Association of sLR11 gene polymorphism with T2DM and carotid atherosclerosis

1. Introduction

The main pathological change of diabetic macrovascular disease is atherosclerosis (AS), and the mechanism involves the induction of smooth muscle cell proliferation, thickening of the arterial wall, atherosclerotic plaque formation and vascular calcification [1]. In the formation of AS, the migration of vascular smooth muscle cells (SMCs) from the media to the intima is a critical step [2]. In 1996, Yamazaki et al. [3] first discovered that low density lipoprotein receptor (LDLR) relative with 11 ligand-binding repeats (LR11) detected by immunological methods in serum is a member of the LDLR family. Furthermore, it was also observed that LR11 was highly expressed on the intima of SMCs, which was followed by medial SMCs near the intimal boundary of the AS lesion [4]. Subsequently, they found that the serum concentration of soluble LDLR relative with soluble LR11 (sLR11) was positively correlated carotid artery intima-media thickness (IMT) in patients with dyslipidemia. Furthermore, multivariate regression analysis revealed that sLR11 levels served as a new quantitative circulatory marker of IMT, and was independent of the classic risk factors for AS [5]. A recent study revealed that sLR11 levels are positively correlated with IMT in middle-aged and non-obese type-2 diabetes mellitus (T2DM) patients. At present, the association of sLR11 gene polymorphism with T2DM and carotid atherosclerosis (CAS) has not been reported in any study worldwide. However, the analysis of single nucleotide polymorphism (SNPs) of the sLR11 gene has been used in studies on the pathogenesis of Alzheimer’s disease (AD). Studies have revealed that the gene polymorphism of apolipoprotein E (APOE) was closely related to T2DM and AD [7, 8, 9, 10]. We inferred that T2DM and AD may have a similar etiology. Therefore, in this study, the positive motif rs3824968 related to AD [11] was selected to investigate the association of the rs3824968 polymorphism of the sLR11 gene with T2DM and CAS in Korean and Han nationalities in the Yanbian area, and provide a genetics theoretical basis for the effective prevention and treatment of AS in diabetes patients.

2. Subjects and methods

3. Results

Diabetes mellitus is a common metabolic disease characterized by hyperglycemia. The risk of cardiovascular disease in patients with diabetes mellitus is 2–4 times of that in normal subjects. The main pathological change of macrovascular disease in patients with T2DM is AS. IMT thickening occurs earlier than AS. IMT is an early quantitative clinical marker of AS in patients with diabetes mellitus. A study revealed that the LR11 (SORL1) gene is located on chromosome 11 (11q24.1), and is the same gene as the sorLA gene in brain, which is consists of 177,491 base containing 51 exons. Studies have confirmed that as a novel circulatory marker for IMT risk of carotid artery, the level of serum sLR11 was correlated with carotid artery ITM thickening and plaque formation, acute coronary syndrome and diabetic retinopathy in T2DM patients [12, 13, 14, 15]. Most experiments have revealed that sLR11 gene polymorphism is associated with the pathogenesis of AD. According to statistical data, approximately 65% of patients with diabetes mellitus have mild-moderate cognitive impairment [16, 17]. The risk of AD in patients with diabetes mellitus is higher than in the normal population. Studies have revealed that in patients with diabetes mellitus and AD, atrophy of the hippocampus and amygdala could be observed by MRI [18, 19]. Therefore, T2DM may be related to the occurrence and development of AD. A study in China revealed that the level of plasma cholesterol was significantly higher in individuals carrying the ApoE ε 4 allele than in individuals carrying the ApoE ε 2 and ApoE ε 3 allele, which increased the risk of AD in T2DM patients. SLR11 is a member of the LDL receptor family, which can bind with inherent LDL receptor-related proteins and ApoE on the endoplasmic reticulum. Therefore, sLR11 may be a susceptible gene following ApoE. At present, studies on the association of the sLR11 gene polymorphism with T2DM and CAS have not been reported. In this study, the AD-related positive motif rs3824968 in an Asian population was selected [11] to investigate the association of sLR11 gene polymorphism with T2DM and CAS in a Korean and Han population in the Yanbian area.

This experiment revealed that a single-base mutation A → T on the rs3824968 locus on the sLR11 gene was detected in the T2DM population and normal population in the Yanbian area, and that the sLR11 gene rs3824968 polymorphism was not associated with T2DM. Recent studies conducted by Chinese scholars revealed that in the Chinese Han population, the sLR11 gene rs2070045 polymorphism was not significantly related to T2DM. Therefore, we considered that the sites associated with AD are not necessarily associated with T2DM. To date, no studies have reported the correlation between the polymorphism of other sites and T2DM. Hence, this is exactly what we are going to do next.

The sLR11 gene polymorphism is affected by genetic heterogeneity. A large number of experiments revealed that sLR11 gene variation is associated with the risk of AD in different ethnic populations. At present, the study subjects are mainly European, Hispanic, African American, Japanese, Israeli Arab and Chinese Han nationality populations. At present, there is still a lack of reports on ethnic differences in sLR11 gene polymorphism among the ethnic minorities in china. In the present study, the genotype and allele frequency distribution of the Korean and Han nationality populations in the Yanbian area were selected and compared. Results revealed that the difference in sLR11 gene rs3824968 polymorphism in the NGT, NCAS and CAS groups between Korean and Han nationalities was not statistically significant. The reason may be related to the small sample size.

At present, it is considered that diabetic hyperglycemia may directly induce the expression of sLR11, and the phenotype of smooth muscle cells in the vascular wall is transformed into the so-called synthetic type. These possibilities are currently being studied [20]. This study revealed that LDL-C and TG are risk factors for CAS. This is consistent with the results of other studies, in which serum sLR11 level was positively correlated with LDL-C and HbA1c [21], and that serum sLR11 level was correlated to the formation of AS [6]. Recent studies have revealed that the deposition of cholesterol and other lipids in the blood vessel wall plays a decisive role in the early formation of AS. These lipid settlings are mainly derived from lipoproteins in the blood, especially from LDL [22]. The concentration of LDL on the surface of the inner wall of blood vessels is proportional to the lipoprotein entering the vessel wall. With the increase in TG and LDL levels, arterial intima thickening and elasticity decrease has become more significant. The present study revealed that in the CAS group, the level order of LDL-C closely correlated to CAS was TT group $>$ AT group $>$ AA group, showing an increasing trend. This suggests that rs3824968 polymorphic T factor may increase the concentration of LDL, allowing more LDL to enter into the arterial intima through the LR. As a result, the peroxidized LDL is engulfed by macrophages, which form foam cells and deposit beneath the intima. This finally makes SMCs to migrate into the intima and proliferate, leading to hyperplasia and hypertrophy of the intima. Then, atherosclerotic plaques are finally formed. Therefore, the rs3824968 polymorphic T factor may increase the risk of CAS in T2DM patients.

In this experiment, no direct risk factors for T2DM were found. However, this study is the first to determine that the sLR11 gene rs3824968 polymorphic of factor T may increase the risk of CAS in T2DM patients by regulating the concentration of LDL-C, showing the dominant mode of inheritance. This would provide some direction and basis for research work in the future.