Hemoglobin Constant Spring among Southeast Asian Populations: Haplotypic Heterogeneities and Phylogenetic Analysis

Abstract

Background: Hemoglobin Constant Spring (Hb CS) is an abnormal Hb caused by a mutation at the termination codon of α2-globin gene found commonly among Southeast Asian and Chinese people. Association of Hb CS with α°-thalassemia leads to a thalassemia intermedia syndrome commonly encountered in the region. We report chromosome background and addressed genetic origins of Hb CS observed in a large cohort of Hb CS among Southeast Asian populations.

Materials and methods: A study was done on 102 Vietnamese (aged 15-49 year-old) and 40 Laotian (aged 18-39 year-old) subjects with Hb CS and results compared with 120 Hb CS genes in Thailand. Hematological parameters were recorded and Hb analysis was performed using capillary electrophoresis. Hb CS mutation and thalassemia genotypes were defined by DNA analysis. Six DNA polymorphisms within α-globin gene cluster including 5'Xba I, Bgl I, Inter-zeta HVR, AccI, RsaI and αPstI 3', were determined using PCR-RFLP assay.

Results: Nine different genotypes of Hb CS were observed. In contrast to the Thai Hb CS alleles which are mostly linked to haplotype (+-S + + -), most of the Vietnamese and the Laotian Hb CS genes were associated with haplotype (+-M + + -), both of which are different from that of the European Hb CS.

Conclusions: Hb CS is commonly found in combination with other thalassemias among Southeast Asian populations. Accurate genotyping of the cases requires both hematologic and DNA analyses. At least two independent origins are associated with the Hb CS gene which could indirectly explain the high prevalence of this Hb variant in the region.

Fig 1. α-Globin gene haplotypes associated with Hb CS genes in Vietnamese and Lao subjects as compared to those described in other populations.

Number of alleles detected in each haplotype is indicated. Plus and minus indicate the presence and absence of each polymorphic site. nd: not determined.

Fig 2. Map of Southeast Asia and China demonstrating distribution of the two major haplotypes associated with Hb CS alleles in the region.

Fig 3. Phylogenetic tree of the Hb CS associated haplotypes constructed using the DendroUPGMA software applying Jaccard (Tanimoto) coefficient with default settings and 100 bootstrap replicates.

Numbers indicate the branch lengths (divergence). Two major Asian Hb CS haplotypes; (+—M + + -) and (+—S + + -) and the predicted Mediterranean haplotype (-—L +—-) are indicated in bold face. The major haplotypes, (+—M + + -) and (+—S + + -), of Hb CS in Asian populations show common ancestors and are separated into each branch. In contrast, the Mediterranean haplotype represents clearly different origin.