Polymorphisms in the human haptoglobin gene cluster: chromosomes with multiple haptoglobin-related (Hpr) genes.

We have found polymorphisms for the number of tandemly arranged haptoglobin-related (Hpr) genes in the haptoglobin gene cluster of Blacks. Genomic mapping and nucleotide sequence analysis indicate that two copies of the Hpr gene first resulted from unequal but homologous crossing-over in a region 3' to the haptoglobin (Hp) and the haptoglobin-related genes. Subsequent increases in the number of Hpr loci have occurred in some chromosomes. Among 25 American Blacks studied (15 were unrelated), 2 related individuals have one extra copy of the Hpr gene and 5 unrelated individuals have more than two extra Hpr genes. None of 26 Whites and one Oriental studied have extra copies. In one of the Blacks, six tandemly arranged Hpr genes were demonstrated in one chromosome by pulsed field gradient electrophoresis. His other chromosome had one Hpr gene. The tandem Hpr genes were found in individuals with the haptoglobin genotypes Hp2/Hp2 (3 of 3 tested) and Hp2/Hp1 (4 of 11 tested), but none were found in the Hp1/Hp1 individuals (11 tested). Fibroblast cell cultures from two Hp2/Hp1 heterozygotes were fused to mouse cells to obtain cell lines retaining a human chromosome 16 on which the haptoglobin gene cluster is located. DNA analysis of the hybrid cells showed that in both individuals the tandemly arranged Hpr genes are linked to the Hp2 allele. These results suggest that the multiple copies are associated with the Hp2 gene.