A Unique Mutational Spectrum of MLC1 in Korean Patients With Megalencephalic Leukoencephalopathy With Subcortical Cysts: p.Ala275Asp Founder Mutation and Maternal Uniparental Disomy of Chromosome 22

Gene therapy rescues brain edema and motor function in a mouse model of megalencephalic leukoencephalopathy with subcortical cysts

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is an ultrarare, infantile-onset leukodystrophy characterized by white matter edema for which there is no treatment. More than 75% of diagnosed cases result from biallelic loss-of-function mutations in the astrocyte-specific gene MLC1, leading to early-onset macrocephaly, cerebellar ataxia, epilepsy, and mild cognitive decline. To develop a gene therapy for MLC, we administered an adeno-associated viral vector capable of crossing the murine blood-brain barrier, delivering the human MLC1 cDNA under the control of a human astrocyte-specific promoter, to 10-month-old Mlc1-/- mice. We observed long-term astrocyte-driven expression of MLC1 up to 1 year after viral vector administration in all brain areas analyzed. Despite the late-stage intervention, in vivo magnetic resonance imaging revealed normalization of water accumulation. Notably, our therapy successfully reversed locomotor deficits in Mlc1-/- mice, as evidenced by improved performance in motor tests assessing cerebellar ataxia-like behaviors. Collectively, these findings not only demonstrate the sustained efficacy of our gene therapy but also highlight the reversibility of vacuolation and motor impairments in Mlc1-/- mice, suggesting that MLC patients could benefit from treatment even after symptom onset.

MLC1 alteration in iPSCs give rise to disease-like cellular vacuolation phenotype in the astrocyte lineage

Background

Megalencephalic leukoencephalopathy with subcortical cysts (MLC), a rare and progressive neurodegenerative disorder involving the white matter, is not adequately recapitulated by current disease models. Somatic cell reprogramming, along with advancements in genome engineering, may allow the establishment of in-vitro human models of MLC for disease modeling and drug screening. In this study, we utilized cellular reprogramming and gene-editing techniques to develop induced pluripotent stem cell (iPSC) models of MLC to recapitulate the cellular context of the classical MLC-impacted nervous system.

Methods

Somatic cell reprogramming of peripheral patient-derived blood mononuclear cells (PBMCs) was used to develop iPSC models of MLC. CRISPR-Cas9 system-based genome engineering was also utilized to create the MLC1 knockout model of the disease. Directed differentiation of iPSCs to neural stem cells (NSCs) and astrocytes was performed in a 2D cell culture format, followed by various cellular and molecular biology approaches, to characterize the disease model.

Results

MLC iPSCs established by somatic cell reprogramming and genome engineering were well characterized for pluripotency. iPSCs were subsequently differentiated to disease-relevant cell types: neural stem cells (NSCs) and astrocytes. RNA sequencing profiling of MLC NSCs revealed a set of differentially expressed genes related to neurological disorders and epilepsy, a common clinical finding within MLC disease. This gene set can serve as a target for drug screening for the development of a potential therapeutic for this disease. Upon differentiation to the more disease relevant cell type-astrocytes, MLC-characteristic vacuoles were clearly observed, which were distinctly absent from controls. This emergence recapitulated a distinguishing phenotypic marker of the disease.

Conclusion

Through the creation and analyses of iPSC models of MLC, our work addresses a critical need for relevant cellular models of MLC for use in both disease modeling and drug screening assays. Further investigation can utilize MLC iPSC models, as well as generated transcriptomic data sets and analyses, to identify potential therapeutic interventions for this debilitating disease.

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.

Homozygous variant of MLC1 results in megalencephalic leukoencephalopathy with subcortical cysts

BACKGROUND

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare, inherited disorder that causes epilepsy, intellectual disorders, and early onset macrocephaly. MLC1 has been identified as a main pathogenic gene.

METHODS

Clinical data such as magnetic resonance imaging (MRI), routine blood tests, and physical examinations were collected from proband. Trio whole-exome sequencing (WES) of the family was performed, and all variants with a minor allele frequency (<0.01) in the exon and canonical splicing sites were selected for further pathogenic evaluation. Candidate variants were validated using Sanger sequencing.

RESULTS

Here, we report a new homozygous variant identified in two children from the same family in the MLC1 gene [NM_015166.4: c.838_843delinsATTTTA, (p.Ser280_Phe281delinsIleLeu)]. This variant is classified as variant of uncertain significance (VUS) according to the ACMG guidelines. Further experiments demonstrate that the newly identified variant causes a decrease of MLC1 protein levels when expressed in a heterologous expression system.

CONCLUSION

Our case expands on this genetic variation and provides new evidence for the clinical diagnosis of MLC1-related MLC.

Transmembrane topology and oligomeric nature of an astrocytic membrane protein, MLC1

MLC1 is a membrane protein mainly expressed in astrocytes, and genetic mutations lead to the development of a leukodystrophy, megalencephalic leukoencephalopathy with subcortical cysts disease. Currently, the biochemical properties of the MLC1 protein are largely unknown. In this study, we aimed to characterize the transmembrane (TM) topology and oligomeric nature of the MLC1 protein. Systematic immunofluorescence staining data revealed that the MLC1 protein has eight TM domains and that both the N- and C-terminus face the cytoplasm. We found that MLC1 can be purified as an oligomer and could form a trimeric complex in both detergent micelles and reconstituted proteoliposomes. Additionally, a single-molecule photobleaching experiment showed that MLC1 protein complexes could consist of three MLC1 monomers in the reconstituted proteoliposomes. These results can provide a basis for both the high-resolution structural determination and functional characterization of the MLC1 protein.

Novel variants causing megalencephalic leukodystrophy in Sudanese families

Mutations in MLC1 cause megalencephalic leukoencephalopathy with subcortical cysts (MLC), a rare form of leukodystrophy characterized by macrocephaly, epilepsy, spasticity, and slow mental deterioration. Genetic studies of MLC are lacking from many parts of the world, especially in Sub-Saharan Africa. Genomic DNA was extracted for 67 leukodystrophic patients from 43 Sudanese families. Mutations were screened using the NGS panel testing 139 leukodystrophies and leukoencephalopathies causing genes (NextSeq500 Illumina). Five homozygous MLC1 variants were discovered in seven patients from five distinct families, including three consanguineous families from the same region of Sudan. Three variants were missense (c.971 T > G, p.Ile324Ser; c.344 T > C, p.Phe115Ser; and c.881 C > T, p.Pro294Leu), one duplication (c.831_838dupATATCTGT, p.Ser280Tyrfs*8), and one synonymous/splicing-site mutation (c.762 C > T, p.Ser254). The segregation pattern was consistent with autosomal recessive inheritance. The clinical presentation and brain MRI of the seven affected patients were consistent with the diagnosis of MLC1. Due to the high frequency of distinct MLC1 mutations found in our leukodystrophic Sudanese families, we analyzed the coding sequence of MLC1 gene in 124 individuals from the Sudanese genome project in comparison with the 1000-genome project. We found that Sudan has the highest proportion of deleterious variants in MLC1 gene compared with other populations from the 1000-genome project.

A Novel Splice-site Mutation on the MLC1 Gene Leading to Exon 9 Skipping and Megalencephalic Leukoencephalopathy with Subcortical Cysts in a Turkish Patient

Megalencephalic leukoencephalopathy (MLC) with subcortical cysts, also known as Van der Knaap disease (MIM #604004) is an autosomal recessive neurological disorder characterized by early onset macrocephaly, epilepsy, neurological deterioration with cerebellar ataxia and spasticity. An 8-month-old boy was admitted to our pediatric neurology clinic with macrocephaly. His brain magnetic resonance imaging (MRI) revealed bilateral, diffuse, symmetric structural white matter abnormalities, relatively sparing the cerebellum and bilateral subcortical temporal cysts. The diagnosis of Van der Knaap disease was suspected based on the clinical features and imaging findings and the genetic analysis revealed a novel homozygous c.768+2T>C mutation of the MLC1 gene. For determination of the novel splice-site mutation's effect, cDNA amplification was performed. cDNA analysis showed that the splice-site c.768+2T>C mutation gave rise to exon 9 skipping.

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.