Some of you may remember the great comedian Rodney Dangerfield who passed away in 2004. While some may remember his roles in Caddy Shack or Back to School (my favorite), he was best known for his catchphrase “I don’t get no respect” which was the basis for many comedy monologues.
When we speak of cardiovascular risk in diabetes, we invoke cholesterol, chronic hyperglycemia, reactive oxygen species (ROS), AGE’s, the presence of kidney disease and perhaps others. But almost no one talks about, measures, or attempts to treat one risk factor that is well recognized and treatable (or at least the effects of it seem to be). Have you guessed it yet?
Haptoglobin (Hp) was first described in 1938. It is an α2-sialoglycoprotein coming mainly from hepatocytes in response to the secretion of cytokines such as IL-6, IL-1 and TNF. Haptoglobin is a tetrameric protein that structurally resembles immunoglobulins because it has two light chains (α) and two heavy chains (β) covalently bound to each other by disulfide bridges (S-S). Its main function is to rapidly bind free hemoglobin from intravascular hemolysis to keep it from oxidatively damaging the vascular endothelium. Although present in all vertebrates, in humans Hp is characterized by molecular heterogeneity caused by genetic polymorphism. In the 1940s and 1950s structural heterogeneity was establish when alleles of different structures and lengths were identified, categorized as Hp 1-1, Hp 2-1, and Hp2-2. While these variants can be found in patients without diabetes, much of the work on their role as risk factors has been done in the context of diabetes (both type 1 and type 2). Biologic functions of Hp include prevention of renal damage from intravascular hemolysis, protection against ‘toxic radicals’, and sparing of nitric oxide when unbound to hemoglobin. It has also been found to have a variety of different effects comprising an immune-modulating action. All of these effects seem to differ with the haptoglobin phenotype, and in general a protective role is seen in patients with the Hp 1-1 variant, and a lack of protective effect or more pathological role is seen with the Hp 2-2 variant. Approximately 40% of patients with diabetes carry the Haptoglobin 2-2 gene variant (1)
Increased oxidative stress in diabetic patients results from oxidation of glucose and the modification of low-density lipoproteins (LDL). These changes may stimulate the production of inflammatory cytokines that have been implicated in the morphological and pathological changes found in macrovascular and microvascular complications, and different degrees of susceptibility to the development of vascular problems have been observed in studies of the antioxidant properties of Hp. In diabetic patients, those with Hp 1-1 show better protection against complications than Hp 2-1 and Hp 2-2 individuals. The Hp 2-2 phenotype has been found more frequently in people developing type 2 diabetes (2). Hp 2-2 has been shown to be a major risk factor in diabetic vascular disease (3), an increased risk of Coronary Heart Disease (4) an increased cardiovascular mortality in type 1 diabetes (5), an increased risk of renal function decline (6), an increased risk of death following stroke (7), retinopathy (8) and many other risks of complications in people with diabetes.
Also, as many of you know, both type 1 and type 2 diabetes are associated with the so called ‘leaky gut’. In type 1 diabetes various antigens are presented to the maturing gut which can stimulate the development of activated T-cells which, over time, destroy pancreatic beta cells. In type 2 diabetes, the ‘leaky gut’ allows lipopolysaccharides into the circulation, releasing cytokines and adipokines causing inflammation, insulin resistance and eventually diabetes with loss of beta cell mass due to glucose and lipid toxicity. A major factor associated with ‘leaky gut’ is zonulin, discovered in 2000 by Fasano. Zonulin is pre-haptoglobin-2. (9) It is fascinating that the haptoglobin 2 allele (especially in those individuals who are homozygous [hp 2-2] would confer risk of diabetes, and risk within diabetes likely due to its role in gut permeability and ongoing inflammation…
The risk associated with the presence of the Hp 2-2 genotype can be mitigated with vitamin E treatment to a striking degree. While vitamin E has not been shown to affect surrogates of risk in individuals with Hp 1-1, its effect is clear for those with Hp 2-2. The summary abstract below speaks for itself:
Clinical trial data from the HOPE, ICARE, and WHS studies is presented showing a pharmacogenomic interaction between the Hp genotype and vitamin E on the development of CVD. Specifically, in individuals with diabetes and the Hp2-2 genotype, vitamin E has been shown to be associated with an approximately 35% reduction in CVD. Cardioprotection by vitamin E in individuals with the Hp2-2 genotype appears to be mediated in part by an improvement in HDL functionality as demonstrated in three independent trials in both type 1 diabetes and type 2 diabetes. (10)
Despite this positive review, and others like it, over 300 articles in PubMed regarding haptoglobin in diabetes, and several presentations at ADA Annual meetings, the current ADA Standards of Medical Care don’t even mention haptoglobin.(11) It truly is the ‘Rodney Dangerfield of risk factors in diabetes’!
So my question to all of you is: With the clear role of haptoglobin genotype 2-2 in the risk of micro and macrovascular complications AND with the clear indication that treatment with Vitamin E can substantially reduce that risk…when was the last time you suggested obtaining a haptoglobin genotype for your patient with diabetes?
1) Changing the Face of Diabetic Care with Haptoglobin Genotype Selection and Vitamin E Nina S. Levy, Ph.D. and Andrew P. Levy, M.D., Ph.D. Rambam Maimonides Med J. 2011 Apr; 2(2): e0047
2) Shi X, et al. Haptoglobin 2-2 Genotype Is Associated with Increased Risk of Type 2 Diabetes Mellitus in Northern Chinese. Genetic Testing and Molecular Biomarkers 2012;16:563-568
3) Asleh R. and Levy AP. In vivo and in vitro studies establishing haptoglobin as a major susceptibility gene for diabetic vascular disease. Vascular Health and Risk Management 2005;1:19–28
4) Cahill LE, et al. The Risk of Coronary Heart Disease Associated With Glycosylated Hemoglobin of 6.5% or Greater Is Pronounced in the Haptoglobin 2-2 Genotype. Journal of the American College of Cardiology 2015;66:1791-9
5) Costacou T. and Orchard TJ. The Haptoglobin genotype predicts cardio-renal mortality in type 1 diabetes. J. Diabetes Complications. 2016;30:221-6
6) Costacou, T, et al. Haptoglobin Genotype and Renal Function Decline in Type 1 Diabetes. Diabetes 58:2904–2909, 2009
7) Ijas P, et al. Haptoglobin Hp2 Variant Promotes Premature Cardiovascular Death in Stroke Survivors. Stroke. 2017;48:1463-1469
8) Mukund R, et al. Haptoglobin2-2 phenotype is an additional risk factor of retinopathy in type 2 diabetes mellitus. Indian Journal of Human Genetic. 2013 Apr-Jun; 19(2): 154–8.
9) Fasano A., Physiological, Pathological, and Therapeutic Implications of Zonulin-Mediated Intestinal Barrier Modulation. The American Journal of Pathology 2008;173:1243-1252
10) Hochberg I, et al. Interaction Between the Haptoglobin Genotype and Vitamin E on Cardiovascular Disease in Diabetes. Current Diabetes Reports 2017_online Jun 17
11) American Diabetes Association. Standards of Medical Care in Diabetes. Diabetes Care 2017; 40: Supplement 1