Prenatal diagnosis of β-thalassemias by amniocentesis: Linkage analysis using multiple polymorphic restriction endonuclease sites

In order to assess the applicability of multiple restriction endonuclease analyses of amniocyte DNA to the prenatal diagnosis of β-thalassemias in general, we studied 12 consecutive couples at risk. DNA of both members of the 12 couples and a previous offspring of each was analyzed for the presence of 4 polymorphic restriction endonuclease sites: the Hpa I site 3' to the β-globin gene, the Hind III site in the (G)γ gene, the Hind III site in the (A)γ gene, and the Bam HI site 3' to the β-gene. Linkage disequilibrium between these sites and β(A) or β(thal) genes was not found, presumably due to the heterogeneity of β(thal) genes. However, the high frequency of polymorphism at these sites allowed differentiation of β(A)-bearing chromosomes from β(thal) or β(S)-bearing chromosomes in both members of 6 couples. In these couples, complete prenatal diagnosis by linkage analysis of amniocyte DNA would be possible. In the remaining 6 couples, β(A) and β(thal) chromosomes could be discriminated in one member. In about 50% of the pregnancies of these couples, exclusion of β-thalassemia is possible by this analysis. These data suggest that when linkage analysis of polymorphic restriction endonuclease sites is carried out, prenatal diagnosis of β-thalassemia states can be accomplished by amniocentesis alone in 75% of pregnancies at risk.