Delineation of the molecular basis of delta- and normal HbA2 beta- thalassemia

In this study, we used cloning and sequence analysis to define the molecular defect in two delta-thalassemia genes, one associated with reduced output of delta-globin chains (delta +thal) from a Sardinian and the other with a complete suppression of delta-chain production from the affected locus (delta zerp thal) from a Southern Italian. Sequence analysis of the delta +thal gene showed a G----T substitution at the first nucleotide of codon 27 (delta +27) which produces an amino acid change (Ala----Ser) and presumably activates a cryptic splice site located at this position. Therefore, only a fraction of the transcript is processed from this site, as indicated by the clinical phenotype of delta +thal. DNA sequencing of the delta zero thal gene revealed a T---- C substitution at position 1 of IVS-1, which abolishes the splicing at this site and thus leads to complete deficiency of normal mRNA explaining the clinical phenotype of delta zero thal. Oligonucleotide analysis was used to confirm the coinheritance of the delta +27 mutation in a group of Sardinians with thalassemia like phenotype and normal HbA2 level who, on the basis of genetic criteria, were supposed to be double heterozygous for delta-thalassemia and beta-thalassemia. The definition of delta-thalassemia defects in each high-risk area facilitates identification of double heterozygotes for delta- and beta- thalassemia by DNA analysis and may thus improve genetic counseling.