β-Thalassemia in Chinese: Use of in vivo RNA analysis and oligonucleotide hybridization in systematic characterization of molecular defects
To perform a systematic analysis of beta-thalassemia genes among Chinese, we have determined the DNA haplotype in the beta-globin gene region of 37 Chinese beta-thalassemia chromosomes. Only four haplotypes were found. Blot hybridization analysis of erythroid RNA from patients homozygous for haplotypes 1, 2, and 3 demonstrated different patterns, suggesting that a different mutation was associated with each haplotype. The mutation associated with haplotype 1 was a C----T substitution at IVS-2, position 654. This mutation produces a new donor splice site and leads to formation of a beta-globin RNA with an insertion of 73 nucleotides. The mutation associated with haplotype 2 was a nucleotide insertion of A between codons 71 and 72, which results in a frameshift and premature termination of beta-globin synthesis. Haplotype analysis suggests that these two mutations may account for up to 85% of beta-thalassemia genes in this ethnic group. The haplotype 3 gene contained a transcriptional "TATA" box mutation that has been previously reported. Oligonucleotide hybridization demonstrated that the mutation associated with haplotype 4 was the same IVS-1 position 5 substitution commonly observed among beta-thalassemia genes in Asian Indians. Since haplotype 4 of Chinese differs at polymorphic sites on either side of the IVS-1 position 5 mutation from the haplotype associated with this mutation in Indians, the mutation presumably arose independently in these two populations.