A Novel Single Gene Deletion (−αMAL3.5) Giving Rise to Silent α Thalassemia Carrier Removing the Entire HBA2 Gene Observed in Two Chinese Patients with Hb H Disease: Case Report of Two Probands

Abstract

We report a novel deletion at the HBA2 presented with Hb H disease in two Malaysian- Chinese patients. The two unrelated probands were diagnosed with Hb H disease in a primary hematological screening for thalassemia. Results from routine molecular analysis with gap-polymerase chain reaction (PCR) method revealed a genotype asynchrony with the observed clinical presentation. Subsequent DNA analysis using a battery of molecular methods such as gap-PCR, multiplex ligation dependent probe amplification, DNA sequencing, confirmed the presence of a novel deletion in both the index cases removing the entire α2 globin gene. We have designated the deletion as (−αMAL3.5). Hematological indices and clinical findings suggest that the deletion has an α+ phenotype. The molecular process of this deletion is the result from misalignment and unequal crossover event between the duplicated homologous Y-boxes within the α globin gene cluster. Uncharacterized deletions, single nucleotide polymorphism and other nucleotide indels at the primer binding sites may impede the optimum condition for its annealing and extension and therefore may invalidate the gap-PCR obscuring the real genotype.我们报告了两例马来西亚华裔患者中随HbH病表现出的一种HBA2的新型缺失。 这两例不相关的先证者在地中海贫血的初步血液筛查中被诊断出患有HbH病。 采用跨越断裂点聚合酶链反应（PCR）方法进行常规分子分析的结果显示出一种与观察到的临床表现不一致的基因型。 后续采用一系列分子方法（如跨越断裂点PCR、多重连接探针扩增、DNA测序）进行的DNA分析证实了这两个指示病例中的新型缺失的存在消除了整个α2珠蛋白基因。 我们将该缺失命名为（-αMAL3.5）。 血液学指标和临床结果提示该缺失具有α+表型。 这种缺失的分子过程是α珠蛋白基因簇内部重复的同源Y-盒之间之间错配和不等交换事件的结果。 未表征的缺失、单核苷酸多态性和其它核苷酸插入/缺失的引物结合位点可能阻碍其退火和延伸的最佳条件，因此可能使跨越断裂点PCR无效，模糊了真实的基因型。