A Newly Identified Deletion of 970 bp at the α-Globin Locus That Removes the Promoter Region of the α1 Gene

A New Intergenic α-Globin Deletion (α-αΔ125) Found in a Kabyle Population

We have identified a deletion of 125 bp (α-α(Δ125)) (NG_000006.1: g.37040_37164del) in the α-globin gene cluster in a Kabyle population. A combination of singlex and multiplex polymerase chain reaction (PCR)-based assays have been used to identify the molecular defect. Sequencing of the abnormal PCR amplification product revealed a novel α1-globin promoter deletion. The endpoints of the deletion were characterized by sequencing the deletion junctions of the mutated allele. The observed deletion was located 378 bp upstream of the α1-globin gene transcription initiation site and leaves the α2 gene intact. In some patients, the α-α(Δ125) deletion was shown to segregate with Hb S (HBB: c.20A>T) and/or Hb C (HBB: c.19G>A) or a β-thalassemic allele. The α-α(Δ125) deletion has no discernible effect on red cell indices when inherited with no other abnormal globin genes. The family study demonstrated that the deletion is heritable. This is the only example of an intergenic α2-α1 non coding DNA deletion, leaving the α2-globin gene and the α1 coding part intact.

A large deletion in the human alpha-globin cluster caused by a replication error is associated with an unexpectedly mild phenotype

We have characterized a newly identified 16.6 kb deletion which removes a significant proportion of the human alpha-globin cluster including the psizeta1, alpha(D), psialpha1 and alpha2-globin genes but leaves the duplicated alpha1 gene intact. This complicated rearrangement results from a combination of slippage and strand switching at sites of microhomology during replication. Functional analysis shows that expression of the remaining alpha1 gene is increased, rather than down-regulated by this deletion. This could be related to its proximity to the remote upstream alpha-globin regulatory elements or reduced competition for these elements in the absence of the dominant alpha2-globin gene. The finding of a very mild phenotype associated with such an extensive deletion in the alpha-globin cluster implies that much of the DNA removed by the deletion is likely to be functionally unimportant. These findings suggest that other than the upstream regulatory elements and promoter proximal elements there are unlikely to be additional positive cis-acting sequences in the alpha-globin cluster.

Long-range regulation of alpha-globin gene expression

Over the past 20 years, there has been an increasing awareness that gene expression can be regulated by multiple cis-acting sequences located at considerable distances (10-1000 kb) from the genes they control. Detailed investigation of a few specialized mammalian genes, including the genes controlling the synthesis of hemoglobin, provide important models to understand how such long-range regulatory elements act. In general, these elements contain a high density of evolutionarily conserved, transcription factor-binding sites and in many ways resemble the upstream regulatory elements found adjacent to the promoters of genes in simpler organisms, differing only in the distance over which they act. We have investigated in detail how the remote regulatory elements of the alpha-globin cluster become activated as hematopoietic stem cells (HSCs) undergo commitment, lineage specification, and differentiation to form red blood cells. In turn, we have addressed how, during this process, the upstream elements control the correct spatial and temporal expression from the alpha-gene promoter which lies approximately 60 kb downstream of these elements. At present too few loci have been studied to determine whether there are general principles underlying long-range regulation but some common themes are emerging.