TTPA基因纯合变异所致共济失调伴维生素E缺乏症1例患者的临床及遗传学分析

Clinical and genetic analysis of a patient with Ataxia and vitamin E deficiency due to homozygous variant of TTPA gene

目的探讨1例TTPA基因变异所致共济失调伴维生素E缺乏症(AVED)患者的临床表型及遗传学特征。方法选择2023年7月于武汉大学中南医院就诊,拟采取辅助生殖技术助孕的1例AVED患者(先证者)作为研究对象。收集先证者的临床资料,并采集先证者及其家系成员(父亲与胞弟、姐、妹)外周静脉血样2 mL,进行血清维生素E水平检测。应用全外显子组测序(WES),对先证者进行遗传学变异检测。应用美国临床相关变异解释的公共档案(ClinVar)等数据库对先证者遗传致病变异进行筛选。应用Sanger测序法对WES检测的先证者相关变异位点于其家系成员中进行验证。根据美国医学遗传学与基因组学学会(ACMG)制定的《遗传变异分类标准与指南》,对变异位点进行致病性评级,并应用多重生物信息学软件(SIFT、Mutation Taster、CADD和SpliceAI),对先证者变异位点有害性进行分析。应用ClinVar数据库、美国单核苷酸多态性数据库(dbSNP)等获取先证者相关变异基因的蛋白结构域信息,并绘制蛋白编码区域变异热点图谱。本研究遵循的研究程序经武汉大学中南医院医学伦理委员会的审查(伦理号:临研伦[2023068K]),并与先证者及其家系成员签署了临床研究知情同意书。结果先证者外周血清维生素E水平为5.186μg/mL,明显低于正常值,其父亲及其胞弟、姐、妹血清维生素E水平均正常。WES检测结果提示,先证者TTPA基因存在c.2T>A(p.0?)纯合错义变异。Sanger测序结果显示,先证者父亲及其胞妹为该变异携带者,其胞姐和胞弟均未携带该变异。依据ACMG制定的《遗传变异分类标准与指南》,TTPA基因c.2T>A(p.0?)错义变异被评为致病性变异(PVS1+PM2_Supporting+PM3)。多重生物信息学软件预测该变异位点位于起始密码子区,可导致TTPA蛋白无法正常合成,为有害变异。TTPA变异区域热点图谱绘制结果显示,ClinVar和dbSNP数据库收录的TTPA基因致病变异在TTPA蛋白的5个结构域区域分布平均,并且TTPA蛋白第1个氨基酸残基位点在不同物种间高度保守。结论TTPA基因c.2T>A(p.0?)纯合变异可能是导致先证者AVED发病的原因。本研究丰富了TTPA基因变异所致AVED患者的变异图谱,并为该家系的诊断、遗传咨询和助孕策略提供了依据。

ObjectiveTo explore the clinical phenotype and genetic characteristics of a patient with Ataxia and vitamin E deficiency syndrome(AVED)due to a variant of TTPA gene.MethodsA patient diagnosed with AVED(proband),intended for assisted reproductive technology for pregnancy in Zhongnan Hospital of Wuhan University in July 2023,was selected as research subject.Clinical data of the proband were collected,and 2 mL of peripheral venous blood samples were collected from the proband and her father and siblings for serum vitamin E level testing.Whole exome sequencing(WES)was carried out.Pathogenic variants were selected based on American public archive of interpretations of clinically relevant variants(ClinVar).Sanger sequencing was performed to validate the candidate variants detected by WES.Pathogenicity of variants was classified based on the guidelines from the American College of Medical Genetics and Genomics(ACMG),and the impact of variants was analyzed using multiple bioinformatics tools including SIFT,Mutation Taster,CADD,and SpliceAI.Information on the protein domains was obtained from ClinVar and dbSNP databases,and a hotspot map for the variants of protein-coding region was constructed.This study was approved by the Medical Ethics Committee of Zhongnan Hospital of Wuhan University(No.2023068K).ResultsThe proband has a significantly low serum level of vitamin E(5.186μg/mL),while her father and siblings were normal.WES revealed that she has harbored a homozygous missense c.2T>A(p.0?)variant of the TTPA gene,for which her father and younger sister were heterozygous carriers.Based on the guidelines from the ACMG,the missense c.2T>A(p.0?)variant of the TTPA gene was classified as pathogenic(PVS1+PM2+PM3).Multiple bioinformatics tools had predicted this variant to be located in the initiation codon region and may lead to abnormal synthesis of the TTPA protein,indicating it was deleterious.The hotspot map based on ClinVar and dbSNP databases showed an even distribution of variants across 5 structural domains of the TTPA protein,with high conservation of the first amino acid residue across various species.ConclusionThe homozygous c.2T>A(p.0?)variant of the TTPA gene probably underlie the AVED in the proband.Above discovery has enriched the mutational spectrum of AVED and provided a basis for the diagnosis,genetic counseling,and assisted reproductive strategies for this family.