Hb A₂ Episkopi – a novel <i>δ</i>-globin chain variant [HBD:c.428C&gt;T] in a family of mixed Cypriot–Lebanese descent

<b>Objectives:</b> Thalassaemia is a potentially lethal inherited anaemia, caused by reduced or absent synthesis of globin chains. Measurement of the minor adult haemoglobin Hb A₂, combining α- with δ-globin, is critical for the routine diagnosis of carrier status for α- or β-thalassaemia. Here, we aim to characterize a novel δ-globin variant, Hb A₂ Episkopi, in a single family of mixed Lebanese and Cypriot ancestry with mild hypochromic anaemia and otherwise normal globin genotype, which also presents with a coincidental 0.78-Mb sequence duplication on chromosome 1 (1q44) and developmental abnormalities. <b>Methods</b>: Analyses included comprehensive haematological analyses, cation-exchange high-performance liquid chromatography (CE-HPLC), cellulose acetate electrophoresis (CAE), Sanger sequencing and structure-based stability predictions for Hb A₂ Episkopi. <b>Results</b>: The GCT &gt; GTT missense mutation, underlying Hb A₂ Episkopi, HBD:c.428C &gt; T, introduces a cd142 codon change in the mature protein, resulting in reduced normal Hb A₂ amounts and a novel, less abundant Hb A₂ variant (HGVS: HBD:p.A143V), detectable as a delayed peak by CE-HPLC. The latter was in line with structure-based stability predictions, which indicated that the substitution of a marginal, non-helical and non-interface residue, five amino acids from the δ-globin chain carboxy-terminus, was moderately destabilizing. <b>Discussion</b>: Detection of the new variant depends on the diagnostic set-up and had failed by CAE and on an independent CE-HPLC system, which, in unfavourable circumstances, may lead to misdiagnoses of β-thalassaemia as α-thalassaemia. Given the mixed background of the affected family, the ethnic origin of the mutation is unclear, and this study thus suggests awareness for possible detection of Hb A₂ Episkopi in both the Cypriot and the Lebanese populations.**研究目标**：地中海贫血（Thalassaemia）是一类潜在致命的遗传性贫血，由珠蛋白链（globin chains）合成减少或缺失所引发。成人次要血红蛋白Hb A₂（将α珠蛋白与δ珠蛋白结合形成）的检测，是常规诊断α或β地中海贫血携带者状态的关键指标。本研究旨在对一个黎巴嫩与塞浦路斯混血血统家族中的新型δ珠蛋白变异体——Hb A₂埃皮斯科皮变异体（Hb A₂ Episkopi）进行特征分析，该家族成员表现为轻度低色素性贫血，且珠蛋白基因型（globin genotype）无异常，但同时存在1号染色体（1q44）上0.78兆碱基的序列重复以及发育异常。 **研究方法**：本研究采用的分析手段包括全面血液学检测、阳离子交换高效液相色谱法（cation-exchange high-performance liquid chromatography, CE-HPLC）、醋酸纤维素薄膜电泳（cellulose acetate electrophoresis, CAE）、桑格测序（Sanger sequencing），以及针对Hb A₂埃皮斯科皮变异体的基于结构的稳定性预测分析。 **研究结果**：导致Hb A₂埃皮斯科皮变异体产生的GCT>GTT错义突变HBD:c.428C>T，会使成熟蛋白的第142位密码子发生改变，造成正常Hb A₂含量降低，并产生一种丰度更低的新型Hb A₂变异体（HGVS命名：HBD:p.A143V），该变异体可通过阳离子交换高效液相色谱法检测到一个延迟洗脱峰。上述结果与基于结构的稳定性预测结果相符：该突变将δ珠蛋白链羧基末端倒数第五位的一个边缘性、非螺旋且非界面残基进行替换，对蛋白结构产生了中等程度的去稳定作用。 **讨论**：该新型变异体的检出效果取决于实验室诊断配置情况，醋酸纤维素薄膜电泳与另一独立的阳离子交换高效液相色谱系统均未能检出该变异体；在不利的临床情境下，这可能导致将β地中海贫血误诊为α地中海贫血。鉴于受累家族的混血血统背景，该突变的种族起源尚不明确，因此本研究提示需在塞浦路斯与黎巴嫩人群中提高对Hb A₂埃皮斯科皮变异体检出的重视程度。