Characterization of molecular mechanisms underlying the axonal Charcot-Marie-Tooth neuropathy caused by MORC2 mutations

Genomics 2 Proteins portal: a resource and discovery tool for linking genetic screening outputs to protein sequences and structures

Recent advances in AI-based methods have revolutionized the field of structural biology. Concomitantly, high-throughput sequencing and functional genomics have generated genetic variants at an unprecedented scale. However, efficient tools and resources are needed to link disparate data types-to 'map' variants onto protein structures, to better understand how the variation causes disease, and thereby design therapeutics. Here we present the Genomics 2 Proteins portal ( https://g2p.broadinstitute.org/ ): a human proteome-wide resource that maps 20,076,998 genetic variants onto 42,413 protein sequences and 77,923 structures, with a comprehensive set of structural and functional features. Additionally, the Genomics 2 Proteins portal allows users to interactively upload protein residue-wise annotations (for example, variants and scores) as well as the protein structure beyond databases to establish the connection between genomics to proteins. The portal serves as an easy-to-use discovery tool for researchers and scientists to hypothesize the structure-function relationship between natural or synthetic variations and their molecular phenotypes.

Intermediate phenotype between CMT2Z and DIGFAN associated with a novel MORC2 variant: a case report

Charcot-Marie-Tooth disease type 2Z is caused by MORC2 mutations and presents with axonal neuropathy. MORC2 mutations can also manifest as developmental delay, impaired growth, dysmorphic facies, and axonal neuropathy (DIGFAN). We report a patient exhibiting an intermediate phenotype between these diseases associated with a novel MORC2 variant. A literature review revealed that the genotype‒phenotype correlation in MORC2-related disorders is complex and that the same mutation can cause a variety of phenotypes.

Morc2a variants cause hydroxyl radical-mediated neuropathy and are rescued by restoring GHKL ATPase

Mutations in the Microrchidia CW-type zinc finger 2 (MORC2) GHKL ATPase module cause a broad range of neuropathies, such as Charcot-Marie-Tooth disease type 2Z; however, the aetiology and therapeutic strategy are not fully understood. Previously, we reported that the Morc2a p.S87L mouse model exhibited neuropathy and muscular dysfunction through DNA damage accumulation. In the present study, we analysed the gene expression of Morc2a p.S87L mice and designated the primary causing factor. We investigated the pathological pathway using Morc2a p.S87L mouse embryonic fibroblasts and human fibroblasts harbouring MORC2 p.R252W. We subsequently assessed the therapeutic effect of gene therapy administered to Morc2a p.S87L mice. This study revealed that Morc2a p.S87L causes a protein synthesis defect, resulting in the loss of function of Morc2a and high cellular apoptosis induced by high hydroxyl radical levels. We considered the Morc2a GHKL ATPase domain as a therapeutic target because it simultaneously complements hydroxyl radical scavenging and ATPase activity. We used the adeno-associated virus (AAV)-PHP.eB serotype, which has a high CNS transduction efficiency, to express Morc2a or Morc2a GHKL ATPase domain protein in vivo. Notably, AAV gene therapy ameliorated neuropathy and muscular dysfunction with a single treatment. Loss-of-function characteristics due to protein synthesis defects in Morc2a p.S87L were also noted in human MORC2 p.S87L or p.R252W variants, indicating the correlation between mouse and human pathogenesis. In summary, CMT2Z is known as an incurable genetic disorder, but the present study demonstrated its mechanisms and treatments based on established animal models. This study demonstrates that the Morc2a p.S87L variant causes hydroxyl radical-mediated neuropathy, which can be rescued through AAV-based gene therapy.

A Novel Heterozygous De Novo MORC2 Missense Variant Causes an Early Onset and Severe Neurodevelopmental Disorder

Microrchidia CW-type zinc finger protein 2 (MORC2) is an ATPase-containing nuclear protein which regulates transcription through chromatin remodelling and epigenetic silencing. MORC2 may have a role in the development of neurones, and dominant variants in this gene have recently been linked with disorders including Charcot-Marie-Tooth type 2Z disease, spinal muscular atrophy and, more recently, a neurodevelopmental syndrome consisting of developmental delay, impaired growth, dysmorphic facies, and axonal neuropathy (DIGFAN), presenting with hypotonia, microcephaly, brain atrophy, intellectual disability, hearing loss, faltering growth, and craniofacial dysmorphism. Notably, variants in MORC2 have shown clinical features overlapping with those of Cockayne and Leigh syndromes. Here, we report a case of MORC2-related DIGFAN syndrome in a female infant caused by a novel heterozygous de novo variant. The condition was early onset and severe, further expanding the range of genotypes associated with this disorder. Clinical features included unilateral hearing loss, developmental delay and regression within the first year of life, microcephaly, severe feeding difficulties, and faltering growth, resulting in death at 13 months of age.

Genetic Heterogeneity Underlying Phenotypes with Early-Onset Cerebellar Atrophy

Cerebellar atrophy (CA) is a frequent neuroimaging finding in paediatric neurology, usually associated with cerebellar ataxia. The list of genes involved in hereditary forms of CA is continuously growing and reveals its genetic complexity. We investigated ten cases with early-onset cerebellar involvement with and without ataxia by exome sequencing or by a targeted panel with 363 genes involved in ataxia or spastic paraplegia. Novel variants were investigated by in silico or experimental approaches. Seven probands carry causative variants in well-known genes associated with CA or cerebellar hypoplasia: SETX, CACNA1G, CACNA1A, CLN6, CPLANE1, and TBCD. The remaining three cases deserve special attention; they harbour variants in MAST1, PI4KA and CLK2 genes. MAST1 is responsible for an ultrarare condition characterised by global developmental delay and cognitive decline; our index case added ataxia to the list of concomitant associated symptoms. PIK4A is mainly related to hypomyelinating leukodystrophy; our proband presented with pure spastic paraplegia and normal intellectual capacity. Finally, in a patient who suffers from mild ataxia with oculomotor apraxia, the de novo novel CLK2 c.1120T>C variant was found. The protein expression of the mutated protein was reduced, which may indicate instability that would affect its kinase activity.

Leigh syndrome

Leigh syndrome, or subacute necrotizing encephalomyelopathy, was initially recognized as a neuropathological entity in 1951. Bilateral symmetrical lesions, typically extending from the basal ganglia and thalamus through brainstem structures to the posterior columns of the spinal cord, are characterized microscopically by capillary proliferation, gliosis, severe neuronal loss, and relative preservation of astrocytes. Leigh syndrome is a pan-ethnic disorder usually with onset in infancy or early childhood, but late-onset forms occur, including in adult life. Over the last six decades it has emerged that this complex neurodegenerative disorder encompasses more than 100 separate monogenic disorders associated with enormous clinical and biochemical heterogeneity. This chapter discusses clinical, biochemical and neuropathological aspects of the disorder, and postulated pathomechanisms. Known genetic causes, including defects of 16 mitochondrial DNA (mtDNA) genes and approaching 100 nuclear genes, are categorized into disorders of subunits and assembly factors of the five oxidative phosphorylation enzymes, disorders of pyruvate metabolism and vitamin and cofactor transport and metabolism, disorders of mtDNA maintenance, and defects of mitochondrial gene expression, protein quality control, lipid remodeling, dynamics, and toxicity. An approach to diagnosis is presented, together with known treatable causes and an overview of current supportive management options and emerging therapies on the horizon.

Expanding the phenotypic variability of MORC2 gene mutations: From Charcot-Marie-Tooth disease to late-onset pure motor neuropathy

MORC2 gene encodes a ubiquitously expressed nuclear protein involved in chromatin remodeling, DNA repair, and transcriptional regulation. Heterozygous mutations in MORC2 gene have been associated with a spectrum of disorders affecting the peripheral nervous system such as Charcot-Marie-Tooth (CMT2Z), spinal muscular atrophy-like with or without cerebellar involvement, and a developmental syndrome associated with impaired growth, craniofacial dysmorphism and axonal neuropathy (DIGFAN syndrome). Such variability in clinical manifestations associated with the increasing number of variants of unknown significance detected by next-generation sequencing constitutes a serious diagnostic challenge. Here we report the characterization of an in vitro model to evaluate the pathogenicity of variants of unknown significance based on MORC2 overexpression in a neuroblastoma cell line SH-EP or cortical neurons. Likewise, we show that MORC2 mutants affect survival and trigger apoptosis over time in SH-EP cell line. Furthermore, overexpression in primary cortical neurons increases apoptotic cell death and decreases neurite outgrowth. Altogether, these approaches establish the pathogenicity of two new variants p.Gly444Arg and p.His446Gln in three patients from two families. These new mutations in MORC2 gene are associated with autosomal dominant CMT and with adult late onset proximal motor neuropathy, further increasing the spectrum of clinical manifestations associated with MORC2 mutations.

Protein misfolding and clearance in the pathogenesis of a new infantile onset ataxia caused by mutations in PRDX3

Peroxiredoxin 3 (PRDX3) encodes a mitochondrial antioxidant protein which is essential for the control of reactive oxidative species (ROS) homeostasis. So far, PRDX3 mutations are involved in mild-to-moderate progressive juvenile onset cerebellar ataxia. We aimed to unravel the molecular bases underlying the disease in an infant suffering from cerebellar ataxia that started at 19 months old and presented severe cerebellar atrophy and peripheral neuropathy early in the course of disease. By whole exome sequencing, we identified a novel homozygous mutation, PRDX3 p.D163E, which impaired the mitochondrial ROS defense system. In mouse primary cortical neurons, the exogenous expression of PRDX3 p.D163E was reduced and triggered alterations in neurite morphology and in mitochondria. Mitochondrial computational parameters showed that p.D163E led to serious mitochondrial alterations. In transfected HeLa cells expressing the mutation, mitochondria accumulation was detected by correlative light electron microscopy (CLEM). Mitochondrial morphology showed severe changes, including extremely damaged outer and inner membranes with a notable cristae disorganization. Moreover, spherical structures compatible with lipid droplets were identified, which can be associated with a generalized response to stress and can be involved in the removal of unfolded proteins. In the patient's fibroblasts, PRDX3 expression was nearly absent. The biochemical analysis suggested that the mutation p.D163E would result in an unstable structure tending to form aggregates that trigger unfolded protein responses via mitochondria and endoplasmic reticulum. Altogether, our findings broaden the clinical spectrum of the recently described PRDX3-associated neurodegeneration and provide new insight into the pathological mechanisms underlying this new form of cerebellar ataxia.

MORC2/β-catenin signaling axis promotes proliferation and migration of breast cancer cells

Although Microrchidia 2 (MORC2) is overexpressed in many types of human cancer, its role in breast cancer progression remains unknown. Here, we report that the chromatin remodeler MORC2 expression positively correlates with β-catenin expression in breast cancer cell lines and patients. Overexpression of MORC2 augmented the expression of β-catenin and its target genes, cyclin D1 and c-Myc. Consistent with these results, we found MORC2 knockdown resulted in decreased expression of β-catenin and its target genes. Surprisingly, we observed that c-Myc, the target gene of β-catenin, regulated the MORC2-β-catenin signaling axis through a feedback mechanism. We demonstrated that MORC2 regulates β-catenin expression and function by modulating the phosphorylation of AKT. In addition, we observed reduced proliferation and migration of MORC2 overexpressing breast cancer cells upon β-catenin inhibition. Overall, our results demonstrate that MORC2 promotes breast cancer cell proliferation and migration by regulating β-catenin signaling.

Microrchidia CW-Type Zinc Finger 2, a Chromatin Modifier in a Spectrum of Peripheral Neuropathies

Microrchidia CW-type zinc finger 2 (MORC2) gene encodes a protein expressed in all tissues and enriched in the brain. MORC2 protein is composed of a catalytic ATPase domain, three coil-coiled domains allowing dimerization or protein complex interaction, a zinc-finger CW domain allowing DNA interaction, and a CHROMO-like (CHRromatin Organization Modifier) domain. Recently, de novo or dominantly inherited heterozygous mutations have been associated with a spectrum of disorders affecting the peripheral nervous system such as the Charcot-Marie-Tooth disease, spinal muscular atrophy-like phenotype disorder, or a neurodevelopmental syndrome associated with developmental delay, impaired growth, dysmorphic facies, and axonal neuropathy (DIGFAN). In this review, we detail the various mutations of MORC2 and their consequences on clinical manifestations. Possible genotype-phenotype correlations as well as intra and inter-family variability are discussed. MORC2 molecular functions such as transcriptional modulation, DNA damage repair, and lipid metabolism are then reviewed. We further discuss the impact of MORC2 mutations on the epigenetic landscape in the neuromuscular system and hypothesize probable pathophysiological mechanisms underlying the phenotypic variability observed.

Structural and Functional Attributes of Microrchidia Family of Chromatin Remodelers

Chromatin remodelers affect the spatio-temporal dynamics of global gene-expression by structurally modulating and/or reorganizing the chromatin. Microrchidia (MORC) family is a relatively new addition to the four well studied families of chromatin remodeling proteins. In this review, we discuss the current understanding of the structural aspects of human MORCs as well as their epigenetic functions. From a molecular and systems-level perspective, we explore their participation in phase-separated structures, possible influence on various biological processes through protein-protein interactions, and potential extra-nuclear roles. We describe how dysregulation/dysfunction of MORCs can lead to various pathological conditions. We conclude by emphasizing the importance of undertaking integrated efforts to obtain a holistic understanding of the various biological roles of MORCs.

HSP90 N-terminal inhibitors target oncoprotein MORC2 for autophagic degradation and suppress MORC2-driven breast cancer progression

Aims

MORC family CW‐type zinc finger 2 (MORC2), a GHKL‐type ATPase, is aberrantly upregulated in multiple types of human tumors with profound effects on cancer aggressiveness, therapeutic resistance, and clinical outcome, thus making it an attractive drug target for anticancer therapy. However, the antagonists of MORC2 have not yet been documented.

Methods and Results

We report that MORC2 is a relatively stable protein, and the N‐terminal homodimerization but not ATP binding and hydrolysis is crucial for its stability through immunoblotting analysis and Quantitative real‐time PCR. The N‐terminal but not C‐terminal inhibitors of heat shock protein 90 (HSP90) destabilize MORC2 in multiple cancer cell lines, and strikingly, this process is independent on HSP90. Mechanistical investigations revealed that HSP90 N‐terminal inhibitors disrupt MORC2 homodimer formation without affecting its ATPase activities, and promote its lysosomal degradation through the chaperone‐mediated autophagy pathway. Consequently, HSP90 inhibitor 17‐AAG effectively blocks the growth and metastatic potential of MORC2‐expressing breast cancer cells both in vitro and in vivo, and these noted effects are not due to HSP90 inhibition.

Conclusion

We uncover a previously unknown role for HSP90 N‐terminal inhibitors in promoting MORC2 degradation in a HSP90‐indepentent manner and support the potential application of these inhibitors for treating MORC2‐overexpressing tumors, even those with low or absent HSP90 expression. These results also provide new clue for further design of novel small‐molecule inhibitors of MORC2 for anticancer therapeutic application.

Structural insights into Charcot-Marie-Tooth disease-linked mutations in human GDAP1

Charcot-Marie-Tooth disease (CMT) is the most common inherited peripheral polyneuropathy in humans, and its different subtypes are linked to mutations in dozens of different genes. Mutations in ganglioside-induced differentiation-associated protein 1 (GDAP1) cause two types of CMT, demyelinating CMT4A and axonal CMT2K. The GDAP1-linked CMT genotypes are mainly missense point mutations. Despite clinical profiling and in vivo studies on the mutations, the etiology of GDAP1-linked CMT is poorly understood. Here, we describe the biochemical and structural properties of the Finnish founding CMT2K mutation H123R as well as CMT2K-linked R120W, both of which are autosomal dominant mutations. The disease variant proteins retain close to normal structure and solution behaviour, but both present a significant decrease in thermal stability. Using GDAP1 variant crystal structures, we identify a side chain interaction network between helices ⍺3, ⍺6, and ⍺7, which is affected by CMT mutations, as well as a hinge in the long helix ⍺6, which is linked to structural flexibility. Structural analysis of GDAP1 indicates that CMT may arise from disruption of specific intra- and intermolecular interaction networks, leading to alterations in GDAP1 structure and stability, and eventually, insufficient motor and sensory neuron function.

Rare among Rare: Phenotypes of Uncommon CMT Genotypes

(1) Background: Charcot–Marie–Tooth disease (CMT) is the most frequent form of inherited chronic motor and sensory polyneuropathy. Over 100 CMT causative genes have been identified. Previous reports found PMP22, GJB1, MPZ, and MFN2 as the most frequently involved genes. Other genes, such as BSCL2, MORC2, HINT1, LITAF, GARS, and autosomal dominant GDAP1 are responsible for only a minority of CMT cases. (2) Methods: we present here our records of CMT patients harboring a mutation in one of these rare genes (BSCL2, MORC2, HINT1, LITAF, GARS, autosomal dominant GDAP1). We studied 17 patients from 8 unrelated families. All subjects underwent neurologic evaluation and genetic testing by next-generation sequencing on an Ion Torrent PGM (Thermo Fischer) with a 44-gene custom panel. (3) Results: the following variants were found: BSCL2 c.263A > G p.Asn88Ser (eight subjects), MORC2 c.1503A > T p.Gln501His (one subject), HINT1 c.110G > C p.Arg37Pro (one subject), LITAF c.404C > G p.Pro135Arg (two subjects), GARS c.1660G > A p.Asp554Asn (three subjects), GDAP1 c.374G > A p.Arg125Gln (two subjects). (4) Expanding the spectrum of CMT phenotypes is of high relevance, especially for less common variants that have a higher risk of remaining undiagnosed. The necessity of reaching a genetic definition for most patients is great, potentially making them eligible for future experimentations.

MORC protein family-related signature within human disease and cancer

The microrchidia (MORC) family of proteins is a highly conserved nuclear protein superfamily, whose members contain common domain structures (GHKL-ATPase, CW-type zinc finger and coiled-coil domain) yet exhibit diverse biological functions. Despite the advancing research in previous decades, much of which focuses on their role as epigenetic regulators and in chromatin remodeling, relatively little is known about the role of MORCs in tumorigenesis and pathogenesis. MORCs were first identified as epigenetic regulators and chromatin remodelers in germ cell development. Currently, MORCs are regarded as disease genes that are involved in various human disorders and oncogenes in cancer progression and are expected to be the important biomarkers for diagnosis and treatment. A new paradigm of expanded MORC family function has raised questions regarding the regulation of MORCs and their biological role at the subcellular level. Here, we systematically review the progress of researching MORC members with respect to their domain architectures, diverse biological functions, and distribution characteristics and discuss the emerging roles of the aberrant expression or mutation of MORC family members in human disorders and cancer development. Furthermore, the illustration of related mechanisms of the MORC family has made MORCs promising targets for developing diagnostic tools and therapeutic treatments for human diseases, including cancers.

Infantile-Onset Charcot-Marie-Tooth Disease With Pyramidal Features and White Matter Abnormalities Due to a De novo MORC2 Gene Variant: A Case Report and Brief Review of the Literature

Background: Charcot–Marie–Tooth (CMT) is the most frequent group of inherited neuropathies and includes several heterogeneous phenotypes. Over 80 causative genes have been described so far. Variants in the microrchidia family CW-type zinc finger 2 (MORC2) gene have been described in several axonal polyneuropathy (CMT2) patients with childhood or adult onset. Occasionally more complex phenotypes with delayed milestones, severe hypotonia, intellectual disability, dystonic postures, pyramidal signs, and neuroimaging abnormalities have been reported.

Case Presentation: We report on a patient with a de novo MORC2 gene variant (c.1181A>G p.Tyr394Cys) with a history of developmental delay, axial hypotonia, progressive gait disorder with dystonic features, and intentional tremor. At the age of 8 years, he showed bilateral pyramidal signs (clonus, increased tendon reflexes, and Babinski sign) and bilateral pes cavus. The first neuroimaging performed at the age of 3 years demonstrated white matter abnormalities in the posterior periventricular zone, in the frontal lobes bilaterally and at the midbrain, stable during childhood and adolescence. Nerve conduction studies (NCS) were negative until the age of 15 years, when a sensory axonal neuropathy appeared. The association between pyramidal signs and neuropathy due to the MORC2 gene variant is increasingly being highlighted, although a neuroradiological correlate is evident only in about half of the cases. Longitudinal nerve conduction velocity (NCV) are helpful to identify late-onset features and provide useful information for diagnosis in patients with rare neurogenetic disorders.

Conclusions: Characterization of complex neurological disorders is important to delineate the expanding phenotypic spectrum of MORC2-related disease, to confirm if possible the pathogenicity of the variants and to deepen the genotype–phenotype correlation.

Morc2a p.S87L mutant mice develop peripheral and central neuropathies associated with neuronal DNA damage and apoptosis

The microrchidia (MORC)-family CW-type zinc finger 2 (MORC2) gene is related to DNA repair, adipogenesis and epigenetic silencing via the human silencing hub (HUSH) complex. MORC2 missense mutation is known to cause peripheral neuropathy of Charcot-Marie-Tooth disease type 2 Z (CMT2Z). However, there have been reports of peripheral and central neuropathy in patients, and the disease has been co-categorized with developmental delay, impaired growth, dysmorphic facies and axonal neuropathy (DIGFAN). The etiology of MORC2 mutation-mediated neuropathy remains uncertain. Here, we established and analyzed Morc2a p.S87L mutant mice. Morc2a p.S87L mice displayed the clinical symptoms expected in human CMT2Z patients, such as axonal neuropathy and skeletal muscle weakness. Notably, we observed severe central neuropathy with cerebella ataxia, cognition disorder and motor neuron degeneration in the spinal cord, and this seemed to be evidence of DIGFAN. Morc2a p.S87L mice exhibited an accumulation of DNA damage in neuronal cells, followed by p53/cytochrome c/caspase 9/caspase 3-mediated apoptosis. This study presents a new mouse model of CMT2Z and DIGFAN with a Morc2a p.S87L mutation. We suggest that neuronal apoptosis is a possible target for therapeutic approach in MORC2 missense mutation. This article has an associated First Person interview with the first author of the paper.

MORC2 gene de novo mutation leads to Charcot-Marie-Tooth disease type 2Z: A pediatric case report and literature review

RATIONALE

Mutations of the MORC2 gene have most commonly been associated with autosomal-dominant Charcot-Marie-Tooth disease type 2Z (CMT 2Z), while the impact of MORC2 mutations in CMT 2Z on neuronal biology and their phenotypic consequences in patients remain to be clarified.

PATIENT CONCERNS

We reported a 27-month-old child with a developmental lag of more than 1 year. He had progressive fatigue for 4 months, accompanied by dysphagia, choking while eating, and progressive aggravation. A genetic study revealed a de novo variant of MORC2, which has not yet been reported.

DIAGNOSIS

According to the child's clinical manifestations, genetic pattern, and American College of Medical Genetics and Genomics pathogenicity analysis, the patient was diagnosed with CMT 2Z caused by MORC2 gene mutation.

INTERVENTIONS

Mitochondrial cocktail therapy (arginine, vitamin B1 tablets, vitamin B2 tablets, coenzyme Q10 capsules, L-carnitine oral liquid, idebenone tablets, etc) was given.

OUTCOMES

Mitochondrial cocktail therapy did not significantly improve the child's condition, head magnetic resonance imaging lesions were not significantly improved at outpatient follow-up more than 1 month later, and the lesions were basically unchanged.

LESSONS

The clinical manifestations of the disease were similar to those of Leigh syndrome, and they were not significantly improved by cocktail therapy. This site has not been reported in the literature domestically or abroad, and the pathogenesis of CMT 2Z caused by this site mutation is indeed not related to mitochondrial dysfunction. Our study is helpful for clinicians with regard to the differential diagnosis of Leigh syndrome and CMT 2Z and improvement of clinicians' understanding of CMT 2Z disease.

The Pathological Features of Common Hereditary Mitochondrial Dynamics Neuropathy

Objectives

Mitofusin 2 and ganglioside-induced differentiation-associated protein 1 are two main mitochondrial dynamics-related proteins. Dysfunction of these two proteins leads to different subtypes of Charcot–Marie–Tooth disease type 2A (CMT2A) and CMT2K. This study aims to report the pathological difference between CMT2A and CMT2K in a large cohort.

Methods

Thirty patients with molecularly confirmed CMT2A and nine with CMT2K were identified by next-generation sequencing. Sural nerve biopsies were performed in 29 patients.

Results

The patients with both diseases showed length-dependent neuropathy with distal weakness, sensory loss, and no deep tendon reflex. Optic neuropathy appeared in 3/30 (10%) patients with CMT2A. Tendon contracture appeared in 4/9 (50.0%) patients with CMT2K. Sural biopsy revealed the loss of both myelinated and unmyelinated nerve fibers. Closely packed, irregularly oriented neurofilaments were observed in axons of unmyelinated nerve fibers in both diseases. Another important finding was the ubiquitous presence of smaller, rounded, and fragmented mitochondria in CMT2A and elongated mitochondria in CMT2K in the myelinated and unmyelinated axons.

Conclusion

This study confirmed large diversity in phenotypes between CMT2A and CMT2K. Mitochondrial dynamics-related variations can induce different mitochondrial morphological changes and neurofilament accumulation in axons.

Charcot-Marie-Tooth disease due to MORC2 mutations in Spain

BACKGROUND AND PURPOSE

MORC2 mutations have been described as a rare cause of axonal Charcot-Marie-Tooth disease (CMT2Z). The aim of this work was to determine the frequency and distribution of these mutations throughout Spain, to provide a comprehensive phenotypical description and, if possible, to establish a genotype-phenotype correlation.

METHODS

Retrospectively, data on patients diagnosed with CMT2Z in Spain were collected and clinical, electrophysiological and muscle imaging information were analysed.

RESULTS

Fifteen patients with CMT2Z were identified throughout Spain, seven of them belonging to a single kindred, whilst the rest were sporadic. The most common mutation was p.R252W, and four new mutations were identified. Eleven patients were categorized as having a scapuloperoneal phenotype, with asymmetric muscle weakness, early proximal upper limb involvement and frequent spontaneous muscular activity with distal sensory impairment and pes cavus, whilst two presented with a more classic length dependent sensory motor phenotype. This distinction was corroborated by the distribution of muscle fatty infiltration in muscle imaging. Two other patients were classified as having a neurodevelopmental phenotype consisting in congenital or early onset, delay in motor milestones, and global developmental delay in one of them. Nerve conduction studies revealed an unequivocally axonal neuropathy with frequent spontaneous activity, and serum creatine kinase levels were increased in 50% of the patients.

CONCLUSIONS

MORC2 mutations are a rare cause of CMT in Spain, but in-depth phenotyping reveals a recognizable phenotypic spectrum that will be clinically relevant for future identification of this disease.

Characterization of genotype-phenotype correlation with MORC2 mutated Axonal Charcot-Marie-Tooth disease in a cohort of Chinese patients

BACKGROUND

Charcot-Marie-Tooth (CMT) disease is an exciting field of study, with a growing number of causal genes and an expanding phenotypic spectrum. The microrchidia family CW-type zinc finger 2 gene (MORC2) was newly identified as a causative gene of CMT2Z in 2016. We aimed to describe the phenotypic-genetic spectrum of MORC2-related diseases in the Chinese population.

METHODS

With the use of Sanger sequencing and Next Generation Sequencing (NGS) technologies, we screened a cohort of 284 unrelated Chinese CMT2 families. Pathogenicity assessments of MORC2 variants were interpreted according to the ACMG guidelines. Potential pathogenic variants were confirmed by Sanger sequencing.

RESULTS

We identified 4 different heterozygous MORC2 mutations in four unrelated families, accounting for 1.4% (4/284). A novel mutation c.1397A>G p. D466G was detected in family 1 and all affected patients presented with later onset axonal CMT with hyperCKemia. The patient in family 2 showed a spinal muscular atrophy (SMA)-like disease with cerebellar hypoplasia and mental retardation, with a hot spot de novo mutation c.260C>T p. S87L. The twin sisters in family 3 were identified as having the most common mutation c.754C>T p. R252W and suffered from axonal motor neuropathy with high variability in disease severity and duration. The patient in family 4 developed an early onset axonal motor and sensory neuropathy, with a reported mutation c.1220G>A p.C407Y. All identified mutations associated with MORC2-related neuropathies are localized in the N-terminal ATPase module.

CONCLUSIONS

Our study confirmed that MORC2-related neuropathies exist in the Chinese population at a relatively high mutation rate. We revealed a complex genotype-phenotype correlation with MORC2 mutations. This report adds a new piece to the puzzle of the genetics of CMT and contributes to a better understanding of the disease mechanisms.

A recurrent MORC2 mutation causes Charcot-Marie-Tooth disease type 2Z

We found a p.Ala406Val (c.1217C > T) mutation in MORC2 in three individuals, from two families. All three individuals were evaluated and clinical electrophysiology was completed. The neuropathy began in childhood to early adulthood, with distal weakness progressing to proximal weakness. Vinblastine (for Hodgkin lymphoma) acutely worsened the weakness in one patient. This finding confirms that that the p.Ala406Val mutation in MORC2 causes severe neuropathy. In addition, we report the first case of vinblastine neurotoxicity in Charcot-Marie-Tooth disease type 2Z.

[A case of Charcot-Marie-Tooth disease type 2Z caused by MORC2 S87L mutation mimicking spinal muscular atrophy]

A 33-year-old man with an unremarkable family history has had limb muscle weakness, joint contracture and skeleton deformation from early childhood. He was diagnosed with spinal muscular atrophy (SMA) by a pediatrician. He needed assistance and used orthoses in his daily life. There was no subjective sensory disturbance. However, physical examination showed slight sensory impairment, and nerve conduction study indicated sensory motor axonal neuropathy. This finding suggested Charcot-Marie-Tooth disease (CMT). Gene analysis detected MORC2 S87L mutation, leading to a diagnosis of CMT type 2Z. Patients with MORC2 S87L mutation are known to exhibit a severe phenotype, and may mimic SMA. It is important to demonstrate subclinical sensory neuropathy in patients with MORC2 S87L mutation mimicking SMA.

Mutations in GDAP1 Influence Structure and Function of the Trans-Golgi Network

Charcot-Marie-Tooth disease (CMT) is a heritable neurodegenerative disease that displays great genetic heterogeneity. The genes and mutations that underlie this heterogeneity have been extensively characterized by molecular genetics. However, the molecular pathogenesis of the vast majority of CMT subtypes remains terra incognita. Any attempts to perform experimental therapy for CMT disease are limited by a lack of understanding of the pathogenesis at a molecular level. In this study, we aim to identify the molecular pathways that are disturbed by mutations in the gene encoding GDAP1 using both yeast and human cell, based models of CMT-GDAP1 disease. We found that some mutations in GDAP1 led to a reduced expression of the GDAP1 protein and resulted in a selective disruption of the Golgi apparatus. These structural alterations are accompanied by functional disturbances within the Golgi. We screened over 1500 drugs that are available on the market using our yeast-based CMT-GDAP1 model. Drugs were identified that had both positive and negative effects on cell phenotypes. To the best of our knowledge, this study is the first report of the Golgi apparatus playing a role in the pathology of CMT disorders. The drugs we identified, using our yeast-based CMT-GDAP1 model, may be further used in translational research.

One Multilocus Genomic Variation Is Responsible for a Severe Charcot-Marie-Tooth Axonal Form

Charcot-Marie-Tooth (CMT) disease is a heterogeneous group of inherited disorders affecting the peripheral nervous system, with a prevalence of 1/2500. So far, mutations in more than 80 genes have been identified causing either demyelinating forms (CMT1) or axonal forms (CMT2). Consequentially, the genotype-phenotype correlation is not always easy to assess. Diagnosis could require multiple analysis before the correct causative mutation is detected. Moreover, it seems that approximately 5% of overall diagnoses for genetic diseases involves multiple genomic loci, although they are often underestimated or underreported. In particular, the combination of multiple variants is rarely described in CMT pathology and often neglected during the diagnostic process. Here, we present the complex genetic analysis of a family including two CMT cases with various severities. Interestingly, next generation sequencing (NGS) associated with Cov'Cop analysis, allowing structural variants (SV) detection, highlighted variations in MORC2 (microrchidia family CW-type zinc-finger 2) and AARS1 (alanyl-tRNA-synthetase) genes for one patient and an additional mutation in MFN2 (Mitofusin 2) in the more affected patient.

Identification of Candidate Genes Associated with Charcot-Marie-Tooth Disease by Network and Pathway Analysis

Charcot-Marie-Tooth Disease (CMT) is the most common clinical genetic disease of the peripheral nervous system. Although many studies have focused on elucidating the pathogenesis of CMT, few focuses on achieving a systematic analysis of biology to decode the underlying pathological molecular mechanisms and the mechanism of its disease remains to be elucidated. So our study may provide further useful insights into the molecular mechanisms of CMT based on a systematic bioinformatics analysis. In the current study, by reviewing the literatures deposited in PUBMED, we identified 100 genes genetically related to CMT. Then, the functional features of the CMT-related genes were examined by R software and KOBAS, and the selected biological process crosstalk was visualized with the software Cytoscape. Moreover, CMT specific molecular network analysis was conducted by the Molecular Complex Detection (MCODE) Algorithm. The biological function enrichment analysis suggested that myelin sheath, axon, peripheral nervous system, mitochondrial function, various metabolic processes, and autophagy played important roles in CMT development. Aminoacyl-tRNA biosynthesis, metabolic pathways, and vasopressin-regulated water reabsorption were significantly enriched in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway network, suggesting that these pathways may play key roles in CMT occurrence and development. According to the crosstalk, the biological processes could be roughly divided into a correlative module and two separate modules. MCODE clusters showed that in top 3 clusters, 13 of CMT-related genes were included in the network and 30 candidate genes were discovered which might be potentially related to CMT. The study may help to update the new understanding of the pathogenesis of CMT and expand the potential genes of CMT for further exploration.

De Novo Variants in the ATPase Module of MORC2 Cause a Neurodevelopmental Disorder with Growth Retardation and Variable Craniofacial Dysmorphism

MORC2 encodes an ATPase that plays a role in chromatin remodeling, DNA repair, and transcriptional regulation. Heterozygous variants in MORC2 have been reported in individuals with autosomal-dominant Charcot-Marie-Tooth disease type 2Z and spinal muscular atrophy, and the onset of symptoms ranges from infancy to the second decade of life. Here, we present a cohort of 20 individuals referred for exome sequencing who harbor pathogenic variants in the ATPase module of MORC2. Individuals presented with a similar phenotype consisting of developmental delay, intellectual disability, growth retardation, microcephaly, and variable craniofacial dysmorphism. Weakness, hyporeflexia, and electrophysiologic abnormalities suggestive of neuropathy were frequently observed but were not the predominant feature. Five of 18 individuals for whom brain imaging was available had lesions reminiscent of those observed in Leigh syndrome, and five of six individuals who had dilated eye exams had retinal pigmentary abnormalities. Functional assays revealed that these MORC2 variants result in hyperactivation of epigenetic silencing by the HUSH complex, supporting their pathogenicity. The described set of morphological, growth, developmental, and neurological findings and medical concerns expands the spectrum of genetic disorders resulting from pathogenic variants in MORC2.

Acetylation of MORC2 by NAT10 regulates cell-cycle checkpoint control and resistance to DNA-damaging chemotherapy and radiotherapy in breast cancer

MORC family CW-type zinc finger 2 (MORC2) is an oncogenic chromatin-remodeling enzyme with an emerging role in DNA repair. Here, we report a novel function for MORC2 in cell-cycle checkpoint control through an acetylation-dependent mechanism. MORC2 is acetylated by the acetyltransferase NAT10 at lysine 767 (K767Ac) and this process is counteracted by the deacetylase SIRT2 under unperturbed conditions. DNA-damaging chemotherapeutic agents and ionizing radiation stimulate MORC2 K767Ac through enhancing the interaction between MORC2 and NAT10. Notably, acetylated MORC2 binds to histone H3 phosphorylation at threonine 11 (H3T11P) and is essential for DNA damage-induced reduction of H3T11P and transcriptional repression of its downstream target genes CDK1 and Cyclin B1, thus contributing to DNA damage-induced G2 checkpoint activation. Chemical inhibition or depletion of NAT10 or expression of an acetylation-defective MORC2 (K767R) forces cells to pass through G2 checkpoint, resulting in hypersensitivity to DNA-damaging agents. Moreover, MORC2 acetylation levels are associated with elevated NAT10 expression in clinical breast tumor samples. Together, these findings uncover a previously unrecognized role for MORC2 in regulating DNA damage-induced G2 checkpoint through NAT10-mediated acetylation and provide a potential therapeutic strategy to sensitize breast cancer cells to DNA-damaging chemotherapy and radiotherapy by targeting NAT10.