Characterization of embryonic globin gene expansions in homo sapiens : mechanistic and evolutionary implications

Abstract

This research focused on the characterization of aberrant ζ-globin expansions as an important model system in the study of genetic recombination and gene rearrangements in Homo sapiens. There are normally two embryonic ζ-globin genes in the α-globin multigene family on chromosome 16p13.3. In this study, thirteen triple ζ-globin and two novel quadruple ζ-globin chromosomes from Laotian and Filipino populations in Hawaii were discovered and characterized by Southern blot gene mapping, PCR amplification and sequencing analyses in an effort to elucidate the structure and processes of gene duplication in the evolution of these amplified regions. Attempts to clone the reduplicated genes in genomic library constructs were unsuccessful, presumably due to sequence-specific idiosyncrasies and problems with methylation. Restriction patterns generated from a battery of enzymes in single and double digests indicate a similar homologous recombination event was responsible for the expansion haplotypes. The high frequencies (10-15%) of triplicated ζ-genes in the populations studied increases the likelihood of a homologous crossover event involving a triple ζ-globin chromosome in the generation of the quadruple arrangement. The identification of two distinct triple ζ-gene chromosomal arrangements, one in linkage with normal α-genes, and another associated with a - α3.7 deletion, indicates that the triple arrangement has undergone two recombination events in the Filipino population. Further, gene mapping and sequence analyses of PCR products of the ζ-gene expansions indicate that the recombinant genes are hybrids of 5'ψζ1 and 3'ζ2 genes, and that the ζ-globin sequences are highly conserved to date. The absence of single ζ-gene chromosomes in these populations suggests that the reciprocal chromosome in the recombinant expansion may be detrimental or lethal, similar to the α-thalassemia deletions. These analyses are suggestive of structural constraints within the molecule which lead to the production of similar zeta expansions in this region. As no correlating phenotypic advantage has been determined to date, the frequencies observed for the reduplicated ζ-globin genotypes are probably maintained by random genetic drift in these populations. The ζ-globin expansions may be an important event in the evolution of new genes or new gene functions in the α-globin complex, and their characterization is relevant to studies of molecular etiology of disease states and evolution in the human genome.