Compound heterozygosity for α thalassemia-2 and hemoglobin seal rock produces mild Hb H disease: The α-globin chain mutation in Hb seal rock is a TAA→GAA transversion in the termination codon of the 5′ α2-globin gene

Hb Seal Rock (142 Term→Glu) is one of four different termination codon mutants that cause the phenotype of α thalassemia. These mutations introduce an amino acid residue in place of the normal termination codon (TAA) and extend the N-terminat portion of the α-globin chain by 31 amino acids. Since the mRNA for these elongated chains is unstable, the phenotype of α thalassemia results. We studied an African-American family where the proband had a Hb of 10 g/dl, 10% reticulocytes, microcytosis, and Hb H inclusion bodies. On hemoglobin electrophoresis, she had about 4% of a hemoglobin variant that migrated slightly anodic to normal Hb A₂. The variant hemoglobin was isolated by HPLC and its amino acid sequence determined by N terminal Edman degradation. At position 142, after the expected C-terminal arginine, a glutamic acid residue was inserted, followed by 30 addition amino acids that corresponded exactly to the structure of Hb Constant Spring, a termination codon variant common in Asians. DNA was isolated from blood leukocytes and Southern blot analysis, using an α thalassemia-specific probe, showed that the patient was heterozygous for the common α^-3.7 "rightward" deletion. Family studies indicated that the Hb Seal Rock and α thalassemia-2 gene were in trans. To determine which of the linked α-globin genes were affected, the α2 and α1 genes were specifically amplified using PCR, and, following asymmetric reamplification, their 3′ ends were sequenced. The expected TAA→GAA transversion was present in the termination codon of the sole remaining α2 gene. Combined heterozygosity for these mutations produces a more severe phenotype than expected because the α1 gene on the chromosome with the Hb Seal Rock mutation is not "up regulated" in the presence of a dysfunctional gene as it is when the α2 gene is deleted.