1. Introduction
Atherogenesis and dyslipidemia are key risk factors for coronary artery disease (CAD), the leading cause of mortality in the world [
1]. Atherosclerosis is an inflammatory process that encompasses the formation of plaque in the artery walls contributing to cardiovascular disease (CVD), hypertension, stroke, and coronary artery disease [
2]. Atherosclerosis is complex and multifactorial, involving genetics, the environment, lipid levels such as low-density lipoprotein cholesterol (LDL-C), total triglycerides (TG), and high-density lipoprotein cholesterol (HDL-C), the comorbidities, diabetes, dyslipidemia, hypercholesterolemia, and the accumulation of oxidized LDL [
3]. Increasing age is highly correlated to atherosclerosis and current epidemiological studies suggest that early detection would lead to prevention and the deployment of interventions for treatment and control prior to cardiovascular disease onset. However, the characterization of early molecular phenotypes atherosclerotic key events remain to be fully elucidated [
4]. Dyslipidemia, one of the main underlying factors in atherogenesis, is highly prevalent worldwide; for instance, it reaches 53% in the United States, 49% in China [
5], and 80% in Turkey [
6]. In Mexico City, the CARMELA study determined a prevalence of 50.5% for hypercholesterolemia and 32.5% for hypertriglyceridemia, and hypotheses are that nearly every two out of three city dwellers might have some type of dyslipidemia [
7,
8].
The early identification of atherosclerosis and dyslipidemia is urgently needed to pinpoint targeting therapies and preventive interventions. Lipids levels and lipid indexes have been of some help because of their association to cardiovascular risk. For example, LDL-C, TG, and HDL-C are strongly correlated to coronary heart disease (CHD) although their underlying genetics has not been fully characterized [
9]. A genetic profile that may aid in the identification of dyslipidemia, and CVD or its predisposition could propel earlier diagnosis and interventions.
Up-today, the association between genetics and blood lipids has aided in the search for reliable markers of CVD, atherosclerosis, and dyslipidemia since in part, these have a heritable basis and the literature evidences their association with an extensive collection of genetic loci [
10,
11]. For example, single nucleotide variants (SNVs) on
APOE,
CETP,
LPL,
PCSK9, and
GCKR have been significantly associated with lipid levels and dyslipidemia in different populations [
10,
11]. Despite the high prevalence of CVD and dyslipidemia in Mexico, only a few studies have investigated its relationship with genetic variation. One report found SNVs on
APOA5,
GCKR,
LPL, and
NPC1 associated with hypertriglyceridemia [
12], while polymorphisms on
ABCA1,
CETP,
LIPC, and
LOC55908 have been associated with hypoalphalipoproteinemia, many of these variants are shared by different populations but some seem to be unique to certain geographical ancestries [
10,
13].
Recent studies have highlighted the accuracy and relevance of lipid ratios/indexes to better assess dyslipidemia and cardiovascular risk. The atherogenic index of plasma (AIP=log TG/HDL-C) can accurately predict hypertension, metabolic syndrome, and ischemic stroke even when HDL-C and TG levels seem normal or when isolated values of TG or HDLC-C cannot assess this risk [
14,
15]. The direct measurement of HDL-C and LDL-C has shown bias in assessing cardiovascular health (16) but their ratio i.e., the CI2 (LDL-C/HDL-C, CI2), although less cited, has been confirmed as a better predictor of cardiovascular risk [
9,
16,
17]. Hence, there is an apparent, but not as frequently acknowledged, value of AIP and CI2 indexes to identify cardiovascular risk. It is possible that AIP and C2 ratios together with genetics could improve the clinical assessment of CVD risk and dyslipidemia. Nevertheless, little is known about the direct relationship between these indexes and genetic variation. Therefore, here we investigated the potential association between lipid indexes, AIP and CI2, and genetic variants in Mexican adults free of cardiovascular disease.
4. Discussion
The identification of a quantitative relationship between genetics and CVD surrogates such as AIP and CI2 is of health transcendence due to the high mortality associated with cardiovascular disease underlied by atherogenesis. Several lipid levels and their indexes have attempted to predict cardiovascular risk and support prevention strategies. There are a couple of studies that associate lipids and lipoproteins measurements with genetic loci in different populations, but high interindividual or population variability has clouded their interpretation and potential application [
30,
31]. The AIP and CI2 indexes have emerged as surrogate markers of cardiovascular health as they have been reliably correlated with cardiovascular risk, lipoprotein size [
17,
29], or plasma atherogenicity [
30]. They have demonstrated to be better CVD predictors compared to TG/HDL-C alone [
31,
32]. M. Dobiasova and J. Frohlich showed that the AIP index closely correlates to lipoprotein particle size and fractional esterification rate of HDL-C which in turn is a predictor of coronary artery disease risk [
33,
34], cerebrovascular accident [
35], the thickness of the carotid intima-media, statin response, and ischemic stroke [
36]. Current reports have provided valuable molecular insights into lipid metabolic pathways and dyslipidemia, but no study has identified the relationship between the ratios, AIP and CI2 with genetics [
10].
Here, we report statistically significant associations between gene variants and lipid indexes AIP and CI2 previously reported as relevant for lipid levels and CVD including,
APOC3/APOA1, 10q.21.3 rs1125117,
KCND3, and
VLDR. Evidence from other fields suggests that adding genetic information to clinical CVD prevention may fine-tune the utility of lipid indexes for disease prediction [
20], likely facilitating the development of specific laboratory tests and algorithms. Below, we discuss the relevance of our findings in the scope of novel and previous genetic associations.
The identification of variants on
APOC3 and
VLDR associated to atherogenic indexes confirmed previous inferences, since these genes are well known to impact lipid levels.
APOC3 has been repeatedly associated with dyslipidemia [
37] and blood lipids. Several studies confirm a variety of loci, not always in LD, mapping on the
APOC1,
APOC3, and
APOA5 clusters and its relation to blood lipids [
38,
39,
40]. Variant
APOC3 rs147210663 has been reported over 40 times associated to dyslipidemia, cholesterol, and BMI, it is in LD with
APOC3 rs5128 here identified, and whose association with triglyceride levels in Pima Amerindians has been reported as a founder mutation [
41]. Also, a recent multi-ancestry analysis on 170,000 exomes including 16,440 individuals of “Hispanic” origin reported that
APOC3 is a relevant gene for HDL-C and the TG/HDL-C ratio [
11]. To further delve into the relevance of the
APOC3 and chromosome 11 loci, Jurado-Camacho et al. described the
APOA1/C3/A5-ZPR1-BUD13 cluster and its impact on several lipid traits including, HDL-C and TG [
41]. These observations agree with our results of the intron variant
APOC3 rs5128 as significantly associated to the AIP index in Mexican adults highlighting that the connection of
APOC3 and blood triglycerides is likely population independent [
42]. Also associated with AIP were intronic variants,
ARRB1 rs11236389 and
CYBA rs12709102 the former codes for the cytosolic protein, arrestin beta 1, with immune functions but no clinical reports were found. The second one is part of the microbicidal oxidase system of phagocytes that has also been related to CAD, the thickness of the carotid intima media, and as a direct indicator of atherogenicity and obesity validating in part our observations [
43]. Although the link between lipid metabolism and variants rs11236389 and rs12709102 here identified have not been previously reported, it might not necessarily be an unexpected observation since these genes, or their paralogs seem to bear variants in relation to cardiovascular risk [
1,
43].
TTN/CCDC141 rs10497525 is an intron variant of the large sarcomeric protein, titin, variations on this gene cause muscle disorders and cardiomyopathies [
44].
TTN/
CCDC141 is highly expressed in the heart [
45] suggesting its potential role in biochemical pathways and cardiovascular health, but not yet discussed under the scope of dyslipidemia and atherogenic indexes. Our results may give rise to the biochemical connection between heart health, blood lipid levels and genetics identified in adults under 53 years.
The last variant associated with the AIP index was
KCND3 rs6703437, this gene codes for a potassium channel responsible for smooth muscle contraction and it is associated with the Brugada syndrome and cardiac conduction [
46].
KCND3 rs6703437 is 0.6 Kb apart and in partial LD with variant rs672757, this latter directly associated with obesity in patients with asthma [
47], hinting towards a potential role of heart disease under a lipid imbalance.
Overall and according to the recent literature, the variants here associated with the AIP index may be indicative of cardiovascular health, heart function, lipid transport, and metabolism. Our observations confirm previous correlations between lipid levels and genes, APOC3, TTN/CCDC141, KCND3, CYBA, and ARRB1 and attest for the first time to a genetic relationship with AIP.
For the CI2, we identified six variants on intergenic and non-coding loci, four of them on chromosome 12 with few or no reports of their clinical relevance. We identified DIPK2B rs4294309 an intron variant located on chromosome Xp11.3. DIPK2B codes for a protein kinase domain 2B and known variants influence autism and intestinal carcinoma, but its relation to lipid metabolism or cardiovascular health has not been previously reported. However, DIPK2B maybe indirectly related to lipid and lipid indexes since its association with autism has been linked to alterations of cholesterol levels, decreased HDL-C, apolipoprotein A1 (ApoA1), and apolipoprotein B (ApoB) (48,49), suggesting a potential lipid-gene-autism relationship that may possibly pinpoint to a genetic marker. On chromosome 12 we identified four variants associated with CI2, rs6582413, rs12817366, rs34115639, and rs10880344 the two former on the Long Intergenic Non-Protein Coding RNA 2451, LINC02451, and the two latter in intergenic regions. Genome-wide linkage and meta-analyses of chromosome 12 have confirmed the presence of variants relevant to premature myocardial infarction and atherogenic plaque of the carotid intimal media, but these reported loci do not appear to be in close LD with the variants here listed. It is possible that several regions on chromosome 12 point towards genetic regulation or coding genes correlated to the CI2 and cardiovascular health that together may be considered as a polygenic cluster on chromosome 12.
Genetic variation on 10q21.3 here, rs7762658, rs11251177, have been associated with coronary artery disease in the GENOA study and in a pedigree of familial hypercholesterolemia [
50]. Loci 10q.21.3 has been suggested to harbor genes with a role in subclinical coronary atherosclerosis [
1], we identified this same locus as 10q21.3 rs1125117 with the highest statistical significance and size effect associated to CI2 (p-value 1.07e-7). Lange L. et al. mentioned that this variant is enriched in families with hypertension which is one of the future goals of the present cohort, i.e., the identification of markers predictive of hypertension and cardiovascular health supporting the relevance of cluster 10q21.3 in particular variant rs1125117.
Other variants associated with CI2 were observed on
KCND3,
DIPK2B, and
LIPC/ALDH1A2 which have already been identified in lipoprotein and dyslipidemia studies. Here,
KCND3 rs6703437 was associated to both, AIP and CI2 indexes (p-value 2.06e-6 and 1.7e-5) suggesting a concomitant association of this variant with HDL-C and triglycerides. Current reports on
KCND3 indicate a link between genetic variation and cognitive impairment [
51,
52] suggesting a shared relationship between neurological disorders and lipids and hence the importance of monitoring genetic variation associated with lipid indexes as potential predictors of several physiological systems.
Associated to CI2 was also the gene
LIPC/ALDH1A2 with the dual function of triglyceride hydrolase and ligand/bridging factor for receptor-mediated lipoprotein uptake.
LIPC/ALDH1A2 rs261342 is located on the 5’ promoter region of
LIPC and has been strongly associated with HDL-C in women as part of a haplotype [
53] and used to assess CVD risk and its relation to lipids and apolipoproteins [
54]. We found this variant associated with CI2 confirming its relationship to HDL-C levels, in males and females, i.e., in a sex independent manner opposed to the sexual dimorphism listed in previous results [
53]. For CI2, we corroborate previous associations between this atherogenic index and 10q21.3 rs11251177, and to loci clustered on chromosomes 12 and 6. Also, we confirmed that gene variation on
LIPC and
KCND3 impact CI2 interindividual variation and the latter may have a stronger health impact as we found it associated with both, AIP and CI2.