Genetic polymorphism of APOB is associated with diabetes mellitus in sickle cell disease

Abstract

Environmental variations have strong influences in the etiology of type 2 diabetes mellitus. In this study, we investigated the genetic basis of diabetes in patients with sickle cell disease (SCD), a Mendelian disorder accompanied by distinct physiological conditions of hypoxia and hyperactive erythropoiesis. Compared to the general African American population, the prevalence of diabetes as assessed in two SCD cohorts of 856 adults was low, but it markedly increased with older age and overweight. Meta-analyses of over 5 million single-nucleotide polymorphisms (SNPs) in the two SCD cohorts identified a SNP, rs59014890, the C allele of which associated with diabetes risk at P = 3.2 × 10(-8) and, surprisingly, associated with decreased APOB expression in peripheral blood mononuclear cells (PBMCs). The risk allele of the APOB polymorphism was associated with overweight in 181 SCD adolescents, with diabetes risk in 592 overweight, non-SCD African Americans ≥ 45 years of age, and with elevated plasma lipid concentrations in general populations. In addition, lower expression level of APOB in PBMCs was associated with higher values for percent hemoglobin A1C and serum total cholesterol and triglyceride concentrations in patients with Chuvash polycythemia, a congenital disease with elevated hypoxic responses and increased erythropoiesis at normoxia. Our study reveals a novel, environment-specific genetic polymorphism that may affect key metabolic pathways contributing to diabetes in SCD.

Figure 1

Age and BMI dependency of diabetes prevalence in SCD. Percent overweight is lower in SCD than in control individuals in (A) younger (34% vs 69%, P<1×10^-16) and (B) older (54% vs 78%, P=1.8×10^-15) individuals. Percent overweight increased with age in SCD (34% vs 54%, P=4.3×10^-8) and in control individuals (69% vs 78%, P=2.8×10^-8). (C) Percent diabetes increased with overweight in younger SCD subjects (0.25% in non-overweight vs 1.5% in overweight, P=0.11) and in control individuals (1.4% in non-overweight vs 4.5% in overweight, P=0.0057). (D) Percent diabetes increased sharply with overweight in older SCD individuals (1.7% in non-overweight vs 17% in overweight, P=1.5×10^-5), whereas it barely doubled in control individuals (16% in non-overweight vs 29% in overweight, P= 4.4×10^-6).

Figure 2

Genetic association of diabetes in SCD. (A) Manhattan plot of GWAS scan. The dashed horizontal line represents the significance threshold. (B) Association between the C allele of rs59014890 and APOB gene expression in UIC (black points) and Howard (red points) cohorts. Residual expression and residual allele dosage values, after adjusting for age, gender, HBB genotype, hydroxyurea treatment and population stratification, are plotted. (C) Heatmap of −log₁₀P from pair-wise comparison for enrichment of DNase hypersensitivity peaks within LD block of rs59014890. astrocyte_C: astrocytes-cerebellar; astrocyte_H: astrocytes-hippocampal; hypatocyte: primary hepatocytes; pancreatic duct: pancreatic duct cells immortalized with E6E7 gene of HPV; pancretic islet: dedifferentiated human pancreatic islets; stellate: hepatic stellate cells; skeletal muscle: skeletal muscle cells.

Figure 3

Correlation between APOB gene expression in PBMCs with percent hemoglobin A1C (A), serum total cholesterol (B) and triglyceride (C) concentrations in Chuvash polycythemia. Residual plots of metabolite measurements against gene expression levels are shown, with r and P from Pearson's one-sided correlation test.