Tumour Necrosis Factor Alpha
TNFA -308 G>A
The 1000 Genomes Project Database and the genome Aggregation Database (gnomAD) reports global frequencies of 9% and 14.62% respectively for the A allele (NIH).
A meta-analysis from Feng et al. (2011) reports that the distribution of the minor A-allele is different depending on the ethnic population for example, 9% in Chinese, 16% in French and Scandinavian, 18% in German and 24% in Australian populations. Joffe et al. (2010) reported minor A-allele frequencies of 14-20% in their South African study population, but also as 13-23% in Caucasian, 14% in African-American, 10-13% in other African populations.
TNFA Gene Detail
Tumour necrosis factor alpha (TNFα) is described as an inflammatory cytokine expressed by the TNFA gene. Increased TNFα levels have been implicated in the development of obesity, dyslipidemia and obesity-related insulin resistance.
The -308 G>A polymorphism has been associated with increased obesity risk, raised levels of triglycerides (TG) and low levels of high-density lipoprotein (HDL), insulin resistance, type 2 diabetes mellitus (T2DM) as well as an increased risk for metabolic disease and some auto-immune diseases such as type 1 diabetes mellitus (T1DM).
Dietary fatty acids i.e. saturated fatty acids (SFA) and the omega-3 and omega-6 polyunsaturated fatty acids (PUFA) are said to have a nutrigenomic interaction, impacting the expression of TNFA – so influencing obesity risk and serum lipid profiles.
TNFA -308 G>A
The TNFA gene is located within the HLA III region in chromosome 6p21 and gives rise to TNFα, the first cytokine associated with inflammation. Within adipose tissue, TNFα plays a role in regulating adipogenesis, lipid metabolism and insulin signalling while it is also essential in regulating inflammatory pathways via its interaction with pro-inflammatory cytokines and anti-inflammatory adipokines to favour an overall inflammatory state.
In terms of lipid metabolism, TNFα is said to increase the production of free fatty acids by inducing lipolysis; it also modulates cholesterol metabolism with elevated TNFα levels associated with dyslipidemia. It has been observed that cholesterol-lowering statin drugs also reduce TNFα levels and that the obstruction of TNFα production improves lipid metabolism.
Obesity is described as a low-grade inflammatory state and TNFα is reportedly over-expressed in the adipose tissue of obese individuals with greater expression seen in visceral fat compared to subcutaneous fat. Elevated circulating TNFα levels have been observed in obese individuals with a decline associated with weight loss. It has initially been thought that the source of this elevated TNFα level was due to the adipocytes; now it is well-known that TNFα is produced in abundance by macrophages in the stroma vascular fraction of adipose tissue, making macrophages the primary source of obesity-induced inflammation.
The -308 G>A polymorphism, located in the promoter region of the TNFA gene, is reported to cause a 2-fold increase in TNFA transcription, thus a subsequent increase in TNFα production. Several studies have found the A-allele associated with measures of adiposity such as elevated BMI and body fat percentage, increased obesity risk, serum lipids and leptin levels, although some provide conflicting evidence. Previous papers have found an independent association between the -308 G>A SNP and increased TG levels as well as lower HDL concentrations in A-allele carriers.
Inflammation has been widely known as an important feature of T2DM, with high levels of pro-inflammation cytokines, including TNFα. Elevated levels of TNFα have been thought to play a fundamental role in the development of T2DM as it can impair insulin signalling pathways and lead to the damaging of pancreatic β-cells. Previous studies have reported increased insulin resistance in A-allele carriers, although some found no correlation. A large-scale meta-analysis from Feng et al. considered 18 studies and reported no significant association between the -308 G>A SNP and T2DM risk in Caucasian and Asian populations. Similarly, Rodrigues et al. (2017) reported no differences in genotype and allele frequencies when comparing T2DM and control groups in a Brazilian population, neither any correlation between TNFα plasma levels and BMI, waist circumference or fasting glucose levels in the T2DM group.
In contrast, a later meta-analysis from Zhao et al. (2014) reported that the A-allele could be a risk factor for the development of T2DM in Asian subjects and Golshani et al. (2015) found that AA and GA genotypes were associated with higher T2DM risk in an Iranian population.
The -308 G>A SNP has been linked to some metabolic disorders and T1DM. Feng et al. (2011) reported on an earlier meta-analysis investigating the relation between this polymorphism and metabolic syndrome; it reported that A-allele carriers had significantly higher fasting insulin levels, systolic arterial blood pressure, higher obesity risk and possibly HOMA-IR but no significant association with BMI was found.
Joffe et al. Tumor Necrosis Factor-a Gene -308 G/A Polymorphism Modulates the Relationship between Dietary Fat Intake, Serum Lipids, and Obesity Risk in Black South African Women. J. Nutr. 2010; 140: 901–907.
It is reported that both the quality and quantity of dietary fatty acids can modulate the relationship between TNFA on obesity and serum lipid profiles. It is suggested that the presence of the A allele by itself does not confer risk but rather may be indicative of a greater responsiveness or sensitivity to changes in dietary intake.
In summary of the findings below, it is suggested that A-allele carriers should monitor total fat intake, especially dietary saturated fats and omega 6 PUFAs. The World Health Organisation (WHO) recommends a (n-6):(n-3) PUFA ration of 5-10:1. High dosages of fish oil supplementation (5g/d) may not be recommended.
For GG genotypes, following a hypocaloric diet (when overweight / obese for weight loss), increasing PUFA intake (including omega 3 FA, in the above mentioned ratio recommended by the WHO), as well as limiting carbohydrate intake (33% total energy) when glucose / insulin levels are elevated, may be considered beneficial.
Total Fat Intake
A South African (SA) based study from Joffe et al. (2010) found that the obesity risk in A-allele carrying black SA women increased with total dietary fat intake, whereas no independent association between the rs1800629 SNP and BMI, insulin resistance and serum lipid concentrations were found. At dietary fat intakes < 40% total energy, the odds of A-allele carriers being obese were lower than for the GG genotype, whereas obesity risk in A-allele carriers increased compared to the GG genotypes, as fat intake increased.
Similar to the study from Joffe et al. (2010), Nieters et al. (2002) previously found no association between the -308 G>A SNP and obesity in German Caucasian men and women, but when dietary fat intake was considered, female A-allele carriers in the highest third for linoleic and arachidonic acid intake (omega 6 PUFAs) had an increased risk for obesity.
In South African A-allele carriers, increasing PUFA intake (as % total energy intake) resulted in elevated LDL-cholesterol levels, whereas increased α-linolenic acid (ALA) intake led to a decrease in total cholesterol:HDL-cholesterol ratio in black SA women.
De Luis et al. (2013) investigated the effect of the -308 G>A SNP on weight loss and metabolic parameters when participants followed a hypocaloric diet, either predominantly high in MUFAs (23% of total energy; daily inclusion of 30-40ml extra virgin olive oil and 40-50g walnuts or almonds) or PUFAs (7% of total energy; daily inclusion of 30-40ml sunflower oil and 3 servings of oily fish per week). There was also the inclusion of a 60-minute aerobic exercise class 3 times per week. Although there was a significant decline in weight, BMI, waist circumference and fat mass in both diet groups after the 3-month intervention, no significant differences were found between G-allele and A-allele carriers in this effect on anthropometrics before or after the intervention. Both genotype and diet groups displayed a significant decline in leptin levels post-intervention. Only in G-allele carriers however, following the high PUFA diet, were there significant decreases in glucose levels, insulin levels, total cholesterol, LDL-cholesterol and triglycerides after the 3-month intervention – GG homozygotes demonstrated better metabolic responses than GA heterozygotes.
In a previous study from De Luis et al.(2013), similar metabolic improvements were reported in G-allele carriers when following a low-fat diet (27% total energy) – the authors speculated that this may be because of a similar distribution of PUFAs in both studies (18% of total fat intake versus 22.7% of total fat intake).
No significant changes in gene expression or plasma TNFα levels were observed in participants who completed a 6-week n-3 FA supplementation with 5 g/day of fish oil. There was however a gene-diet interaction seen with the modulation of plasma inflammatory biomarker levels – A-allele carriers, when compared to the GG genotype, had significantly higher CRP levels after fish oil supplementation. Research does however report that marine omega-3 FAs have been shown to decrease expression levels of inflammation-related genes as well as plasma concentrations of cytokine and C-reactive protein (CRP). Omega-3 PUFA is said to impact inflammation through altered eicosanoid production, but possibly also by influencing cell signalling and gene expression.
Decrease Saturated Fat Intake
A diet high in saturated fatty acids (SFA) has been linked to increased inflammatory markers, especially in overweight and obese individuals. This inflammatory effect is said to be through the activation of the toll-like receptor 4 (TLR4) pathway, which is expressed in both subcutaneous and visceral adipose tissue, where SFA serve as ligands for TLR4. Experimental studies have demonstrated that adipocytes incubated with SFA, but not with MUFA, oleic acid, omega-3 PUFA or DHA, could increase TNFA expression and circulating TNFα concentrations.
Another study from De Luis et al. (2013) examined the effect of the -308 G>A SNP on weight loss and metabolic parameters when participants followed a high-protein, low carbohydrate hypocaloric diet (Diet HP – 1050 kcal/d and 33% carbohydrates of total energy) versus a standard hypocaloric diet (Diet S – 1093 kcal/d and 53% carbohydrates of total energy). A significant decrease in BMI, weight, waist circumference, fat mass, leptin levels and systolic blood pressure was observed in both genotypes and both diet groups. With both diet groups, but only in GG homozygotes, total cholesterol, LDL-cholesterol and TG levels significantly decreased. Only in the Diet HP group, and only in the GG genotype, insulin levels and HOMA-R significantly decreased. Similarly, a previous study from De Luis et al. reported that the significant improvement of glucose and insulin levels was only observed in GG genotypic participants following a low carbohydrate, hypocaloric diet (but not a low-fat hypocaloric diet).
Expression and Sequence Variants of Inflammatory Genes; Effects on Plasma Inflammation Biomarkers Following a 6-Week Supplementation with Fish Oil
Cormier et al, 2016.