鸟苷二磷酸甘露糖焦磷酸化酶β亚基基因突变致肢带型肌营养不良合并先天性肌无力综合征

Guanosine diphosphate-mannose pyrophosphorylase-B gene mutations causing limb-girdle muscular dystrophy overlapping with congenital myasthenic syndrome

目的报道4例鸟苷二磷酸甘露糖焦磷酸化酶β亚基（GMPPB）基因突变所致继发性肌营养不良蛋白聚糖病的病例,并结合文献对本病的临床表现、肌肉影像、分子病理及遗传学特点进行分析.方法收集2009—2017年在我院门诊就诊的继发性肌营养不良蛋白聚糖病2个家系共4例患者的详细病史、体格检查、肌电图和其他辅助检查结果,分析家系1和家系2的2例先证者的肌肉影像、肌肉活体组织检查病理特点,并对该2例患者行靶向二代基因测序.结果4例患者来自2个家系（其中3例来自同一家系）,男性2例,女性2例,发病年龄为17-18岁.临床上符合肢带型肌营养不良（LGMD）和先天性肌无力综合征（CMS）重叠表型,表现为四肢近端肌无力,且具有日间波动性,晨轻暮重.患者早期跑跳不如同龄人,体育成绩差.体检双下肢近端肌力Ⅰ-Ⅱ级,双上肢近端肌力Ⅲ-Ⅳ级,四肢远端肌力Ⅴ级.脊旁肌和四肢近端肌萎缩,腓肠肌肥大.实验室检查：肌酸激酶明显升高（1877-5275U/L）,针极肌电图呈肌源性损害,重频刺激试验示低频递减.肌肉MRI表现为大腿后群肌（腘绳肌）显著脂肪变性;肌肉病理呈肌营养不良改变.靶向二代基因测序均显示GMPPB基因复合杂合突变,家系1突变位点为c.860G〉T（p.R287L）/c.851T〉C（p.V284L）;家系2突变位点为c.1097A〉G（p.N366S）/c.589G〉T（p.V197F）.上述4个突变位点均为新发突变,经家系验证提示2个突变分别来自患者父母.免疫组织化学及蛋白质电泳分析证实2例先证者肌肉组织α-肌营养不良蛋白聚糖的表达水平均较正常对照明显下降.4例患者经溴吡斯的明治疗后肌无力症状均有不同程度的改善.结论GMPPB基因突变可致肌肉和神经肌肉接头同时受累,临床表现为LGMD与CMS重叠.胆碱酯酶抑制剂能部分改善此类患者肌无力症状.

Objective To report four patients with secondary α-dystroglycanopathy caused by guanosine diphosphate-mannose pyrophosphorylase-B （ GMPPB ） gene mutations and review the literature aiming to analyze the clinical manifestations , muscle image , molecular pathology and genetic characteristics of the disease.Methods The medical history , physical examination , electromyography and other clinical data of four patients with secondary α-dystroglycanopathy from two families were collected and retrospectively reviewed from 2009 to 2017.Case 1 （ proband of pedigree 1） and case 2 （ proband of pedigree 2） were then further analyzed with muscle imaging , muscle pathology and targeted next generation gene sequencing （NGS）.Results Four patients came from two families （three from the same pedigree）, two males and two females, with an onset age of 17 -18 years.All four cases presented as limb-girdle muscular dystrophy （LGMD） overlapping with congenital myasthenic syndrome （CMS） characterized by evident proximal limb weakness in early adulthood and fluctuating muscle weakness .They all had delayed motor milestone and did not perform well in physical education since childhood . Serum creatine kinase was elevated markedly （1877-5275 U/L）.Myogenic changes on electromyography and marked attenuation on three Hz repetitive nerve stimulation were observed in all patients .Muscle MRI showed prominent involvement of bilateral hamstrings in case 1 and case 2.Muscular dystrophic patterns were demonstrated on muscle biopsies . Targeted NGS revealed two compound heterozygous missense mutations in GMPPB for each case .Case 1 carried c.860G〉T （p.R287L）/c851T〉C （p.V284L）.Case 2 and his two affected sisters （case 3 and case 4） carried c.1097A 〉G （ p.N366S）/c.589G 〉T （ p.V197F） .All of these mutations were novel variants and pedigree analysis suggested that the two mutations were from parents .Compared with normal controls, immunohistochemistry and Western blotting showed significantly decreased expression of α-dystroglycan in the muscle tissue from case 1 and case 2.The myasthenic symptoms of all four patients were improved to varying degrees after treatment with pyridostigmine bromide . Conclusions Mutations in GMPPB can lead to dysfunction both in muscle and in neuromuscular transmission causing overlapping between LGMD and CMS phenotypes . Cholinesterase inhibitors can partly improve the symptoms of myasthenia in such patients .