Alpha-thalassemia caused by a large (62 kb) deletion upstream of the human alpha globin gene cluster

Abstract

We describe a family in which alpha-thalassemia occurs in association with a deletion of 62 kilobases from a region upstream of the alpha globin genes. DNA sequence analysis has shown that the transcription units of both alpha genes downstream of this deletion are normal. Nevertheless, they fail to direct alpha globin synthesis in an interspecific hybrid containing the abnormal (alpha alpha)RA chromosome. It seems probable that previously unidentified positive regulatory sequences analogous to those detected in a corresponding position of the human beta globin cluster are removed by this deletion.

Contrasting effects of alpha and beta globin regulatory elements on chromatin structure may be related to their different chromosomal environments.

“…Human a globin mRNA expression was evaluated in each of four hybrids carrying a different, mutated copy of chromosome 16. As previously reported (Hatton et al, 1990), a basal level of expression was seen in a hybrid containing the (aa)RA chromosome (Figure 6b) but in the others, human a globin mRNA, if present, was below the level of detection by the RNase protection assay. We found no evidence for aberrant expression of the 4 globin gene which remains intact on the (aa)IJ chromosome (data not shown).…”

“…Hybrids representing the abnormal chromosomes studied here were designated as follows (aa)RA, H-101 (Hatton et al, 1990); (aa)CMO, CMOSN (Flint et al, 1994); (aa)TAT, TATA (Wilkie et al, 1990); --MC, LCSN (Vickers and Higgs, 1989).…”

“…To analyse the effect of HS -40 on long range chromatin structure and to determine the degree to which formation of DNase I hypersensitive sites at the a promoters and HS -40 are interdependent, we analysed interspecific hybrids containing a normal copy of chromosome 16 or a copy of chromosome 16 with deletions which removed either HS -40 (Hatton et al, 1990;Liebhaber et al, 1990;Wilkie et al, 1990;Flint et al, 1994) or the a gene promoters . We have previously shown that four mutant chromosomes, (aa)RA, (aa)IJ, (aa)TAT and (aa)CMo, have deletions of between 35 and >150 kb of DNA that all include HS -40 ( Figure 1).…”

“…Naturally occurring deletions (Kioussis et al, 1983;Hatton et al, 1990;Stamatoyannopoulos et al, 1994) first demonstrated that, in each cluster, activated gene expression is dependent on remote, positive regulatory elements which have been localized to erythroid-specific DNase I hypersensitive sites lying upstream of the structural genes. In the a cluster, a single element (HS -40) is located 40 kb upstream of the 4 globin gene .…”

1990 MACK FORSTER PRIZE LECTURE REVIEW: The Molecular Genetics of the a globin gene family

“…5). We have now demonstrated that the (aa)RA chromosome accounts for the a thalassaemia phenotype in this family [30].…”