Molecular characterization of a high A2 ß thalassemia by direct sequencing of single strand enriched amplified genomic DNA

Abstract

Two families, one of Anglo-Saxon-Dutch descent, and the other, West Indian black, have an atypical ß thalassemia characaterized by an unusually high level of Hb A2 in the heterozygous state. Restriction endonuclease mapping showed a deletion of about 1.35 kilobase (kb) in the 5' region of the ß globin gene. Direct sequencing of a specific region of genomic DNA amplified by a new modification of the polymerase chain reaction defined the deletion to be 1,393 base pairs (bp) and to be the same in both families. The deletion extends from 485 bp 5' to the mRNA CAP site to the middle of the second intervening sequence. This deletion, together with three others previously described that remove the 5' end of the ß gene but leave the d gene intact, are all associated with unusually high levels of Hb A2 in the heterozygous state.