DNA methylation in cardiovascular diseases is gaining attention; however, its role in AAA has not been comprehensively studied. It is well known that smoking is a significant risk factor for AAA, and smoking has a crucial impact on the growth and rupture of AAA (21). As mentioned earlier, DNA methylation is controlled by DNMTs. Nicotine itself has been shown to not only affect the expression of DNMT1 but also influence promoter methylation levels in GABA-ergic neurons (22). Atherosclerosis is a complex pathological process involving vascular wall cells and inflammatory cells. As one of the risk factors for AAA, the mechanisms of atherosclerosis may be correlated with AAA. Endothelial dysfunction is the pathological basis of atherosclerosis and is accompanied by changes in vascular wall permeability. Its pathological process is closely related to abnormal DNA methylation (23). Other risk factors for AAA, such as hypertension and diabetes, may also involve similar mechanisms in AAA, but further correlational research is needed Figure 2.

Early studies have demonstrated a significant elevation in plasma homocysteine (Hcy) levels in AAA patients, and hyperhomocysteinemia (HHcy) is associated with the expansion rate of AAA (29). AAt the same time, a study's findings indicate that Hcy does not alter the protein levels of DNMT1 and DNMT3 but selectively reduces the activity of DNMT1 by 29% (30). Elevated Hcy levels may promote the development of AAA through multiple interrelated mechanisms, including endothelial dysfunction, protein hydrolysis, endoplasmic reticulum stress, enhanced inflammation, and increased cell apoptosis (31), which may indicate that HHcy in AAA may play a crucial role in AAA progression by reducing DNA methylation. Although research has shown an association between Hcy level and the growth rate of AAA, the conclusions drawn are relatively weak due to AAA being a multifactorial disease. However, a studies have indicated that patients with HHcy exhibit a faster expansion rate compared to those with normal Hcy levels, with a considerable number of patients demonstrating rapid expansion (>10 mm/year), thereby increasing the risk of rupture (32). Another study aimed at exploring the direct causal relationship between AAA and HHcy also suggests that HHcy may exacerbate AAA formation at least partially through the activation of peripheral fibroblast NADPH oxidase 4 (33). These two studies further confirm the high association of AAA with HHcy. A cohort study by Kristina Sundquist et al. provides an alternative perspective by confirming overall high methylation in AAA cases and concluding a significant correlation between overall DNA methylation, Hcy, and the baseline diameter of AAA (34). However, this study did not establish a correlation between overall DNA methylation and plasma Hcy levels. In the future, further studies may be carried out to further explore the specific mechanism of HHcy in promoting the growth and rupture of AAA by mediating DNA methylation.

Moreover, the diminished inhibitory effect of regulatory T cells (Tregs) may contribute to the progression of AAA, which is closely related to DNA methylation. Research suggests a reduced expression of FOXP3 (a transcriptional regulatory factor) in peripheral CD4CD25 Tregs of AAA patients, leading to functional deficiencies in overall CD4CD25 Tregs, indicating impaired immune regulation by Tregs, which may contribute to the disease progression of AAA (35). Another study confirms significantly higher DNA methylation levels in Tregs of AAA patients compared to healthy subjects, indicating a transcriptionally suppressed state affecting cellular functions (36). Furthermore, changes in DNA methylation have been shown to impact the expression of various genes related to vascular smooth muscle cell (VSMC) apoptosis, inflammation, and ECM degradation. This imbalance may be associated with the dysregulation of MMPs and TIMPs, closely linking them to AAA (17). These findings warrant further research for a comprehensive understanding of the mechanistic role of DNA methylation in AAA.

In addition, there have been some exciting advances in genome-wide DNA methylation. Evan J. Ryer et al. determined the genome-wide DNA methylation in peripheral blood mononuclear cells (PBMCs) of abdominal aortic aneurysm (AAA) patients, discovering significant differences in DNA methylation at specific CpG islands (CGIs). However, the observed differential methylation was noted to be potentially age-related rather than AAA-related (37). Additionally, the correlation between DNA methylation and gene silencing increases with the density of CpG dinucleotides at the promoter region (38). The human genome has fewer CpGs, but more CGIs. CGIs, relatively small 5′ end promoter regions, may experience gene suppression due to methylation of the fifth carbon of cytosine in this region (39). Aberrant CGI hypermethylation is common in tumor progression and may lead to abnormal gene silencing (40). Furthermore, the variations in 5mC, 5hmC, TET family proteins, and DNMTs at the genome-wide level differ depending on the type of cancer (24, 41) potentially serving as relevant interrogation sites for DNA methylation. Toghill BJ et al. used next-generation sequencing (NGS) on VSMCs collected from individual aortic tissues, pioneering the determination of CpG methylation status in regulatory regions of genes located at AAA risk loci identified in genome-wide association studies (GWAS) (42). With technological advancements, epigenome-wide association studies (EWAS) have evolved after GWAS, providing a systematic approach to revealing epigenetic variations underlying common diseases. EWAS, applied for a decade to analyze DNA methylation variations in complex diseases, has unveiled new molecular mechanisms for various common diseases, such as rheumatoid arthritis, metabolic syndrome, breast cancer, Alzheimer's disease, etc. It has made the application of epigenetic variations as biomarkers possible (43). This approach may lead to innovative research perspectives on the mechanisms of DNA methylation in AAA, offering molecular-level advancements in understanding the pathogenesis of AAA.

Research on histone acetylation and methylation modifications in abdominal aortic aneurysm (AAA) has been extensive. In the related studies of histone acetylation, Han et al. compared human AAA tissue with healthy aortic tissue, analyzing the differences in histone acetylation and the expression of corresponding HATs. The results showed significant overexpression of three lysine acetyltransferase (KAT) family members in the AAA vascular wall. Some lysine acetyltransferases, such as KAT2B, were found to be correlated with AAA diameter. High expression of KAT2B, KAT3B, and KAT6B was also observed in inflammatory cells (44). Another study found higher levels of H3 and H3K14 acetylation in T lymphocytes of AAA patients (45). However, contrasting results have been reported. As mentioned earlier, HDAC/HAT constitutes a reversible enzyme pair regulating histone acetylation PTM. Galán M et al. found that HDACs corresponding to KAT were significantly upregulated in AAA. In a mouse model, HDAC inhibitors were observed to restrict aneurysm progression. The study suggested that HDAC may promote processes such as ECM degradation, increased inflammatory mediators, and VSMC apoptosis (46). While some studies in mouse models have demonstrated inhibitory effects of HDAC inhibitors on MMP expression, the specific mechanisms require further investigation (47). Qian Xia et al. also identified a decrease in acetylation rates in H3 and H3K9, attributing it to the elevated influence of HDAC1 and HDAC5. HDAC1 and HDAC5 were found to inhibit the transcriptional activity of regulatory T cells (Tregs) (36). Additionally, Jacob Greenway et al. detected downregulation of H3K4me1, H3K9me3, and H3K56ac levels in two animal AAA models. The study revealed dynamic changes in histone H3 modifications during AAA formation, with no observed upregulation in H3 modifications (48). The discrepancies in results may be attributed to the levels of HAT and HDAC expression and their alterations during AAA development. Moreover, differences in modifications at specific sites vs. global modifications, as well as species variations, risk factors, and control tissue, need further in-depth research to address these disparities and determine the processes regulating histone methylation and acetylation patterns in AAA Figure 3.

In addition, there are novel findings regarding histone methylation. Research has revealed that histone demethylase JMJD3 stimulates the pro-inflammatory monocyte/macrophage phenotype in AAA tissue by selectively removing inhibitory histone H3K27me3 methylation. This leads to vascular remodeling and aortic dilation, promoting the progression of AAA (54). Another study reported that SETDB2, a histone methyltransferase, specifically trimethylates lysine 9 on histone H3 (H3K9me3), reducing the expression of inhibitory histones. This results in the loss of TIMP expression, causing dysregulation of MMP activity, aortic wall degeneration, and AAA formation (55). Numerous studies have demonstrated that, apart from histone acetylation, methylation, and phosphorylation, histone modifications also include ubiquitination and butyrylation. These modifications not only participate in the progression of metabolic diseases (56), but also impact the tumor microenvironment (57).

Furthermore, histone modifications, particularly histone methylation and acetylation, which have been extensively studied, play a regulatory role in cardiovascular diseases, including AAA risk factors such as atherosclerosis and hypertension (16). These study of histone modifications will play some inspiring roles in the study of the mechanism of AAA disease progression. In addition, histone modifications also include phosphorylation mentioned above and some novel modifications studied in cancer, such as lactatation, ubiquitination, lysine, citrullination, etc. These modifications may be closely related to metabolites (58). Exploring other histone modifications may provide new directions for studying the pathogenesis and treatment of AAA and requires further investigation. On this basis, since nucleosomes formed after DNA binding to histones control the structure of chromatin, the levels of DNA methylation and histone modification can also be expanded, and the microscopic to macroscopic study can be carried out at the level of chromatin remodeling, so as to further explore the disease progression of AAA.

Approximately only 2% of the human genome encodes proteins, while non-coding RNAs (ncRNAs) are RNA molecules that do not encode proteins. In addition to their roles in transcription, ncRNAs may play complex and crucial epigenetic regulatory roles in higher animals. These ncRNAs, often referred to as regulatory RNAs, participate in biological processes such as gene expression regulation and chromatin structure modulation by interacting with DNA, RNA, and proteins (59). This section focuses on the above three ncRNAs that have been studied in AAA so far Figure 4.

In recent years, an increasing body of research has highlighted the crucial role of epigenetic factors in the onset and progression of cardiovascular diseases (95–98). RNA methylation is a significant form of epigenetic modification, belonging to the fields of epitranscriptomics and epigenomics, which collectively regulate gene expression in eukaryotes (99, 100), this includes 6-methyladenosine (m6A), 5-methylcytosine (m5C), and N-7-methylguanosine (m7G). Currently, studies regarding the association between RNA methylation and AAA are limited, with the primary focus on m6A.

The PCSK family consists of nine proteases, with PCSK9 being demonstrated as crucial in the regulation of CVD (134). In a 2023 study, a potential therapeutic target, Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9), was identified through AAA's genome-wide association meta-analysis of 121 independent risk loci. When elastase was introduced into PCSK9 mice, AAA growth decreased, indicating a unique role for PCSK9 in AAA (135). Besides, Inhibitors of PCSK9 can reduce low-density lipoprotein (LDL) cholesterol levels, potentially decreasing the risk of severe cardiovascular disease symptoms associated with atherosclerosis (136). A large-scale study involving 100,000 individuals suggested that elevated lipoprotein levels may increase the risk of AAA 2–3 times (137). Given PCSK's role in regulating lipid metabolism, this represents the feasibility of PCSK as a therapeutic target, necessitating more preclinical research and clinical trials to assess its efficacy in AAA-related outcomes.

Meanwhile, epigenetic mechanisms likely play a role in some functional aspects of PCSK genes. For example, sirt6 deacetylase can inhibit PCSK9 gene expression by enriching the transcription factor Foxo3 in the proximal promoter region of the PCSK9 gene, leading to histone H3 deacetylation (138). Increased expression of has-miR-335 and has-miR-6825 can lower PCSK9 mRNA expression in myocardial cells (139). Additionally, the histone acetyltransferase P300 may increase PCSK9 expression (140). Non-coding RNAs (ncRNA) play a crucial role in the progression of inflammation in atherosclerosis by targeting genes related to the PCSK9 pathway at the post-transcriptional level (141). Further research is needed to uncover the pathway-related mechanisms of PCSK9 in AAA, potentially providing new insights and breakthroughs into the complex pathogenesis of AAA.

While PCSK9's Epigenetic importance is established, the specific mechanism of other PCSK family protein in AAA are still under exploration. Members of the PCSK family share structural similarities, but their functional roles vary. In cancer research, other PCSK family members have been reported to activate cytokines, influencing cell proliferation, migration, and extracellular matrix remodeling—important features in the progression of AAA (142). A study categorized PCSK analysis into plaque expression, biochemical blood parameters, morphological characteristics, and patient symptoms. The cumulative data from each section suggested the feasibility of PCSK6 as a new therapeutic target, followed by PCSK5, PCSK7, and FURIN, all showing potential for CVD treatment within this family (143). This, of course, is also closely related to the treatment of AAA. Therefore, the research on PCSK family proteins may be more in-depth and comprehensive.

The transforming growth factor-beta (TGF-β) family members include TGF-beta, pathway factors, and bone morphogenetic proteins (BMP), among other genes, with extensive research in cancer. TGFβs can activate various pathways, including the Sma- and Mad-related family proteins (Smad), mitogen-activated protein kinase (MAPK), and phosphoinositide 3-kinase (PI3K) pathways. Their association with AAA may lie in their ability to ultimately inhibit inflammatory cell infiltration, reduce ECM degradation, limit VSMC apoptosis, promote ECM formation, and be involved in tissue repair, fibrosis, extracellular matrix remodeling, cell proliferation, and migration (144, 145). It's worth mentioning that the Smad pathway, in terms of epigenetics, is regulated by various miRNAs during the AAA process. Notably, miRNA-29b-mediated Smad targeting facilitates AAA by influencing key matrix metalloproteinases (MMP-2 and MMP-9). Inhibiting miRNA-26a increases gene expression of SMAD-1 and SMAD-4, promoting vascular smooth muscle cell proliferation, inhibiting cell differentiation and apoptosis, altering TGF-β pathway signaling. Additionally, miR-424/322 analogs modulate Smad2/3/runt (146–149). Moreover, miRNA-195, miRNA-155, and miR-143/145 intersect with the Smad pathway in the TGF-β pathway (150). A genetic association study in a Dutch population analyzed single nucleotide polymorphisms (SNPs) in TGF-β receptor genes TGFBR1 and TGFBR2, revealing potential correlations similar to TAA in AAA (151). This may be evidence that some TGF-β is related to genetics or epigenetics. In the TGF-β pathway, multiple genes coordinate final outcomes, and aside from the Smad pathway, the other two pathways may form a complex and mutually influencing regulatory system with various non-coding RNAs. Furthermore, there is limited research on histone modification and DNA methylation in this context, necessitating further exploration of the pathways and discovery of new targets.

Receptor tyrosine kinases (RTKs) form the largest class of enzyme-linked receptors, serving both as receptors for growth factors and as enzymes that catalyze downstream target protein phosphorylation. RTKs play crucial regulatory roles not only in normal cells but also have pivotal functions in the progression of various cancers, including lung cancer, gastric cancer, thyroid cancer, etc (152). Mutations in receptor tyrosine kinases lead to the activation of a series of signal cascades, impacting protein expression, including epigenetic effects, although specific research in this regard is currently lacking. RTKs can modulate various downstream signaling pathways such as MAPK, PI3K/Akt, and JAK/STAT. Various types of RTKs may be overexpressed in AAA, including epidermal growth factor receptors (EGFRs) (153), vascular endothelial growth factor receptors (VEGFRs) (154), platelet-derived growth factor receptors (PDGFRs) (155), insulin-like growth factor receptors (IGFRs) (156) and fibroblast growth factor receptors (FGFRs) (157). Research indicates that caspase recruitment domain and membraneassociated guanylate kinaselike domain protein 3 (CARMA3) recruits two downstream signaling molecules, BCL10 and MALT1, through its N-terminal effector CARD domain, forming the CARD11-BCL10-MALT1 (CBM) complex (158), involved in the NF-kB signaling pathway induced by G protein-coupled receptor (GPCR) and RTK. Inflammation is a critical feature in the pathogenesis of cardiovascular diseases, including Abdominal Aortic Aneurysm (159). During the inflammatory process, blood vessels and the surrounding connective tissue are important regulatory factors (160). In endothelial cells of mice, CARMA3 assembles BCL10 and MALT1 to form the CBM signalosome, triggering NF-kB activation induced by CXCL8/IL8. CXCL8/IL8 is a crucial chemokine involved in promoting angiogenesis and inflammation. This process controls VEGF expression and promotes autocrine activation of VEGF receptors (161). The CBM complex also mediates coagulation-induced NF activation (162). However, how CARMA3 connects to GPCR and regulates the NF-kB signaling pathway remains a puzzle. Future studies can explore how RTKs, through epigenetic mechanisms, influence NF-kB to impact the occurrence and development of AAA, thereby identifying new therapeutic targets.

The three aforementioned gene pathways represent only a minute fraction of the vast gene regulatory network. In addition to the previously discussed pathways, such as the inflammatory response pathways, including the Interleukin-6 (IL-6) pathway, and the MMP pathway responsible for degrading cellular tissue matrix, there is limited research on the interplay with epigenetics. Besides refining the epigenetic aspects of these pathways, further exploration and research are needed on gene pathways related to cellular autophagy, ECM, and VSMC proliferation, and their associations with epigenetic modifications.

Due to studies demonstrating a common coexistence and positive correlation between Chlamydia pneumoniae infection and AAA progression (164, 165), antibiotics were once considered as potential candidates for clinical treatment. However, results from two large clinical studies showed that doxycycline treatment did not reduce aneurysm growth, nor did it delay the need for AAA surgery or the timing of surgical repair (166, 167). The most commonly used AAA animal model involves inducing abdominal aortic aneurysm formation with angiotensin II, and clinical trials are often conducted using ACE inhibitors (ACEIs) and angiotensin II receptor blockers (ARBs). Additionally, there are numerous clinical trials for anti-inflammatory drugs targeting immune cell inflammatory infiltration in AAA, statin drugs selected from observational experiments for their potential correlation with AAA growth and rupture, and antiplatelet drugs that weaken blood clots. It is noteworthy that metformin, targeting the risk factor of diabetes for AAA, is still under clinical trial. However, AAA growth is a long-term process influenced by multiple factors, including statistical considerations. Since age is also a risk factor for AAA, elderly AAA patients often end up with the outcome of loss to follow-up due to death. As a result, the statistical results of clinical trials are often censored, making them less convincing (9, 168, 169). Currently, most drug clinical trials involve repurposing basic drugs from other diseases. These trials test the impact of drugs on limiting AAA growth, but there is still no clinical trial data providing conclusive evidence of drug efficacy in restricting AAA progression. Further research may require larger sample sizes, diverse populations, and clinical trial designs that account for gender differences. Future exploration may also involve personalized targeting of drug development for AAA to translate the effects of AAA-related drugs into therapeutic outcomes.

Since the cellular aberrations observed in abdominal aortic aneurysms (AAA) involving proliferation, apoptosis regulation, inflammatory infiltration, and fibrosis are similar to those observed in cancer, the therapeutic significance of epigenetic modification in cancer may be instructive for AAA. It is plausible that similar treatment approaches could be applied. For example, Brd4, as an epigenetic regulator of multiple programmed cell death, has some untapped potential in cancer. This may indicate that it may serve as a potential direction for epigenetic therapy of AAA (170). In addition, m6A in RNA modification may play some crucial roles in tumor glycolysis, and the glycolytic reprogramming of cancer cells is a major characteristic of cancer, which may become a special therapeutic mechanism affecting the occurrence and development of tumors. It may represent that AAA may also have similar epigenetic mechanisms related to proliferation to be further studied (171). Immunological treatments targeting cell-specific epigenetic modifiers such as histone demethylase JMJD3 and SET domain bifurcated protein lysine methyltransferase 2 (SETDB2) have shown promise in effectively intervening in AAA progression (54, 55).

Exploring well-established pathways in cancer research for potential treatment targets is anotheravenue. For instance, the Wnt/β-catenin pathway has been found to be significantly activated in human and experimental animal AAA models, suggesting the presence of potential therapeutic targets, despite the exclusion of specific anti-tumor drugs for arterial disease in animal models (172). Due to the intricate genetic actions of pathways, in-depth research is necessary to determine whether improving arterial aneurysm development is achievable by targeting specific components of these pathways. The presence of ncRNAs may fulfill this role. Recent studies on lncRNA have shown that inhibiting H19 expression could serve as a novel molecular therapeutic target, preventing or slowing AAA progression by suppressing IL-6 expression (65). The role of miRNAs in AAA treatment has been demonstrated (Table 1), but a combination with specific pathways and target gene functions is still required. It is essential to explore the intersections of various pathways and ncRNAs in detail to identify critical therapeutic targets for AAA progression.

Another perspective for epigenetic treatment is drawing inspiration from risk factor diseases or other cardiovascular diseases similar to AAA. For example, research has shown that stem cells have potential in treating cardiovascular diseases, with mesenchymal stem cells (MSCs) effectively inhibiting cell senescence in cardiovascular disease (196). Recent results in epigenetic research indicate that upregulating miR-19b-3p can enhance the anti-aging effect of extracellular vesicles from MSCs (MSC-EXO) isolated from AAA patients. This occurs through the regulation of the MST4/ERK/Drp1 pathway, inhibiting mitochondrial fission in VSMCs (177). In addition, there are a number of epigenetic therapeutic drugs in clinical trials in cardiovascular diseases, such as DNMT inhibitors (azacytidine, decitabine, and hydroalazine), HDAC inhibitors (vorinostat, sodium valproate), histone methylation inhibitors (GSK126, EPZ-5676), and histone methylation inhibitors (azacytidine, decitabine, and hydroalazine). Bromodomain and exo-terminal motif (BET) inhibitor (RVX208) (197). They have different effects on cancer and cardiovascular diseases, and may be used as a treatment for AAA in the future.

Our exploration of epigenetic modifications extends beyond this. In addition to translating established epigenetic modification mechanisms observed in animals into clinical trial directions, other types of epigenetic modifications that have not been well-explored in AAA, such as chromatin remodeling and other types of RNA modifications, are also considered. Two studies on chromatin remodeling focused on the unique subunits BAF60a and BAF60c of the SWI/SNF chromatin remodeling complex, determining that BAF60a promotes epigenetic regulation of VSMC inflammation and BAF60c plays a crucial role in maintaining VSMC homeostasis (198, 199). However, there are still few studies on the specific mechanisms of chromatin remodeling in AAA and even cardiovascular diseases. In the future, more comprehensive studies are expected to make the effects of histone and DNA remodeling at the chromatin level on AAA more clear. With the development of epigenetic technologies, new sequencing and editing methods can reveal complex network regulations of many genes controlled by epigenetic mechanisms. The application of these epigenetic technologies in AAA research can provide a genetic-level understanding of epigenetic modifications as targets and signaling pathways for new drug development and offer critical insights into potential therapeutic strategies for AAA.

However, despite the potential therapeutic advantages of epigenetic therapy, it also comes with limitations. Epigenetic treatments may induce unnecessary changes or off-target effects outside the intended target, potentially leading to adverse reactions or even other diseases. Furthermore, uncertainties remain regarding the long-term impacts and safety of epigenetic therapy, necessitating further research and validation. It is also important to note that most epigenetic research is limited to animal models, in vitro studies on human cells, or bioinformatics-based screenings. Effective translation into clinical trials may face obstacles, such as delivery methods or cost and accessibility.