Lancet Neurol：氯离子通道基因突变致髓鞘水肿性白质脑病

突变小鼠模型显示氯离子通道CLC-2对离子和水平衡有重要作用，但是此作用在人体内未得到确认。来自法国的Christel Depienne等学者试图揭示一类新的疾病体，该类疾病MRI显示不明原因的白质脑病，并进一步研究该类疾病体的基因改变。研究结果发表在2013年7月的Lancet Neurol杂志上。该类疾病白质脑病的突出特点是脑白质水肿，提示水代谢和离子代谢紊乱。

在此观察性分析研究中，招募的患者为MRI显示不明原因的白质脑病，尤其是在内囊后支、中脑的大脑脚及小脑中脚的信号异常。使用外显子测序揭示相关突变基因，通过Sanger测序方法、mRNA分析检测候选基因，随后研究突变导致功能变异情况。最后，在没有神经系统症状的尸体脑组织中使用免疫组化及电镜研究ClC-2的分布情况。

研究共纳入7例符合标准的患者，3例成年时发作，4例儿童期发作。在3例成人患者和3例儿童患者中发现纯合子或复合杂合子。研究发现CLCN2基因突变导致ClC-2功能丧失。剩下的一例儿童患者有X连锁遗传家族史及GJB1基因突变，此基因编码连接蛋白32.临床表现各有差异，表现为小脑性共济失调、痉挛、脉络膜视网膜病变伴视野缺损、视神经病变、认知障碍和头痛。MRI显示弥散受限，提示髓鞘空泡变性，此改变在成人患者局限在特定的白质部位，而在儿童患者为脑白质的弥漫性改变。通过免疫组化发现ClC-2存在于胶质突触复合体各部分，在血管周围星形细胞足突、胶质界膜和室管膜细胞中广泛存在。

该研究证实ClC-2参与脑内水代谢及离子代谢。CLCN2基因的常染色体隐性突变导致以脑内水和离子代谢紊乱及髓鞘内水肿为特点的白质脑病。

Brain white matter oedema due to ClC-2 chloride channel deficiency: an observational analytical study.
BACKGROUND
Mutant mouse models suggest that the chloride channel ClC-2 has functions in ion and water homoeostasis, but this has not been confirmed in human beings. We aimed to define novel disorders characterised by distinct patterns of MRI abnormalities in patients with leukoencephalopathies of unknown origin, and to identify the genes mutated in these disorders. We were specifically interested in leukoencephalopathies characterised by white matter oedema, suggesting a defect in ion and water homoeostasis.
METHODS
In this observational analytical study, we recruited patients with leukoencephalopathies characterised by MRI signal abnormalities in the posterior limbs of the internal capsules, midbrain cerebral peduncles, and middle cerebellar peduncles from our databases of patients with leukoencephalopathies of unknown origin. We used exome sequencing to identify the gene involved. We screened the candidate gene in additional patients by Sanger sequencing and mRNA analysis, and investigated the functional effects of the mutations. We assessed the localisation of ClC-2 with immunohistochemistry and electron microscopy in post-mortem human brains of individuals without neurological disorders.
FINDINGS
Seven patients met our inclusion criteria, three with adult-onset disease and four with childhood-onset disease. We identified homozygous or compound-heterozygous mutations in CLCN2 in three adult and three paediatric patients. We found evidence that the CLCN2 mutations result in loss of function of ClC-2. The remaining paediatric patient had an X-linked family history and a mutation in GJB1, encoding connexin 32. Clinical features were variable and included cerebellar ataxia, spasticity, chorioretinopathy with visual field defects, optic neuropathy, cognitive defects, and headaches. MRI showed restricted diffusion suggesting myelin vacuolation that was confined to the specified white matter structures in adult patients, and more diffusely involved the brain white matter in paediatric patients. We detected ClC-2 in all components of the panglial syncytium, enriched in astrocytic endfeet at the perivascular basal lamina, in the glia limitans, and in ependymal cells.
INTERPRETATION
Our observations substantiate the concept that ClC-2 is involved in brain ion and water homoeostasis. Autosomal-recessive CLCN2 mutations cause a leukoencephalopathy that belongs to an emerging group of disorders affecting brain ion and water homoeostasis and characterised by intramyelinic oedema.
FUNDING
European Leukodystrophies Association, INSERM and Assistance Publique-Hôpitaux de Paris, Dutch Organisation for Scientific Research (ZonMw), E-Rare, Hersenstichting, Optimix Foundation for Scientific Research, Myelin Disorders Bioregistry Project, National Institute of Neurological Disorders and Stroke, and Genetic and Epigenetic Networks in Cognitive Dysfunction (GENCODYS) Project (funded by the European Union Framework Programme 7).