Interaction of a rare illegitimate recombination event and a poly A addition site mutation resulting in a severe form of α thalassemia

Abstract

The clinical diversity of thalassemia depends on interaction of diverse genetic defects. We have characterized a severe form of α thalassemia caused by coinheritance of a rare α-globin gene deletion and a nondeletional defect in a southern Italian family. The proband, a 7-year-old girl, exhibited an abnormal hemoglobin electrophoresis pattern with hemoglobin H and hemoglobin Barts, indicating inheritance of a severe form of α thalassemia. Southern blot analysis of DNA showed normal as well as aberrant α-globin gene fragments indicating heterozygosity for a deletional form of α thalassemia in the proband and her mother. The coinheritance of a nondeletional form of α thalassemia (αα(T)) was suspected because of the severity of the proband's phenotype and the presence of normal α-globin gene fragments in the father. Selective polymerase chain reaction of the paternal α1- and α2-globin genes in the proband followed by DNA sequence analysis showed an AATAAA to AATGAA mutation in the polyadenylation signal sequence of the α2- globin gene. Genomic DNA mapping and sequence analysis of a unique polymerase chain reaction product generated across the deletion breakpoint of the maternal allele showed a 5,201-bp deletion extending from 870 nucleotides 5' of the α2-globin gene to nucleotide +519 in the α1-globin gene. This deletion is similar to that previously suggested by blotting studies in a Greek family (Pressley et al, Nucleic Acids Res 8:4889, 1980) and removes the entire α2-globin gene and a portion of the 5' end of the α1-globin gene. Sequence characterization of the resultant aberrant truncated α1-globin gene from the proband showed a 27 nucleotide duplication corresponding to the 3' end of the α-globin gene IVS-2 region separated by the insertion of a tetranucleotide (GGTT), suggesting that this deletion is caused by an illegitimate recombination event.