| Background and PurposeThalassemia is one of the most common lethal genetic disease caused by unbalanced synthesis betweenαandβglobin.It is widely distributed in the tropics and subtropics.The carrier rate ofβ-thalassemia all around the world was found to be 4.83%according to recent research data.In China,the most prevalence region was located at the tropical and subtropical provinces which lie to the south of the Yangtze River.Thalassemia was divided toα-thalassemia andβ-thalassemia by the type of abnormally synthesized globin.The 4-bp deletion(-CTTT) at codon 41/42(CD41/42) of the humanβ-globin gene represents one of the most commonβ-thalassemia mutation in East Asia and Southeast Asia.It accounts for about 40%of all theβ-thalassemia carriers in Southern China,and even reaches as high as 90%in some population(such as Li minority in Hainan province,China)Thalassemia is historically afflicted with endemic malaria,thus hypothetically evolving under natural selection by malaria infection.The malaria selection hypothesis was raised by JBS Haldane as early as 1940s.Such hypothesis was raised not only based on the similarity of geographical distribution between both diseases, but like Glucose-6-phosphate dehydrogenase(G6PD) deficiency,HbS,HbE and HbC also based on the demonstration of malaria resistance on disease-carrier in epidemiological and molecular pathological investigations.Several examples of positive selection for hemoglobinopathies,such as sickle cell trait,α-thalassemia trait hemoglobin E(Hb-E) trait,and homozygous Hb-C,have been demonstrated by novel haplotype-based molecular evolution analysis.However despite some epidemiological and cellular evidence that may explain protection against falciparum-malaria in theβ-thalassemia trait,molecular evidence,in terms of the pattern of variation that is consistent with the hypothesis of balancing selection,has not been well documented.The purpose of this paper is to report the results of our molecular study which is based largely on a population sample from China.Such a sample allows us to carry out not only some conventional analyses but also those based on population genetics theory,thereby,expanding our understanding of the mechanism of evolution for theβ-globin gene cluster,particularly the 4-bp deletions(-CTTT) that are associated withβ-thalassemia.Materials and methodsTo understand the evolutionary process of generating theβCD41/42 allele and its maintenance,including the effect of natural selection on theβCD41/42 chromosome,we sequenced a 15.933 kb region spanning 20.693 kb of theβ-globin cluster surrounding the 4-bp deletion using a sample from a Chinese population consisting of 24 normal individuals and 16 heterozygotes for the deletion.Forty-nine polymorphic sites were found.We then used Phase2.1.1 software to infer the haplotype and recombination hotspot,and applied population genetics analysis such as Tajima's D neutrality test,four gametes test,linkage disequilibrium calculation and median network construction based on the inferred haplotype.The softwares are Arlequin3.01,DnaSP4.0 and Network4.1.1.2.Results and discussionThe neutrality test resulted in a significant positive Tajima's D for theβ-globin locus which is consistent with the existence of balancing selection.This suggests that the 4-bp deletion occurred at this locus may be an event that is subject to natural selection,due to malaria which leads to the heterozygote advantage,spread widely with further help by conversion and migration.Analysis of the data,using a variety of methods including formal and visual approaches suggest that the spread of the 41/42 (-CTTT) deletion is most likely mediated by inter-allelic gene conversion although independent deletions in different lineages are also possible.The evolutionary process of this mutant through gene conversion that could conceivably take place between the 4-bp deletion and normal sequence in the respective region was discussed in details. |
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