Deletions of both alpha-globin genes on one chromosome (alpha-thalassemia-1) of the Southeast Asian type were observed in 5 cases, and 3 alpha-globin gene triplications were identified.
There were 14 heterozygotes and two homozygotes for alpha(+)-thalassemia; the remaining test subject carried a deletion of both alpha-globin genes (alpha(0) -thalassemia of the Southeast Asian type) on one chromosome 16, and triple alpha-globin genes on the other.
In this population sample we did not find either heterozygous carriers of alpha 0 thalassemia (deletion of both alpha genes in "cis") or individuals showing hemolytic anemia due to inactivation of three alpha-globin genes (Hb H disease).
Hemolysates from adults with the alpha-thalassemia-1 haplotype due to the greater than 17.5-kb deletion of both alpha-globin genes from the same chromosome were found to contain embryonic zeta (zeta)-globin chains (alpha-globin-like chains), as determined by a specific and sensitive radioimmunoassay and an electrophoretic technique. zeta-Globin chains were not present in hemolysates from adults with deletion of a single alpha-globin gene from one or both chromosomes.
A total of 664 DNA samples were screened for alpha-thalassemia 2 and alpha-thalassemia 1 caused respectively by either deletion of one or both of the duplicated alpha-globin genes. alpha-Thalassemia 2 was detected in high frequencies in coastal and lowland regions where malaria has been holo- to hyperendemic but in low frequencies in non-malarious highland regions.
The deletions in the zeta-alpha globin gene cluster in two infants with the hemoglobin Bart's hydrops fetalis syndrome (homozygous alpha thalassemia 1) have been mapped by restriction endonuclease analysis using a zeta-specific probe.
In 10 families with clinically well-defined alpha-thalassemia-1 (alpha-thal-1), hydribization of alpha-globin cDNA was reduced to the same level as that found in Asians with alpha-thal-1, where two of the four normally present alpha-globin genes are deleted.
In alpha-thalassemia-1 and the nondeletion type of hemoglobin-H disease the two alpha-globin genes are at two loci on one chromosome and none reside on the other chromosome.
Molecular hybridization studies with synthetic radioactive DNA's complementary to alpha-globin mRNA sequences show that in addition to the usual deletion defect, a nondeletion defect produces the phenotype of alpha-thalassemia-1.