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In this study, 32 patients with Hb H (beta(4)) disease have been identified. Three different alpha-thalassemia-1 (thal) determinants; nine with the -17.4 kb (MED I) type, 12 with the -20.5 kb type and 10 with the -26.5 kb (MED II) type were characterized. Of the 32 patients, 19 had the 3.7 kb deletion and one had the 4.2 kb deletion in trans to alpha-thal-1 determinants. Only one patient, homozygous for the polyadenylation signal (poly A) site (PA 1) mutation, was identified to be associated with Hb H disease. The other patient had the poly A (PA 1) mutation in trans to the MED I (-17.4 kb) determinant. The 5 nt (nucleotide) deletion was present in three patients, two of them in the same family; this mutation was found in association with the MED II (26.5 kb deletion). The other patient had the -5 nt mutation in trans to the MED I (-17.4 kb) determinant. An unstable hemoglobin (Hb) variant [Hb Adana, codon 59 (CA)] was present in association with the alpha-thal-1 deletion (20.5 kb) in two adults and caused a severe type of Hb H disease. Five patients with Hb H disease had the genotype - -(MED II)/alpha(PA 2)alpha one had a Hb S heterozygosity (- -(MED II)/alpha(PA 2)alpha + Hb AS). A patient with Hb H disease (- -(MED I)/-alpha(3.7)) also had Hb S trait.

A total of 25 unrelated Hb H patients were studied at the DNA level. Ten different genotypes were found to be responsible for the disease. The most prevalent alpha-thalassemia-2 determinant was the alpha alpha/-alpha (3.7) kb deletion (56%) which was followed by a nondeletional type of alpha-thalassemia, namely the pentanucleotide deletion in the 5' first intervening sequence splice junction [alpha(-5nt) alpha] (16%). The two most frequent alpha-thalassemia-1 determinants were alpha alpha/-20.5 kb and alpha alpha/-17.5 kb (MED-I) deletions. In two patients, homozygosity for the polyadenylation signal mutation [alpha (PA-2)alpha] was found to be responsible for Hb H disease. Clinical and hematological expression seems more severe in patients with the alpha (-5nt) alpha deletion at the donor site of the first intervening sequence and the alpha(PA-2) alpha mutation in trans to an alpha-thalassemia-1 determinant. Homozygosity for the alpha (PA-2)alpha mutation was also found to be associated with severe phenotype.