Ph.D Thesis

Ph.D StudentAsleh Rabea
SubjectElucidation of the Mechanisms Responsible for the
Interaction between the Haptoglobin Polymorphism
and Diabetic Cardiovascular Disease
DepartmentDepartment of Medicine
Supervisor PROF. Andrew Peter Levy


There exist two classes of alleles at the haptoglobin (Hp) locus, denoted class 1 and class 2, and three potential genotypes denoted Hp1-1, Hp2-1 and Hp2-2. The chief function of Hp is to bind free hemoglobin (Hb) and thereby prevent Hb-induced oxidative tissue damage. We have recently demonstrated in multiple independent populations based longitudinal and cross sectional analyses that the Hp2-2 genotype is associated with an increased risk for diabetic cardiovascular disease (CVD). The mechanism whereby the Hp polymorphism confers susceptibility to CVD and the reason why this interaction is specific to the diabetic state is not understood.

Through this study, we sought to understand functional differences between the different Hp types in regard to their antiatherogenic function and to understand why these differences may only be of importance in the diabetic state. We used multiple in vitro and in vivo systems to demonstrate at different levels that the Hp2 allele is associated with a significant decrease in antiatherogenic properties as compared to the Hp1 allele, specifically in the setting of diabetes. First, we demonstrated that the antioxidant activity of Hp is severely impaired against glycated Hb, and that the Hp1-1 protein is superior to Hp2-2 protein in clearance of free Hb into macrophages via CD163 scavenger receptor. Second, we demonstrated that the Hp-Hb complex can activate cells expressing CD163 in a Hp type-dependent manner, leading to elevation in intracellular markers of macrophage activation. Third, we found that Hp2-2 fails to stabilize Hb-derived iron when Hb is glycated, resulting in a significantly increased amount of redox active iron and intracellular oxidative stress. In vivo, we observed significantly elevated levels of labile plasma iron (LPI) in Hp2 diabetic mice as well as in diabetic individuals with the Hp2 allele as compared to those with the Hp1 allele. Finally, we demonstrated that LPI is dramatically elevated in diabetic patients as compared to non-diabetic patients after myocardial infarction (MI), and that elevated LPI is an independent powerful predictor of 30-day mortality after MI in diabetes.

Taken together, we present in this study evidence in support of the mechanisms underlying the increased incidence of CVD in diabetic individuals with the Hp 2 allele. The model presented here provides simple in vitro an in vivo paradigms in which to test different therapeutic agents to reduce morbidity and mortality from diabetic cardiovascular complications attributed to the Hp polymorphism.