in microRNAs (miRNAs), but the interacting mechanism is not fully understood [28]. Although there have been many studies on the relationship between m7G modifications and different biological processes, the mechanisms of their interactions are not yet fully clear. In addition, despite the large number of m7G modification sites identified in the mammalian transcriptome, systematic and genome-wide studies are still lacking, especially with regard to variability across tissues and cell types. We listed the evidence on the topic of m7G modifications in the tumorigenesis of BC in the following sections.

m7G modifications-associated proteins

It was found that METTL1/WDR4 can stabilize tRNA tertiary structures by modulating m7G modifications. In mammals, studies have identified METTL1 as the most well-known m7G-related protein, which associates with the accessory factor WDR4 to mediate modifications in multiple RNA species including tRNAs, miRNAs, and mRNAs. Proper functioning of mRNA translation and genealogical differentiation requires the presence of the METTL1/WDR4 methyltransferase complex [29]. Mutations in the WDR4 gene can lead to impaired tRNAm7G modification, reducing the expression of multiple genes associated with biological functions. In addition, miRNAs also undergo m7G modifications via modulating the METTL1/WDR4 complex, functioning to promote miRNA formation [28]. METTL1 serves as a methyltransferase of m7G, promoting m7G modification in targeted-mRNAs, whereas WDR4 enhances the formation of heterodimeric complexes to target mRNAs [16].

Another common m7G regulator in humans is the WBSCR22/TRMT112 complex, which facilitates m7G modification at eukaryotic 18S rRNA. WBSCR22, in conjunction with its metabolic stabilizer TRMT112, is involved in 18S rRNA precursor processing. WBSCR22 is independent of the catalytic activity of the precursor subunit of 40S rRNA, which is required only to provide efficient nuclear export to promote ribosome biogenesis [30]. Consequently, the m7G regulatory factor may serve as a quality control mechanism during ribosome biogenesis. The RNA guanine-7 methyltransferase (RNMT) enzyme, in conjunction with the RNMT-activating mini-protein (RAM) complex, mediates the addition of m7G caps during the early stages of eukaryotic transcription [31]. RAM contributes to maintaining RNMT structural stability and recruiting target RNAs, which then undergo m7G modification to form the 5’ m7GpppX cap structure. This modification not only shields the RNA from degradation by nucleic acid exonucleases but also influences RNA processing, nuclear export, and translational efficiency [32]. Taken together, the m7G modification combined with its methyltransferase complexes (i.e., METTL1/WDR4) play essential roles in various biological activities of humans, including oncogenesis, development, and progression of cancer cells. Figure 1 illustrates the possible molecular mechanisms by which m7G modification influences cancer cell proliferation and differentiation through targeting miRNA or tRNA molecules. m7G modification is involved in both the processing of primary microRNA (pri-miRNA) into mature miRNA and the degradation of mRNA through structural regulation of cellular components. Also, tRNA can be methylated by m7G-mediated METTL1/WDR4 complex, causing the reduction of ribosome pausing, which are translated into the regulatory targeted proteins. However, although studies have revealed the role of the METTL1/WDR4 complex in multiple cancers, including BC, its specific mechanism is still not fully understood. Future studies should further explore the role of METTL1/WDR4 in different types of tumors, especially in the mechanisms of immune escape and drug resistance.

Fig. 1.

Mechanisms of m7G modification-mediated miRNA and tRNA affecting tumorigenesis. m7G modification-associated miRNA and tRNA involve in the proliferation and differentiation of cancer cells. m7G modification plays roles in the process of pri-miRNA into a mature miRNA as well as the mRNA degradation process by regulating cellular structure. Also, tRNA can be methylated by m7G-mediated METTL1/WDR4 complex, causing the reduction of ribosome pausing, which are translated into the regulatory targeted proteins

Within the topic of this review, we focus on the progress of m7G modification research in tumorigenesis and the progression of BC. Analysis of 13 relevant studies revealed that the underlying molecular pathways connecting m7G modification and BC progression are primarily mediated by m7G regulators (i.e., P27, METTL1/WDR4, RNMT/FAM103A1, AGO2, EIF4E, DPCS, EIF4E3, NCBP1, TCP1, and ANKRD36), m7G-related long non-coding RNAs (lncRNAs), and m7G-related miRNAs. Here are the molecular biological mechanisms derived from the published studies reporting the potential biological functioning of m7G modification in the development of BC.

METTL1/WDR4

As aforementioned, the METTL1/WDR4 complex plays an important role in modifying tRNAs, mRNAs, and miRNAs with m7G [33]. WDR4 functions as a key scaffold for METTL1 and tRNA T-arm, thus regulating m7G methylation. The WDR4 scaffold facilitates METTL1 binding to the T-arm of tRNA, activating m7G methylation [34]. Three included studies reported the roles of METTL1/WDR4 complex in BC development. Xu et al. [35] performed computational analysis on RNA-seq data derived from the TCGA (The Cancer Genome Atlas) database. With the help of the establishment of a miRNA risk model, they found that six m7G-related miRNA signatures had a predictive value of the overall survival and immune landscape of TNBC. Among these miRNAs, miR-4505 was the most significant factor for predicting the survival of TNBC with a hazard ratio (HR) of 5.4048 (95%CI: 1.9407 to 15.0528, P = 0.0012). The authors observed that METTL1 and WDR4 were overexpressed and interplayed in TNBC (correlation, r = 0.36, P < 0.001). The areas under the curves (AUCs) values for 1-, 3-, and 5-year overall survival in TNBC patients all exceeded approximately 0.7. In the multivariate Cox analysis, it was suggested that stage N (P < 0.001) combined with the risk score (P = 0.0184) were the independent factors for predicting the OS of TNBC. Finally, this study revealed a strong correlation between tumor-infiltrating lymphocytes (TILs) and regulatory T cells (Tregs) with the m7G miRNA-based risk model (all P < 0.01). Four immune checkpoint molecules (CD28, ICOS, CTLA-4, and TNFRSF9) have been identified as critical targets for immunotherapy in TNBC patients with poor survival outcomes. This integrative computational and statistical analysis revealed that m7G-related miRNAs associated with prognosis could offer novel insights into the diagnostic and prognostic assessment of TNBC.

The tumor microenvironment (TME) and immune regulation are the important factors that influence the efficiency of BC immunotherapy [36]. Li TJ et al. [37] implied that m7G modification was remarkably linked to the TME of BC. They demonstrated that a strong association was observed between METTL1/WDR4 upregulation and poor prognosis, as well as impaired immunotherapy response, in BC cohorts. Further, tumor cells in the high-m7G regulator-related genes (MGRRGs)-score group had elevated proliferative, invasive, and angiogenic abilities than the low-MGRRGs score group. The authors also reported that BC patients verified to have high m7G levels and low TME scores had survival times that were shorter than those of any other groups. It was found that low TME scores favor aggressive cancer cells, inhibit antitumor immunity, and facilitate tumor progression [38]. For predicting the therapeutic efficiency of immune checkpoint inhibitors (ICIs), the authors found that the m7G-TME classifier might be a promising tool in assessing patient’s responses to ICI treatments. This study demonstrated that the integration of m7G modifications enhanced prognostication and treatment response prediction in BC. However, we should acknowledge that the m7G-TME signature of Li et al.’s study was not verified by BC tumor samples (biopsies) and a large ICI treatment cohort.

To detect the m7G-regulated genes linking the prognosis of BC samples, Li JR et al. [39] analyzed 29 m7G-related genes from the RNA sequence data in the TCGA database. After establishing the prognosis of m7G-regulated gene signature and performing the functional enrichment and tumor mutation burden analyses, 5 differentially expressed m7G-related genes (CYFIP1, NCBP1, EIF4E3, EIF4E, and WDR4) might be correlated to the OS of BC patients. Specifically, NCBP1 is associated with an unfavorable prognosis and exhibits high diagnostic significance. The authors further found that the main function of NCPB1 in BC is to enhance neuroactive interactions between ligands and receptors as well as transduction. The level of NCBP1 was found to be correlated to the reprogramming of the immunosuppressive microenvironment in BC. Since NCBP1 was an important target of m7G, Li et al. [39] constructed a cell line model with knocked-down NCBP1 by transfecting siRNA. The results demonstrated that reducing the expression of NCBP1 (knockdown) impeded the malignancy of BC. Specifically, it led to the suppression of the proliferation, migratory, and invasive properties of BC cell lines. Aberrant expression of NCBP1 was found in various types of cancers (i.e., lung cancer and lymphoma) [40, 41]. In this study, Li et al. firstly identified the potential role of NCBP1 in BC development. In summary, this study offered a new insight into BC prognosis prediction by specific m7G-related genes, especially for the NCBP1 gene. Recently, Luo et al. [42] demonstrated that the inhibition of WDR4 led to a significant impairment in breast cancer progression both in vitro and in vivo. Additionally, WDR4 played a crucial role in regulating the cell cycle and the mTORC1 signaling pathway, primarily through the modification of m7G. However, METTL1/WDR4 has also been found to serve as a tumor suppressor in BC. For example, Du et al. [43] conducted their research through both a clinical study and an experimental research. They found that METTL1/WDR4 were down-regulated in BC tissues. METTL1 functionally suppressed BC cell proliferation and cell cycle progression. It elevated the m7G modifications of 19 tRNAs, influencing the translation of growth arrest and DNA damage 45 alpha (GADD45A) and retinoblastoma protein 1 (RB1) in a codon-dependent fashion linked to m7G. METTL1 overexpression potentiated the anti-tumor effects of Abemaciclib, a CDK4/6 inhibitor. The above recent relevant studies indicated that METTL1/WDR4-associated m7G exhibited dual functions, acting as both an oncogene and a tumor suppressor depending on the cellular context or specific conditions.

m7G-related lncRNAs

There has been a substantial of literature demonstrating the crucial roles of lncRNAs in BC development, including the occurrence, cell growth, migration, recurrence, metastasis, and chemotherapy resistance [44, 45]. Recently, mounting studies implied that m7G-related LncRNAs could predict the tumorigenesis of multiple malignancies, i.e., clear cell renal carcinoma [46], hepatocellular carcinoma [47], ovarian cancer [48], and gastric cancer [49]. In this review, three relevant studies provided data on the effects of m7G-related LncRNAs in BC. Cao et al. [50] reported that seven m7G associated LncRNAs (EGOT, TFAP2A-AS1, Z68871.1, LINC01871, AC245297.3, SEMA3B-AS1, and AP003469.4) were independently correlated to the OS of the patients with BC. Using data from the TCGA database, through co—expression network construction and Cox regression analysis, the area under the curve (AUC) values for predicting 1—year, 3—year, and 5—year survival rates all exceeded 0.7. The ssGSEA study results demonstrated that BC patients having low—risk m7G—related LncRNAs responded better to PD1/PD—L1 immunotherapy than the patients with high—risk m7G—related LncRNAs. By analyzing the co-expression network of m7G-associated genes and lncRNAs, Cao et al. [50] identified seven m7G-related lncRNAs that could be applied for predicting the prognostic outcomes in BC patients. However, this study only applies the TCGA database for internal confirmation, which needs further investigation.

In line with Cao et al.’s study, Huang et al. [51] demonstrated that eight m7G-associated LncRNAs (NDUFA6-DT, TFAP2AAS1, FARP1-AS1, RERE-AS1, LINC00115, MIR302CHG, BAIAP2-DT, and COL4A2-AS1) were found to be served as the potential diagnostic biomarkers of BC patients. An eight—m7G—related lncRNA—based predictive model was employed to construct a prognostic signature for BC. Metabolism-associated pathways and immune infiltration of the tumor cells were suggested to be the underlying mechanisms for the role of m7G-related lncRNAs in BC development. Results from the expression profile of the eight m7G-related lncRNAs by qRT-PCR examination showed that five lncRNAs were up-regulated and one lncRNA was down-regulated in BC cell line MCF-7. Of note, the eight m7G-related lncRNAs identified in Huang et al.’s study were not the same as those of Cao et al.’s study. Similarly, Huang et al. also failed to verify their findings in other data sets.

In humans, four primary types of RNA methylation modifications exist. These include N6—methyladenosine (m6A), 5—methylcytosine (m5C), N1—methyladenosine (m1A), and m7G [11]. At present, few studies have investigated the signature of four RNA methylation modifications to predict the prognosis of cancers. In this section, several included studies have constructed prognostic models based on m7G-associated lncRNAs through the TCGA database and found them to be potentially valuable in predicting patient survival, immunotherapy response and tumor mutational load. However, although a few studies have constructed prognostic models, most have only used the TCGA database for internal validation, and the lack of independent validation against external datasets may affect the ability of the models to generalize. Also, there are fewer studies on the specific functional mechanisms of m7G-associated lncRNAs in breast cancer, and their roles in the mechanisms of tumorigenesis, metastasis, and drug resistance have not yet been explored in depth. Therefore, future relevant studies of the specific functions of m7G-related lncRNAs may provide a theoretical basis for their use as therapeutic targets in BC.

m7G-related miRNAs

Numerous studies have demonstrated that microRNAs assume a crucial part in the process of carcinogenesis across a wide range of cancers [52]. In recent years, research has revealed that miRNAs associated with m7G can effectively forecast the prognosis and the efficacy of anti-tumor treatments in numerous types of malignancies, including hepatocellular carcinoma [53], lung adenocarcinoma [54], and renal cancer [55]. Mechanistically, m7G-related miRNAs exert their roles in cancers by affecting tumor microenvironment, immune cell infiltration, and immunity-linked pathways. There were three included studies that reported the roles of m7G-related miRNAs in BC. Xu et al. [35] identified six m7G-related miRNA signatures (i.e., miR-421, miR-1915-3p, miR-3177-5p, miR-4326, miR-4505, and miR-5001-3p) through the RNA sequencing from TCGA database. The researchers utilized the five miRNAs to build a risk assessment model in order to evaluate its prognostic significance for patients with BC. They found that Treg and TIL were associated with the risk model, while the high expression of immune checkpoints (i.e., TNFRSF9, CD28, ICOS, and CTLA-4) was identified in the high-risk group. This study demonstrated that m7G-associated miRNAs may act as potential prognostic biomarkers for BC patients.

RNMT, one of the m7G cap methyltransferases, is an important mediator of T-cell activation. T-cell receptor stimulation induces RNMT, which can specifically regulate the production of ribosome and ribosome biogenesis by regulating mRNA, small nucleolar RNA, and rRNA production [56]. The RAM/FAM103A1 protein is a crucial component of the mRNA cap methylation machinery, which functions to maintain mRNA translation and cell viability [57]. Zhang et al. [58] detected a total of 12 miRNAs related to m7G methyltransferase in the TCGA database, including miR-2115-5p, miR-4675, miR-4501, and miR-556-3p, which were used to establish the prognostic model which presented a remarkable degree of accuracy in predicting the survival rates among BC patients (the AUC were almost > 0.7). The authors also found that IGLV1-36 (m7G-related mRNA) was positively associated with good prognosis and immune infiltration of BC patients. GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway enrichment analyses revealed that genes associated with m7G were concentrated in the epidermal growth process and the signaling cascades of neuroactive ligand-receptor interaction.

Breast invasive carcinoma (BRCA) is a subtype of BC, posing a substantial risk to the sufferer’s health for their poor prognosis [59]. Therefore, exploring the molecular mechanisms of the tumorigenesis of BRCA is warranted. By analyzing the related data from the TCGA database, Li AY et al. [60] implied that four m7G-related miRNAs (i.e., miR-7, miR-139, miR-4728, and miR-10b) were the most dominant genes in the prognostic signature of BRCA. The ROC-AUC was 0.737, 0.724, and 0.702 for 1-, 3-, and 5-year survivals in BC patients. According to the Kaplan—Meier survival plot, the low-risk group demonstrated substantially higher survival rates when compared with the high—risk group. Moreover, the authors further found that CD52 showed the greatest positive correlation with immune function cells among the BRCA genes. Previous study shows that CD52 is a key tumor suppressor gene that is activated in BC [61]. A high CD52 expression rate may negatively influence M2 macrophage infiltration but can accelerate anti-cancer immune cell infiltration [61]. Therefore, the high expression of CD52 may protect breast cancer patients from the disease. The m7G-related miRNAs and CD52 provide a means to identify a patient who is at risk of low survival time, allowing further aggressive interventions to be made earlier. The above-included studies implied that the interaction between miRNAs and m7G modifications plays an important role in BC development, prognostic assessment, and immunotherapy response. However, although some studies have proposed prognostic models based on m7G-associated miRNAs, their application in clinical practice is still limited and lacks sufficient clinical trial data to support them. Therefore, it is necessary to conduct more clinical trials to assess the predictive ability of m7G-related miRNA-based prognostic models in BC patients and the guidance of immunotherapy response and to promote their application in the clinic.

Other m7G regulators or m7G-associated genes

Within the thirteen eligible studies, in addition to METTL1/WDR4, m7G-related lncRNAs, and m7G-associated miRNAs, other m7G regulators or m7G-associated genes involving in BC development included P27, AGO2, EIF4E, DPCS, EIF4E3, TCP1, and ANKRD36. As an inhibitor of the cyclin-dependent kinase, P27 (Kip1) exerts a pivotal effect on tumorigenesis and the progression of BC [62]. P27 serves as an unusual tumor suppressor. Its deletion, as well as its binding and suppression of cyclin—CDK expression, results in the disruption of the cancer cell cycle. This occurs because P27's normal function of impeding cyclin-CDK activity is crucial for maintaining the proper regulation of the cell cycle in cancer cells. When P27 is deleted or unable to bind and inhibit cyclin-CDK effectively, the cell cycle of cancer cells becomes dysregulated, facilitating unrestrained cell growth [63]. Eto et al. [64] conducted an in-vitro research and found that a 5'-m7G cap of mRNAs could interact with P27. Anti-cancer agents inhibited the proliferation of the BC cells by up-regulating the expression of p27, thus inhibiting the G1-to-S phase transition of the cancer cells. This research demonstrated that the increase in p27 levels was a fairly accurate indication of the cancer—preventive effects exerted by nutritional substances and chemopreventive agents. Huang et al. identified four m7G-related genes (AGO2, EIF4E, DPCS, and EIF4E3) significantly associated with the TIME of BC. The four prognostic biomarkers could be used for predicting the prognosis and the candidate drugs related to BC. Intriguingly, all four m7G-related genes were previously found to be involved in the tumorigenesis of BC reported by other publications. For example, abnormal AGO2 expression affected the invasive capacity of the BC cells by regulating the expression level of LASP1 and Let-7a (a miRNA) [65]. A high expression of EIF4E was found to be associated with tumor macrophage infiltration, which resulted in a poor prognosis for BC patients [66]. McLean et al. [67] reported that aminoflavone inhibited the growth of BC cells by increasing ROS production and enhancing oxidative DNA damage, which might be partially mediated by its effects on DNA–protein cross-links (DPCs).

In tumorigenesis and immune response, RNA methylation modifications (i.e., m6A and m7G) exert their biological effects through enzymes known as ‘‘writers’’ [68]. Based on the TCGA database and the samples from a cohort, Zhang et al. [69] identified a “writer”, namely RBM15B, together with TCP1 and ANKRD36, which were applied for constructing a prognostic model for BC. The RMW_Score is defined as the “writers” of RNA methylation and the risk score of their associated genes. The authors revealed that lower RMW_Score was correlated to better OS and the immune cells infiltration of BC patients. Therefore, the RMW_Score could potentially serve as a sensitive indicator for forecasting the survival outcomes and the effectiveness of treatments in BC patients. Further machine learning approach identified three key genes, RBM15B, ANKRD36, and TCP1. Furthermore, they also found that TCP1 significantly enhanced the proliferating and migrating ability of BC alisertib-resistant cells. Inconsistent with the aforementioned studies that provided the specific m7G regulators or -related genes, Huang et al. [70] did not particularly point out the m7G-associated genes in their multi‑omics analysis. They found that m7G regulator genes were highly expressed in BC patients. Based on the available data derived from the TCGA database, it was discovered that m7G can act as a new and innovative biomarker for predicting the clinical outcomes and the effectiveness of immunotherapy in BC, presenting with an AUC of 0.81 on predicting the prognosis of BC patients). The aforementioned studies demonstrated that m7G modification plays an important role in BC genesis, progression, immune microenvironment regulation and treatment response. The METTL1/WDR4 complex stabilizes RNA tertiary structure, promotes translation efficiency, and influences cell proliferation, and differentiation by regulating m7G modifications of tRNAs, miRNAs and mRNAs. ​Nevertheless, several limitations need to be acknowledged. For example, many studies on m7G modification in BC rely heavily on public databases like TCGA for preliminary analyses, often lacking independent experimental validation. Some prognostic models exhibit poor generalization across different datasets, with low AUC values, limiting their clinical applicability. Despite identifying multiple m7G-related lncRNAs and miRNAs, their specific molecular mechanisms remain unclear due to insufficient functional studies.

Limitations and perspectives

To our knowledge, our study is the first review aiming to summarize the potential roles of m7G in BC. Based on the findings from the thirteen included studies, we identified that m7G methyltransferases were usually aberrantly expressed in BC, including TNBC and BRCA. Several eligible studies validated the expression and prognostic value of m7G-related genes in breast cancer by RT-qPCR, western blot, immunohistochemistry, and other experimental methods. The function of m7G methyltransferases is to ensure that m7G modifications occur at specific locations on target RNAs, functioning to modulate the structure, processing, development, and maturation of RNA molecules, and thus regulating the translation process of the cancer cells. m7G modifications in mRNA, tRNA, and rRNA can ultimately affect target gene expression (i.e., m7G regulators, m7G-associated genes, m7G-related lncRNAs, and m7G-related miRNAs) and regulate BC-related biological functions. Figure 2 showed the roles of m7G modifications-mediated mRNA transcription, lncRNA/microRNA biosynthesis, tRNA stability, and 18SrRNA maturation in the tumorigenesis of BC. This novel finding indicates that m7G modification and its regulators have a broad application prospect in the future clinical diagnosis and treatment of BC. For example, it was found that mutations in the METTL1 gene resulted in elevated levels of METTL1 protein, which promoted rapid cellular replication and the formation of oncogenic transformation, resulting in highly aggressive tumor tissues [71]. Knockdown of the METTL1 gene inhibited METTL1 protein production and cancer cell growth without damaging healthy cells [71]. This reveals that METTL1 may act as a promising target for a novel treatment option for malignancies. This hypothesis was derived from a previous study reporting a small molecule inhibitor that targeted METTL3 protein (a regulator of m6A) for the treatment of acute myeloid leukemia [71]. Based on the data from the above-included studies, the current relevant research suffers from insufficient validation, poor model generalization, insufficient mechanism study, and sample heterogeneity. Future studies should strengthen independent experimental validation, improve the generalization ability of prognostic models, explore in depth the specific molecular mechanisms of m7G modification in BC, and address the impact of sample heterogeneity. With a deeper understanding of the m7G modification mechanism, m7G-related lncRNAs and miRNAs are expected to become potential biomarkers and therapeutic targets for breast cancer, providing new ideas for individualized treatment of BC.

Fig. 2.

Roles m7G modifications-mediated mRNA transcription, lncRNA/microRNA biosynthesis, tRNA stability, and 18SrRNA maturation in breast cancer. The main m7G methyltransferase complexes in breast cancer are METTL1/WDR4 and WBSCR22/TMRT112, which insert the m7G modifications into target RNA molecules. m7G modifications in mRNA, tRNA, and rRNA can ultimately impact the expression of target genes and governs biological functions associated with BC, collectively contributing to the tumorigenesis of BC

Summary

This review delves into the molecular functions of m7G modifications, examining both physiological and pathological contexts, and their related regulators during the development of BC. Future research should focus on the application of m7G modifications for the identification and implementation of clinically personalized BC treatments. Additionally, further molecular-level investigations are necessary to utilize these molecules for accurately predicting the efficacy and prognosis of BC-related therapies.

Author contributions

TYY and FFW contributed to conceive and design the study. CYC performed the systematic searching. TYY extracted the data. TYY and LY wrote the manuscript. LY and FFW supervised the manuscript.

Funding

This work was supported by the grants from the Science and Technology Program of Traditional Chinese Medicine in Zhejiang Province (No. 2024ZL1272).

Availability of data and materials

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

All Authors read and approved the manuscript for publication.

Competing interests

The authors declare no competing interests.