On the origin and spread of beta-thalassemia: recurrent observation of four mutations in different ethnic groups.

Seven beta-thalassemia genes were characterized after they were identified as candidates for previously undescribed mutations based upon the close association of DNA polymorphism haplotypes in the beta-globin gene cluster with specific ethnic mutations. The molecular defect in four of these genes was identical, a frameshift deletion of four nucleotides (-CTTT) within codons 41 and 42. This gene represents a common Southeast Asian mutation shared by a Laotian beta-thalassemia gene, [framework 1 (FR1)], a Vietnamese (FR1), and two Chinese patients (FR3 Asian and FR1). The deletion has been observed previously in Chinese (FR1) and Asian Indians (FR2) and is an example of independent origins of the same molecular defect, possible interallelic gene conversion (as it is seen on two different beta-globin gene frameworks in Chinese), and mutant gene migration in the Asian countries. A second example of mutant gene migration was identified in an Iranian patient with a nucleotide insertion (G) between codons 8 and 9, the same mutation previously found in an Asian Indian in the same chromosomal background. The last two genes examined represent further strong evidence for independent origins of mutation. A C-to-T substitution at position -88 in an Asian Indian has been identified previously in an American Black on a different beta-globin gene framework, and a G-to-A transition at nucleotide 1 of intervening sequence 2 found in an American Black has been observed previously on a different chromosome background in Mediterraneans. This study suggests that there are not many common beta-thalassemia mutations remaining to be discovered. It also suggests that certain sequences in the beta-globin gene are relatively mutation sensitive.