The + 138 Ins/del A polymorphism is located at nucleotide position + 138 of 5’ untranslated region of EDN1 gene in chromosome 6 (chr6:12290496–12290499, GRCh38.p12) and its reference SNP number is rs1800997. Because it is located in the non-coding region of the EDN1 gene, the amino acid sequence of the endothelin precursor is not affected .
A significant association was observed between the + 138 Ins/del A polymorphism and hypertension in this study (P < 0.05) (Table 2). The results of this study are consistent with a previous study conducted in the UK, which revealed a significant association between the insertion genotype and hypertension (χ2 = 6.202, P = 0.045) . However, some other previous studies have produced conflicting results. In another study conducted in the UK, the + 138 Ins/del A polymorphism was not associated with hypertension (P > 0.05) . Correspondingly, there was no association between + 138 Ins/del A polymorphism and hypertension in the Czech, Chinese and Americans in previous reports [9,10,11, 21]. Hence, it can be proposed that interethnic differences may influence the association of the + 138 Ins/del A polymorphism with hypertension.
The mechanism underlying the association between + 138 Ins/del A polymorphism and hypertension in this study may be the influence of gene expression by the + 138 Ins/del A polymorphism. Popowski et al.  revealed the effect of the + 138 Ins/del A polymorphism on mRNA and protein formation in experimental studies. The 4A4A genotype was significantly associated with elevated mRNA level compared to the 3A3A genotypes (P < 0.001). EDN1 protein expression was also higher among the 4A4A genotypes (P < 0.001). These findings may be due to increased mRNA stability and increased gene expression, because the half-life of mRNA in the 4A4A genotype was long as compared with other genotypes (35.4 ± 7.9 vs. 19.9 ± 4.5 min) . Therefore, the current study agrees with the mechanism explored by Popowski et al. .
The + 5665 G/T polymorphism is located at nucleotide position + 5665 of exon of EDN1 gene on chromosome 6 (chr6:12296022, GRCh38.p12), and its reference SNP number is rs5370 . The transversion of guanine-to-thymine changes lysine to asparagine at amino acid 198 of EDN1 protein. The locus is not in the regulatory region of the EDN1 gene, but in the region encoding prepro-endothelin-1 .
In the present study, there was no significant association between the + 5665 G/T polymorphisms and hypertension (P > 0.05) (Table 3). In the multi-model analysis, no significant association was observed between the + 5665 G/T gene variation and hypertension (P > 0.05). Correspondingly, the alleles were not significantly associated with hypertension (P > 0.05).
These findings were consistent with a previous study on Chinese people conducted by Fang et al. . In a total of 423 hypertensive subjects and 114 healthy control subjects, there was no significant association between genotype frequency and hypertension (OR, 0.84; 95% CI, 0.51–1.37; P = 0.541). In addition, there was no significant association between allele frequency and hypertension (OR, 1.28; 95% CI, 0.81–2.01; P = 0.323) . In contrast, in the study of Dzholdasbekova and Gaipov  consisting of 120 Kazakh hypertensive patients and 80 controls, the T allele and the TT genotype were significantly associated with hypertension (χ2 = 13.81, P = 0.001 in genotypic model; χ2 = 7.27, P = 0.007 in allelic model).
This study did not reveal any association between the + 5665 G/T genotype and hypertension because there may be inter-ethnic differences that affect the association between genetic polymorphism and hypertension. Another possible explanation of the + 5665 G/T polymorphism affecting the blood pressure is that genotype-phenotype association may be influenced by environmental factors such as obesity, fitness level, physical activity and socioeconomic status which were not evaluated in the present study [12, 14, 24,25,26,27]. The sample size required to evaluate gene-environment interactions is much larger than the sample size required to determine genetic or environmental factors alone. Although the current study did not investigate the effect of environmental factors on genotype-phenotype association, it showed no association between the + 5665 G/T polymorphism itself and hypertension and this result can provide information for future research that will consider the interaction with the environmental factors.
In the present study, the D’ between the + 138 Ins/del A and + 5665 G/T polymorphisms was 0.108 and the r2 was 0.009 (Table 4). Hence, in this study, LD between these two loci was weak. In previous data, LD between the two loci was found to be weak in some populations including Gujarati Indian from Houston, Texas; Punjabi from Lahore, Pakistan; Indian Telugu from the UK; Kinh in Ho Chi Minh City, Vietnam; Chinese Dai in Xishuangbanna, China; Sri Lankan Tamil from the UK; and Bengali from the Bangladesh (r2 < 0.05, D’ < 0.05). Meanwhile, some other populations revealed strong LD between the two variants as in Europeans and Africans (r2 > 0.5, D’ > 0.5). These data are currently available by using the interactive LDpop tool online . Based on the results of this study and previous reports, it is clear that the LD between the + 138 Ins/del A and + 5665 G/T polymorphisms shows a population-specific difference. There can be no chromosome-specific differences between these two loci because they are located on the same chromosome (chromosome 6) .
The limitations of this study were that the sample size was relatively small, the selection of control individuals was not strict with ambulatory blood pressure measurements, and environmental factors were not investigated. However, this study can prove that hypertension is significantly related to the + 138 Ins/del A polymorphism of EDN1 gene, while the effect of the + 5665 G/T polymorphism is not significant on hypertension in Burmese people, Myanmar.