Single-Tube Multiplex-PCR Screen for Anti-3.7 and Anti-4.2 α-Globin Gene Triplications

Abstract

The coexistence of α-globin gene triplication (ααα) is an important modulator of the severity of β-thalassemia trait or β-thalassemia intermedia, exacerbating the phenotypic severity of β-thalassemia by causing more globin chain imbalance (1)(2). Typically, the inheritance of a single β-thalassemia allele is associated with mild anemia and hypochromic microcytic red cells. Compared with simple β-heterozygotes, co-inheritance of triplicated or quadruplicated α-globin genes in β-heterozygotes often leads to more significant anemia, splenomegaly, more pronounced red cell abnormalities, the presence of circulating normoblasts, higher hemoglobin F concentrations, and even the presence of inclusion bodies in erythroblasts (3)(4). Because the α- and β-globin gene clusters are physically unlinked and segregate independently, β-thalassemia carriers who also carry triplicated or quadruplicated α-globin genes have a 25% risk of having a similarly affected offspring, although their partners may be entirely normal. Triplicated α-globin genes appear to be ubiquitous and have been found in most populations (2). They result from misalignment and unequal crossover between the homologous X-, Y-, and Z-box segments of the α-globin gene cluster during meiosis (Fig. 1A⇓ ). Generally, two types of triplicated alleles can be generated from an unequal crossover, αααanti3.7 and αααanti4.2. If the crossover occurs between the homologous Z2 and Z1 boxes, also referred to as a “rightward crossover”, this produces a −α3.7 single-gene deletion allele and the reciprocal αααanti3.7 triplicated allele. However, if the crossover occurs between the X2 and X1 boxes (a “leftward crossover”), a −α4.2 single-gene deletion allele and the reciprocal αααanti4.2 triplicated allele are generated (5). A Sri Lankan study of individuals with severe to moderate β-thalassemia revealed a 2% frequency of α-globin gene triplications (6), whereas a preliminary study in Hong Kong suggests that the frequency …