The present study was performed to investigate the association of reported G-2548A polymorphism in the 5, promoter region of LEP gene with obesity-related biomarkers such as increased BMI, waist, hips, SAD, plasma leptin levels, plasma glucose levels, and blood pressure in obese Saudi patients. Analysis of the LEP gene gave three different variants of the genotype: GG for the wild type, GA heterozygous, and AA for the homozygous for the mutant type. Subjects with normotensive ND had 46.0% (GG), 40.0% (GA) and 14.0% (AA). Obese normotensive ND subjects had 40.0% (GG), 38.8% (GA), and 21.2% (AA). Patients with obese hypertensive T2D had 33.0% (GG), 48.7% (GA), and 18.3% (AA).
Our data showed no association between LEP G-2548A variants with BMI and other anthropometric measurements such as waist, hips, and SAD, indicating that this genetic variant is not a relevant marker of obesity. Previous studies analyzing the association between the G-2548A LEP variant and increased BMI have been controversial. Indeed, a lack of association between the LEP G-2548A gene polymorphism and increased BMI was reported in different Caucasian populations including French [31], Spanish [32], Brazilian [33], Tunisian [25], and Romanian obese patients [34].
In contrast to these findings, other studies confirmed this relationship between the LEP G-2548A variant and increased BMI in overweight Europeans [20] and Taiwanese Aborigines with severe obesity [24]. A family heart study by Jiang et al. reported also a significant association between this variant of the LEP G-2548A gene and obesity [35]. Additionally, In the Taiwanese population, Wang et al. reported that the BMI of the GG genotype was significantly higher than that of GA and AA genotypes in extremely obese patients [24]. Since the LEP G-2548A gene polymorphism is not at a conserved region among humans, mice, and rats, therefore it is functional significance is uncertain [36]. On the other hand, this variant is located at the 5' end of the promoter region of this gene [19]. It has been postulated this region might contain inhibitory elements subcontinent [37].
Our data showed that the GA genotype of G-2548A LEP polymorphism had significantly higher plasma leptin levels compared with those carrying GG genotype in the study population and healthy ND control subjects. In contrast, the AA genotype of the LEP G-2548A gene polymorphism had significantly higher plasma glucose levels and HOMA-IR in the study population. However, conflicting reports have been shown. In the Taiwanese population, no association was found between this LEP variant and plasma leptin levels [24]. On the other hand, the French men cohort study found that the AA genotype of the LEP G-2548A gene was associated with increased plasma leptin levels [20]. This finding was confirmed by a Sweden study, which showed that the -2548A allele of this variant was associated with increased leptin messenger RNA (mRNA) levels and increased adipose tissue leptin secretion rate [21]. Conversely, a study carried out in a French population showed that the G-2548A LEP variant could potentially alter leptin expression, and female subjects with the AA homozygote had lower mean leptin levels than girls with other genotypes [31]. The relationship between the AA genotype and lower plasma leptin concentrations was also described in obese Brazilian women [23].
Leptin inhibits the glucose-stimulated insulin secretion, and leptin receptors are present on β-cells as well as on fat cells, thus enabling leptin to modulate both insulin secretion and action [38]. Our study, therefore, found that the AA genotype of the LEP G-2548A gene was significantly associated with increased plasma glucose levels and HOMA-IR. Furthermore the -2458A allele of this gene was significantly associated with elevated plasma glucose levels. Our results are in agreement with data from Julie et al. [39], who suggested that the AA and AG genotype carriers have a significantly higher risk for gestational diabetes mellitus than those carrying the GG genotype. However, these findings are in disagreement with a pervious study in which the LEP -2548G carriers were associated with higher plasm glucose in patients with T2DM [40]. Based on our data, it can be suggested that the AA and GA genotype carriers have a significantly higher risk for T2DM than those carrying the GG genotype, which supports the hypothesis that leptin has a role in the development of insulin resistance and subsequently diabetes.
The role of leptin in the pathogenesis of obesity-related hypertension seems through sympathetic activation in the circulatory system and/or at the renal level [14-15] and being prevented by adrenergic blockade drugs [14]. One of the main consequences of SNS activation in hypertension is the increased renal sympathetic nerve activity, which leads to sodium retention, volume expansion, and increased blood pressure [41]. In addition to SNS activation, leptin induces endothelin-1 synthesis in vascular endothelial cells, increases the expression of endothelin type A receptor in vascular smooth muscle cells [42] as well as smooth muscle cell proliferation [43], thus contributing to the increased peripheral vascular resistance. However, our data showed no association between genotypes and alleles of the LEP G-2548A polymorphism and increased blood pressure. However, previous studies found a positive association systolic and diastolic blood pressure with the LEP G-2548A gene. In Tunisian obese subjects, the AA genotype had significantly higher systolic and diastolic blood pressure [25], whereas, in obese Brazilian patients, the AA genotype of LEP G-2548A gene polymorphism had significantly lower levels of systolic, diastolic, and mean arterial blood pressure [44].
Finally, conflicting in results between our data and previous studies may be due to interactions of LEP G-2548A polymorphism with other variants in leptin and/or leptin receptor genes, as well as other variables such as gender, characteristics of subjects, sample size and population, or the model used in genetics analyses. In conclusion, the present study showed that the genotypes distribution of G-2548A variant of the LEP gene (GA and AA) are associated with increased plasma leptin and glucose levels in a set of Saudi individuals. Moreover, AA and GA genotypes and A allele might be an important risk factor predisposing healthy subjects to T2DM. A larger clinical study should be undertaken with a larger population sample to investigate the real meaning of correlations between the LEP gene polymorphism and diabetes mellitus, supporting evidence for leptin gene polymorphism as a genetic factor on diabetes mellitus risk.