The association between telomere length and non-alcoholic fatty liver disease: a prospective study

Non-alcoholic fatty liver disease (NAFLD), along with its progressive subtype non-alcoholic steatohepatitis (NASH), are chronic metabolic disorders characterized by excessive hepatic steatosis without a history of alcohol abuse or other liver diseases [1, 2]. Currently, NAFLD is the most common chronic liver disease, affecting approximately 29.9–34.1% of adults globally [3]. NAFLD accounts for the majority of liver-related mortality across the globe [4] and has been linked to other chronic diseases, namely diabetes mellitus and hypertension [5]. Uncovering risk factors and pathogenesis of the disease might be advantageous for identifying at-risk individuals and developing effective interventions.

In particular, NAFLD appears more prevalent among the older population [6], indicating a possible linkage between NAFLD and the aging process. Cellular senescence has generally been identified as a significant mechanism of aging-associated dysfunction. Chief among the drivers of cellular senescence is telomere attrition [7]. Telomeres are nucleoprotein complexes attached to the ends of eukaryotic chromosomes [8]. They are fundamental to maintaining genome stability, as with cellular divisions, telomeres shorten, and a critical decrease in telomere length (TL) will initiate cellular senescence [9]. Accordingly, TL is widely acknowledged as a biomarker of senescence.

Previous research has reported evidence on the involvement of telomere homeostasis in NAFLD development. One animal study found that in established models, tert-deficient (deficiency of telomerase reverse transcriptase) mice were more susceptible to hepatocyte injury and steatosis when given liquid high-fat diets, indicating that the absence of telomerase, which is essential for maintaining proliferative capacity and alleviating telomere attrition, could provoke hepatocyte metabolic dysfunction [10]. Meanwhile, population-based investigations have looked into the association between TL and NAFLD via case-control and cross-sectional designs [11, 12]. Nevertheless, the results are inconsistent, and the existing literature lacks longitudinal evidence and cannot infer causal or temporal relationships.

Furthermore, accumulating evidence has demonstrated that NAFLD etiology is attributed to a combination of behavioral, environmental, and genetic factors. Specifically, apart from dietary and exercise-related factors [13, 14], air pollution is also considered to contribute to NAFLD incidence [15]. In addition, genome-wide association studies (GWAS) have uncovered susceptibility loci for NAFLD in European ancestry [16]. However, what still needs to be clarified is how TL contributes to the association between the above-mentioned factors and NAFLD.

Therefore, utilizing the longitudinal design and comprehensive data on lifestyle, air pollution and genetic variations in the UK Biobank, we performed an analysis to examine the association between TL and NAFLD incidence and further assessed how TL contributes to the association of NAFLD with its corresponding risk factors (aging, lifestyle, air pollution and genetic susceptibility).

In this prospective study of over 450,000 individuals in the UK, our findings suggested an inverse association of TL with NAFLD incidence. In addition, the association of age and NAFLD incidence was partly mediated by TL. The highest risk of NAFLD incidence was found in participants with low TL and old age, low TL and high air pollution score, low TL and unfavorable lifestyle, and low TL and high PRS. In addition, we found a positive addictive interaction between high PRS and low TL, accounting for 14.57% (2.51%, 27.14%) of the risk of NAFLD incidence in participants with low telomere length and high genetic susceptibility.

Previously, the results of the association between TL and NAFLD remained inconsistent. For example, a case-control study involving 70 NAFLD patients and 60 controls found that hepatocyte telomeres (hepatocyte telomere mean fluorescent intensity, MFI) tended to be shorter in NAFLD patients (553 vs 1053, P < 0.0001) [12]. In contrast, an age-matched case-control study of 240 diabetics demonstrated that NAFLD patients had significantly longer LTLs (6400.2 ± 71.8 base pairs [bp] vs. 6023.7 ± 49.5 bp, P < 0.001) [33]. Additionally, several case-control studies have identified the inverse association [34‐36], while others found the association not statistically significant [11, 37]. The generalizability of previous epidemiological studies was largely limited due to their study designs, small sample size, and methods used to define NAFLD, leading to biased results. We conducted a prospective study for the first time in this field to evaluate the association between TL and incident NAFLD as well as the first to identify telomere length as a mediator between age and NAFLD using a population-based approach, though we were unable to identify a significant relationship between TL and NASH incidence, potentially due to limited number of cases (n = 465). Numerous studies have suggested the link between TL and progressed liver diseases. In addition to cirrhosis formation being more common in telomerase-deficient mice [38], animal studies have also suggested that telomere system was crucial for hepatocyte regeneration [39]. The associations between TL and other chronic liver diseases have also been revealed in the UK Biobank (alcoholic liver disease and liver cirrhosis) [40]. Population-based research has also found that patients with more advanced stages of liver cirrhosis tended to have shorter TL [41]. The evidence mentioned above, combined with our study, provides valuable insights into the understanding of the potential risk effect of telomere shortening on not only the incidence of NAFLD but also the subsequent progression of advanced chronic liver diseases.

The aging process is accompanied by telomere shortening. Telomere shortening triggers cellular senescence [9] and induces steatosis in hepatocytes via p53-p21 and p16-Rb pathways. IL-1b, IL-6, chemokines, and SASP components are secreted by these senescent cells, causing both tissue degeneration and further senescence. Inflammation was also induced by interleukins and TNF secretion, macrophage activation, and lymphocyte senescence, leading to further progression of NAFLD into NASH [12, 42]. Telomere length in leukocytes is highly correlated with those in other tissues [43], thus serving as a valid proxy for hepatocyte telomere length and providing further explanation for the established association between LTL and NAFLD incidence.

When analyzing the joint effects of TL and other risk factors of NAFLD, we found that participants with low TL and old age, low TL and high air pollution score, low TL and unfavorable lifestyle, and low TL and high PRS exhibited the highest risk of NAFLD onset. The joint effects may be a consequence of common mechanisms including oxidative stress, inflammation, and insulin resistance [13‐16]. In addition, we also found that high PRS and low TL interact synergistically to lead to the development of NAFLD. One possible mechanism underlying the synergistic interaction is that both factors may increase level of oxidative stress and inflammation, induced by accelerated telomere shortening and activation of genes that promote NAFLD. On the one hand, oxidative stress can result from DNA damage and telomere shortening [9]. On the other hand, genetic variations (HSD17B13 and TM6SF2) linked to NAFLD possess the capacity to modulate steroid levels and impact autophagy and mitochondrial function in the liver, which subsequently influence the expression or functionality of genes associated with oxidative stress, inflammation, or telomere maintenance [44‐46]. Overall, these observations could offer insights for identifying those at-risk and individuals who might gain benefits from interventions to reduce air pollution and cultivate healthy lifestyle.

At present, no drug has been approved to treat or prevent NAFLD. Current management of the disease primarily places emphasis on controlling the metabolic condition, with diet and exercise serving as the mainstays of disease prevention and treatment. Existing literature has suggested some effective interventions for telomere extension, such as danazol [47]. Other therapies targeting reactivation of endogenous TERT expression [48] or exogenous delivery [49] have been proposed. Our study provides evidence for the potential of telomere length as a therapeutic target to reduce NAFLD incidence, which has also been proposed by a previous review on this field [50]. However, the benefits of telomere extension should be considered cautiously, since long telomeres have been implicated in the development of multiple cancers [51]. Studies involving animals and populations should be conducted in the future to investigate possible telomere-lengthening therapies that are capable of preventing and treating NAFLD while also safe for the general public’s health.

Strengths of this research included the large sample size, prospective design, reliable outcome measurement, consistency in the sensitivity analysis, and appropriate adjustment for covariates. It is pertinent to note, however, that this study has some limitations. First of all, UK Biobank is not an accurate representation of the UK population [52]. Thus, the estimation of risk is generalizable [53], though summary statistics including NAFLD incidence are unreliable [54]. Second, the ascertainment of NAFLD cases was based on hospital admission records, thus tended to identify more advanced cases, potentially resulting in the underdiagnosis of milder cases. This is a commonly used strategy for NAFLD identification in previous studies [15, 19, 55, 56], and advanced cases have been proven to be more clinically significant, as the severity was positively correlated with subsequent adverse outcomes [57]. Nevertheless, to address this issue, we extended the diagnosis by using primary care records, and the results remained consistent. In addition, we assessed the association between TL and MRI-derived liver PDFF, as a means of detecting moderate NAFLD cases, and found consistent results, suggesting that TL could not only contribute to advanced cases of NAFLD but also less advanced cases. Third, we acknowledge the proposed new terminology in replacement for NAFLD as metabolic dysfunction-associated steatotic liver disease (MASLD) published in June 2023 [58]. The new definition incorporates the inclusion of metabolic syndrome as a criterion, while the old definition solely relies on histology and ultrasound examination. Nevertheless, considering a body of evidence demonstrating comparable prevalence and the fact that almost all patients with NAFLD meet MASLD criteria [59‐62], our findings of the association between TL and NAFLD incidence could be largely extrapolated under the new MASLD definition framework. Still, future studies should be conducted to directly assess the role of TL on MASLD pathogenesis and verify our study. Fourth, air pollution may be underestimated or overestimated since only four air pollutants were measured at participants’ home addresses during a 1-year period (2010) as an indicator of exposure, which was commonly used in previous environmental epidemiological studies [27, 63]. Fifth, lifestyle status was determined using self-reported data, which may result in incorrect classification. Sixth, due to most of participants being of European descent, caution should be exercised when generalizing our results, particularly in relation to genetic susceptibility.

Long telomere length was associated with decreased risk of NAFLD incidence. The association of age and NAFLD incidence was partly mediated by TL. The highest risk of NAFLD incidence was found in participants with low TL and old age, low TL and high air pollution score, low TL and unfavorable lifestyle, and low TL and high PRS. In addition, we found a positive addictive interaction between high PRS and low TL. Future studies targeting diverse populations should be conducted to evaluate the association of TL and NAFLD pathogenesis and further investigate potential telomere-lengthening therapies that that are capable of preventing and treating NAFLD.