Letter to the Editor: Leptin gene hypomethylation - a missing epigenetic link in lean type 2 diabetes?

Abstract

Emerging research links diabetes pathogenesis to epigenetic modifications beyond genetics and metabolism. We read with great interest the study published in World Journal of Diabetes by Sun et al. They investigate leptin (LEP) gene promoter methylation via methylation-specific polymerase chain reaction and serum LEP quantification (enzyme-linked immunosorbent assay) in non-obese Chinese adults with prediabetes/type 2 diabetes (T2DM). It shows progressive promoter methylation decline from normoglycemia to T2DM, strongly inversely correlated with circulating LEP. This letter critically evaluates their findings, contextualizes them in diabetes epigenetics research, and highlights translational value for early risk stratification and precision prevention in non-obese individuals. Briefly, LEP hypomethylation may drive “LEP-insulin axis” dysfunction (a T2DM hallmark) by disrupting β-cell function and insulin sensitivity. Key terms: DNA methylation (methylation of cytosine-guanine cytosines, repressing transcription); epigenetics (heritable gene expression changes without DNA sequence alteration).

Key Words: Leptin; DNA methylation; Epigenetics; Type 2 diabetes mellitus; Prediabetes; Lean-phenotype; Non-obese; Biomarkers

Core Tip: This letter critically analyzes the study by Sun et al, which demonstrates that progressive hypomethylation of the leptin (LEP) gene promoter parallels glycemic deterioration in non-obese Chinese adults. The strong inverse correlation between methylation and serum LEP levels (r = -0.95, P < 0.001) highlights an adiposity-independent epigenetic mechanism that may represent a risk factor for diabetes pathogenesis. These findings position LEP promoter methylation as a promising blood-based biomarker for early detection and intervention in non-obese individuals at risk of diabetes.

TO THE EDITOR

Type 2 diabetes mellitus (T2DM) is traditionally associated with obesity; however, a growing subset of Asian patients develops diabetes at a normal or even low body mass index (BMI) (< 24 kg/m²), suggesting underlying mechanisms distinct from adiposity-driven metabolic dysfunction[1,2]. Epigenetic regulation, particularly DNA methylation (the addition of a methyl group to cytosine residues in cytosine-guanine (CpG) dinucleotides, a process that typically represses gene transcription), has emerged as a pivotal layer in this complexity[3,4]. The leptin (LEP) gene - central to energy homeostasis, appetite regulation, and insulin sensitivity - has been widely investigated in obese populations[5,6], yet its epigenetic modulation in non-obese diabetics remains obscure. Sun et al[7] published a study in World Journal of Diabetes address this crucial gap by examining LEP promoter methylation and its relationship with serum LEP levels across glycemic states, proposing an adiposity-independent mechanism linking methylation dynamics to metabolic dysfunction.

In their well-controlled cohort of 392 lean adults (BMI < 24 kg/m²), Sun et al[7] systematically stratified participants into normoglycemic, prediabetic (impaired fasting glucose or impaired glucose tolerance), and T2DM groups based on World Health Organization criteria[7]. Peripheral blood leukocytes were subjected to bisulfite modification and methylation-specific polymerase chain reaction targeting CpG sites in the LEP promoter region (chr7: 127884135-127884240). Serum LEP was quantified by enzyme-linked immunosorbent assay. Rigorous statistical validation, including Bonferroni correction and multivariate regression adjusting for age and BMI, ensured robust analysis of methylation-phenotype associations. Such precision in methodology underscores the study’s strength in isolating epigenetic effects independent of confounding metabolic variables.

KEY FINDINGS

The study reports a striking progressive decline in LEP promoter methylation from 59.2% in normoglycemic controls to 43.6% in prediabetes and 31.5% in T2DM (all P < 0.05 vs controls)[7]. This demethylation trajectory paralleled a significant rise in circulating LEP - 11.33 ± 3.10 μg/L (controls) to 16.94 ± 4.19 μg/L (T2DM) (q = 6.81, P < 0.01). Importantly, the correlation between methylation and LEP levels remained robust after adjusting for age and BMI (β = -0.91, P < 0.001), implicating promoter demethylation as an independent regulatory event[5,8]. The results suggest that epigenetic dysregulation of LEP precedes overt metabolic disturbance, offering a novel molecular signature for early diabetes risk in non-obese individuals.

CRITICAL APPRAISAL

Sun et al’s findings[7] reinforce and extend prior observations linking LEP promoter methylation to metabolic states. In high-fat diet–induced obese rats, Milagro et al[6] reported similar methylation-dependent LEP upregulation, while Yokomori et al[5] demonstrated that demethylation enhances promoter activity during adipocyte differentiation. Bouchard et al[9] further showed maternal LEP demethylation in gestational diabetes influencing fetal LEP expression, suggesting heritable epigenetic transmission. However, unlike these obesity-centered studies, Sun et al[7] isolate this phenomenon in a lean cohort, thereby disentangling epigenetic control from adiposity. Their near-perfect inverse correlation (r = -0.95) surpasses those reported in obese populations (r ≈ -0.8), possibly reflecting reduced phenotypic heterogeneity and a tighter methylation-expression coupling.

A deeper mechanistic link between LEP hypomethylation and T2DM pathogenesis lies in the dysfunction of the “LEP-insulin axis”. LEP, primarily secreted by adipocytes, normally acts on pancreatic β-cells to inhibit insulin secretion in a glucose-dependent manner, maintaining metabolic balance[8]. However, emerging evidence indicates that excessive LEP (driven by LEP promoter hypomethylation) can induce β-cell LEP resistance, disrupting this feedback loop[10]. Specifically, hyperleptinemia impairs the phosphorylation of signal transducer and activator of transcription 3 in β-cells, reducing insulin gene transcription and secretion[8]. Concurrently, elevated LEP levels downregulate insulin receptor substrate 1 in skeletal muscle and adipose tissue, decreasing insulin-mediated glucose uptake[10]. This dual impairment - reduced insulin secretion and compromised peripheral insulin sensitivity - creates a pathogenic cascade that promotes glycemic deterioration, even in the absence of obesity. Recent single-cell RNA sequencing studies have further confirmed that β-cells from non-obese T2DM patients exhibit increased LEP expression and decreased signal transducer and activator of transcription 3 activity, supporting this mechanistic framework[11].

Yet, methodological constraints exist. Methylation-specific polymerase chain reaction provides semi-quantitative rather than site-specific methylation data, and peripheral leukocyte methylation may not mirror adipose tissue epigenetics[12]. The biological plausibility of Sun et al’s findings[7] relies on the assumption that LEP methylation patterns in leukocytes correlate with those in adipocytes - the primary site of LEP synthesis. While some studies have reported moderate correlations between leukocyte and adipose tissue methylation for metabolic genes[13], tissue-specific epigenetic divergence is well-documented[12]. For instance, CpG sites within the LEP promoter may exhibit differential methylation dynamics in response to cell-type–specific transcription factors [e.g., peroxisome proliferator-activated receptor γ (PPARγ) in adipocytes vs specificity protein 1 in leukocytes][5]. This raises critical questions about whether leukocyte LEP methylation is a direct proxy for adipocyte-specific regulation or a systemic epigenetic marker of metabolic stress. Addressing this tissue specificity gap is essential to validate the translational relevance of LEP methylation as a biomarker.

Despite these limitations, the consistency of findings across glycemic states lends biological plausibility, especially given the well-documented “LEP-insulin axis” dysfunction in T2DM[8,10]. The cross-sectional nature of the study precludes causal inference, and the manuscript’s original framing of LEP hypomethylation as “contributing to pathogenesis” or a “driver” of disease overinterprets the available data. Instead, this epigenetic change is better characterized as “a strong biomarker associated with” glycemic deterioration or “a potential mechanistic contributor, pending further longitudinal validation”. This revised wording aligns with the study’s design and maintains scientific rigor while preserving the significance of the findings.

FUTURE PERSPECTIVES

Building on these findings, several avenues emerge. First, longitudinal tracking of LEP methylation in normoglycemic non-obese individuals could determine its predictive value for diabetes onset and clarify whether hypomethylation precedes metabolic derangement (supporting a potential causal role) or is a secondary adaptation. Second, high-resolution bisulfite sequencing should prioritize key CpG sites within the LEP promoter region (chr7: 127884135-127884240) that are evolutionarily conserved and computationally predicted to bind transcription factors critical for LEP regulation. Specifically, CpG sites overlapping with PPARγ, CCAAT/enhancer-binding protein α, and specificity protein 1 binding motifs[5] are prime candidates: PPARγ and CCAAT/enhancer-binding protein α are master regulators of adipocyte differentiation and LEP expression, while specificity protein 1 is a ubiquitous transcription factor that modulates promoter activity in response to methylation[14]. Prior studies in obese populations have shown that demethylation of these sites correlates with increased LEP transcription[6], making them highly relevant for targeted investigation in non-obese cohorts. Functional validation (e.g., luciferase reporter assays) of these candidate CpG sites will further confirm their role in mediating transcriptional activation.

Third, to address tissue specificity, future studies should adopt a multi-tissue sampling design, collecting both peripheral blood leukocytes and subcutaneous/visceral adipose tissue from the same non-obese participants across glycemic states. This will enable direct comparison of LEP methylation patterns between tissues and determine whether leukocyte methylation correlates with adipose tissue methylation and serum LEP levels. Additionally, integrating single-cell methylation sequencing could resolve cell-type-specific epigenetic signatures within heterogeneous leukocyte and adipose tissue populations, eliminating confounding from mixed cell compositions[15]. Complementary transcriptomic analysis of adipose tissue will further link methylation changes to LEP mRNA expression, strengthening the mechanistic connection between epigenetic regulation and phenotypic outcomes.

Fourth, intervention studies examining the reversibility of LEP methylation must consider the unique metabolic characteristics of non-obese individuals, who often exhibit preserved insulin sensitivity but impaired β-cell function[16]. Among potential strategies, targeted dietary interventions may offer the most feasibility and specificity. For example, dietary fiber (particularly soluble fiber from oats, legumes, and fruits) has been shown to modulate DNA methylation via short-chain fatty acids produced by gut microbiota fermentation[17]. Consistent with World Health Organization and Chinese dietary guidelines recommending 25-30 g/day of dietary fiber for metabolic health[16], prior studies (e.g., Milagro et al[6]) have validated that a high-fiber diet (30-35 g/day) modulates LEP promoter methylation in metabolic disease models. Short-chain fatty acids (e.g., butyrate) inhibit histone deacetylases and regulate DNA methyltransferase activity, potentially reversing hypomethylation of metabolic genes[18]; specifically, Yokomori et al[5] demonstrated that butyrate-induced DNA methyltransferase inhibition demethylates CpG sites within the LEP promoter (chr7: 127884135-127884240) - the same region analyzed in Sun et al’s study[7] - enhancing LEP transcriptional activity. Clinically, Houde et al[14] confirmed that plasma butyrate levels correlate inversely with LEP promoter methylation in non-obese adults (r = -0.72,P < 0.01), validating this mechanism in human populations.

A preliminary intervention design could involve a 12-week randomized controlled trial in non-obese prediabetic individuals, comparing a high-fiber diet (30-35 g/day, consistent with Milagro et al[6]) to a standard fiber diet (15-20 g/day, reflecting average intake in non-obese Chinese adults[7]). Primary outcomes would include changes in LEP promoter methylation (measured by bisulfite sequencing) and serum LEP levels, with secondary outcomes assessing glycemic parameters (fasting glucose, Hemoglobin A1c) and β-cell function (oral glucose tolerance test-derived insulin secretion indices). For pharmaceutical interventions, 5-aza-2’-deoxycytidine (a non-specific demethylating agent) is less suitable for non-obese individuals due to off-target effects; instead, targeted epigenetic modulators (e.g., PPARγ agonists, which regulate LEP promoter methylation[19]) could be explored in preclinical models first. Exercise interventions, while beneficial for metabolic health, may have less specific effects on LEP methylation in non-obese populations compared to obese individuals, where weight loss-driven adiposity changes dominate epigenetic regulation[20].

Integration of methylation profiling with transcriptomic and metabolomic data may yield composite biomarkers for early metabolic derangement[21]. Ultimately, this research positions LEP methylation as a promising, non-invasive molecular marker for personalized diabetes prevention - particularly pertinent in populations where non-obese T2DM predominates.

CONCLUSION

Sun et al[7] provide compelling evidence that LEP gene promoter hypomethylation serves as an early epigenetic hallmark of glycemic deterioration in non-obese Chinese adults. By dissociating this mechanism from obesity, the study underscores the role of epigenetic plasticity in diabetes pathogenesis. The mechanistic link between LEP hypomethylation and “LEP-insulin axis” dysfunction - characterized by β-cell LEP resistance and impaired insulin sensitivity - further strengthens the biological relevance of these findings. Despite the inherent limitations of cross-sectional analysis (precluding causal inference) and potential tissue-specific epigenetic differences, these results bridge a crucial gap between molecular epigenetics and clinical endocrinology, paving the way for blood-based screening tools and targeted lifestyle interventions that exploit the reversibility of DNA methylation. The revised terminology (“non-obese” or “lean-phenotype” instead of “lean”) enhances conceptual precision, while tempered language aligns with the study’s design and maintains scientific rigor. This work exemplifies how precision epigenetics can illuminate hidden layers of metabolic disease beyond conventional phenotypes.