Letter to the Editor: Leptin hypomethylation - an early epigenetic marker of lean-type diabetes

Abstract

We read with interest study by Sun et al published in the World Journal of Diabetes. Type 2 diabetes mellitus (T2DM), a metabolic syndrome characterized by insulin resistance and hyperglycemia, has long been considered closely associated with obesity, which is commonly defined as a body mass index (BMI) ≥ 25 kg/m². However, a large number of patients with T2DM in the Chinese population do not meet the BMI criteria for obesity. Leptin (LEP), a hormone secreted by the adipose tissue, is also closely linked to obesity. Interestingly, several studies have demonstrated that LEP resistance and hyperleptinemia can coexist in patients with T2DM. To date, little attention has been paid to the epigenetic regulation of the LEP gene in Chinese patients with T2DM and low BMI. Therefore, in the present cross-sectional study focusing on LEP disorders in lean diabetes, we explored the relationships between epigenetic regulation of the LEP gene, progression of T2DM, and serum LEP levels.

Key Words: Epigenetic gene regulation; Lean type 2 diabetes; Leptin; DNA methylation; Disease progression prediction

Core Tip: Epigenetic dysregulation of the leptin (LEP) gene in lean Chinese patients with diabetes remains understudied. Sun et al conducted a cross-sectional study to investigate the associations between LEP promoter methylation and serum leptin concentrations in relation to diabetes progression in non-obese individuals [body mass index (BMI) < 24 kg/m²]. The results showed that in lean adult Chinese patients, methylation of the LEP promoter gradually decreased with advancing diabetes, demonstrating a significant inverse correlation with serum leptin levels. These findings imply that LEP promoter methylation may act as a potential biomarker for early risk stratification of diabetes and prediction of its severity in non-obese patients with diabetes.

TO THE EDITOR

This article aims to comprehensively analyze the research findings of Sun et al[1] published in the World Journal of Diabetes, which in combination with existing literature to reveal the important role of leptin (LEP) gene methylation in risk screening and disease progression prediction among lean individuals with type 2 diabetes mellitus (T2DM) in China. The findings highlight that abnormal epigenetic regulation at the LEP promoter is not confined to obesity-related diabetes but is also closely associated with the onset and progression of diabetes in the lean population.

Being a chronic metabolic disorder and one of the most prevalent chronic diseases in adults, T2DM accounts for over 90% of all diabetes cases worldwide[2]. Traditionally, the onset of diabetes has been regarded as closely related to obesity[3], and clinical studies have reported that a large number of young patients diagnosed with T2DM are obese[4]. However, some researchers have also found that Asian patients exhibit a lower average body mass index (BMI) compared with European populations[5]. In Asia, diabetes often develops at a younger age, is characterized by early β-cell dysfunction and insulin resistance, and necessitates early insulin therapy in many patients[5,6].

According to the World Health Organization, obesity among Asian populations is defined as a BMI ≥ 25 kg/m². Numerous studies have explored the mechanism by which obesity contributes to the onset and progression of T2DM[7]. Specifically, immune dysregulation resulting from obesity-induced chronic low-grade inflammation has long served as a key driver underlying the development of insulin resistance and T2DM[8]. Research has demonstrated that obesity promotes intracellular lipid metabolism and glucose uptake within macrophages, thereby inducing their M1 polarization[9]. It has also been reported that 86% of adults with T2DM are overweight, 52% are categorized as obese, and 8.1% have morbid obesity[10].

LEP, secreted by adipose tissue, often acts as a marker of adipocyte differentiation and maturation[11]. During adipocyte maturation, the methylation levels of the LEP gene promoter gradually decrease, reflecting its upregulated expression and elevated serum LEP levels. LEP exerts its effects on the hypothalamus to suppress appetite, enhance energy expenditure, and modulate insulin secretion. LEP can further regulate adipocyte sensitivity to insulin and diminish lipid accumulation[11]. Under physiological conditions, LEP levels exhibit a positive correlation with body fat mass, thereby maintaining energy balance.

In obesity, characterized by excessive adipose tissue, LEP resistance (a state where elevated serum LEP concentrations fail to exert their normal metabolic effects) develops, which impairs LEP-mediated regulation of energy homeostasis and further exacerbates obesity[12]. Serum LEP concentrations are therefore typically elevated in obese individuals[11,13]. Interestingly, recent studies have demonstrated that serum LEP concentrations are also elevated in lean patients, challenging conventional perspectives[14]. This implies that abnormal epigenetic regulation of the LEP gene is implicated in the onset and progression of diabetes. However, research gaps persist regarding the methylation pattern of the LEP promoter in lean diabetes[15].

Given the limited research on the association between the epigenetic regulation of the LEP gene and the onset and progression of diabetes in lean Chinese patients, Sun et al[1] conducted a clinical cross-sectional study focusing on elevated LEP concentrations in this population. Their study enrolled 392 participants aged 40–60 years with a BMI < 24 kg/m², who were divided into three groups: 120 normoglycemic controls, 94 prediabetic individuals (44 with impaired fasting glucose/50 with impaired glucose tolerance), and 178 lean individuals with T2DM. For assessment of LEP promoter methylation, genomic DNA was extracted from peripheral blood leukocytes and subjected to bisulfite conversion, followed by methylation-specific polymerase chain reaction targeting the CpG island region of the LEP promoter (chr7: 127884135-127884240)[23]. Droplet digital polymerase chain reaction was used to validate methylation quantification. Serum LEP levels were quantified via a double-antibody sandwich enzyme-linked immunosorbent assay according to the manufacturer’s instructions. Statistical analyses included analysis of variance, χ² tests, and Pearson correlation analysis with Bonferroni-corrected P values. Age and BMI were included as key covariates, while additional confounding factors (sex, hypertension status, smoking rate) were controlled through study design, as no significant intergroup differences were observed. It was found that LEP promoter methylation progressively decreased with the development of diabetes (59.2% in controls, 43.6% in prediabetes, and 31.5% in T2DM) and was negatively correlated with serum LEP levels (r = -0.95, 95% confidence interval: -0.97 to -0.92, P < 0.001).

Overall, this study contributes to an enhanced understanding of the association between LEP epigenetic modifications and serum LEP levels in diabetes progression, independent of obesity[1].

DECLINE IN LEP PROMOTER METHYLATION IS LINKED TO DISEASE PROGRESSION IN LEAN CHINESE PATIENTS WITH DIABETES MELLITUS

Sun et al’s study[1] broadens the scope of research on LEP methylation. Historically, research on LEP methylation has usually focused on obese individuals[16]. Emerging studies have further indicated that epigenetic regulation, typified by DNA methylation, serves a critical role in the onset and progression of diabetes[17]. Given that previous research on LEP methylation was largely limited to obese individuals or patients with obesity-related diabetes, Sun et al[1] examined changes in LEP promoter methylation during the progression from prediabetes to type 2 diabetes in lean Chinese adults. Notably, their study demonstrated for the first time that LEP promoter methylation progressively decreases with disease onset and progression in this population. This finding addresses an important gap in epigenetic research involving individuals with normal BMI but elevated diabetes risk.

LEP PROMOTER HYPOMETHYLATION CORRELATES WITH SERUM LEP ELEVATION

Using clinically accessible blood samples, Sun et al’s study[1] demonstrated an almost perfect negative correlation between LEP promoter methylation levels and serum LEP concentrations. The relationship remained significant even after adjusting for age and BMI, suggesting that LEP methylation may be associated with the onset and progression of diabetes. This finding challenges the traditional paradigm that obesity is the primary factor associated with methylation alterations. The regulation of LEP expression through promoter methylation may contribute to metabolic dysregulation in diabetes independently of obesity-related factors[18]. This study provides novel insights into methylation-mediated regulation of LEP gene expression in non-obese conditions, providing evidence for a novel metabolic regulatory model[19].

MECHANISM: DNA METHYLATION, LEP LEVEL, AND DIABETES

Previous studies have shown that methylation in the CpG island in gene promoters typically suppresses transcription, while promoter demethylation alleviates this suppression and enhances gene expression[20]. Sun et al[1] observed that in lean individuals, progression of diabetes was associated with gradual demethylation of the LEP promoter, which further upregulated the expression of the LEP gene, resulting in elevated serum LEP levels. These findings indicate that epigenetic demethylation is potentially linked to elevated serum LEP levels independent of conventional fat mass accumulation[21].

In summary, hyperglycemic or hyperinsulinemic conditions could potentially be involved in regulating LEP promoter methylation, which may further contribute to metabolic disorders, though this mechanistic link remains speculative and requires experimental validation. However, these conclusions are solely based on cross-sectional data, with no supporting functional experiments to substantiate their validity. Mechanistic studies to clarify how elevated glucose or insulin levels modulate LEP epigenetic modifications in cellular and animal models would improve the reliability of the findings. Specifically, these include adipocyte culture experiments with glucose/insulin treatment, studies using lean diabetic animal models, and assessment of epigenetic enzyme activity.

While Sun et al’s study[1] links decreased LEP methylation and higher LEP levels to diabetes progression, the mechanisms by which excess LEP aggravates diabetes in lean individuals remain unclear. Drawing on prior research, excessive LEP secretion is hypothesized to trigger functional resistance in downstream signaling pathways, thereby contributing to further elevations in blood glucose levels[22]. Subsequent investigations should incorporate functional experiments to clarify the role of LEP in insulin metabolism in lean individuals with diabetes.

TRANSLATIONAL IMPLICATIONS

Early detection

Low methylation of the LEP promoter is progressively associated with blood glucose abnormalities, highlighting its potential as a blood-based epigenetic marker for early screening. While previous studies have established an association between low LEP mRNA expression and T2DM, Sun et al’s study[1], focusing on lean Chinese adults, demonstrated that LEP promoter methylation levels are correlated with diabetes severity. This finding may facilitate the identification of individuals with normal BMI but high metabolic risk, thereby enhancing early detection of diabetes in the lean Chinese population.

Intervention and monitoring

Epigenetic alterations are both reversible and heritable[23]. Accordingly, interventions such as dietary adjustments or structured exercise regimens may restore LEP promoter methylation homeostasis[24], thereby decelerating diabetes progression in lean populations. As LEP methylation changes in alignment with metabolic shifts, it can act as a biomarker for evaluating the efficacy of lifestyle-based interventions.

RESEARCH LIMITATIONS AND FUTURE EXPECTATIONS

Given its cross-sectional design, Sun et al’s study[1] demonstrates an association between LEP promoter methylation levels and diabetes severity rather than a causal link. In the future, a long-term follow-up cohort should be established to further examine whether LEP methylation can predict diabetes onset and progression, clarify whether LEP hypomethylation precedes elevated serum LEP levels, and validate the predictive value of LEP methylation to boost its clinical credibility.

With regard to research subjects, methylation analysis was limited to peripheral blood leukocytes, whereas LEP is mainly expressed in adipocytes. Future studies should therefore explore the relationship between methylation patterns in peripheral blood and adipose tissue, along with the correlation between adipose tissue status and diabetes progression.

Sun et al’s work[1] focuses on LEP gene promoter methylation in lean Chinese patients with diabetes but lacks detailed validation of the process by which LEP gene promoter methylation leads to an increase in serum LEP levels in a hyperglycemic environment. Future studies could assess LEP transcriptional activity in combination with metabolomic detection and adipocyte function assessment to examine the activity of diabetes-related metabolic enzymes and establish a more complete causal framework for diabetes progression. Future studies should also explore whether metabolic stress correlates with methylation changes, which, in turn, may be linked to increased serum LEP levels and diabetes progression.

In terms of sample sources, Sun et al’s study[1] was conducted at a single center and restricted to one hospital and region. Expanding both the sample size and the geographical scope would strengthen the credibility of the findings. Furthermore, clinical trials are needed to evaluate how interventional strategies affect LEP methylation levels. It is also essential to investigate how lifestyle modifications or epigenetic-targeted drugs influence the reversibility of LEP methylation and to clarify their therapeutic efficacy.