Identification of a regulatory SNP in the retinol binding protein 4 gene associated with type 2 diabetes in Mongolia

Abstract

Increased levels of retinol binding protein 4 (RBP4) in serum is associated with insulin resistance. To examine this further, the genomic region of RBP4 was genetically surveyed in Mongolian people, who as a group are suffering from a recent rapid increase in diabetes. The RBP4 gene was screened by DHPLC system, and the PCR fragments which showed heteroduplex peaks in multiple samples were followed by direct sequencing to identify common polymorphisms in 48 Mongolian diabetic samples. Identified single nucleotide polymorphisms (SNPs) were genotyped in 511 control and 281 type 2 diabetes samples. The functions of SNPs in the regulatory region were assessed by reporter gene assay and electrophoretic mobility shift assay. Possible association between functional SNPs and serum RBP4 levels or metabolic parameters was statistically assessed. Nine SNPs were identified in the RBP4 gene. A case-control study revealed that the rare alleles of four SNPs were associated with increased risk of diabetes, even after Bonferroni correction (-803, G > A, P = 0.0054; +5169, C > T, P = 0.0025; +6969, G > C, P = 0.0015; +7542, T > del, P = 0.0015). The -803 G > A SNP influenced the transcription efficiency in a hepatocarcinoma cell line as well as the binding efficiency of hepatocyte nuclear factor 1 alpha to the motif. In addition, the -803 A allele was associated with increased serum RBP4 levels in diabetic patients. We have identified a functional SNP in the RBP4 gene associated with type 2 diabetes in Mongolian people.

Effects of Genetic Variation in the Human Retinol Binding Protein-4 Gene (RBP4) on Insulin Resistance and Fat Depot–Specific mRNA Expression

“…In conclusion, consistent with previously reported findings in mice and in human studies (1,6,7), several RBP4 SNPs and their haplotypes are likely to affect measures of insulin resistance (fasting plasma insulin and 2-h plasma glucose) and related traits (BMI, WHR, and circulating FFAs), as well as RBP4 mRNA levels in visceral adipose tissue in humans. These effects may ultimately result in type 2 diabetes, which is in line with the observed association of the A-G-G-T-G-C haplotype with increased risk of type 2 diabetes in the present study.…”

“…The highly conserved region surrounding rs3758539 matches human transcriptional binding sites for MAZ (11) and R1/R2/Sp1 for the major allele G (12,13) and c-Ets-2 for the minor allele A (14). Furthermore, this SNP seems to influence the transcription efficiency in a hepatocarcinoma cell line as well as the binding efficiency of hepatocyte nuclear factor 1␣ to the motif (6). Regarding the association of the RBP4 haplotype with mRNA levels, this also indicates a potential functional relevance of the noncoding RBP4 variants in the putative promoter region.…”

“…Despite known physiology as well as chromosomal location, to date, very few studies on the effects of genetic variation in the RBP4 gene on increased metabolic risk in humans have been reported (6,7). We therefore investigated whether genetic variants within the RBP4 gene might be responsible for observed changes in RBP4 mRNA expression and whether it may affect obesity/type 2 diabetes and pathophysiologically relevant traits in humans.…”

RBP4: a controversial adipokine

“…Two haplotypes (SNP (risk allele): rs3758538(A), rs3758539(G), rs10882283(G), c248C44TOC(T), rs12265684(G), rs10882273(C); and rs3758539(G), 390COG(G), 406TOC(T), rs36014035(G), rs12265684(G), rs34571439(T), rs12766992(G), rs10882273(C)) are significantly increased in Caucasians with T2D when compared with healthy controls (33,48). Four SNPs in Mongolian population (SNP (risk allele): rs3758539(A), rs7094671(T), rs12265684(C), and 7542TOdel(del)) and a haplotype in South Indian population (rs3758538(C), rs3758539(T), rs36014035(G), and rs34571439(T)) have also been determined that are associated with T2D (31,49). Haplotype (SNP (risk allele)) rs3758538(A), rs3758539(G), rs10882283(G), c248C44TOC(T), rs12265684(G), rs10882273(C)) is also associated with higher mean fasting plasma insulin and 2 h plasma glucose in OGTT in non-diabetic Caucasians (33).…”

“…Haplotype (SNP (risk allele)) rs3758538(A), rs3758539(G), rs10882283(G), c248C44TOC(T), rs12265684(G), rs10882273(C)) is also associated with higher mean fasting plasma insulin and 2 h plasma glucose in OGTT in non-diabetic Caucasians (33). In Mongolians, SNP in the regulatory region of the RBP4 gene (SNP (risk allele) rs3758539(A)) is associated with increased circulating RBP4 levels in T2D subjects (49). Recently, three SNPs (SNP (risk allele): rs974456 (T), rs736118 (A), and rs4886578 (A)) and two haplotypes (SNP (risk allele): rs974456(C), rs736118(G), rs4886578(G) and (rs974456(T), rs736118(A), rs4886578(A)) in the gene encoding a high-affinity cell surface receptor for RBP4, STRA6, have been identified that are associated with T2D (31).…”