The molecular basis of α-thalassemias: Frequent occurrence of dysfunctional α loci among non-Asians with Hb H disease

Abstract

Study of Asians has previously indicated that deletion of α-globin structural genes is the predominant lesion in α-thalassemias and that Hb H disease occurs when three of four normal α loci per cell are deleted. To test the generality of this model, Hb H disease DNAs of both Asian and non-Asian origin were analyzed by restriction endonuclease mapping using the technique of Southern (1975). Whereas in normal DNA, α sequences are present in a single Eco RI fragment of cellular DNA approximately 22.5 kb long, fragments of 22.5, 20 and 2.6 kb were found in various Hb H disease DNAs. The 20 kb Eco RI fragment alone, in which a single α-globin structural locus resides, was found in Asian Hb H disease DNA. This finding is consistent with the deletion model of α-thalassemia. In contrast, seven of eight non-Asian Hb H disease DNAs displayed a more complex molecular composition. The fragment patterns observed were 22.5 kb alone, 22.5 plus 2.6 kb, 20 plus 2.6 kb and 20 kb alone. Non-Asian Hb H disease DNAs contained one, two or three α loci per cell in contrast to the one locus predicted by the simple deletion model of α-thalassemia. The data are best explained by the existence of defective α loci in certain individuals with α-thalassemia, particularly outside the Asian population. Restriction mapping of the 20 kb Eco RI fragment found in Asian and some non-Asian Hb H disease DNAs demonstrated a striking similarity in the placement of restriction sites about the single α gene compared with sites about the two genes in the 22.5 kb Eco RI fragment seen in normal DNA. These data are consistent with origin of the 20 kb fragment from the 22.5 kb normal Eco RI fragment by either unequal crossing-over or a deletion event. The molecular heterogeneity and frequent occurrence of defective α loci in non-Asian Hb H disease DNAs described here may explain, in part, the clinical heterogeneity of α-thalassemias and the absence of the homozygous deletion state (hydrops fetalis) in non-Asians. Further study of cellular DNA fragments containing the defective α loci identified in this work may indicate the types of specific mutations responsible for abnormal globin gene expression and complement similar studies on abnormal β genes in β-thalassemias.