The present study investigated the genetic polymorphisms of rs74650330 in patients with CAD and controls. The results obtained in the current study suggested that the rs74650330 SNPs exhibited no significantly different genotype or allele frequencies between patients with CAD and controls.
From the literature [12], we knew that rs13107325 of the SLC39A8 gene was associated with NT-proBNP levels in patients with acute coronary syndrome. We therefore wanted to investigate whether this SNP was associated with CAD in the Chinese population. Our data showed that the rs13107325 polymorphism was not present in the Han Chinese participants enrolled in the current study. However, there were several reports in the literature about a missense mutation in the SLC39A8 gene (rs13107325) that was associated with schizophrenia, Crohn’s disease and severe idiopathic scoliosis in European populations [19-21]. Exploratory analyses suggested that the rs13107325 SNP is monomorphic in individuals of Asian and African descent, while it is prevalent in patients of European descent, which is related to the selective pressure of colder climate in Europe, leading to an increase in the frequency of the T allele so that humans can adapt to the environment [22].
In the analysis of PCR product sequencing results, a single base deletion SNP (subsequently confirmed as rs74650330 in NCBI’s SNP database) was observed at 261 bp for rs13107325. rs74650330 is an intron mutation, and further investigation is required to elucidate its effects. From the Regulome DB (https://www.regulomedb.org/regulome-search/) and HaploReg (https://pubs.broadinstitute.org/mammals/haploreg/haploreg.php) databases, we could not obtain annotation information for the rs74650330 SNP. Most of the risk SNPs detected by GWAS are located in the noncoding regions of the genome, indicating that these SNPs play their functional roles mainly by regulating gene expression [23]. Introns may regulate gene expression through splicing and "exon shuffling" during evolution. If a mutation occurs in splice junctions, the splicing process is usually disrupted, as the splicing machinery is unable to recognize the sequence. Furthermore, Pagani et al revealed that mutations in the middle of intron sequences may result in changes to splicing patterns [24]. Sometimes, lethality is attributed to intron mutations rather than missense mutations [25].
To explore the potential effect on CAD, we further compared clinical features in the case and control groups according to the rs74650330 genotype. The data showed that statistically significant differences in the serum HDL-C and LDL-C levels were present between the C/C and C/del genotypes in the control group (P<0.05). The mutant allele carriers had lower HDL-C (1.167-fold decrease) and higher LDL-C levels (1.126-fold increase) than the C/C homozygotes, which suggested that the CC genotype could be a protective genotype in healthy people. However, no statistically significant differences in the lipid profiles of C allele and mutation allele carriers in the CAD group were observed (P>0.05). Comparing the lipid levels between the two groups, for the C/C genotype, CAD patients had higher TG levels and lower HDL-C and LDL-C levels, and for the C/del and del/del genotypes, CAD patients had lower TC and LDL-C levels.
CAD has a complex pathogenesis that may be caused by risk factors other than cholesterol levels, including smoking, hypertension, glucose intolerance, dyslipidemia and obesity [26]. This meaningful result was not observed in the CAD population, which may be related to the possibility of CAD patients taking statin lipid-lowering drugs. Statins are comprehensive lipid-regulating drugs that not only strongly reduce TC and LDL-C levels but also lower TG levels to some extent. The effect of the drugs may mask the protective effect of the CC genotype. In addition, according to our results, we speculate that SLC39A8 with the C mutation could regulate the expression of genes that promote the development of atherosclerosis and CAD or inhibit the expression of cardiovascular-protective genes. These negative effects might outweigh the blood fat-lowering benefits of statins, which may explain why the C deletion of rs74650330 increases the risk of CAD in patients with low LDL-C levels. However, how it regulates the expression of genes associated with CAD needs further study.
Furthermore, the results of this study indicated that the C/del genotype did not affect the risk of CAD when the LDL-C level was equal to or greater than 2.365 mmol/L. The possible reason is that atherogenic dyslipidemia, characterized by increased levels of LDL particles, is considered to be a major factor in CAD risk [27]. Compared with the strong effect of LDL on CAD, the pathogenic effect size of the rs74650330 C→del mutation may be smaller. However, since there is a risk, and it is more than seven times higher, this mutation should not be clinically ignored.
Study strengths and limitations
In this study, we introduced a method for detecting the SNPs (rs13107325 and rs74650330) of SLC39A8 in one tube by base-quenched probe technique. And, the potential relationship between rs74650330 and lipid levels in CAD patients from Han population in Jiangsu (China) had been further revealed. However, the present study had a number of limitations. Certain patients may receive lipid-lowering therapy prior to diagnosis with CAD, suggesting why the TC levels were lower in patients with CAD than in the control group subjects in the current study, which differed from previously published results [28]. We had a limited sample size to detect SNPs, and weak effects can be observed when considering multiple corrections. In addition, our sample was limited to the Han population. The control group may contain some volunteers whose coronary arteries have stenosis but no symptoms. Further research is warranted on patients with large sample sizes and other ethnic groups as well as patients enrolled from multiple sources to confirm our findings.