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In a black family with members having alpha-thalassemia and hemoglobin H (HbH) disease, a deletion of an AG dinucleotide at the 3' end of exon 1 near the junction with intron 1 was shown previously to produce a dysfunctional alpha-thalassemia gene with a reading frame-shift and a nonsense codon (Safaya S, Rieder RF: J Biol Chem 263:4328-4332, 1988). We have found that the same mutation is responsible for alpha-thalassemia and HbH disease in a second unrelated black family (Bellevue R, Dosik H, Rieder RF: Br J Haematol 41:193-202, 1979). Despite the loss of two nucleotides from the consensus sequence at the 5' splice donor site of intron 1, studies employing an in vitro plasmid-based expression system indicated that the mutant alpha-globin mRNA was spliced normally and expressed in amounts equal to normal alpha-globin mRNA in COS-7 cells. The correct processing of the mRNA in these studies is probably due to the presence of a tandem repeat of the affected AG dinucleotide. However, in reticulocytes from subjects bearing the mutant gene, we were unable to detect any of the abnormal mRNA. These findings suggest that there is accelerated post-transcriptional loss of mRNA bearing a premature terminator codon.

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