Association of UCP1 and UCP2 variants with diabetic retinopathy susceptibility in type-2 diabetes mellitus patients: a meta-analysis

Diabetic retinopathy (DR), a common sight-threatening microvascular complication among patients with diabetes mellitus (DM), is the major cause of irreversible blindness and visual impairment in working-age adults [1]. Though the pathophysiological mechanisms of DR remain elusive, increasing evidence suggests that long duration of DM, poor control of blood glucose and high blood pressure mainly contribute to the pathogenesis and development of DR [2]. However, DR could also occur in patients with short duration of DM, well control of blood glucose and normal blood pressure. Besides, epidemiological studies revealed familial inheritance and ethnic variations in DR [3], indicating that genetic factors could play a role in the pathogenesis and development of DR [4].

The elevation of oxidative stress has been suggested contributing to the development of DM complications [5], which is caused by reactive oxygen species (ROS) overproduction, mainly the mitochondrial ROS [5‐7]. Excessive ROS resulted from hyperglycemia causes retinal mitochondrial dysfunction with serious damage to the oxidative phosphorylation complexes and abolished adenosine triphosphate biosynthesis [8], and induces capillary endothelial cell apoptosis, which subsequently leads to the dysregulation of the angiogenesis-related genes [9] and diabetic microvascular complications, including DR [10, 11]. Ion channels physiologically play a role in signal transmission and visual processing, but also link to induced oxidative stress and significantly contribute to a wide spectrum of ocular diseases [12]. Uncoupling proteins (UCPs) belong to a group of proton carrier transporters (H⁺) in the inner membrane of mitochondria [13]. UCPs are able to uncouple the oxidized substrates and dissipate the potential energy on the inner membrane as heat to reduce ROS overproduction from mitochondria [14‐17]. The overproduced ROS could cause increases proton conductance by UCP1–3, leading to decease in superoxide radicals through the mitochondria respiratory chain reaction [18]. In human genome, there were five different UCPs, named UCP1 to 5, with various tissue distributions and functions [19]. Uncoupling protein 1 (UCP1) gene is located on chromosome 4q31.1 and found to be expressed in brown adipose tissue, endothelial cells and pericytes of retina [20]. UCP1 mainly plays a role in the maintenance of body temperature in a cold environment through non-shivering thermogenesis [17]. It has been shown that elevated of glucose levels upregulates UCP1 expression, protecting cells from glucose-induced ROS damage [21]. Uncoupling protein 2 (UCP2) and 3 (UCP3) genes are both located in the same cluster on chromosome 11q13.4. UCP2 is ubiquitously expressed across different tissues in the body, whereas UCP3 is mainly expressed in the skeletal muscle tissue [22]. In UCP2 knockout mice, ROS production increases in macrophages and pancreatic islets [23, 24], whereas overexpression of UCP2 inhibits mitochondrial death pathway in cardiomyocytes [25], indicating that UCP2 could be involved in cell protection from ROS damage. UCP2 and UCP3, together with SLC25A27 (UCP4) and BMCP1 (UCP5), exert cytoprotective effects by reducing oxidative stress under certain conditions [22].

Since UCPs are involved in the pathophysiology of glucose-related ROS cell damage, it is reasonable to hypothesize that the UCPs variants could be related to the susceptibility of DR. Yet, inconsistent results have been reported on the association analysis of UCPs variants with the risk of DR [26‐28]. Herein, we aimed to conduct a meta-analysis to clarify the association of different UCPs variants with the susceptibility of DR.

DR is one of the most common microvascular complications in DM patients. The hyperglycemia-induced ROS is considered as one of the initial and major pathways causing the damage to the endothelial cells. The UCPs are anion carrier proteins in mitochondrial inner membrane. UCPs function to reduce mitochondrial ROS, especially hyperglycemia-induced oxidative stress, and protect endothelial cells from oxidative stress by balancing the proton motive force across the mitochondrial inner membrane [22]. Therefore, UCPs could possibly participate in the development and pathogenesis of DR.

This meta-analysis verified the association of the reported UCPs variants with the susceptibility of DR. Our results showed that UCP1 rs1800592 variant was not significantly associated with DR in type-2 DM patients in the pooled effects analysis (Table 3 and Fig. 2); yet, in the subgroup analysis, UCP1 rs1800592 was significantly associated with PDR in type-2 DM patients in the allelic and homozygous models (Table 4 and Fig. 4). The patients carrying allele G of UCP1 rs1800592 variant have 26% higher risk developing PDR than those carrying allele A. A previous study demonstrated that the carriers of rs1800592 GG genotype exhibited higher UCP1 gene expression than those with AA genotype in the retina samples [20]. This could suggest that higher UCP1 expression in retina by allele G of rs1800592 variant could be associated with higher susceptibility of PDR. Conversely, UCP1 expression was lower in carriers of GG genotype than those with AA genotype in intraperitoneal adipose cells, indicated the tissue-specific effect of rs1800592 on UCP1 expression activity [38]. Moreover, allele G of UCP1 rs1800592 also showed elevated expression of MnSOD2 gene, which is another major scavenger for mitochondrial ROS [20, 39]. Our discovery was resulted from 2 reported studies, and our approach is similar to that from our previous studies [40‐42]. Nevertheless, further studies with larger cohorts in different populations are needed to verify its association with PDR.

UCP2 is the most widely distributed uncoupling protein and most frequently studied in DM and DR, and itis associated with the increased oxidative stress and negatively regulates the insulin secretion [43, 44]. Total 4 UCP2 variants, UCP2 rs659366, UCP2 rs660339 (p.A55V), UCP2 45-bp Ins/Del and UCP2 rs1800849, were reported in the association analysis with DR; however, only UCP2 rs659366 variant comprised enough studies for the meta-analysis, and other UCP2 variants have not been further analyzed in this study. UCP2 rs659366 has been reported to be associated with type-2 DM [45]; however, in this meta-analysis, we demonstrated that UCP2 rs659366 variant showed no pooled association with DR in the type-2 DM patients (Table 3 and Fig. 3). The elevation of UCP2 expression could be induced by high glucose treatment in epithelial cell of human vein, and the A allele of UCP2 rs659366 increases promoter activity as compared to the G allele, which can be exacerbated under hyperglycemic condition to exert a protective effect [46]. The negative association of UCP2 rs659366 variant with DR in this meta-analysis might indicated that UCP2 gene variation may not be contributed to the development of DR. Nevertheless, it is of worth to note that, in the F-SNP database analyses, UCP2 rs660339 is strongly linked with UCP2 rs659366, and partially linked with UCP2 45-bp Ins/Del variant [28]. One report showed that the haplotype of 3 differentUCP2 variants [Ins (45-bp Ins/Del), A (rs659366) and Ala (rs660339)] is associated with the decreased UCP2 gene expression in human retina [47]. This could be an independent risk factor for PDR in both type-1 and 2 DM patients [28]. Additional association studies are necessary in order to confirm the association of all 4 UCP2 variants with DR in different ethnic groups.

We conducted the subgroup analyses on ethnicity in this meta-analysis. There was no significant association in different ethnic group, which could be due to the limited and sample sizes after stratification. Thus, the ethnicity-specific effects of these variants need to be determined with larger sample sizes in additional cohort studies.

There are several limitations in this meta-analysis. First, the number of reported studies for each UCPs variant was still limited. Second, the lack of original clinical information would be difficult to adjust the relevant variables, such as duration of diabetes, medications and other chronic diseases.

In summary, our meta-analysis revealed no significant pooled association of UCP1 rs1800592 and UCP2 rs659366 with DR in DM patients; yet allele G of UCP1 rs1800592 variant could be associated with the increased risk of PDR in type-2 DM patients. Our results suggest that UCP1 rs1800592 variant could be clinically applied as a genetic marker for PDR prediction and risk analysis in DM clinics.