Expanding the spectrum of megalencephalic leukoencephalopathy with subcortical cysts in two patients with GLIALCAM mutations

Intragenic homozygous duplication in HEPACAM is associated with megalencephalic leukoencephalopathy with subcortical cysts type 2A

Disease-causing variants in HEPACAM are associated with megalencephalic leukoencephalopathy with subcortical cysts 2A (MLC2A, MIM# 613,925, autosomal recessive), and megalencephalic leukoencephalopathy with subcortical cysts 2B, remitting, with or without impaired intellectual development (MLC2B, MIM# 613,926, autosomal dominant). These disorders are characterised by macrocephaly, seizures, motor delay, cognitive impairment, ataxia, and spasticity. Brain magnetic resonance imaging (MRI) in these individuals shows swollen cerebral hemispheric white matter and subcortical cysts, mainly in the frontal and temporal regions. To date, 45 individuals from 39 families are reported with biallelic and heterozygous variants in HEPACAM, causing MLC2A and MLC2B, respectively. A 9-year-old male presented with developmental delay, gait abnormalities, seizures, macrocephaly, dysarthria, spasticity, and hyperreflexia. MRI revealed subcortical cysts with diffuse cerebral white matter involvement. Whole-exome sequencing (WES) in the proband did not reveal any clinically relevant single nucleotide variants. However, copy number variation analysis from the WES data of the proband revealed a copy number of 4 for exons 3 and 4 of HEPACAM. Validation and segregation were done by quantitative PCR which confirmed the homozygous duplication of these exons in the proband and carrier status in both parents. To the best of our knowledge, this is the first report of an intragenic duplication in HEPACAM causing MLC2A.

Megalencephalic leukoencephalopathy with subcortical cysts: a variant update and review of the literature

The leukodystrophy megalencephalic leukoencephalopathy with subcortical cysts (MLC) is characterized by infantile-onset macrocephaly and chronic edema of the brain white matter. With delayed onset, patients typically experience motor problems, epilepsy and slow cognitive decline. No treatment is available. Classic MLC is caused by bi-allelic recessive pathogenic variants in MLC1 or GLIALCAM (also called HEPACAM). Heterozygous dominant pathogenic variants in GLIALCAM lead to remitting MLC, where patients show a similar phenotype in early life, followed by normalization of white matter edema and no clinical regression. Rare patients with heterozygous dominant variants in GPRC5B and classic MLC were recently described. In addition, two siblings with bi-allelic recessive variants in AQP4 and remitting MLC have been identified. The last systematic overview of variants linked to MLC dates back to 2006. We provide an updated overview of published and novel variants. We report on genetic variants from 508 patients with MLC as confirmed by MRI diagnosis (258 from our database and 250 extracted from 64 published reports). We describe 151 unique MLC1 variants, 29 GLIALCAM variants, 2 GPRC5B variants and 1 AQP4 variant observed in these MLC patients. We include experiments confirming pathogenicity for some variants, discuss particularly notable variants, and provide an overview of recent scientific and clinical insight in the pathophysiology of MLC.

Mycobacterium avium subsp. paratuberculosis Antigens Elicit a Strong IgG4 Response in Patients with Multiple Sclerosis and Exacerbate Experimental Autoimmune Encephalomyelitis

Neuroinflammation can be triggered by microbial products disrupting immune regulation. In this study, we investigated the levels of IgG1, IgG2, IgG3, and IgG4 subclasses against the heat shock protein (HSP)70_533-545 peptide and lipopentapeptide (MAP_Lp5) derived from Mycobacterium avium subsp. paratuberculosis (MAP) in the blood samples of Japanese and Italian individuals with relapsing remitting multiple sclerosis (MS). Additionally, we examined the impact of this peptide on MOG-induced experimental autoimmune encephalomyelitis (EAE). A total of 130 Japanese and 130 Italian subjects were retrospectively analyzed using the indirect ELISA method. Furthermore, a group of C57BL/6J mice received immunization with the MAP_HSP70_533-545 peptide two weeks prior to the active induction of MOG_35-55 EAE. The results revealed a significantly robust antibody response against MAP_HSP70_533-545 in serum of both Japanese and Italian MS patients compared to their respective control groups. Moreover, heightened levels of serum IgG4 antibodies specific to MAP antigens were correlated with the severity of the disease. Additionally, EAE mice that were immunized with MAP_HSP70_533-545 peptide exhibited more severe disease symptoms and increased reactivity of MOG_35-55-specific T-cell compared to untreated mice. These findings provide evidence suggesting a potential link between MAP and the development or exacerbation of MS, particularly in a subgroup of MS patients with elevated serum IgG4 levels.

Clinicoradiologic Correlation in 22 Egyptian Children With Megalencephalic Leukoencephalopathy With Subcortical Cysts

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare genetic form of cerebral white matter disease whose clinicoradiologic correlation has not been completely understood. In this study, we investigated the association between clinical and brain magnetic resonance imaging (MRI) features in 22 Egyptian children (median age 7 years) with MLC. Gross motor function was assessed using the Gross Motor Function Classification System, and evaluation of brain MRI followed a consistent scoring system. Each parameter of extensive cerebral white matter T2 hyperintensity, moderate-to-severe wide ventricle/enlarged subarachnoid space, and greater than 2 temporal subcortical cysts was significantly associated (P < .05) with worse Gross Motor Function Classification System score, language abnormality, and ataxia. Having >2 parietal subcortical cysts was significantly related to a worse Gross Motor Function Classification System score (P = .04). The current study indicates that patients with MLC manifest signification association between certain brain MRI abnormalities and neurologic features, but this should be confirmed in larger studies.

Structural basis for the dominant or recessive character of GLIALCAM mutations found in leukodystrophies

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a type of leukodystrophy characterized by white matter edema, and it is caused mainly by recessive mutations in MLC1 and GLIALCAM genes. These variants are called MLC1 and MLC2A with both types of patients sharing the same clinical phenotype. In addition, dominant mutations in GLIALCAM have also been identified in a subtype of MLC patients with a remitting phenotype. This variant has been named MLC2B. GLIALCAM encodes for an adhesion protein containing two immunoglobulin (Ig) domains and it is needed for MLC1 targeting to astrocyte–astrocyte junctions. Most mutations identified in GLIALCAM abolish GlialCAM targeting to junctions. However, it is unclear why some mutations behave as recessive or dominant. Here, we used a combination of biochemistry methods with a new developed anti-GlialCAM nanobody, double-mutants and cysteine cross-links experiments, together with computer docking, to create a structural model of GlialCAM homo-interactions. Using this model, we suggest that dominant mutations affect different GlialCAM–GlialCAM interacting surfaces in the first Ig domain, which can occur between GlialCAM molecules present in the same cell (cis) or present in neighbouring cells (trans). Our results provide a framework that can be used to understand the molecular basis of pathogenesis of all identified GLIALCAM mutations.

Megalencephalic Leukoencephalopathy: Insights Into Pathophysiology and Perspectives for Therapy

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare genetic disorder belonging to the group of vacuolating leukodystrophies. It is characterized by megalencephaly, loss of motor functions, epilepsy, and mild mental decline. In brain biopsies of MLC patients, vacuoles were observed in myelin and in astrocytes surrounding blood vessels. It is mainly caused by recessive mutations in MLC1 and HEPACAM (also called GLIALCAM) genes. These disease variants are called MLC1 and MLC2A with both types of patients sharing the same clinical phenotype. Besides, dominant mutations in HEPACAM were also identified in a subtype of MLC patients (MLC2B) with a remitting phenotype. MLC1 and GlialCAM proteins form a complex mainly expressed in brain astrocytes at the gliovascular interface and in Bergmann glia at the cerebellum. Both proteins regulate several ion channels and transporters involved in the control of ion and water fluxes in glial cells, either directly influencing their location and function, or indirectly regulating associated signal transduction pathways. However, the MLC1/GLIALCAM complex function and the related pathological mechanisms leading to MLC are still unknown. It has been hypothesized that, in MLC, the role of glial cells in brain ion homeostasis is altered in both physiological and inflammatory conditions. There is no therapy for MLC patients, only supportive treatment. As MLC2B patients show an MLC reversible phenotype, we speculated that the phenotype of MLC1 and MLC2A patients could also be mitigated by the re-introduction of the correct gene even at later stages. To prove this hypothesis, we injected in the cerebellar subarachnoid space of Mlc1 knockout mice an adeno-associated virus (AAV) coding for human MLC1 under the control of the glial-fibrillary acidic protein promoter. MLC1 expression in the cerebellum extremely reduced myelin vacuolation at all ages in a dose-dependent manner. This study could be considered as the first preclinical approach for MLC. We also suggest other potential therapeutic strategies in this review.

Glial Chloride Channels in the Function of the Nervous System Across Species

In the nervous system, the concentration of Cl- in neurons that express GABA receptors plays a key role in establishing whether these neurons are excitatory, mostly during early development, or inhibitory. Thus, much attention has been dedicated to understanding how neurons regulate their intracellular Cl- concentration. However, regulation of the extracellular Cl- concentration by other cells of the nervous system, including glia and microglia, is as important because it ultimately affects the Cl- equilibrium potential across the neuronal plasma membrane. Moreover, Cl- ions are transported in and out of the cell, via either passive or active transporter systems, as counter ions for K+ whose concentration in the extracellular environment of the nervous system is tightly regulated because it directly affects neuronal excitability. In this book chapter, we report on the Cl- channel types expressed in the various types of glial cells focusing on the role they play in the function of the nervous system in health and disease. Furthermore, we describe the types of stimuli that these channels are activated by, the other solutes that they may transport, and the involvement of these channels in processes such as pH regulation and Regulatory Volume Decrease (RVD). The picture that emerges is one of the glial cells expressing a variety of Cl- channels, encoded by members of different gene families, involved both in short- and long-term regulation of the nervous system function. Finally, we report data on invertebrate model organisms, such as C. elegans and Drosophila, that are revealing important and previously unsuspected functions of some of these channels in the context of living and behaving animals.

Identification in Chinese patients with GLIALCAM mutations of megalencephalic leukoencephalopathy with subcortical cysts and brain pathological study on Glialcam knock-in mouse models

BACKGROUND

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare neurological degenerative disorder caused by the mutations of MLC1 or GLIALCAM with autosomal recessive or autosomal dominant inheritance and a different prognosis, characterized by macrocephaly, delayed motor and cognitive development, and bilateral abnormal signals in cerebral white matter (WM) with or without cysts on magnetic resonance imaging (MRI). This study aimed to reveal the clinical and genetic features of MLC patients with GLIALCAM mutations and to explore the brain pathological characteristics and prognosis of mouse models with different modes of inheritance.

METHODS

Clinical information and peripheral venous blood were collected from six families. Genetic analysis was performed by Sanger sequencing of GLIALCAM. Glialcam^Arg92Trp/+ and Glialcam^{Lys68Met/Thr132Asn} mouse models were generated based on mutations from patients (c.274C>T(p.Arg92Trp) (c.203A>T(p.Lys68Met), and c.395C>A (p.Thr132Asn))). Brain pathologies of the mouse models at different time points were analyzed.

RESULTS

Six patients were clinically diagnosed with MLC. Of the six patients, five (Pt1-Pt5) presented with a heterozygous mutation in GLIALCAM (c.274C>T(p.Arg92Trp) or c.275G>C(p.Arg92Pro)) and were diagnosed with MLC2B; the remaining patient (Pt6) with two compound heterozygous mutations in GLIALCAM (c.203A>T (p.Lys68Met) and c.395C>A (p.Thr132Asn)) was diagnosed with MLC2A. The mutation c.275C>G (p.Arg92Pro) has not been reported before. Clinical manifestations of the patient with MLC2A (Pt6) progressed with regression, whereas the course of the five MLC2B patients remained stable or improved. The Glialcam^Arg92Trp/+ and Glialcam^{Lys68Met/ Thr132Asn} mouse models showed vacuolization in the anterior commissural WM at 1 month of age and vacuolization in the cerebellar WM at 3 and 6 months, respectively. At 9 months, the vacuolization of the Glialcam^{Lys68Met/ Thr132Asn} mouse model was heavier than that of the Glialcam^Arg92Trp/+ mouse model. Decreased expression of Glialcam in Glialcam^Arg92Trp/+ and Glialcam^{Lys68Met/ Thr132Asn} mice may contribute to the vacuolization.

CONCLUSIONS

Clinical and genetic characterization of patients with MLC and GLIALCAM mutations revealed a novel mutation, expanding the spectrum of GLIALCAM mutations. The first Glialcam mouse model with autosomal recessive inheritance and a new Glialcam mouse model with autosomal dominant inheritance were generated. The two mouse models with different modes of inheritance showed different degrees of brain pathological features, which were consistent with the patients' phenotype and further confirmed the pathogenicity of the corresponding mutations.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Ten Novel Mutations in Chinese Patients with Megalencephalic Leukoencephalopathy with Subcortical Cysts and a Long-Term Follow-Up Research

Objective

Megalencephalic leukoencephalopathy with subcortical cysts (MLC, OMIM 604004) is a rare neurological deterioration disease. We aimed to clarify clinical and genetic features of Chinese MLC patients.

Methods

Clinical information and peripheral venous blood of 20 patients and their families were collected, Sanger-sequencing and Multiple Ligation-dependent Probe Amplification were performed to make genetic analysis. Splicing-site mutation was confirmed with RT-PCR. UPD was detected by haplotype analysis. Follow-up study was performed through telephone for 27 patients.

Results

Out of 20 patients, macrocephaly, classic MRI features, motor development delay and cognitive impairment were detected in 20(100%), 20(100%), 17(85%) and 4(20%) patients, respectively. 20(100%) were clinically diagnosed with MLC. 19(95%) were genetically diagnosed with 10 novel mutations in MLC1, MLC1 and GlialCAM mutations were identified in 15 and 4 patients, respectively. Deletion mutation from exon4 to exon9 and a homozygous point mutation due to maternal UPD of chromosome22 in MLC1 were found firstly. c.598-2A>C in MLC1 leads to the skip of exon8. c.772-1G>C in MLC1 accounting for 15.5%(9/58) alleles in Chinese patients might be a founder or a hot-spot mutation. Out of 27 patients in the follow-up study, head circumference was ranged from 56cm to 61cm in patients older than 5yeas old, with a median of 57cm. Motor development delay and cognitive impairment were detected in 22(81.5%) and 5(18.5%) patients, respectively. Motor and cognitive deterioration was found in 5 (18.5%) and 2 patients (7.4%), respectively. Improvements and MRI recovery were first found in Chinese patients. Rate of seizures (45.5%), transient motor retrogress (45.5%) and unconsciousness (13.6%) after head trauma was much higher than that after fever (18.2%, 9.1%, 0%, respectively).

Significance

It’s a clinical and genetic analysis and a follow-up study for largest sample of Chinese MLC patients, identifying 10 novel mutations, expanding mutation spectrums and discovering clinical features of Chinese MLC patients.

Regulatory-auxiliary subunits of CLC chloride channel-transport proteins

The CLC family of chloride channels and transporters is composed by nine members, but only three of them, ClC-Ka/b, ClC-7 and ClC-2, have been found so far associated with auxiliary subunits. These CLC regulatory subunits are small proteins that present few common characteristics among them, both structurally and functionally, and their effects on the corresponding CLC protein are different. Barttin, a protein with two transmembrane domains, is essential for the membrane localization of ClC-K proteins and their activity in the kidney and inner ear. Ostm1 is a protein with a single transmembrane domain and a highly glycosylated N-terminus. Unlike the other two CLC auxiliary subunits, Ostm1 shows a reciprocal relationship with ClC-7 for their stability. The subcellular localization of Ostm1 depends on ClC-7 and not the other way around. ClC-2 is active on its own, but GlialCAM, a transmembrane cell adhesion molecule with two extracellular immunoglobulin (Ig)-like domains, regulates its subcellular localization and activity in glial cells. The common theme for these three proteins is their requirement for a proper homeostasis, since their malfunction leads to distinct diseases. We will review here their properties and their role in normal chloride physiology and the pathological consequences of their improper function.

Leukoencephalopathy with thalamus and brainstem involvement and high lactate caused by novel mutations in the EARS2 gene in two siblings

Leukoencephalopathy with thalamus and brainstem involvement, and high lactate (LTBL) is a recently identified disease related to mutations in the EARS2 gene encoding glutamyl-tRNA synthetase. We report clinical and radiological findings for two siblings with new pathogenic mutations in the EARS2 gene. Both patients showed symptoms of mild-type disease, but there were clinical differences between the two siblings. While the older brother had hypotonia and delayed developmental milestones, the younger brother had seizures and spasticity in the lower extremities. Brain magnetic resonance imaging (MRI) findings were quite similar for the two siblings. MRI findings were specific to LTBL. MRI lesions of the older sibling had regressed over time. Clinical and radiological improvement, as in the previously reported patients with LTBL, may be an important clue for diagnosis.

Metabolic, endocrine, and other genetic disorders

Metabolic, endocrine, and genetic diseases of the brain include a very large array of disorders caused by a wide range of underlying abnormalities and involving a variety of brain structures. Often these disorders manifest as recognizable, though sometimes overlapping, patterns on neuroimaging studies that may enable a diagnosis based on imaging or may alternatively provide enough clues to direct further diagnostic evaluation. The diagnostic workup can include various biochemical laboratory or genetic studies. In this chapter, after a brief review of normal white-matter development, we will describe a variety of leukodystrophies resulting from metabolic disorders involving the brain, including mitochondrial and respiratory chain diseases. We will then describe various acidurias, urea cycle disorders, disorders related to copper and iron metabolism, and disorders of ganglioside and mucopolysaccharide metabolism. Lastly, various other hypomyelinating and dysmyelinating leukodystrophies, including vanishing white-matter disease, megalencephalic leukoencephalopathy with subcortical cysts, and oculocerebrorenal syndrome will be presented. In the following section on endocrine disorders, we will examine various disorders of the hypothalamic-pituitary axis, including developmental, inflammatory, and neoplastic diseases. Neonatal hypoglycemia will also be briefly reviewed. In the final section, we will review a few of the common genetic phakomatoses. Throughout the text, both imaging and brief clinical features of the various disorders will be discussed.

Structural determinants of interaction, trafficking and function in the ClC-2/MLC1 subunit GlialCAM involved in leukodystrophy

KEY POINTS

The extracellular domain of GlialCAM is necessary for its targeting to cell junctions, as well as for interactions with itself and MLC1 and ClC-2. The C-terminus of GlialCAM is not necessary for interaction but is required for targeting to cell junctions. The first three residues of the transmembrane segment of GlialCAM are required for GlialCAM-mediated ClC-2 activation.

ABSTRACT

Mutations in the genes encoding the astrocytic protein MLC1, the cell adhesion molecule GlialCAM or the Cl(-) channel ClC-2 underlie human leukodystrophies. GlialCAM binds to itself, to MLC1 and to ClC-2, and directs these proteins to cell-cell contacts. In addition, GlialCAM dramatically activates ClC-2 mediated currents. In the present study, we used mutagenesis studies combined with functional and biochemical analyses to determine which parts of GlialCAM are required to perform these cellular functions. We found that the extracellular domain of GlialCAM is necessary for cell junction targeting and for mediating interactions with itself or with MLC1 and ClC-2. The C-terminus is also necessary for proper targeting to cell-cell junctions but is not required for the biochemical interaction. Finally, we identified the first three amino acids of the transmembrane segment of GlialCAM as being essential for the activation of ClC-2 currents but not for targeting or biochemical interaction. Our results provide new mechanistic insights concerning the regulation of the cell biology and function of MLC1 and ClC-2 by GlialCAM.

MLC1 protein: a likely link between leukodystrophies and brain channelopathies

Megalencephalic leukoencephalopathy with subcortical cysts (MLCs) disease is a rare inherited, autosomal recessive form of childhood-onset spongiform leukodystrophy characterized by macrocephaly, deterioration of motor functions, epileptic seizures and mental decline. Brain edema, subcortical fluid cysts, myelin and astrocyte vacuolation are the histopathological hallmarks of MLC. Mutations in either the MLC1 gene (>75% of patients) or the GlialCAM gene (<20% of patients) are responsible for the disease. Recently, the GlialCAM adhesion protein was found essential for the membrane expression and function of the chloride channel ClC-2 indicating MLC disease caused by mutation in GlialCAM as the first channelopathy among leukodystrophies. On the contrary, the function of MLC1 protein, which binds GlialCAM, its functional relationship with ClC-2 and the molecular mechanisms underlying MLC1 mutation-induced functional defects are not fully understood yet. The human MLC1 gene encodes a 377-amino acid membrane protein with eight predicted transmembrane domains which shows very low homology with voltage-dependent potassium (K⁺) channel subunits. The high expression of MLC1 in brain astrocytes contacting blood vessels and meninges and brain alterations observed in MLC patients have led to hypothesize a role for MLC1 in the regulation of ion and water homeostasis. Recent studies have shown that MLC1 establishes structural and/or functional interactions with several ion/water channels and transporters and ion channel accessory proteins, and that these interactions are affected by MLC1 mutations causing MLC. Here, we review data on MLC1 functional properties obtained in in vitro and in vivo models and discuss evidence linking the effects of MLC1 mutations to brain channelopathies.

Disrupting MLC1 and GlialCAM and ClC-2 interactions in leukodystrophy entails glial chloride channel dysfunction

Defects in the astrocytic membrane protein MLC1, the adhesion molecule GlialCAM or the chloride channel ClC-2 underlie human leukoencephalopathies. Whereas GlialCAM binds ClC-2 and MLC1, and modifies ClC-2 currents in vitro, no functional connections between MLC1 and ClC-2 are known. Here we investigate this by generating loss-of-function Glialcam and Mlc1 mouse models manifesting myelin vacuolization. We find that ClC-2 is unnecessary for MLC1 and GlialCAM localization in brain, whereas GlialCAM is important for targeting MLC1 and ClC-2 to specialized glial domains in vivo and for modifying ClC-2's biophysical properties specifically in oligodendrocytes (OLs), the cells chiefly affected by vacuolization. Unexpectedly, MLC1 is crucial for proper localization of GlialCAM and ClC-2, and for changing ClC-2 currents. Our data unmask an unforeseen functional relationship between MLC1 and ClC-2 in vivo, which is probably mediated by GlialCAM, and suggest that ClC-2 participates in the pathogenesis of megalencephalic leukoencephalopathy with subcortical cysts.