A novel LRSAM1 mutation is associated with autosomal dominant axonal Charcot-Marie-Tooth disease. Eur J Hum Genet. 2013;21:190-194. [PMID: 22781092 DOI: 10.1038/ejhg.2012.146]

Two case reports of a novel missense mutation in the PNPLA6 gene in two siblings with chorioretinal dystrophy, hypogonadotropic hypogonadism, and cerebellar ataxia

BACKGROUND

Boucher Neuhäuser Syndrome (BNS) is a rare disease with autosomal recessive inheritance defined by the classical triad; early-onset ataxia, hypogonadism and chorioretinal dystrophy.

CASE PRESENTATION

We present two siblings diagnosed with BNS at midlife, identified with homozygous state of a novel PNPLA6 missense mutation. One healthy sibling and the mother were heterozygous carriers of the mutation. The proband presented with the classical triad and the other sibling presented with visual problems at first. The proband was referred to our department by a private Neurologist, in early adulthood, because of hypogonadism, cerebellar ataxia, axonal neuropathy, and chorioretinal dystrophy for further evaluation. The sibling was referred to our department for evaluation, at childhood, due to visual problems. Later, the patient displayed the triad of ataxia, hypogonadotropic hypogonadism, and chorioretinal dystrophy. The unusual medical history of the two siblings led to further examinations and eventually the diagnosis of the first BNS cases in Cyprus. WES-based ataxia in silico gene panel analysis revealed 15 genetic variants and further filtering analysis revealed the PNPLA6 c.3323G > A variant. Segregation analysis in the family with Sanger sequencing confirmed the PNPLA6 homozygous variant c.3323G > A, p.Arg1108Gln in exon 29.

CONCLUSIONS

This highlights the importance of considering rare inherited causes of visual loss, spinocerebellar ataxia, or/and HH in a neurology clinic and the significant role of genetic sequencing in the diagnostic process.

The phenotypic spectrum of pathogenic ATP1A1 variants expands: the novel p.P600R substitution causes demyelinating Charcot-Marie-Tooth disease

BACKGROUND

Charcot-Marie-Tooth disease (CMT) is a genetically and clinically heterogeneous group of inherited neuropathies. Monoallelic pathogenic variants in ATP1A1 were associated with axonal and intermediate CMT. ATP1A1 encodes for the catalytic α1 subunit of the Na⁺/ K⁺ ATPase. Besides neuropathy, other associated phenotypes are spastic paraplegia, intellectual disability, and renal hypomagnesemia. We hereby report the first demyelinating CMT case due to a novel ATP1A1 variant.

METHODS

Whole-exome sequencing on the patient's genomic DNA and Sanger sequencing to validate and confirm the segregation of the identified p.P600R ATP1A1 variation were performed. To evaluate functional effects, blood-derived mRNA and protein levels of ATP1A1 and the auxiliary β1 subunit encoded by ATP1B1 were investigated. The ouabain-survival assay was performed in transfected HEK cells to assess cell viability, and two-electrode voltage clamp studies were performed in Xenopus oocytes.

RESULTS

The variant was absent in the local and global control datasets, falls within a highly conserved protein position, and is in a missense-constrained region. The expression levels of ATP1A1 and ATP1B1 were significantly reduced in the patient compared to healthy controls. Electrophysiology indicated that ATP1A1^p.P600R injected Xenopus oocytes have reduced Na⁺/ K⁺ ATPase function. Moreover, HEK cells transfected with a construct encoding ATP1A1^p.P600R harbouring variants that confers ouabain insensitivity displayed a significant decrease in cell viability after ouabain treatment compared to the wild type, further supporting the pathogenicity of this variant.

CONCLUSION

Our results further confirm the causative role of ATP1A1 in peripheral neuropathy and broaden the mutational and phenotypic spectrum of ATP1A1-associated CMT.

C698R mutation in Lrsam1 gene impairs nerve regeneration in a CMT2P mouse model

Missense mutation C694R in the RING domain of the LRSAM1 gene results in a dominantly inherited polyneuropathy, Charcot-Marie-Tooth disease type 2P (CMT2P). We have generated and characterized a Lrsam1^C698R knock-in mouse model produced through CRISPR/Cas9 technology. Both heterozygous (Lrsam1^+/C698R) and homozygous (Lrsam1^C698/C698R) knock-in mice exhibited normal motor functions on behavioral tests as well as normal on nerve conduction studies. Axonal density and myelin thickness were not significantly different between mutants and wild-type mice by sciatic nerve morphometric analysis up to 17 months of age. In line with these normal findings, protein–protein interactions between mutant LRSAM1 and RNA-binding proteins (such as FUS and G3BP1) were still present in mouse cells, which differs from the disrupted interactions between these proteins in human CMT2P cells. However, after crush nerve injury, Lrsam1^+/C698R mice had a mild, but statistically significant, reduced compound nerve action potential and conduction velocity during recovery. Therefore, C698R mutation results in a mild impaired nerve regeneration in mice. We speculate that repetitive nerve injuries may, at least partially, contribute to the slowly progressive axonal loss in CMT2P.

A Novel CLN6 Variant Associated With Juvenile Neuronal Ceroid Lipofuscinosis in Patients With Absence of Visual Loss as a Presenting Feature

The neuronal ceroid lipofuscinoses (NCLs), also known as Batten disease, are a group of autosomal recessive lysosomal storage disorders that are characterized by neurodegeneration, progressive cognitive decline, motor impairment, ataxia, loss of vision, seizures, and premature death. To date, pathogenic variants in more than 13 genes have been associated with NCLs. CLN6 encodes an endoplasmic reticulum non-glycosylated transmembrane protein, which is involved in lysosomal acidification. Mutations in CLN6 cause late-infantile juvenile NCL (JNCL) adult-onset NCL, and Kufs disease. Members from two available families with JNCL were clinically evaluated, and samples were collected from consenting individuals. The molecular investigation was performed by whole-exome sequencing, Sanger sequencing, and family segregation analysis. Furthermore, in silico prediction analysis and structural modeling of the identified CLN6 variants were performed. We report clinical and genetic findings of three patients from two Greek-Cypriot families (families 915 and 926) with JNCL. All patients were males, and the first symptoms appeared at the age of 6 years. The proband of family 926 presented with loss of motor abilities, ataxia, spasticity, seizure, and epilepsy. The proband of family 915 had ataxia, spasticity, dysarthria, dystonia, and intellectual disability. Both probands did not show initial signs of vision and/or hearing loss. Molecular analysis of family 926 revealed two CLN6 biallelic variants: the novel, de novo p.Tyr295Cys and the known p.Arg136His variants. In family 915, both patients were homozygous for the p.Arg136His CLN6 variant. Prediction analysis of the two CLN6 variants characterized them as probably damaging and disease-causing. Structural modeling of the variants predicted that they probably cause protein structural differentiation. In conclusion, we describe two unrelated Cypriot families with JNCL. Both families had variants in the CLN6 gene; however, they presented with slightly different symptoms, and notably none of the patients has loss of vision. In silico prediction and structural analyses indicate that both variants are most likely pathogenic.

LRSAM1 and the RING domain: Charcot-Marie-Tooth disease and beyond

In the past decade, mutations in LRSAM1 were identified as the genetic cause of both dominant and recessive forms of axonal CMT type 2P (CMT2P). Despite demonstrating different inheritance patterns, dominant CMT2P is usually characterized by relatively mild, slowly progressive axonal neuropathy, mainly involving lower limbs, with age of onset between the second and fifth decades of life. Asymptomatic individuals were identified in several pedigrees exemplifying the strong phenotypic variability of these patients requiring serial clinical evaluation to establish correct diagnosis; in this respect, magnetic resonance imaging of lower-limb musculature showing fatty atrophy might be helpful in detecting subclinical gene mutation carriers. LRSAM1 is a universally expressed RING-type E3 ubiquitin protein ligase catalysing the final step in the ubiquitination cascade. Strikingly, TSG101 remains the only known ubiquitination target hampering our mechanistic understanding of the role of LRSAM1 in the cell. The recessive CMT mutations lead to complete loss of LRSAM1, contrary to the heterozygous dominant variants. These tightly cluster in the C-terminal RING domain highlighting its importance in governing the CMT disease. The domain is crucial for the ubiquitination function of LRSAM1 and CMT mutations disrupt its function, however it remains unknown how this leads to the peripheral neuropathy. Additionally, recent studies have linked LRSAM1 with other neurodegenerative diseases of peripheral and central nervous systems. In this review we share our experience with the challenging clinical diagnosis of CMT2P and summarize the mechanistic insights about the LRSAM1 dysfunction that might be helpful for the neurodegenerative field at large.

Impaired Mitochondrial Mobility in Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is a progressive, peripheral neuropathy and the most commonly inherited neurological disorder. Clinical manifestations of CMT mutations are typically limited to peripheral neurons, the longest cells in the body. Currently, mutations in at least 80 different genes are associated with CMT and new mutations are regularly being discovered. A large portion of the proteins mutated in axonal CMT have documented roles in mitochondrial mobility, suggesting that organelle trafficking defects may be a common underlying disease mechanism. This review will focus on the potential role of altered mitochondrial mobility in the pathogenesis of axonal CMT, highlighting the conceptional challenges and potential experimental and therapeutic opportunities presented by this "impaired mobility" model of the disease.

Location matters - Genotype-phenotype correlation in LRSAM1 mutations associated with rare Charcot-Marie-Tooth neuropathy CMT2P

More than 80 genes are known to be associated with Charcot-Marie-Tooth disease (CMT). Mutations of LRSAM1 were identified as a rare cause and define the subgroup of axonal neuropathy CMT2P. We identified additional 14 patients out of 12 families. Clinical and electrophysiological data confirm a late-onset axonal neuropathy with a predominance of sensorimotor impairment. The patients harbored ten different variants in LRSAM1, seven of which were novel. Due to variable inheritance patterns and clustering of pathogenic variants in 3´-prime exons, interpretation of genetic variants in LRSAM1 is challenging. The majority follows dominant inheritance, whereas recessive inheritance has been described for one variant. Variants at the 3`end may or may not escape from nonsense-mediated decay, thereby defining the pattern of inheritance. Our data emphasize the importance of the C-terminal RING domain, which exerts a dominant-negative effect on protein function, whenever affected by an altered or truncated protein. In conclusion, CMT2P is a rare, but nevertheless relevant cause of adult-onset axonal and painful neuropathy. ACMG (American College of Medical Genetics and genomics) criteria should be carefully applied in variant interpretation, with special attention to premature termination codon-introducing variants and their location within the gene.

The genetic landscape of axonal neuropathies in the middle-aged and elderly: Focus on MME

OBJECTIVE

To test the hypothesis that monogenic neuropathies such as Charcot-Marie-Tooth disease (CMT) contribute to frequent but often unexplained neuropathies in the elderly, we performed genetic analysis of 230 patients with unexplained axonal neuropathies and disease onset ≥35 years.

METHODS

We recruited patients, collected clinical data, and conducted whole-exome sequencing (WES; n = 126) and MME single-gene sequencing (n = 104). We further queried WES repositories for MME variants and measured blood levels of the MME-encoded protein neprilysin.

RESULTS

In the WES cohort, the overall detection rate for assumed disease-causing variants in genes for CMT or other conditions associated with neuropathies was 18.3% (familial cases 26.4%, apparently sporadic cases 12.3%). MME was most frequently involved and accounted for 34.8% of genetically solved cases. The relevance of MME for late-onset neuropathies was further supported by detection of a comparable proportion of cases in an independent patient sample, preponderance of MME variants among patients compared to population frequencies, retrieval of additional late-onset neuropathy patients with MME variants from WES repositories, and low neprilysin levels in patients' blood samples. Transmission of MME variants was often consistent with an incompletely penetrant autosomal-dominant trait and less frequently with autosomal-recessive inheritance.

CONCLUSIONS

A detectable fraction of unexplained late-onset axonal neuropathies is genetically determined, by variants in either CMT genes or genes involved in other conditions that affect the peripheral nerves and can mimic a CMT phenotype. MME variants can act as completely penetrant recessive alleles but also confer dominantly inherited susceptibility to axonal neuropathies in an aging population.

Cracking the Monoubiquitin Code of Genetic Diseases

Ubiquitination is a versatile and dynamic post-translational modification in which single ubiquitin molecules or polyubiquitin chains are attached to target proteins, giving rise to mono- or poly-ubiquitination, respectively. The majority of research in the ubiquitin field focused on degradative polyubiquitination, whereas more recent studies uncovered the role of single ubiquitin modification in important physiological processes. Monoubiquitination can modulate the stability, subcellular localization, binding properties, and activity of the target proteins. Understanding the function of monoubiquitination in normal physiology and pathology has important therapeutic implications, as alterations in the monoubiquitin pathway are found in a broad range of genetic diseases. This review highlights a link between monoubiquitin signaling and the pathogenesis of genetic disorders.

The Multifaceted Roles of the Tumor Susceptibility Gene 101 (TSG101) in Normal Development and Disease

The multidomain protein encoded by the Tumor Susceptibility Gene 101 (TSG101) is ubiquitously expressed and is suggested to function in diverse intracellular processes. In this review, we provide a succinct overview of the main structural features of the protein and their suggested roles in molecular and cellular functions. We then summarize, in more detail, key findings from studies using genetically engineered animal models that demonstrate essential functions of TSG101 in cell proliferation and survival, normal tissue homeostasis, and tumorigenesis. Despite studies on cell lines that provide insight into the molecular underpinnings by which TSG101 might function as a negative growth regulator, a biologically significant role of TSG101 as a tumor suppressor has yet to be confirmed using genuine in vivo cancer models. More recent observations from several cancer research teams suggest that TSG101 might function as an oncoprotein. A potential role of post-translational mechanisms that control the expression of the TSG101 protein in cancer is being discussed. In the final section of the review, we summarize critical issues that need to be addressed to gain a better understanding of biologically significant roles of TSG101 in cancer.

Ubiquitin ligase LRSAM1 suppresses neurodegenerative diseases linked aberrant proteins induced cell death

Accumulation of aberrant misfolded proteins is a major hallmark of several neurodegenerative diseases. Intracellular accumulations of such abnormal proteins are selectively cleared by the ubiquitin-proteasome system (UPS). But how the failure of misfolded protein degradation cause proteinopathies is still an unanswered question?. Previous studies have suggested that few selective quality control (QC) E3 ubiquitin ligase from the UPS can selectively target insoluble aggregated proteins for their intracellular degradation. Few reports suggest that lack or aberrant functions of QC E3 ubiquitin ligases can be a possible causative factor of neurodegeneration and aging. Earlier findings indicated that leucine-rich repeat and sterile alpha motif containing-1 (LRSAM1) is associated with Charcot-Marie-Tooth Type 2P (CMT2P) disease in which loss of LRSAM1 function sensitizes peripheral axons for degeneration. Here, our current study for the first time demonstrates that E3 ubiquitin Ligase LRSAM1 is a really interesting new gene (RING) class protein which suppresses the accumulation of misfolded protein aggregates and also alleviates their deleterious cytotoxic effects. We have also observed that LRSAM1 expression is elevated under neurodegenerative stress conditions, and partial depletion of LRSAM1 endogenous levels aggravates mitochondrial abnormalities and severely affects cell survival during proteotoxic insults. Overall, our current finding indicates that LRSAM1 can alleviate cytotoxic insults mediated by a variety of neurodegeneration linked proteotoxic stress events, and most likely LRSAM1 interplay a significant role in between different components of cellular protein quality control mechanism. This study will also allow us to better comprehend the problem of proteinopathies linked with aberrant protein accumulation and open new possibilities to better elucidate the molecular mechanisms involved in the pathologies of neurodegeneration and aging.

LRSAM1 variants and founder effect in French families with ataxic form of Charcot-Marie-Tooth type 2

Currently only 25-30% of patients with axonal forms of Charcot-Marie-Tooth disease (CMT) receive a genetic diagnosis. We aimed to identify the causative gene of CMT type 2 in 8 non-related French families with a distinct clinical phenotype. We collected clinical, electrophysiological, and laboratory findings and performed genetic analyses in four different French laboratories. Seventy-two patients with autosomal dominant inheritance were identified. The disease usually started in the fourth decade and the clinical picture was dominated by sensory ataxia (80%), neuropathic pain (38%), and length-dependent sensory loss to all modalities. Electrophysiological studies showed a primarily axonal neuropathy, with possible isolated sensory involvement in milder phenotypes. Disease severity varied greatly but the clinical course was generally mild. We identified 2 novel variants in LRSAM1 gene: a deletion of 4 amino acids, p.(Gln698_Gln701del), was found in 7 families and a duplication of a neighboring region of 10 amino acids, p.(Pro702_Gln711dup), in the remaining family. A common haplotype of ~450 kb suggesting a founder effect was noted around LRSAM1 in 4 families carrying the first variant. LRSAM1 gene encodes for an E3 ubiquitin ligase important for neural functioning. Our results confirm the localization of variants in its catalytic C-terminal RING domain and broaden the phenotypic spectrum of LRSAM1-related neuropathies, including painful and predominantly sensory ataxic forms.

LRSAM1 E3 ubiquitin ligase: molecular neurobiological perspectives linked with brain diseases

Cellular protein quality control (PQC) plays a significant role in the maintenance of cellular homeostasis. Failure of PQC mechanism may lead to various neurodegenerative diseases due to accumulation of aberrant proteins. To avoid such fatal neuronal conditions PQC employs autophagy and ubiquitin proteasome system (UPS) to degrade misfolded proteins. Few quality control (QC) E3 ubiquitin ligases interplay an important role to specifically recognize misfolded proteins for their intracellular degradation. Leucine-rich repeat and sterile alpha motif-containing 1 (LRSAM1) is a really interesting new gene (RING) class protein that possesses E3 ubiquitin ligase activity with promising applications in PQC. LRSAM1 is also known as RING finger leucine repeat rich (RIFLE) or TSG 101-associated ligase (TAL). LRSAM1 has various cellular functions as it modulates the protein aggregation, endosomal sorting machinery and virus egress from the cells. Thus, this makes LRSAM1 interesting to study not only in protein conformational disorders such as neurodegeneration but also in immunological and other cancerous disorders. Furthermore, LRSAM1 interacts with both cellular protein degradation machineries and hence it can participate in maintenance of overall cellular proteostasis. Still, more research work on the quality control molecular functions of LRSAM1 is needed to comprehend its roles in various protein aggregatory diseases. Earlier findings suggest that in a mouse model of Charcot-Marie-Tooth (CMT) disease, lack of LRSAM1 functions sensitizes peripheral axons to degeneration. It has been observed that in CMT the patients retain dominant and recessive mutations of LRSAM1 gene, which encodes most likely a defective protein. However, still the comprehensive molecular pathomechanism of LRSAM1 in neuronal functions and neurodegenerative diseases is not known. The current article systematically represents the molecular functions, nature and detailed characterization of LRSAM1 E3 ubiquitin ligase. Here, we review emerging molecular mechanisms of LRSAM1 linked with neurobiological functions, with a clear focus on the mechanism of neurodegeneration and also on other diseases. Better understanding of LRSAM1 neurobiological and intracellular functions may contribute to develop promising novel therapeutic approaches, which can also propose new lines of molecular beneficial targets for various neurodegenerative diseases.

Downregulation of microRNA-431-5p promotes enteric neural crest cell proliferation via targeting LRSAM1 in Hirschsprung's disease

BACKGROUND

Hirschsprung's disease (HSCR) is characterized by missing of enteric neurons in the terminal areas of the whole gut, which is causally related to poor proliferation of enteric neural crest cells (ENCCs). Our aim is to explore how miR-431-5p interacts with its target gene in regulation of proliferation of ENCCs in HSCR.

METHODS

Mouse model of HSCR was established by Benzalkonium chloride (BAC) treatment. Quantitative Real-time PCR and western blotting were performed to determine the miR-431-5p and the LRSAM1 expression in colon tissues of the HSCR group (n = 8) and the control group (n = 8) and in ENCCs isolated from colon tissues. CCK-8 assay was performed to detect the proliferation of ENCCs of HSCR. ENCCs after transfection with miR-431-5p mimics or miR-431-5p inhibitor. Luciferase reporter assay was conducted to clarify the connections between miR-431-5p and LRSAM1.

RESULTS

Upregulation of miR-431-5p and downregulation of LRSAM1 were found in ENCCs of HSCR. Downregulation of miR-431-5p could promote cell proliferation of ENCCs. LRSAM1 was proved to be the target gene of miR-431-5p by luciferase reporter assay. Moreover, proliferation of ENCCs was increased in the miR-431-5p inhibitor group and was suppressed after knocking down LRSAM1.

CONCLUSION

Downregulation of miR-431-5p promoted proliferation of ENCCs via targeting LRSAM1, which provides an innovative and candidate target for treatment of HSCR.

Deregulation of LRSAM1 expression impairs the levels of TSG101, UBE2N, VPS28, MDM2 and EGFR

CMT is the most common hereditary neuromuscular disorder of the peripheral nervous system with a prevalence of 1/2500 individuals and it is caused by mutations in more than 80 genes. LRSAM1, a RING finger ubiquitin ligase also known as TSG101-associated ligase (TAL), has been associated with Charcot-Marie-Tooth disease type 2P (CMT2P) and to date eight causative mutations have been identified. Little is currently known on the pathogenetic mechanisms that lead to the disease. We investigated the effect of LRSAM1 deregulation on possible LRSAM1 interacting molecules in cell based models. Possible LRSAM1 interacting molecules were identified using protein-protein interaction databases and literature data. Expression analysis of these molecules was performed in both CMT2P patient and control lymphoblastoid cell lines as well as in LRSAM1 and TSG101 downregulated SH-SY5Y cells.TSG101, UBE2N, VPS28, EGFR and MDM2 levels were significantly decreased in the CMT2P patient lymphoblastoid cell line as well as in LRSAM1 downregulated cells. TSG101 downregulation had a significant effect only on the expression of VPS28 and MDM2 and it did not affect the levels of LRSAM1. This study confirms that LRSAM1 is a regulator of TSG101 expression. Furthermore, deregulation of LRSAM1 significantly affects the levels of UBE2N, VPS28, EGFR and MDM2.

LRSAM1 Depletion Affects Neuroblastoma SH-SY5Y Cell Growth and Morphology: The LRSAM1 c.2047-1G>A Loss-of-Function Variant Fails to Rescue The Phenotype

Objective

Deleterious variants in LRSAM1, a RING finger ubiquitin ligase which is also known as TSG101-associated ligase (TAL), have recently been associated with Charcot-Marie-Tooth disease type 2P (CMT2P). The mechanism by which mutant LRSAM1 contributes to the development of neuropathy is currently unclear. The aim of this study was to induce LRSAM1 deficiency in a neuronal cell model, observe its effect on cell growth and morphology and attempt to rescue the phenotype with ancestral and mutant LRSAM1 transfections.

Materials and Methods

In this experimental study, we investigated the effect of LRSAM1 downregulation on neuroblastoma SH-SY5Y cells by siRNA technology where cells were transfected with siRNA against LRSAM1. The effects on the expression levels of TSG101, the only currently known LRSAM1 interacting molecule, were also examined. An equal dosage of ancestral or mutant LRSAM1 construct was transfected in LRSAM1-downregulated cells to investigate its effect on the phenotype of the cells and whether cell proliferation and morphology could be rescued.

Results

A significant reduction in TSG101 levels was observed with the downregulation of LRSAM1. In addition, LRSAM1 knockdown significantly decreased the growth rate of SH-SY5Y cells which is caused by a decrease in cell proliferation. An effect on cell morphology was also observed. Furthermore, we overexpressed the ancestral and the c.2047-1G>A mutant LRSAM1 in knocked down cells. Ancestral LRSAM1 recovered cell proliferation and partly the morphology, however, the c.2047-1G>A mutant did not recover cell proliferation and further aggravated the observed changes in cell morphology.

Conclusion

Our findings suggest that depletion of LRSAM1 affects neuroblastoma cells growth and morphology and that overexpression of the c.2047-1G>A mutant form, unlike the ancestral LRSAM1, fails to rescue the phenotype.

Charcot-Marie-Tooth disease: genetic subtypes in the Sardinian population

Charcot-Marie-Tooth disease (CMT) is characterised by great variability of genetic subtypes. This study aimed to assess the genetic subtypes of CMT disease in the Sardinian population. Genetic screening was performed for CMT cases (CMT1, CMT2, and hereditary neuropathy with susceptibility to pressure palsies [HNPP]). A total of 1,043 subjects (119 index cases) were evaluated. In CMT1 index cases (69/119; 58%), PMP22 duplication at 17p11.2 was the most frequent genetic diagnosis (60/69; 87%), followed by mutations in the GJB1 gene (5/69; 7.2%), in the SH3TC2 gene (3/69; 4.4%) and PMP22 Gly107Val point mutation (1/69; 1.4%). The CMT2 group (24/119; 20.1%) comprised 10/24 (41.6%) patients carrying MPZ gene Ser44Phe mutation, 6/24 (25%) with mutations in MFN2 and HSPB1, and 1/24 (4.2%) in GJB1 and LRSAM1. In the HNPP group (26/119; 21.9%), the majority of patients reported the PMP22 deletion (25/26; 96.2%). Further studies are needed to comprehend the overall picture of the disease in Mediterranean area.

A novel missense mutation of CMT2P alters transcription machinery

OBJECTIVE

Charcot-Marie-Tooth type 2P (CMT2P) has been associated with frameshift mutations in the RING domain of LRSAM1 (an E3 ligase). This study describes families with a novel missense mutation of LRSAM1 gene and explores pathogenic mechanisms of CMT2P.

METHODS

Patients with CMT2P were characterized clinically, electrophysiologically, and genetically. A neuronal model with the LRSAM1 mutation was created using CRISPR/Cas9 technology. The neuronal cell line along with fibroblasts isolated from the patients was used to study RNA-binding proteins.

RESULTS

This American family with dominantly inherited axonal polyneuropathy reveals a phenotype similar to those in previously reported non-US families. The affected members in our family cosegregated with a novel missense mutation Cys694Arg that alters a highly conserved cysteine in the RING domain. This mutation leads to axonal degeneration in the in vitro neuronal cell line. Moreover, using protein mass spectrometry, we identified a group of RNA-binding proteins (including FUS, a protein critically involved in motor neuron degeneration) that interacted with LRSAM1. The interactions were disrupted by the Cys694Arg mutation, which resulted in reduction of intranuclear RNA-binding proteins.

INTERPRETATION

Our findings suggest that the mutant LRSAM1 may aberrantly affect the formation of transcription machinery. Given that a similar mechanism has been reported in motor neuron degeneration of amyotrophic lateral sclerosis, abnormalities of RNA/RNA-binding protein complex may play a role in the neuronal degeneration of CMT2P. Ann Neurol 2016;80:834-845.

A LRSAM1 mutation links Charcot-Marie-Tooth type 2 to Parkinson's disease

LRSAM1 mutations have been found in recessive and dominant forms of Charcot–Marie–Tooth disease. Within one generation of the original Dutch family in which the dominant LRSAM1 mutation was identified, three of the five affected family members have developed Parkinson's disease between ages 50 and 65 years, many years after neuropathy onset. We speculate that this late‐onset parkinsonism is part of the LRSAM1 phenotype, thus associating a hitherto peripheral nerve disease with a central nervous system phenotype. How the mutated Lrsam1 protein, which normally has E3 ubiquitin ligase activity and is expressed in the nervous system, impacts on substantia nigra neurons is unclear.

PHF23 (plant homeodomain finger protein 23) negatively regulates cell autophagy by promoting ubiquitination and degradation of E3 ligase LRSAM1

Autophagy is a multistep process that involves the degradation and digestion of intracellular components by the lysosome. It has been proved that many core autophagy-related molecules participate in this event. However, new component proteins that regulate autophagy are still being discovered. At present, we report PHF23 (PHD finger protein 23) with a PHD-like zinc finger domain that can negatively regulate autophagy. Data from experiments indicated that the overexpression of PHF23 impaired autophagy, as characterized by decreased levels of LC3B-II and weakened degradation of endogenous and exogenous autophagic substrates. Conversely, knockdown of PHF23 resulted in opposite effects. Molecular mechanism studies suggested that PHF23 interacts with LRSAM1, which is an E3 ligase key for ubiquitin-dependent autophagy against invading bacteria. PHF23 promotes the ubiquitination and proteasome degradation of LRSAM1. We also show that the PHD finger of PHF23 is a functional domain needed for the interaction with LRSAM1. Altogether, our results indicate that PHF23 is a negative regulator associated in autophagy via the LRSAM1 signaling pathway. The physical and functional connection between the PHF23 and LRSAM1 needs further investigation.

A novel mutation in LRSAM1 causes axonal Charcot-Marie-Tooth disease with dominant inheritance

Background

Charcot-Marie-Tooth disease (CMT) refers to a heterogeneous group of genetic motor and sensory neuropathies. According to the primary site of damage, a distinction is made between demyelinating and axonal forms (CMT1 and 2, respectively, when inherited as an autosomal dominant trait). Leucine-rich repeat and sterile alpha motif-containing protein 1 (LRSAM1) is a ubiquitin-protein ligase with a role in sorting internalised cell-surface receptor proteins. So far, mutations in the LRSAM1 gene have been shown to cause axonal CMT in three different families and can confer either dominant or recessive transmission of the disease.

Case presentation

We have identified a novel mutation in LRSAM1 in a small family with dominant axonal CMT. Electrophysiological studies show evidence of a sensory axonal neuropathy and are interesting in so far as giant motor unit action potentials (MUAPs) are present on needle electromyography (EMG), while motor nerve conduction studies including compound motor action potential (CMAP) amplitudes are completely normal. The underlying mutation c.2046+1G >T results in the loss of a splice donor site and the inclusion of 63 additional base pairs of intronic DNA into the aberrantly spliced transcript. This disrupts the catalytically active RING (Really Interesting New Gene) domain of LRSAM1.

Conclusions

Our findings suggest that, beyond the typical length-dependent degeneration of motor axons, damage of cell bodies in the anterior horn might play a role in LRSAM1-associated neuropathies. Moreover, in conjunction with other data in the literature, our results support a model, by which disruption of the C-terminal RING domain confers dominant negative properties to LRSAM1.

Advances in the molecular diagnosis of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth (CMT) disease or hereditary motor and sensory neuropathy is the most common inherited neuromuscular disorder affecting at least 1 in 2500. CMT disease is pathologically and genetically heterogeneous and is characterized by a variable age of onset, slowly progressive weakness and muscle atrophy, starting in the lower limbs and subsequently affecting the upper extremities. Symptoms are usually slowly progressive, especially for the classic and late-onset phenotypes, but can be rather severe in early-onset forms. CMT is grouped into demyelinating, axonal and intermediate forms, based on electrophysiological and pathological findings. The demyelinating types are characterized by severely reduced motor nerve conduction velocities (MNCVs) and mainly by myelin abnormalities. The axonal types are characterized by normal or slightly reduced MNCVs and mainly axonal abnormalities. The intermediate types are characterized by MNCVs between 25 m/s and 45 m/s and they have features of both demyelination and axonopathy. Inheritance can be autosomal dominant, X-linked, or autosomal recessive. Mutations in more than 30 genes have been associated with the different forms of CMT, leading to major advancements in molecular diagnostics of the disease, as well as in the understanding of pathogenetic mechanisms. This editorial aims to provide an account that is practicable and efficient on the current molecular diagnostic procedures for CMT, in correlation with the clinical, pathological and electrophysiological findings. The most frequent causative mutations of CMT will also be outlined.

Loss of the E3 ubiquitin ligase LRSAM1 sensitizes peripheral axons to degeneration in a mouse model of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous condition characterized by peripheral axon degeneration with subsequent motor and sensory deficits. Several CMT gene products function in endosomal sorting and trafficking to the lysosome, suggesting that defects in this cellular pathway might present a common pathogenic mechanism for these conditions. LRSAM1 is an E3 ubiquitin ligase that is implicated in this process, and mutations in LRSAM1 have recently been shown to cause CMT. We have generated mouse mutations in Lrsam1 to create an animal model of this form of CMT (CMT2P). Mouse Lrsam1 is abundantly expressed in the motor and sensory neurons of the peripheral nervous system. Both homozygous and heterozygous mice have largely normal neuromuscular performance and only a very mild neuropathy phenotype with age. However, Lrsam1 mutant mice are more sensitive to challenge with acrylamide, a neurotoxic agent that causes axon degeneration, indicating that the axons in the mutant mice are indeed compromised. In transfected cells, LRSAM1 primarily localizes in a perinuclear compartment immediately beyond the Golgi and shows little colocalization with components of the endosome to lysosome trafficking pathway, suggesting that other cellular mechanisms also merit consideration.