Molecular analysis of deletions in the human beta-globin gene cluster: deletion junctions and locations of breakpoints.

DNA fragments that contain the deletion junction regions of four independent deletions involving the human beta-globin gene cluster have been isolated and cloned. The fragments were isolated from individuals with the conditions referred to as Sicilian (delta beta)zero-thalassemia, Turkish G gamma+(A gamma delta beta)zero-thalassemia, Black G gamma+(A gamma delta beta)zero-thalassemia, and HPFH-2. The sequences of the deletion junctions and of the normal DNA surrounding their 3' breakpoints were determined and compared to the previously determined sequences of normal DNA surrounding their 5' breakpoints. These comparisons show that the deletions were the result of nonhomologous recombinational events. Two of the deletion junctions contain "orphan" nucleotides, while the other two show very limited amounts of "junctional homology." Both types of junctions are common among recombination events in mammalian cells and we discuss a simple joining scheme that could account for the junctions reported here. Unlike other deletions in this cluster and in other gene clusters, none of the eight deletion breakpoints examined here occurred within Alu family repeats. To examine the significance of deletion breakpoints within various sequence categories, we analyzed the data from a well-defined set of deletions within this locus. In contrast to deletions in the alpha-globin gene cluster, the occurrence of breakpoints in Alu family repetitive sequences is not statistically significant within the beta-globin gene cluster. However, breakpoints do occur within transcriptional units of the beta-globin gene cluster more frequently than expected by chance alone. We conclude from our analysis that the mechanisms of DNA joining are not locus or location specific, but at least a portion of the mechanisms of chromosomal breakages do show locus specificity.