Determining the cause of patchwork HBA 1 and HBA 2

The human α-globin gene cluster is located on chromosome 16 pterp13.3, and is arranged in the order, 5’ζ2-ψζ1-ψα2-ψα1-α2-α1-θ1-3’. Recent reexamination of the ψα2 gene revealed that it is expressed at a very low level, and was renamed as mu-globin gene. The cluster was thought to result from duplication events that occurred more than 300 million years ago. Unequal homologous recombinations are common between the two ζand two α-globin genes of this cluster, due to the extensive sequence homology between the HBZ (ζ2-globin) and HBZP (ψζ1-globin) genes and between the HBA1 (α1-globin) and HBA2 (α2-globin) genes, respectively. Despite their ancient origin, the HBA1 and HBA2 genes have remained very similar, comprising three segments of homology (the X, Y, and Z boxes) that are punctuated with non-homologous regions (I, II, and III). They are identical throughout the 868 base pairs upstream of the cap site, except for two positions: nucleotide -634 is an A in HBA1 and a G in HBA2, while nucleotide –733 is a C in HBA1 and a T in HBA2. Both genes are also identical in their 5’untranslated regions, the first intron (IVSI) and all three coding exons. In the second intron (IVSII), only two sites of difference exist: a single nucleotide difference at IVSII,55 (G in HBA1 and T in HBA2), and the substitution of an octanucleotide in HBA1 (positions 119-126, 5’-CTCGGCCC-3’) with a single G at position 119 in HBA2. The 3’ untranslated regions of the two genes share 78% homology, preceding a short region of 100% identity adjacent to the polyadenylation site. Several models have been suggested for this concerted evolution of the α-globin gene cluster, the most prominent being gene conversion and crossover fixation. Evidence for mispairing and reciprocal crossing over between HBA1 and HBA2 can be inferred from the existence of the single α-globin gene deletions (-α and -α) and their reciprocal triplicated alleles (ααα and ααα). The demonstration of α-globin gene deletions in transformed E. coli bacteria, identical to those found in humans, further reinforces the theory that sequence homology at the α-globin gene cluster promotes unequal crossing over. Although Department of Pediatric Medicine, KK Women’s & Children’s Hospital, Singapore 229899, Singapore (HLaw, ISN); Departments of Medicine and Pathology, Boston University School of Medicine, Boston, MA 02118, USA (HLuo, MHS, DHC); Department of Pediatrics, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 119074, Singapore (WW, JFH, SSC); Kariminejad-Najmabadi Pathology & Genetics Center, 14665/154, Tehran, Iran (HN).