Molecular Genetic Analysis of the Silent Carrier of Beta Thalassemia

The silent carrier of (beta) thalassemia has a decreased (beta)/(alpha) globin synthesis ratio, but normal Hb A(,2) and Hb F levels and erythrocytic indices. A molecular genetic analysis has been performed using as subjects an Albanian family in which the father is a silent carrier, the mother has high Hb A(,2) (beta)-thalassemia trait, and both children have (beta) thalassemia. The relative excess (alpha)-globin in this family is not due to an increase in (alpha)-globin gene number. The maternal and paternal (beta)-globin genes were cloned from the daughter\u27s genomic DNA and characterized. The maternal gene contains a splice junction mutation in IVS-1 which results in (beta)(\u270) thalassemia. Nucleotide sequence analysis of the paternal gene failed to reveal any base changes of functional significance. When introduced into HeLa cells the gene was expressed at normal levels with proper processing of RNA. Haplotype analysis revealed that the affected son and daughter inherited different (epsilon)(gamma)(delta)(beta)-globin gene clusters from the father. However, the father is homozygous for all polymorphic restriction sites downstream from a Taq I site approximately 3 kb 5\u27 to the (delta)-globin gene, suggesting that either recombination within primordial germ cells of the father had occurred downstream from the Taq I site or the paternal silent carrier allele is not linked to the (beta)-globin gene cluster. In either case, structural and functional analyses demonstrate that the silent carrier allele is not due to a mutation within the (beta)-globin structural gene or its immediate flanking regions and as such represents a novel form of (beta)(\u27+) thalassemia. In addition, during the course of these studies a novel Rsa I polymorphism was identified approximately 550 base pairs 5\u27 to the (beta)-globin cap site. Population screening demonstrated the presence of this site in DNA from individuals of northern European, Mediterranean, Middle Eastern, African, Southeast Asian and Asian Indian descent, who also carried (beta)A, (beta)E, (beta)S, and (beta)-thalassemia alleles at the (beta)-globin locus