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HAPLOTYPE STRUCTURE OF THE ENPP1 GENE AND NOMINAL ASSOCIATION OF THE K121Q POLYMORPHISM WITH GLYCEMIC TRAITS IN THE FRAMINGHAM HEART STUDY

¹Center for Human Genetic Research and Diabetes Unit, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts
²Program in Medical and Population Genetics, Broad Institute of Harvard and MIT, Cambridge, Massachusetts
³Department of Medicine, Harvard Medical School, Boston, Massachusetts
⁴Department of Biostatistics, Boston University School of Public Health, Boston, Massachusetts
⁵Division of Endocrinology, Diabetes, and Hypertension, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts
⁶the National Heart, Lung, and Blood Institute's Framingham Heart Study, Framingham, Massachusetts; and
⁷General Medicine Division, Massachusetts General Hospital, Boston, Massachusetts

Objective: A recent meta-analysis demonstrated a nominal association of the ectonucleotide pyrophosphate phosphodiesterase 1 (ENPP1) K121Q polymorphism with type 2 diabetes. We set out to confirm the association of ENPP1 K121Q with hyperglycemia, expand this association to insulin resistance traits, and determine whether the association stems from K121Q or another variant in linkage disequilibrium with it.

Research Design and Methods: We characterized the haplotype structure of ENPP1 and selected 39 tag SNPs that captured 96% of common variation in the region (minor allele frequency 5%) with an r²0.80. We genotyped the SNPs in 2,511 Framingham Heart Study participants and used age-sex-adjusted linear mixed effects models to test for association with quantitative metabolic traits. We also examined whether interaction between K121Q and BMI affected glycemic trait levels.

Results: The Q allele of K121Q (rs1044498) was associated with increased fasting plasma glucose (FPG), HbA_1c, fasting insulin, and insulin resistance by homeostasis model assessment (HOMA-IR; all P=0.01-0.006). Two non-coding SNPs (rs7775386, rs7773477) demonstrated similar associations, but linear mixed effect models indicated that their effects were not independent from K121Q. We found no association of K121Q with obesity, but interaction models suggested that the effect of the Q allele on FPG and HOMA-IR was stronger in those with a higher BMI (P=0.008 and 0.01 for interaction, respectively).

Conclusions: The Q allele of ENPP1 K121Q is associated with hyperglycemia and insulin resistance in whites. We found an adiposity-SNP interaction, with a stronger association of K121Q with diabetes-related quantitative traits in people with a higher BMI.

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