Lipids, including cholesterols and triglycerides (TG), play vital roles in many physiological processes ^[1][8]. The cholesterols form a part of the plasma membrane and regulate membrane properties, such as thickness, internal curvature and permeability ^[2][9]. Cholesterols are also required to synthesize various molecules, such as bile acids and steroid hormones, and function as a regulator in neuronal signaling pathways. On the other hand, TG is the energy source for muscle and adipose tissues. Given the hydrophobic characteristics of TG and cholesterols, the molecules are transported in lipoproteins and chylomicrons. The carriers consist of a core hydrophobic containing a variable amount of cholesterol esters and TG enveloped by phospholipids, free cholesterol and apolipoproteins ^[1][8]. Although essential, abnormal lipid levels, commonly described as dyslipidemia, are detrimental in that it increases the risk of many diseases, including cerebrovascular and cardiovascular diseases ^[1][3][2,8]. Dyslipidemia can be characterized by an abnormal value of TG of ≥1.70 mmol/L, high-density lipoprotein (HDL) of <1.03 mmol/L for males and <1.29 mmol/L for females, respectively ^[4][10]. Although the syndrome increases the risk of cardiovascular diseases, each component of the lipid profile is an independent risk predictor for Metabolic syndrome (MetS). Numerous population studies reported that TC, low-density lipoprotein (LDL)LDL and HDL are independently correlated with various cardiovascular diseases, including myocardial infarction ^[5][6][11,12].

Apolipoproteins play a pivotal role in TG and cholesterol transport and metabolism. Apo B is a major protein component of all types of pro-atherogenic lipoproteins, including very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL) and LDL ^[7][13]. There are two Apo B types, Apo B100 and Apo B48, encoded by the same gene. However, their molecular sizes are different. Apo B48 is smaller and has a relative molecular weight of approximately 48% of the Apo B100, as determined by sodium dodecyl sulphate gel electrophoresis ^[8][14]. Moreover, Apo B48 is synthesized in the intestines, constituting the primary apolipoprotein present in chylomicrons ^[8][9][14,15]. In contrast, Apo B100 is predominantly expressed in the liver, although a small fraction of Apo B100 is expressed in the intestines ^[9][15].

The gene encoding Apo B, known as the APOB gene, is located in the short arm of chromosome 2 ^[10][16]. The gene is 43 kb long and comprises 28 introns and 29 exons variably scattered in three thrombolytic peptides (T2, T3, T4) ^[11][17]. The intron interruptions on the coding sequences occur mainly in peptide T4, followed by T2 and T3, with the intron amounts 24, 3, and 1, respectively ^[11][17]. Variable lengths of introns and exons have been determined, ranging from 107–3000 and from 39–7572 base pairs, respectively ^[11][17]. To date, more than 18,500 SNPs records of the APOB gene are available in the National Library of Medicine ^[12][18].

The elevation of Apo B is an essential cardiovascular disease biomarker ^[13][19]. A recent mendelian randomization study reported that high Apo B predicted a lower life span and a more significant risk of heart disease ^[14][20]. Various single nucleotide polymorphisms (SNPs) in the APOB gene cause amino acid substitution or loss of restriction sites by affecting specific endonucleases, such as RsaI and EcoRI. Other SNPs cause no changes in translating the same amino acids, a condition known as silent mutation ^[11][17]. Although there are no changes in amino acids, silent mutations of specific SNPs are attributable to increased cardiovascular risks ^[15][21]. For instance, an rs693 polymorphism has been reported to cause an altered lipid profile ^[10][15][16][16,21,22]. The SNP rs693 is located on the most extended exon of the APOB gene, with 7572 base pairs ^[11][17]. Two allele forms, C- and T-alleles, contribute to polymorphism in rs693. The latter is a minor allele, postulated as the risk allele ^[10][16]. The presence of the T-allele changes the genetic code. However, the same amino acid, threonine, forms during translation. Interestingly, the rs693 silent mutation is associated with various changes in metabolic profile ^[10][16].

A meta-analysis conducted by Niu et al. (2017), which included the articles published before December 2016, had reported a positive association between the rs693 T-allele carrier with TC (n = 41,764), TG (n = 22,128), LDL (n = 22,286) and Apo B (n = 12,364) levels than the non-T-allele carrier. In contrast, the T-allele carriers of rs693 negatively correlated with HDL (n = 39,292) levels compared to non-T-allele carriers ^[10][16]. Numerous studies published from 2017 onwards reported inconsistent results, secondary to various limitations, including sample size, age and comorbidities. Although multiple limitations exist, the discussion of individual studies provides different perspectives regarding specific populations. Table 1 summarizes the genetic studies that address the association between specific polymorphisms and lipid parameters in the specific populations.

A study in an elderly Brazilian cohort reported significant associations between SNP rs693 and TC, total lipid and LDL levels, with homozygous TT demonstrating higher levels than the C-allele carriers ^[15][21]. Moreover, a case-control study on an Iranian population reported that T-allele carriers had a significantly higher risk of familial hypercholesterolemia than the non-carriers ^[17][23]. Familial hypercholesterolemia is a common genetic disease, affecting 1 person in 311 globally ^[18][24]. Cardiovascular disease is estimated to develop in 1 in 17 individuals with familial hypercholesterolemia ^[18][24]. Apo B polymorphism is one of three primary defects attributable to familial hypercholesterolemia ^[19][25]. Genetic testing in the disease is essential for risk prediction. It facilitates the prevention and treatment plans for individuals at risk of cardiovascular disease.

Alghamdi et al. (2021) reported a significant association between the rs693 AG genotype and TC levels and the rs693 GG genotype and TG levels in a young female group with MetS compared to controls; however, there was no significant association in other genotypes (AA/AG/GG) and lipid parameters, including LDL, HDL and Apo B100 levels, between the MetS and control groups ^[16][22]. Similarly, a case-control study involving patients with acute coronary syndrome and healthy controls found no correlation between SNP rs693 polymorphisms and apolipoprotein B levels ^[20][26]. In contrast, negligible associations were reported in a study in a Colombian Caribbean population between T-allele and C-allele carriers in TC, TG, LDL and HDL levels regardless of genetic models (dominant, recessive, co-dominant and additive) ^[21][27].

How silent mutation of rs693 affects lipid profiles remains obscured. A possible explanation is the association of the SNP rs693 with other alleles in the APOB gene or other genes at different loci, a condition known as linkage disequilibrium ^[15][22][23][21,28,29]. For instance, Li et al. (2020) reported negligible associations between the SNP rs693 and multiple APOB genetic variations in a case-control study involving participants with and without obstructive sleep apnea ^[23][29]. Interestingly, positive associations with TC, LDL and Apo B were reported when rs693 was included in a group of Apo B SNPs. MetS components, including high TG and low HDL levels, are essential risk factors for obstructive sleep apnea ^[24][30]. To summarize, the SNP rs693 polymorphisms are linked with various components of metabolic syndrome, including LDL and HDL. In addition, SNP rs693 polymorphisms are associated with other lipid parameters, including TC, TG, total lipid and Apo B100.

Another SNP in the APOB gene widely studied is rs17240441, located in exon 1, which consists of 210 base pairs ^[10][11][16,17]. Two allele forms, insertion (ins) and deletion (del), contribute to the polymorphism of rs17240441. The possession of the del-allele causes deletion of the nine-nucleotide sequence (GCAGCGCCA), resulting in the loss of 3 (leucine-alanine-leucine) out of 27 amino acid residues ^[10][16]. The removal may alter the hydrophobicity levels and Apo B processing as the sequence might be located in the leader peptide region ^[25][31]. The entry and translocation across the plasma membrane of the proteins are two stages regulated by the leader peptide before protein release. The structural changes due to genetic polymorphism may affect the export process, particularly in the translocation process, resulting in altered Apo B processing and export ^[26][32].

Cardiovascular risks are attributable to the del-allele ^[27][33]. A meta-analysis study involving 23 studies reported higher levels of TC (n = 7875), LDL (n = 5658) and Apo B (n = 5047) in the del-allele carriers than in the non-del allele carriers (I). However, negligible associations were reported in TG (n = 7411) and HDL (n = 5124) levels ^[10][16]. Moreover, associations between the SNP rs17240441 and lipid profile are reported in various populations, including a group of teenagers with essential hypertension (with or without hypercholesterolemia) ^[28][34], human-deficiency virus (HIV)-infected patients on anti-retroviral treatment ^[29][35] and healthy people ^[30][36].

Higher levels of TC in the del/del genotype than the ins-allele carriers were reported in the teenagers with essential hypertension both with and without hypercholesterolemia. Nevertheless, the genotype del/del was associated with a higher LDL than the ins-allele carriers only in the teenage group with essential hypertension without hypercholesterolemia ^[28][34]. In contrast, Vimaleswaran et al. (2015) reported that ins-allele carriers had significantly higher fasting TC, TG and LDL but lower HDL compared to homozygotes del/del. Interestingly, measurement of post-prandial TG in the same study found that homozygotes ins/ins had significantly higher TG than the del-allele carriers ^[30][36].

In HIV-infected patients on anti-retroviral treatment, higher levels of TC and LDL were reported in the rs17240441 genotype del/del than the genotype ins/ins adjusted for age, gender and lipid-lowering agents use. However, only LDL levels remained significant after multiple testing corrections ^[29][35]. Previous studies demonstrated that the protease inhibitors (e.g., indinavir and ritonavir) used in HIV-infected individuals altered lipid profiles as characterized by increased TC ^[31][32][37,38], TG ^[31][32][37,38] and LDL ^[31][37]. However, the genetic study reported no significant association between rs17240441 polymorphisms and lipid profiles ^[29][35]. One mechanism of protease inhibitors-induced lipid abnormalities is due to endoplasmic reticulum stress and autophagy inhibition in adipocytes, resulting in lipid metabolism abnormality. Deranged lipid profiles, characterized by low HDL levels but high TG levels, are components of metabolic syndrome. These findings supported the role of the SNP rs17240441 in the pathophysiology of metabolic syndrome via the modulation of various lipid components, including Apo B, TC and LDL.

Opposite to Apo B, which is the primary apolipoprotein found in pro-atherogenic lipoproteins, Apo A1 is the primary component of an anti-atherogenic lipoprotein, HDL ^[33][39]. The Apo A1 protection against atherogenesis is attributable to its inhibitory effects on platelet aggregation via the synergistic effect with prostacyclin ^[33][34][39,40]. Stabilization of the prostacyclin by Apo A1 enhances the anti-aggregatory effect, preventing thrombus formation at the injured vascular loci ^[33][39]. In addition, Apo A1 plays an essential role in the reverse cholesterol transport from peripheral tissues back to the liver via interactions with various receptors ^[33][39].

Other common SNPs contributing to dyslipidemia are located in the APOA5-A4-C3-A1 gene complex or near the complex. The SNP rs964184 is located near the complex ^[35][41]. Woestijne et al. (2014) reported that G-allele (minor allele) carriers had significantly higher TG levels and Apo B but lower HDL levels. Furthermore, the study found BMI as a predictor for TG levels in heterozygous genotypes of rs964184 ^[35][41]. Moreover, an interregional study involving more than 100,000 people of European origin in the US, Europe and Australia reported that rs964184 was significantly associated with numerous lipid parameters, including TC, TG, LDL and HDL levels ^[36][42]. However, a meta-analysis of non-European cohorts reported mixed results. Only HDL and TG levels were significantly associated with SNP rs964184 in the East Asian cohort (n = 15,046), and TG levels were the only lipid parameter associated with SNP rs964184 in the South Asian cohort (n = 9705). In contrast, no significant association was found between SNP rs964184 and lipid parameters in the African American group (n = 8061) ^[36][42].

Replicative studies in other populations in China further supported the role of SNP rs964184 in lipid parameters ^[23][37][29,43]. A study in the obstructive sleep apnea population found a significant inverse association between SNP rs964184 and Apo A1 levels ^[23][29]. Interestingly, more significant correlations were reported when SNP rs964184 was analyzed as a cluster with other Apo A1 SNPs, with positive associations with HDL, LDL and Apo A1 levels but a negative association with TG levels ^[23][29]. Qiu et al. (2018) conducted a study on Han and Maonan Chinese populations, two populations with distinct characteristics in terms of geographical terrain, cultures and lifestyles. The study reported lower HDL levels in the Maonan Chinese group with the risk allele carrier. An inverse correlation was only reported in the male subgroup ^[37][43]. In contrast, a positive association between the risk allele carrier of rs964184 and TG levels but negative associations with Apo A1 levels and Apo A1/Apo B ratio were reported in the Han Chinese population. Intriguingly, the significance of associations changed during subgroup analyses, with TG levels remaining significant for females and the Apo A1 levels and Apo A1/Apo B ratio for males. However, association direction (positive or inverse) was not affected ^[37][43].

Due to the fact that we spend most of our awake time in a post-prandial state, Wojczynski et al. (2015) performed a genome-wide association study in European origin and Amish populations to investigate the role of genetic variants on post-prandial TG levels. In that study, they found a remarkable association between rs964184 and post-prandial TG levels. Interestingly, the significance association diminished when the baseline TG values were controlled, suggesting that SNP rs964184 is the primary determinant for baseline rather than post-prandial TG levels ^[38][44]. Similarly, Alcala-Diaz et al. (2022) reported higher post-prandial TG levels in the risk G-allele carrier at baseline. A three-year dietary intervention with a low-fat diet significantly lowered the post-prandial TG levels, comparable to the CC genotype ^[39][45]. The finding suggests that SNP rs964184 can be modulated by environmental factors, such as diet in the study, possibly through the gene–environment interaction.

The mechanism of how SNP rs964184 affects the lipid parameters might be attributable to the location of the SNP in a three-prime untranslated region (3-UTR) of the zinc finger 1 (ZPR1) gene ^[39][45]. Although the part is not translated, the 3-UTR plays a critical role in the structural and functional aspects of mRNA and proteins ^[40][46]. The cholesterol regulation of the ZPR1′s promoter part is attributable to the capability of the region to interact with peroxisome proliferator-activated receptor gamma (PPARG) proteins 1 and 2 ^[39][45]. PPARG1 is expressed in most tissues, but PPARG2 is primarily expressed in adipose tissue ^[41][47]. Without stimulus, the PPARs present as a complex with a co-repressor molecule. Activating PPARs with specific ligands promote the PPAR-coactivator complex to bind to the DNA promoter region, such as the ZPR1 region, resulting in the activation or inhibition of specific genes ^[39][41][45,47]. For instance, the genes activated may be essential in cholesterol metabolism through activating hepatocyte nuclear factor-4 alpha ^[39][42][45,48]. In summary, SNP rs964184 polymorphisms correlate with various lipid components, such as TC, LDL, Apo A1 and Apo B levels, as well as components of metabolic syndrome TG and HDL.

LPL, which is the major plasma triglyceride lipase, is attached to vascular endothelium via glycophosphatidylinositol (GPI)-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) ^[44][50]. In lipid metabolism, LPL performs an important role in the hydrolysis of circulating triglyceride-rich lipoproteins, such as chylomicrons and VLDL ^[45][51]. Angiopoietin-like proteins (ANGPTL), which are a family of proteins with similar structure to angiopoietin, are also involved in lipoprotein metabolism, specifically related to LPL. These proteins are associated with the ability to inhibit LPL enzymatic activities and increases LPL cleavage. The inhibition of lipoprotein lipase activity by ANGPTL may lead to an increase in circulating lipid levels, especially triglyceride ^[46][52]. A structural interaction between ANGPTL8 and ANGPTL3 will form a complex that is a potent endogenous inhibitor of LPL ^[47][48][53,54]. In addition, another type of ANGPTL, namely ANGPTL4, has also been indicated to serve as a potent inhibitor of the LPL enzyme. ANGPTL4 can both attach and inactivate LPL complexed to GPIHBP1. The inactivation of LPL by ANGPTL4 causes reduction affinity of LPL towards GPIHBP1, causing dissociation ^[49][55].

Previous studies demonstrated that abnormal lipoprotein lipase, including deficiency and mutation, was firmly associated with the incidence of dyslipidemia, leading to consequences such as atherosclerosis and stroke ^[45][51]. The relationship between LPL and dyslipidemia in different populations are tabulated in Table 2. In a Saudi population, there were associations of LPL polymorphisms, namely HindIII with CAD, while LPL polymorphisms of PvuII and Ser447Ter demonstrated no association with CAD. Meanwhile, there were no significant values between the genotypes of the HindIII, PvuII and Ser447Ter polymorphisms in terms of TG, TC, HDL-c and LDL-c ^[50][56]. This result was also in parallel with a previous study conducted in a Macedonian population where the presence of LPL-PvuII polymorphism did not represent a statistically significant risk factor for CAD, thus, indicating a lack of association between this polymorphism and CAD ^[51][57]. LPL HindIII has also been investigated in an Iraqi smoking male population, which demonstrated associations between the lipid parameters of the smokers. Specifically, the genotypes of LPL HindIII polymorphism, H+H+ genotype group demonstrated significantly higher TG and VLDL-c concentrations while a significantly lower HDL-C concentration than those of the HeH- genotype ^[52][58]. Therefore, it seems that there were inconsistent findings in the LPL polymorphisms in terms of lipid parameters in the different populations.

In a more recent study, the association of other LPL gene polymorphisms, including rs1534649 and rs28645722 with plasma lipid levels, were examined. In this study, the T-allele of rs1534649 polymorphism demonstrated significantly low HDL-c, while the rs28645722 polymorphism revealed no association between plasma lipid levels ^[53][59]. Common polymorphism has affected the effectiveness of fibrate therapy, a commonly used drug for lowering TG and increasing HDL-c. The LPL synonymous rare variants were significantly associated with absolute HDL-c change and TG percent change in the patients treated with fibrate. This study indicated that individuals with dyslipidemia carrying rare synonymous variants within the LPL gene had an attenuated response to the fibrate therapy ^[54][60]. In summary, individuals carrying rs1534649 polymorphism may have higher risk in developing MetS, as it was associated with low HDL-C, which is one of the MetS components. Meanwhile, individuals who did not respond well to fibrate therapy might possess the rare synonymous LPL gene variants and alternative treatments should be considered.