ELISA for Determination of the Haptoglobin Phenotype

An enzyme linked immunosorbent assay (ELISA) for the determination of the human haptoglobin phenotype

BACKGROUND

Haptoglobin (Hp) is an abundant serum protein which binds extracorpuscular hemoglobin (Hb). Two alleles exist in humans for the Hp gene, denoted 1 and 2. Diabetic individuals with the Hp 2-2 genotype are at increased risk of developing vascular complications including heart attack, stroke, and kidney disease. Recent evidence shows that treatment with vitamin E can reduce the risk of diabetic vascular complications by as much as 50% in Hp 2-2 individuals. We sought to develop a rapid and accurate test for Hp phenotype (which is 100% concordant with the three major Hp genotypes) to facilitate widespread diagnostic testing as well as prospective clinical trials.

METHODS

A monoclonal antibody raised against human Hp was shown to distinguish between the three Hp phenotypes in an enzyme linked immunosorbent assay (ELISA). Hp phenotypes obtained in over 8000 patient samples using this ELISA method were compared with those obtained by polyacrylamide gel electrophoresis or the TaqMan PCR method.

RESULTS

Our analysis showed that the sensitivity and specificity of the ELISA test for Hp 2-2 phenotype is 99.0% and 98.1%, respectively. The positive predictive value and the negative predictive value for Hp 2-2 phenotype is 97.5% and 99.3%, respectively. Similar results were obtained for Hp 2-1 and Hp 1-1 phenotypes. In addition, the ELISA was determined to be more sensitive and specific than the TaqMan method.

CONCLUSIONS

The Hp ELISA represents a user-friendly, rapid and highly accurate diagnostic tool for determining Hp phenotypes. This test will greatly facilitate the typing of thousands of samples in ongoing clinical studies.

A review of haptoglobin typing methods for disease association study and preventing anaphylactic transfusion reaction

Haptoglobin, the product of the Hp gene, is a glycoprotein involved in the scavenging of free hemoglobin. Haptoglobin levels increase or decrease in response to various acquired conditions, and they are also influenced by genetic predisposition. There were 2 major alleles, Hp (1) and Hp (2), and 1 minor allele, Hp (del) . Many researchers have attempted to study the haptoglobin types and their association with disease; however, no definitive conclusions have been reached yet. It is reported that patients who are genetically deficient in haptoglobin are at risk of anaphylaxis against blood components containing haptoglobin. Haptoglobin genotypes also affect the reference intervals of haptoglobin levels. Many studies have attempted to establish simple and accurate typing methods. In this paper, we have broadly reviewed several methods for haptoglobin typing-phenotyping, Southern blotting, conventional PCR, real-time PCR, and loop-mediated isothermal amplification. We discuss their characteristics, clinical applications, and limitations. The phenotyping methods are time consuming and labor intensive and not designed to detect patients harboring Hp (del) . The rapid and robust haptoglobin genotyping may help in preventing fatal anaphylactic reactions and in establishing the relationships between the haptoglobin phenotypes and diseases.

Toxicological consequences of extracellular hemoglobin: biochemical and physiological perspectives

Under normal physiology, human red blood cells (RBCs) demonstrate a circulating lifespan of approximately 100-120 days with efficient removal of senescent RBCs taking place via the reticuloendothelial system, spleen, and bone marrow phagocytosis. Within this time frame, hemoglobin (Hb) is effectively protected by efficient RBC enzymatic systems designed to allow for interaction between Hb and diffusible ligands while preventing direct contact between Hb and the external environment. Under normal resting conditions, the concentration of extracellular Hb in circulation is therefore minimal and controlled by specific plasma and cellular (monocyte/macrophage) binding proteins (haptoglobin) and receptors (CD163), respectively. However, during pathological conditions leading to hemolysis, extracellular Hb concentrations exceed normal plasma and cellular binding capacities, allowing Hb to become a biologically relevant vasoactive and redox active protein within the circulation and at extravascular sites. Under conditions of genetic, drug-induced, and autoimmune hemolytic anemias, large quantities of Hb are introduced into the circulation and often lead to acute renal failure and vascular dysfunction. Interestingly, the study of chemically modified Hb for use as oxygen therapeutics has allowed for some basic understanding of extracellular Hb toxicity, particularly in the absence of functional clearance mechanisms and in circulatory antioxidant depleted states.

TaqMan-based real-time PCR for genotyping common polymorphisms of haptoglobin (HP1 and HP2)

BACKGROUND

The haptoglobin gene (HP) has 2 common codominant alleles (HP(1) and HP(2)) that account for 3 phenotypes. HP(2) is generated by a 1.7-kb intragenic duplication of HP(1).

METHODS

We used the real-time TaqMan PCR system to develop an effective method for HP genotyping that allows us to evaluate the relative number of copies of the HP(2) allele-specific junctional region of the 1.7-kb gene duplication (HP2) by comparing the intensity of the amplification signals to those of the HP promoter region (HP5'), which was used as the internal control. The difference in threshold cycles (DeltaCt) between HP2 and HP5' was used to assess HP(2) copy number. In addition, the assay detects the HP deletion (HP(del)) at the same time.

RESULTS

The mean 2(-DeltaDeltaCt) values (the HP2/HP5' ratio) obtained from 123 samples of known HP genotypes clearly differentiated 2 nonoverlapping intervals that correspond to the HP genotypes. Ratios for HP(2)/HP(1) samples ranged from 0.34-0.50, HP(2)/HP(2) samples ranged from 0.79-0.98, and the absence of an HP(2) allele signal was defined as HP(1)/HP(1). We simultaneously detected HP(del). The assay produces results in <1 h.

CONCLUSIONS

The TaqMan-based real-time PCR method was successfully applied to HP genotyping. The method is easy to use in a molecular diagnosis laboratory, and its robustness and rapidity make it suitable for high-throughput analysis of large populations.

Proteotyping of human haptoglobin by MALDI-TOF profiling: Phenotype distribution in a population of toxic oil syndrome patients

Toxic oil syndrome (TOS) is a disease that appeared in Spain in 1981 as a consequence of the ingestion of an aniline-adulterated oil illegally marketed as edible. TOS affected more than 20 000 people and produced over 400 deaths in the first 18 months after the outbreak. There is evidence that genetic factors could play a role in the susceptibility of individuals towards the disease. Recently, we suggested that haptoglobin (Hp) polymorphism could also play a role in TOS. To provide a rapid method for high-throughput Hp phenotyping, we developed a two-step MALDI-TOF procedure that allows specific identification of the three common Hp alpha chains. Resolution of the homologous alpha-1s and alpha-1f chains, which have a mass difference of only 0.043 Da, is obtained after guanidination of the protein with O-methylisourea. We applied this procedure to the study of the distribution of the Hp alleles HP(1s), HP(1f), HP(2) in a control versus a TOS-affected population, both originally exposed to the toxic oil. The MALDI-TOF proteotyping method was validated by a parallel analysis of the serum samples by 2-DE. Data obtained from 54 TOS cases and 48 control individuals indicate significant differences in the distribution of Hp phenotypes in the two populations.