Although the statistically significant SNPs were no longer significant after Bonferroni’s correction, logistic regression still suggested that 5 candidate SNPs consistent with the chi-square tests were associated with hypertension (adjust sex, age, BMI, TG, LDL, and HDL).
VDCCs mediate the entry of calcium ions into excitable cells. Their function involves the contraction of vascular smooth muscle, the release of neurotransmitters or hormones and the expression of genes. The VDCC is a multi-protein complex composed of a variety of subunits, including α1, β, α2/δ and γ. The activity of VDCCs is mainly directed by the pore-forming α1 subunit, which is the target of calcium antagonist (CCBs)[22, 23],and can be divided into different types, such as α1A, α1B, α1C, α1D, α1E and α1S. In this study, the SNPs selected from the genes encoded subunits alpha 1A (CACNA1A), alpha 1C (CACNA1C), alpha 1S (CACNA1S), and beta (CACNB2).
Previous studies have shown that the upregulation of VDCC expression can lead to the enhancement of calcium influx and thereby increase the contractility of VSMCs, which partly explains the pathogenesis of hypertension. On the other hand, VDCCs not only regulate the contraction of VSMCs but also affect differentiation, which is also associated with various cardiovascular diseases. Kudryavtseva O et al. investigated the effect of VDCCs on VSMC differentiation by injecting alpha 1C-siRNA into mice to reduce the expression of VDCCs. Their research showed that the decrease in VDCC expression promoted the change in VSMCs from contractile to non-contractile (the mRNA expression of myosin heavy chain 11 (Myh11), α-actin (Acta2), h-caldesmon was significantly reduced). Therefore, whether the upregulation or downregulation of VDCC expression can cause a change in VSMC tone.
We found one positive SNP (rs10425859), which suggest an increased risk for the people carried the Aminor allele on this SNP. (ORAdditive = 1.269 CI95%Additive = 1.030-1.563), in CACNA1A in the Dai population after logistic regression analyses, in CACNA1A in the Dai population after logistic regression analyses. The α1A (CACNA1A) subunit is mainly expressed in the nervous tissue of the brain and involves the transmission of synaptic signals in the central nervous system. Research on rat models has shown that abnormal expression of the CACNA1A gene can lead to the desensitization of calcitonin gene-related peptide (CGRP) receptors, which are critical in maintaining the systolic response of cerebrovascular smooth muscle[27, 28].In addition, Hu Z et al. also reported the association between CACNA1A polymorphisms (rs8182538) and hypertension in Chinese Han.
In Dai people, another positive SNP (rs2365293), which suggest an increased risk for the people carried the Tminor allele on this SNP (ORDominant = 1.335 CI95%Dominant= 1.002-1.778). It was found in CACNA1S, which encodes the α1S subunit expressed in skeletal muscle cells. According to the database of Online Mendelian Inheritance in Man (OMIM), the mutations in this gene are associated with hypokalemic periodic paralysis, thyrotoxic periodic paralysis, and malignant hyperpyrexia. A report has shown that this gene is associated with hypertension in genome-wide homozygous imbalance analysis, but the specific mechanism of hypertension has not been reported.
Association analyses in China,Lithuanianand Europehave reported the association between CACNB2 polymorphisms (rs4373814 in Chinese Han, rs12258967 in Lithuanian and rs18748804 in Europe) and hypertension. In our study, we found three other positive SNPs (rs17539088, Gminor, ORDominant = 0.732 CI95%Dominant= 0.554-0.966, ORAdditive = 0.769 CI95%Additive = 0.616-0.960; rs16917217, Gminor, ORDominant = 0.737 CI95%Dominant= 0.558-0.973, ORAdditive = 0.776 CI95%Additive = 0.622-0.969; and rs61839222, Aminor, ORDominant = 0.737 CI95%Dominant= 0.558-0.973, ORAdditive = 0.776 CI95%Additive = 0.622-0.969 ), which suggest a decreased risk, in CACNB2 after logistic regression analyses. Although the auxiliary subunit coded by CACNB2 is not the main structure of VDCC activity, it is equally important to maintain the normal structure and function of the whole protein complex. For example, the molecular signals released by the β subunit guide α1 subunit transport to the plasma membrane to construct the correct functional protein complex.In addition, Durairaj Pandian V et al. found that the overexpression of CACNB2 can upregulate the gene expression of the RAS-MAPK pathway, which is one of the key chain reactions of hypertension.
The identification of pathogenic genes of complex diseases such as hypertension has always been difficult. Generally, association analyses are used to screen risk genetic variations of complex diseases. One of the difficulties in association research is that the genetic background of the samples used in the analysis is complex. Due to ethnic and cultural habits, most Dai people are likely to inhabit their villages and rarely intermarry with other ethnic groups in Yunnan Province, leading to the relatively homogenous genetic background of the Dai people. Compared with the Han people who have a complex genetic heterogeneity, the Dai have a genetical homogeneity and smaller pool of susceptibility genes. The disease gene spectrum of hypertension may be narrow in this population. A few disease gene mutations responded for the hypertension in Dai people, which provides an advantage in finding susceptibility variations of hypertension in this special group.
The positive SNPs showed in this study all locate in the intron. These SNPs may be linked to susceptible mutations, which make them show positive results in association analysis. The frequency of susceptible mutations may be low, so it is difficult to obtain significant positive results by association analysis. However, during evolution, especially in these ethnic groups with a relatively narrow genetic background, the mutations are likely to be linked to Tag SNPs, so that they can be detected in our study. These findings may help us understand the genetic causes of hypertension from a unique perspective.
There are some limitations in the current study. First, as a cross-sectional study, we were not able to determine the causal relationship between VDCCs and hypertension. Second, since all the participants were Dai people, the generalisability of the results may not extend to non-Dai populations. Although the main constraint of this study was the small sample size of the Dai group, the statistical power is enough for the number of analysed SNPs. Further functional studies and association analyses in larger samples and other populations should be conducted to confirm our results.