The A54T polymorphism at the intestinal fatty acid binding protein 2 is associated with insulin resistance in glucose tolerant Caucasians

Abstract

An A54T polymorphism at the fatty acid binding protein 2 (FABP2) locus was found to be associated with insulin resistance in non-diabetic Pima Indians. To see whether this association is present in other populations, we performed a cross sectional study to examine the role of this polymorphism on insulin resistance in 55 healthy and normotensive Caucasian subjects with normal glucose tolerance. Insulin sensitivity (%S) and beta cell function (%B) were assessed using the Homeostasis Model Assessment (HOMA). Their genotypes were determined using a polymerase chain reaction-restriction fragment length polymorphism assay. The relationship between the genotypes and the phenotypes was examined. After genotyping, we identified 24 AA, 27 AT and 4 TT subjects. The TT subjects were combined with the AT subjects during the analysis due to its small sample size. No differences were noted in gender distribution, clinical features, and fasting lipid profile between the two genotypic groups (AA vs. AT/TT). The AT/TT group had a higher fasting plasma insulin concentration and a lower %S than the AA group (p = 0.0444 and p = 0.0461, respectively). However, no differences were noted in plasma glucose concentrations and %B. Univariate analysis revealed that this polymorphism explained 7.3% of the variation in %S. Multivariate analysis revealed that the polymorphism was an independent determinant for %S (p = 0.0434) and with body mass index accounted for 28.7% of the variation in %S. In contrast, this polymorphism had no impact on %B. The A54T polymorphism at the FABP2 locus is a risk factor for insulin resistance in a Caucasian population.

Background

An A54T polymorphism at the fatty acid binding protein 2 (FABP2) locus was found to be associated with insulin resistance in non-diabetic Pima Indians. To see whether this association is present in other populations, we performed a cross sectional study to examine the role of this polymorphism on insulin resistance in 55 healthy and normotensive Caucasian subjects with normal glucose tolerance. Insulin sensitivity (%S) and beta cell function (%B) were assessed using the Homeostasis Model Assessment (HOMA). Their genotypes were determined using a polymerase chain reaction-restriction fragment length polymorphism assay. The relationship between the genotypes and the phenotypes was examined.

Results

After genotyping, we identified 24 AA, 27 AT and 4 TT subjects. The TT subjects were combined with the AT subjects during the analysis due to its small sample size. No differences were noted in gender distribution, clinical features, and fasting lipid profile between the two genotypic groups (AA vs. AT/TT). The AT/TT group had a higher fasting plasma insulin concentration and a lower %S than the AA group (p = 0.0444 and p = 0.0461, respectively). However, no differences were noted in plasma glucose concentrations and %B. Univariate analysis revealed that this polymorphism explained 7.3% of the variation in %S. Multivariate analysis revealed that the polymorphism was an independent determinant for %S (p = 0.0434) and with body mass index accounted for 28.7% of the variation in %S. In contrast, this polymorphism had no impact on %B.

Conclusions

The A54T polymorphism at the FABP2 locus is a risk factor for insulin resistance in a Caucasian population.

Introduction

].

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], we examined the relationship of this polymorphism with insulin sensitivity in 55 healthy and normotensive Caucasians with normal glucose tolerance.

Results

. Using the PCR-RFLP assay, we identified 24 AA, 27 AT, and 4 TT subjects. In this Caucasian population, the allele frequency was 68% for the A allele and 32% for the T allele. The distribution of genotypes was in compliance with the Hardy-Weinberg equilibrium (p = 0.8321).

Clinical features of the studied subjects

arithmetic means

. Although the AT/TT subjects had a higher fasting plasma insulin concentration than the AA subjects (p = 0.0444), no differences were noted in fasting plasma glucose concentrations and postchallenged plasma glucose and insulin concentrations. We estimated insulin sensitivity (%S) and beta cell function (%B) using the average of the three fasting plasma glucose and insulin concentrations. While no difference was noted in %B, the AT/TT subjects were more insulin resistant (a lower %S) than the AA subjects (p = 0.0461).

Clinical features and glycemic parameters by the FABP2 genotypes

p = 0.0461.

). This polymorphism and body mass index explained 28.7% of the variation in %S and this polymorphism was an independent determinant of %S (p = 0.0434). However, this polymorphism had no impact on %B. Age, gender and waist hip ratio accounted for 19.5% of the variation in %B.

Multivariate analysis

Discussions

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Published studies of FABP2 and A54T

the present study

. The negative quantitative studies could be the result of other confounding factors, such as the inclusion of diabetic, impaired glucose tolerant or hypertensive subjects in the study. Therefore, we only enrolled glucose tolerant and normotensive subjects in the present study for the reasons described previously.

.

In summary, we examined the role of the A54T polymorphism in the pathogenesis of insulin resistance in 55 glucose tolerant and normotensive healthy Caucasians. We found that this polymorphism had an independent, but very modest influence (7.3%) on insulin sensitivity (%S), which was assessed by the HOMA. However, it had no impact on beta cell function (%B). To our knowledge, this is the very first report of a positive association between this polymorphism and insulin sensitivity in a Caucasian population.

Subjects

]. They were calculated from the average of three fasting plasma glucose and insulin concentrations (mM and mU/L, respectively) using the following formulae: %S = 22.5 / (insulin x glucose) and %B = 20 x insulin / (glucose - 3.5). The study was approved by the Institutional Review Board and written informed consents were obtained at the entry of the study from each participant. We confirm that the study has complied with the recommendations of the Declaration of Helsinki.

Genotyping

, 5% glycerol, 0.275 U Taq polymerase, 50 mM KC1, 10 mM Tris-HCl pH 8.3, and 0.1 μg of genomic DNA. The region of interest was amplified by an initial denaturation at 94°C for 5 minutes, 35 cycles of denaturation at 94°C for 30 seconds, annealing at 60°C for 30 seconds, and extension at 72°C for 30 seconds, and concluded with a final extension at 72°C for 10 minutes. Then, 5 μl of the PCR product (375 base pairs) was digested in a 15-μl reaction volume containing 1 U of Hha I (New England Biolabs Inc., Beverly, Massachusetts, USA) with the buffer supplied by the vender. The digested PCR products were resolved on 2.0% agarose gels. Hha I digested the wild type, Alanine (GCT), which yielded two products, 200 and 175 base pairs (A allele). The G to A substitution (Threonine, ACT) destroyed the Hha I site (T allele).

Statistical analysis

= 0.3014, p < 0.0001), they were removed from the multivariate analysis based on their p values as indicated in each analysis. A nominal P value of less than 0.05 was considered significant. SYSTAT 8.0 for Windows from SPSS, Inc. (Chicago, Illinois) was used for the statistical analysis.

Acknowledgements

We thank Mohammad F. Saad, M.D. and the staff of the General Clinical Research Center at the University of California, Los Angeles for their continued support. We also thank George P. Tsai, Sonya Wilsterman, Jennifer M. Ryu, Jennifer L. McGullam, and Jennifer E. McCarthy for their laboratory assistance. The work was supported in parts by grants from USPHS M01RR00865 (UCLA-GCRC), NIH/NIDDK R01DK52337-01 (KCC), Diabetes Action Research and Education Foundation (KCC), and American Diabetes Association (KCC).