The Molecular Basis of HB H Disease in Turkey

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 α-thalassemia-2 determinant was the αα/-α(3.7) kb deletion (56%) which was followed by a nondeletional type of α-thalassemia, namely the pentanucleotide deletion in the 5′ first intervening sequence splice junction [α(-5nt)α] (16%). The two most frequent α-thalassemia-1 determinants were αα/-20.5 kb and αα/-175 kb (MED-I) deletions. In two patients, homozygosity for the polyadenylation signal mutation [α(PA-2)α] was found to be responsible for Hb H disease. Clinical and hematological expression seems more severe in patients with the α(-5nt)α deletion at the donor site of the first intervening sequence and the α(PA-2)α mutation in trans to an α-thalassemia-1 determinant. Hornozygosity for the α(PA-2)α mutation was also found to be associated with severe phenotype.