Nienke L. Postma

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).

Exome sequencing reveals mutated SLC19A3 in patients with an early-infantile, lethal encephalopathy

To accomplish a diagnosis in patients with a rare unclassified disorder is difficult. In this study, we used magnetic resonance imaging pattern recognition analysis to identify patients with the same novel heritable disorder. Whole-exome sequencing was performed to discover the mutated gene. We identified seven patients sharing a previously undescribed magnetic resonance imaging pattern, characterized by initial swelling with T2 hyperintensity of the basal nuclei, thalami, cerebral white matter and cortex, pons and midbrain, followed by rarefaction or cystic degeneration of the white matter and, eventually, by progressive cerebral, cerebellar and brainstem atrophy. All patients developed a severe encephalopathy with rapid deterioration of neurological functions a few weeks after birth, followed by respiratory failure and death. Lactate was elevated in body fluids and on magnetic resonance spectroscopy in most patients. Whole-exome sequencing in a single patient revealed two predicted pathogenic, heterozygous missense mutations in the SLC19A3 gene, encoding the second thiamine transporter. Additional predicted pathogenic mutations and deletions were detected by Sanger sequencing in all six other patients. Pathology of brain tissue of two patients demonstrated severe cerebral atrophy and microscopic brain lesions similar to Leighs syndrome. Although the localization of SLC19A3 expression in brain was similar in the two investigated patients compared to age-matched control subjects, the intensity of the immunoreactivity was increased. Previously published patients with SLC19A3 mutations have a milder clinical phenotype, no laboratory evidence of mitochondrial dysfunction and more limited lesions on magnetic resonance imaging. In some, cerebral atrophy has been reported. The identification of this new, severe, lethal phenotype characterized by subtotal brain degeneration broadens the phenotypic spectrum of SLC19A3 mutations. Recognition of the associated magnetic resonance imaging pattern allows a fast diagnosis in affected infants.

Hyaluronan accumulation and arrested oligodendrocyte progenitor maturation in vanishing white matter disease.

Vanishing white matter disease is a genetic leukoencephalopathy caused by mutations in eukaryotic translation initiation factor 2B. Patients experience a slowly progressive neurological deterioration with episodes of rapid clinical worsening triggered by stress. The disease may occur at any age and leads to early death. Characteristic neuropathological findings include cystic degeneration of the white matter with feeble, if any, reactive gliosis, dysmorphic astrocytes and paucity of myelin despite an increase in oligodendrocytic density. These features have been linked to a maturation defect of astrocytes and oligodendrocytes. However, the nature of the link between glial immaturity and the observed neuropathological features is unclear. We hypothesized that the defects in maturation and function of astrocytes and oligodendrocytes are related. Brain tissue of seven patients with genetically proven vanishing white matter disease was investigated using immunohistochemistry, western blotting, quantitative polymerase chain reaction and size exclusion chromatography. The results were compared with those obtained from normal brain tissue of age-matched controls, from chronic demyelinated multiple sclerosis lesions and from other genetic and acquired white matter disorders. We found that the white matter of patients with vanishing white matter disease is enriched in CD44-expressing astrocyte precursor cells and accumulates the glycosaminoglycan hyaluronan. Hyaluronan is a major component of the extracellular matrix, and CD44 is a hyaluronan receptor. We found that a high molecular weight form of hyaluronan is overabundant, especially in the most severely affected areas. Comparison between the more severely affected frontal white matter and the relatively spared cerebellum confirms that high molecular weight hyaluronan accumulation is more pronounced in the frontal white matter than in the cerebellum. High molecular weight hyaluronan is known to inhibit astrocyte and oligodendrocyte precursor maturation and can explain the arrested glial progenitor maturation observed in vanishing white matter disease. In conclusion, high molecular weight species of hyaluronan accumulate in the white matter of patients with vanishing white matter disease, and by inhibiting glial maturation and proper function, they may be a major determinant of the white matter pathology and lack of repair.

Megalencephalic leucoencephalopathy with cysts: defect in chloride currents and cell volume regulation

Megalencephalic leucoencephalopathy with subcortical cysts is a genetic brain disorder with onset in early childhood. Affected infants develop macrocephaly within the first year of life, after several years followed by slowly progressive, incapacitating cerebellar ataxia and spasticity. From early on, magnetic resonance imaging shows diffuse signal abnormality and swelling of the cerebral white matter, with evidence of highly increased white matter water content. In most patients, the disease is caused by mutations in the gene MLC1, which encodes a plasma membrane protein almost exclusively expressed in brain and at lower levels in leucocytes. Within the brain, MLC1 is mainly located in astrocyte-astrocyte junctions adjacent to the blood-brain and cereborspinal fluid-brain barriers. Thus far, the function of MLC1 has remained unknown. We tested the hypothesis that MLC1 mutations cause a defect in ion currents involved in water and ion homeostasis, resulting in cerebral white matter oedema. Using whole-cell patch clamp studies we demonstrated an association between MLC1 expression and anion channel activity in different cell types, most importantly astrocytes. The currents were absent in chloride-free medium and in cells with disease-causing MLC1 mutations. MLC1-dependent currents were greatly enhanced by hypotonic pretreatment causing cell swelling, while ion channel blockers, including Tamoxifen, abolished the currents. Down regulation of endogenous MLC1 expression in astrocytes by small interfering RNA greatly reduced the activity of this channel, which was rescued by overexpression of normal MLC1. The current-voltage relationship and the pharmacological profiles of the currents indicated that the channel activated by MLC1 expression is a volume-regulated anion channel. Such channels are involved in regulatory volume decrease. We showed that regulatory volume decrease was hampered in lymphoblasts from patients with megalencephalic leucoencephalopathy. A similar trend was observed in astrocytes with decreased MLC1 expression; this effect was rescued by overexpression of normal MLC1. In the present study, we show that absence or mutations of the MLC1 protein negatively impact both volume-regulated anion channel activity and regulatory volume decrease, indicating that megalencephalic leucoencephalopathy is caused by a disturbance of cell volume regulation mediated by chloride transport.

Mice with megalencephalic leukoencephalopathy with cysts: a developmental angle

Megalencephalic leukoencephalopathy with cysts (MLC) is a genetic disease characterized by infantile onset white matter edema and delayed onset neurological deterioration. Loss of MLC1 function causes MLC. MLC1 is involved in ion–water homeostasis, but its exact role is unknown. We generated Mlc1‐null mice for further studies.

Megalencephalic leukoencephalopathy with cysts: the Glialcam-null mouse model

Megalencephalic leukoencephalopathy with cysts (MLC) is a genetic infantile‐onset disease characterized by macrocephaly and white matter edema due to loss of MLC1 function. Recessive mutations in either MLC1 or GLIALCAM cause the disease. MLC1 is involved in astrocytic volume regulation; GlialCAM ensures the correct membrane localization of MLC1. Their exact role in brain ion‐water homeostasis is only partly defined. We characterized Glialcam‐null mice for further studies.

Eight novel mutations in MLC1 from 18 Iranian patients with megalencephalic leukoencephalopathy with subcortical cysts

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) (MIM #604004) is a rare autosomal recessive neurological disorder characterized by macrocephaly, motor and cognitive decline, ataxia, spasticity and occasional seizures. Magnetic resonance imaging (MRI) shows diffusely abnormal and swollen white matter of the cerebral hemispheres and subcortical cysts in the anterior temporal and frontoparietal region. Mutations in MLC1(22q13.33) and GLIALCAM have been identified in patients with MLC. Mutations in MLC1 account for approximately 75% of the cases. MLC was suspected in eighteen Iranian patients from sixteen families based on positive clinical findings including macrocephaly beginning in the first year, neurocognitive deterioration, seizure or loss of consciousness after minor head trauma. All except two were born to consanguineous parents. Brain MRI images were compatible with MLC and confirmed the diagnosis. Sequencing of entire coding region of MLC1 was performed for seventeen patients and mutations in MLC1 were detected in all of them. Eight novel mutations and seven previously reported mutations were identified. This report shows that MLC is relatively common in Iranian population, as expected for rare diseases with high inbreeding, with a surprisingly high frequency of novel mutations.

Interferon-α and the calcifying microangiopathy in Aicardi-Goutieres syndrome.

Aicardi–Goutières syndrome is a leukoencephalopathy with calcifications and increased cerebrospinal fluid interferon‐α. The relation between interferon‐α and brain pathology is poorly understood. We report a patient with mutations in the disease‐associated gene SAMHD1. Neuropathology showed an extensive microangiopathy with calcifications consistently associate with blood vessels. In an in vitro model of the microangiopathy, interferon‐α enhanced vascular smooth muscle cell‐derived calcifications. The noninfarcted white matter harbored apoptotic oligodendrocytes and increased numbers of oligodendrocyte progenitors. These findings better define the white matter pathology and provide evidence that interferon‐α plays a direct pathogenetic role in the calcifying angiopathy typical of this disease.

OP3 – 2232: Phenotype and genotype in 212 patients with megalencephalic leukoencephalopathy with subcortical cysts: New insights in the disease spectrum

Objective Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare inherited disorder characterized by white matter oedema. The disease starts with macrocephaly, generally followed by neurological deterioration. Patients with recessive MLC1 or GLIALCAM mutations have the classic, deteriorating phenotype while patients with dominant GLIALCAM mutations show remarkable recovery. Our aim was to better delineate the phenotypic spectrum and identify potential discriminating clinical and MRI features in patients with different genetic defects. Methods We performed a multi-institutional cross-sectional observational study of the clinical, radiological and genotypic characteristics of the genetically confirmed MLC patients in our database. Results Pathogenic mutations were found in 212 patients (183 families). Recessive MLC1 mutations were present in 80% of patients; GLIALCAM mutations were recessive in 7% and dominant in 13%. The median age at the latest clinical observation was 5 years (range 7 months–56 years). Seven patients were deceased (age range 3–56 years). Epilepsy was common and usually well controlled. Few patients however, had refractory epilepsy and some had status epilepticus, sometimes fatal. Interestingly, we found that also within the group of patients with recessive mutations, there were oligosymptomatic and improving cases. Several patients with the classic phenotype were sensitive to mild head trauma, leading to seizures or prolonged unconsciousness. This was not the case for patients with dominant mutations. Systematic MRI review revealed that autosomal dominant improving MLC could be distinguished from classical MLC on the basis of absence of cerebellar white matter abnormalities at any stage. Furthermore, autosomal dominant patients were more likely to only show rarefied subcortical white matter instead of actual subcortical cysts. Conclusion MLC is a well recognisable disease associated with three different gene defects. We studied a relatively large cohort of patients and provided new insights in the phenotypic characteristics.