Association of telomere length and telomerase methylation with n-3 fatty acids in preschool children with obesity

The rapidly increased prevalence of childhood obesity and its associated comorbidities including obstructive sleep apnea syndrome, psychological problems, cardiovascular diseases, type 2 diabetes, cancers, etc., are seriously threatening short-term and long-term health [1]. The underlying mechanisms for the obesity associated complications are attributable to the chronic inflammation, formation of reactive oxygen species (ROS), lipid peroxidation, and resulted DNA instability or damage [2, 3]. Telomeres, specialized DNA-protein structures located at the ends of eukaryotic chromosomes, have been found to be negatively correlated to obesity in length, being with an important heterogeneity [4, 5]. In children, studies have brought discrepant results, with some showing that obesity is related to shorter telomeres [6‐8] but others finding no association [9, 10].

Telomeres are composed of repetitive DNA with highly conserved sequences (5′-TTAGGG-3′), and associated proteins [11], and play a crucial role in cellular survival, by maintaining chromosome stability and limiting progressive loss of genomic information caused by semi-conservative replication of DNA during the cycle of cell division [12, 13]. Telomere maintenance requires the active telomerase that is a ribonucleic acid-protein complex (RNP) composed of a single long non-coding telomerase RNA (TER), telomerase reverse transcriptase (TERT), and other proteins that vary among organisms, and functions to add telomeric repeat DNA to chromosome ends [14, 15]. It has been demonstrated that telomerase activity is mostly undetectable in somatic cells several weeks after birth, and high in cells with high generative potentials, such as germ line cells, hematopoietic and few types of stem cells, most cancer cells, and during embryonic development [16‐18]. Mutations in many of the core telomerase RNP subunits, promoter elements, as well as the maturation factors lead to telomerase deficiency diseases such as dyskeratosis congenita or aplastic anemia, while its aberrant upregulation is a prerequisite for the immortal phenotype of most cancer cells [19‐22]. In cancers, DNA hypermethylation with the TERT promoter is a prevalent telomerase-activating mechanism that can act independently of or in conjunction with promoter mutations [19, 20].

Cumulative evidences show that telomere length can be affected by many factors, such as age, gender, genetic variation, physiological stress, lifestyle factors (smoking, obesity, and lack of exercise), environmental exposure to carcinogens, and diseases (cancer, etc.) [14, 23]. Most of the telomere loss happens in the first 4 years of age, plateauing by age 4, with the greatest amount of attrition within the first years of life [18], followed by a slower attrition rate throughout adulthood. In obesity, telomere length is negatively associated with body weight and fat mass in American Indians, independently of chronological age, lifestyle factors and obesity-related inflammation or comorbidities [4]. Regarding the effects of diet and nutrients, most studies indicate that a healthy diet characterized by a high intake of dietary fiber and unsaturated lipids exerts a protective role on telomere health, whereas high consumption of sugar and saturated lipids accelerates telomere attrition [24]. National Health and Nutrition Examination Survey indicated that higher mineral and vitamin consumption is associated with longer telomeres among adults in the United States [25]. Vitamin D supplementation is found to be able to increase peripheral lymphocyte telomerase activity in overweight African Americans [26].

Since the 1960’s, indiscriminate recommendations have been made to substitute vegetable oils (high in n-6 PUFAs and low in n-3 PUFAs) for saturated fats, resulting in the increased ratio of n-6/n-3 PUFAs. This dietary fat change has been considered to be related to the increased prevalence of chronic non-communicable diseases including obesity, diabetes, cardiovascular diseases in modern society [27]. Under intake of n-3 PUFAs and its imbalanced ratio to n-6 PUFAs in early life may be an important determinant for adipocyte differentiation and growth and may relate to obesity pathogenesis in later life [28, 29]. Proposed mechanisms by which n-3 PUFAs improve body composition and counteract obesity-related metabolic changes include modulating lipid metabolism, adipokine expression and adipogenesis, alleviating adipose tissue inflammation and oxidative stress, and altering epigenetics [29‐32]. Therefore, this study aimed to investigate changes in telomere length and TERT promoter DNA methylation, and further to determine their correlation with n-3 PUFAs in preschool children with obesity.

A systematic review and meta-analysis has demonstrated a negative correlation between adult obesity and telomere length with weak to moderate statistical significance for the main research, and an important heterogeneity [5]. In child population, studies investigating telomere length and obesity, and their causal relationship proved inconclusive [6‐10]. In this study, in Chinese preschool children with obesity, leukocyte telomeres were shown being shortened and had a negative association with the BMI. This may increase the risk of metabolic complications in later life, because leukocyte telomere length has been found to predict onset of cardiometabolic diseases in adults, specifically diabetes mellitus and cardiovascular disease [39‐41]. Also, the shorter telomere length in preschool age is associated with obesity at age 9 in Latino Children [9].

Although the molecular mechanisms by which obesity affects telomere length have not been described clearly, current research findings suggest that the shortened telomere was probably caused by the increased chronic inflammation and oxidative stress in obesity [42]. It is reported that expression of TERT, the key enzyme for maintaining telomere length, induced by cytokines is organized through the PI3K/AKT and NF/kB signaling pathways [22]. In the present study, we found that DNA methylation fractions of the TERT promoter were increased in children with obesity. Epigenetic changes including DNA methylation have been recognized to be the key players in governing gene expression, and higher promoter DNA methylation may down-regulate gene expression [43, 44]. The data presented here suggested that DNA hypermethylation at the TERT promoter might reduce mRNA expression of TERT and its activity, and thus lead to telomere attrition in obesity. Consistently, hypermethylation of the TERT promoter alleles signals transcriptional repression of those alleles, leading to attenuation of TERT activation in cancer cells [45]. In contrast, many studies reported that DNA hypermethylation of the TERT promoter is positively related to telomerase activity in cancers [20, 46, 47].

The available evidence suggests that some antioxidant nutrients, the consumption of fruits and vegetables, and Mediterranean diet are mainly associated with longer telomeres [48]. With regard to macronutrients and telomeres, total protein and carbohydrates have not been clearly associated with telomere length; whereas the quality of carbohydrates, and particularly dietary fiber, may have a potential beneficial effect on telomere health [49, 50]. The effects of dietary fats on telomeres have been studied in more detail than the other macronutrients. Although the overall relation of monounsaturated fatty acids and PUFAs with telomere length is inconsistent [48], most studies have demonstrated a positive relation between n-3 PUFAs and telomere length in adult chronic diseases [51‐53]. Differently, a study on Chinese adults reported that n-6 PUFAs are positively, but n-3 PUFAs are negatively associated with leukocyte telomere [54]. Herein we found that erythrocyte DHA content was decreased in obese children, and had a positive association to leukocyte telomere length. Unfortunately, no association between erythrocyte DHA and DNA methylation of the TERT promoter was shown, although DHA functions to regulate DNA and histone methylation through affecting methyl group metabolism [31, 32, 55]. Therefore, the mechanisms by which DHA affects telomere length may reside in its anti-inflammatory or anti-oxidative stress effects [29, 30], but is independent of its function in epigenetic modification.

With respect to saturated fat, it has been reported that women with SFAs intake in the lowest compared to highest quartile had significantly longer telomeres, and that in men, high total fat, SFAs, and butter intake was inversely associated with telomere length [56, 57]. In contrast, we found that erythrocyte total SFAs were increased in children with obesity, and were positively correlated with telomere length. The underlying reasons could not be answered herein. It may be involved in more total fat and SFAs required for infants and toddlers than older children and adults. High fat diets are important during early life because of very high energy needed for growth and for the rapid development of the nervous system, and fat restrictions may affect body composition and increase obesity risk in later life [58]. To note, in addition to fatty acids, many other factors reported can affect telomere length, such as age, gender, physiological and psychological stress, sedentary lifestyle, exposure to environmental carcinogens, etc. [14, 23]. Thus, more researches are needed to clarify the underlying mechanisms for shortened telomere length in the future.

The duration of obesity could have a significant influence on the observed changes in telomeres. Unfortunately, the data after birth at fixed time points, reflecting the development of obesity, were not available and thus its effects on telomere length could not be investigated in the current study. This question should be highlighted as a future research perspective. Meanwhile, the severity of obesity may also affect telomeres. It could be intriguing to divide the children into moderate obese and severe obese, but the smaller number could not allow further analysis and satisfactions. Future studies with a large sample size may be helpful for this hypothesis test.

In summary, preschool children with obesity had shorter leukocyte telomeres, accompanied by DNA hypermethylation at the TERT promoter. A possible relationship between the shortened leukocyte telomeres and reduced body DHA content and increased AA/DHA ratio in obesity is suggested. However, the association of DHA with telomere length might not be involved in its modification on the TERT promoter DNA methylation.