Beta-thalassemia syndromes.

In summary, the beta-thalassemias are due to defects in or around the structural beta-globin gene. In some Indian patients, there is deletion of sequence at the 3' end of the beta-globin gene. Most commonly, single nucleotide mutations cause beta(+)- and beta(0) -thalassemia. More than 30 such mutations have been identified. Defects in the promoter region 5' to the gene as far 5' as -87 and closer to the gene at -27 and -28 in the ATA sequence can cause beta (+)-thalassemia. Single nucleotide changes in coding regions leading to termination or nonsense codons commonly cause beta (0)-thalassemia. In addition, beta(0)-thalassemia can be due to single nucleotide changes in the invariant GT at the 5' splice junction in IVS 1 and 2 and in the AG at the 3' end of IVS 2. Additionally, single nucleotide mutations can occur within IVS that result in both beta(+)- and beta(0)-thalassemia. New splice sites are usually the result of these single nucleotide mutations, and they lead to new, abnormal splicing patterns. In some instances, beta (+)-thalassemia results when a new splice signal created within IVS is still associated with some continued normal splicing as well as with abnormal splicing. The abnormal splicing leads to abnormal mRNA, while the normal splicing leads to some normal mRNA and the beta (+)-pheno-single In other cases, such as with the defect as position 705 of IVS 2, the single nucleotide change within the IVS allows only abnormal mRNA splicing, and it results in beta (0)-thalassemia.(ABSTRACT TRUNCATED AT 250 WORDS)