ELISA for Determination of the Haptoglobin Phenotype

Abstract

The haptoglobin genetic locus at 16q22 is polymorphic with two known classes of alleles, denoted 1 and 2 (1). The polymorphism is extremely common, with worldwide frequencies of the two alleles being approximately equal. However, there is considerable geographic and ethnic variation in the distribution of haptoglobin phenotypes (1). Over the past 3–4 years our laboratory has demonstrated that haptoglobin is a major susceptibility gene for the development of diabetic vascular complications in multiple longitudinal and cross-sectional population studies (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13). Diabetic individuals homozygous for the haptoglobin 2 allele were shown to be at five times greater risk of developing cardiovascular disease compared with diabetic individuals homozygous for the haptoglobin 1 allele, with an intermediate risk for the heterozygote (8). The increased susceptibility to vascular complications conferred by the Hp 2 allele has recently been recapitulated in a transgenic animal model, which showed direct linkage of the polymorphism with disease susceptibility (R. Lotan et al., manuscript submitted). Mechanistic studies using the purified protein products of the Hp 1 and Hp 2 alleles have identified profound differences in antioxidant and immunomodulatory activity (14)(15). Functional as well as structural differences exist between the various haptoglobin allelic protein products (1). The Hp 2 allele appears to have arisen by an intragenic duplication event of exons 3 and 4 of the Hp 1 allele, which leads to the duplication of a multimerization domain in exon 3. Consequently, the Hp 1 allele protein product forms dimers only. The Hp 2 allele has two copies of exon 3; therefore, Hp 2 allele protein products combine to form cyclic polymers three monomers and larger in size. In heterozygotes, linear polymers containing both allelic …