Correction of HDL dysfunction in individuals with diabetes and the haptoglobin 2-2 genotype

OBJECTIVE—Pharmacogenomics is a key component of personalized medicine. The Israel Cardiovascular Events Reduction with Vitamin E Study, a prospective placebo-controlled study, recently demonstrated that vitamin E could dramatically reduce CVD in individuals with diabetes and the haptoglobin (Hp) 2-2 genotype (40% of diabetic individuals). However, because of the large number of clinical trials that failed to demonstrate benefit from vitamin E coupled with the lack of a mechanistic explanation for why vitamin E should be beneficial only in diabetic individuals with the Hp 2-2 genotype, enthusiasm for this pharmacogenomic paradigm has been limited. In this study, we sought to provide such a mechanistic explanation based on the hypothesis that the Hp 2-2 genotype and diabetes interact to promote HDL oxidative modification and dysfunction.RESEARCH DESIGN AND METHODS—Hb and lipid peroxides were assessed in HDL isolated from diabetic individuals or mice with the Hp 1-1 or Hp 2-2 genotypes. HDL function was assessed based on its ability to promote cholesterol efflux from macrophages. A crossover placebo-controlled study in Hp 2-2 diabetic humans and in Hp 1-1 and Hp 2-2 diabetic mice assessed the ability of vitamin E to favorably modify these structural and functional parameters.RESULTS—Hb and lipid peroxides associated with HDL were increased and HDL function was impaired in Hp 2-2 diabetic individuals and mice. Vitamin E decreased oxidative modification of HDL and improved HDL function in Hp 2-2 diabetes but had no effect in Hp 1-1 diabetes.CONCLUSIONS—Vitamin E significantly improves the quality of HDL in Hp 2-2 diabetic individuals.Pharmacogenomics is a key component of personalized medicine (1). Therapy targeted to a specific patient based on his or her genetically determined pathophysiology responsible for the disease offers the possibility of significantly improving patient care and reducing costs. However, despite the clear public health and economic benefits that would be attained by such an approach, this paradigm has not been successfully applied to a common disease.Cardiovascular disease (CVD) is responsible for 75% of deaths among individuals with diabetes, and yearly expenditures for CVD in diabetes exceed $200 billion (2). Neither conventional risk factors nor the degree of glycemic control adequately predict which individuals with diabetes develop CVD, suggesting the existence of genetic susceptibility factors.A polymorphism in the haptoglobin (Hp) gene may define which individuals with diabetes are at greatest risk of CVD. There exist two classes of alleles at the Hp locus denoted 1 and 2 with three possible Hp genotypes 1-1, 2-1, and 2-2 (3). In five independent longitudinal studies performed in ethnically and geographically diverse groups, individuals with the Hp 2-2 genotype and diabetes were found to have a two- to fivefold increased risk of CVD compared with diabetic individuals without the Hp 2-2 genotype (4–8). The prevalence of the Hp 2-2 genotype in the diabetic population in most Western countries is ∼40%, making this a common polymorphism.The Hp polymorphism differs from nearly all polymorphisms being assessed in genome-wide association studies because it is a functional polymorphism (3). Understanding functional differences between the Hp 1 and Hp 2 allelic protein products, particularly in diabetes, may provide insight into why Hp 2-2 diabetic individuals have more CVD and how this increased burden of disease might be reduced. The most well-understood function of Hp is to bind Hb released from erythrocytes (3). Each day, >6 g Hb is released into the bloodstream due to turnover of erythrocytes, and heme iron in this Hb is a powerful oxidizing agent (9,10). Hp, which is present in a 400-fold molar excess to free Hb under normal conditions, binds Hb, reducing its ability to mediate oxidative modifications and directing its removal from the blood via the monocyte/macrophage CD163 Hp-Hb scavenger receptor (11).More than 5 years ago, motivated by in vitro studies demonstrating that the Hp 2-2 protein provides inferior protection against Hb-mediated oxidative stress (9,10), coupled with the suggested importance of oxidative stress in diabetic atherosclerosis (12), we sought to determine whether antioxidant therapy might be particularly beneficial to the Hp 2-2 diabetic cohort. We first tested this hypothesis by examining stored samples from the Heart Outcomes Prevention Evaluation (HOPE) study, which had failed to demonstrate benefit from vitamin E (13). We found that myocardial infarction and CVD death were reduced by >40 and 50%, respectively, in Hp 2-2 diabetic HOPE participants who received vitamin E (14). To prospectively test the hypothesis, we initiated a double-blind randomized placebo-controlled study of vitamin E in 1,434 Hp 2-2 diabetic individuals (Israel Cardiovascular Events Reduction with Vitamin E [ICARE] Study). We found that vitamin E supplementation was associated with a >50% reduction in the combined primary outcome of stroke, myocardial infarction, and cardiovascular death in Hp 2-2 diabetes (7).Enthusiasm for these findings, despite the considerable public health and economic benefits that they suggest, has been muted. Our study comes in the wake of numerous large clinical trials that failed to demonstrate that vitamin E provides any protection against CVD and may be harmful (13,15–20). Further hampering acceptance of this paradigm is the lack of a rational pathophysiological and pharmacogenomic mechanism to explain why Hp 2-2 diabetic individuals have an increased risk of CVD and how vitamin E mitigates this risk. In this study, we sought to provide a rationale for the pharmacogenomic application of the Hp genotype to prevent CVD in diabetes by elucidating the unique structural modifications and dysfunctional nature of HDL in Hp 2-2 diabetic individuals and how these structural and functional changes in HDL are rapidly reversed with vitamin E.