A nullimorphic ERLIN2 mutation defines a complicated hereditary spastic paraplegia locus (SPG18)

Review of the Genetic Spectrum of Hereditary Spastic Paraplegias in the Middle East and North Africa Regions

Background and Objectives

Hereditary spastic paraplegias (HSPs) are inherited neurodegenerative disorders, and their classification is based on inheritance mode, allelic variants, and clinical presentation. Despite global occurrence, research, especially in the Middle East and North Africa (MENA) regions, is lacking, underscoring the need for further investigation. The objective of this study was to improve the regions' clinical practice and public health, and this study aims to gather data on HSP prevalence, pathogenic variants, and patient characteristics in MENA countries.

Methods

A systematic literature review encompassing PubMed, MEDLINE, and Google Scholar was conducted. Quality assessment was performed on the included studies. Data extraction and analysis provided insights into HSP's current status in the region.

Results

Iran had the highest number of patients with HSP, followed by Tunisia. SPG11 (19.8%), FA2H (8.5%), and ZFYVE26 (7.7%) were the most frequently found genes in the cases. Autosomal recessive HSP with thin corpus callosum was common among the affected patients, with SPG11 identified as the primary cause.

Discussion

Our analysis highlights genetic diversity and regional prevalence variations. Despite limited research in MENA countries, we stress the importance of further investigation to address gaps in understanding and improve patient care and public health initiatives.

Biallelic variants in SREBF2 cause autosomal recessive spastic paraplegia

Hereditary spastic paraplegias (HSPs) refer to a genetically and clinically heterogeneous group of neurodegenerative disorders characterized by the degeneration of motor neurons. To date, a significant number of patients still have not received a definite genetic diagnosis. Therefore, identifying unreported causative genes continues to be of great importance. Here, we perform whole exome sequencing in a cohort of Chinese HSP patients. Three homozygous variants (p.L604W, p.S517F, and p.T984A) within the sterol regulatory element-binding factor 2 (SREBF2) gene are identified in one autosomal recessive family and two sporadic patients, respectively. Co-segregation is confirmed by Sanger sequencing in all available members. The three variants are rare in the public or in-house database and are predicted to be damaging. The biological impacts of variants in SREBF2 are examined by functional experiments in patient-derived fibroblasts and Drosophila. We find that the variants upregulate cellular cholesterol due to the overactivation of SREBP2, eventually impairing the autophagosomal and lysosomal functions. The overexpression of the mature form of SREBP2 leads to locomotion defects in Drosophila. Our findings identify SREBF2 as a causative gene for HSP and highlight the impairment of cholesterol as a critical pathway for HSP.

Association of novel ERLIN2 gene variants with hereditary spastic paraplegia

Two ERLIN2 variants (NM_007175.8:c.660delA and NM_007175.8:c.869C>T) were detected in a Spanish patient with hereditary spastic paraplegia via whole-exome sequencing and software-based pathogenic variant selection. Segregation analysis revealed that the patient's two affected siblings carried both variants, whereas their offspring, carrying only one variant, were asymptomatic, indicating the autosomal recessive nature of the disease. These findings suggest that the identified variants can be classified as pathogenic when they are present as compound heterozygous variants.

Biallelic variants in ERLIN1: a series of 13 individuals with spastic paraparesis

Biallelic variants in the ERLIN1 gene were recently reported as the cause of two motor neuron degeneration diseases, SPG62 and a recessive form of amyotrophic lateral sclerosis. However, only 12 individuals from five pedigrees have been identified so far. Thus, the description of the disease remains limited. Following the discovery of a homozygous pathogenic variant in a girl with SPG62, presenting with intellectual disability, and epilepsy, we gathered the largest series of SPG62 cases reported so far (13 individuals) to better understand the phenotype associated with ERLIN1. We collected molecular and clinical data for 13 individuals from six families with ERLIN1 biallelic variants. We performed RNA-seq analyses to characterize intronic variants and used Alphafold and a transcripts database to characterize the molecular consequences of the variants. We identified three new variants suspected to alter the bell-shaped ring formed by the ERLIN1/ERLIN2 complex. Affected individuals had childhood-onset paraparesis with slow progression. Six individuals presented with gait ataxia and three had superficial sensory loss. Aside from our proband, none had intellectual disability or epilepsy. Biallelic pathogenic ERLIN1 variants induce a rare, predominantly pure, spastic paraparesis, with possible cerebellar and peripheral nerve involvement.

Expanding SPG18 clinical spectrum: autosomal dominant mutation causes complicated hereditary spastic paraplegia in a large family

BACKGROUND

SPG18 is caused by mutations in the endoplasmic reticulum lipid raft associated 2 (ERLIN2) gene. Autosomal recessive (AR) mutations are usually associated with complicated hereditary spastic paraplegia (HSP), while autosomal dominant (AD) mutations use to cause pure SPG18.

AIM

To define the variegate clinical spectrum of the SPG18 and to evaluate a dominant negative effect of erlin2 (encoded by ERLIN2) on oligomerization as causing differences between AR and AD phenotypes.

METHODS

In a four-generation pedigree with an AD pattern, a spastic paraplegia multigene panel test was performed. Oligomerization of erlin2 was analyzed with velocity gradient assay in fibroblasts of the proband and healthy subjects.

RESULTS

Despite the common p.V168M mutation identified in ERLIN2, a phenoconversion to amyotrophic lateral sclerosis (ALS) was observed in the second generation, pure HSP in the third generation, and a complicated form with psychomotor delay and epilepsy in the fourth generation. Erlin2 oligomerization was found to be normal.

DISCUSSION

We report the first AD SPG18 family with a complicated phenotype, and we ruled out a dominant negative effect of V168M on erlin2 oligomerization. Therefore, our data do not support the hypothesis of a relationship between the mode of inheritance and the phenotype, but confirm the multifaceted nature of SPG18 on both genetic and clinical point of view. Clinicians should be aware of the importance of conducting an in-depth clinical evaluation to unmask all the possible manifestations associated to an only apparently pure SPG18 phenotype. We confirm the genotype-phenotype correlation between V168M and ALS emphasizing the value of close follow-up.

Hereditary spastic paraparesis type 18 (SPG18): new ERLIN2 variants in a series of Italian patients, shedding light upon genetic and phenotypic variability

INTRODUCTION

Hereditary spastic paraparesis (HSP) is a group of central nervous system diseases primarily affecting the spinal upper motor neurons, with different inheritance patterns and phenotypes. SPG18 is a rare, early-onset, complicated HSP, first reported as linked to biallelic ERLIN2 mutations. Recent cases of late-onset, pure HSP with monoallelic ERLIN2 variants prompt inquiries into the zygosity of such genetic conditions. The observed relationship between phenotype and mode of inheritance suggests a potential dominant negative effect of mutated ERLIN2 protein, potentially resulting in a milder phenotype. This speculation suggests that a wider range of HSP genes could be linked to various inheritance patterns.

PURPOSE AND BACKGROUND

With documented cases of HSP loci exhibiting both dominant and recessive patterns, this study emphasizes that the concept of zygosity is no longer a limiting factor in the establishment of molecular diagnoses for HSP. Recent cases have demonstrated phenoconversion in SPG18, from HSP to an amyotrophic lateral sclerosis (ALS)-like syndrome.

METHODS AND RESULTS

This report highlights two cases out of five exhibiting HSP-ALS phenoconversion, discussing an observed prevalence in autosomal dominant SPG18. Additionally, the study emphasizes the relatively high incidence of the c.502G>A variant in monoallelic SPG18 cases. This mutation appears to be particularly common in cases of HSPALS phenoconversion, indicating its potential role as a hotspot for a distinctive SPG18 phenotype with an ALS-like syndrome.

CONCLUSIONS

Clinicians need to be aware that patients with HSP may show ALS signs and symptoms. On the other hand, HSP panels must be included in genetic testing methods for instances of familial ALS.

ERLIN1/2 scaffolds bridge TMUB1 and RNF170 and restrict cholesterol esterification to regulate the secretory pathway

Complexes of ERLIN1 and ERLIN2 (ER lipid raft-associated 1 and 2) form large ring-like cup-shaped structures on the endoplasmic reticulum (ER) membrane and serve as platforms to bind cholesterol and E3 ubiquitin ligases, potentially defining functional nanodomains. Here, we show that ERLIN scaffolds mediate the interaction between the full-length isoform of TMUB1 (transmembrane and ubiquitin-like domain-containing 1) and RNF170 (RING finger protein 170). We identify a luminal N-terminal conserved region in TMUB1 and RNF170, which is required for this interaction. Three-dimensional modelling shows that this conserved motif binds the stomatin/prohibitin/flotillin/HflKC domain of two adjacent ERLIN subunits at different interfaces. Protein variants that preclude these interactions have been previously linked to hereditary spastic paraplegia. Using omics-based approaches in combination with phenotypic characterization of HeLa cells lacking both ERLINs, we demonstrate a role of ERLIN scaffolds in limiting cholesterol esterification, thereby favouring cholesterol transport from the ER to the Golgi apparatus and regulating Golgi morphology and the secretory pathway.

Novel ERLIN2 variant expands the phenotype of Spastic Paraplegia 18

BACKGROUND

The Brazilian Northeast region is notable for its high prevalence of consanguineous marriages and isolated populations, which has led to a significant prevalence of rare genetic disorders. This study describes the clinical presentation of four affected individuals from the same family, comprising two siblings and their cousins, with ages ranging from 11 to 20 years.

METHODS

In a small and isolated community in Northeastern Brazil, affected individuals initially underwent a clinical assessment. Subsequently, written consent was obtained from their legal guardians, and an extensive clinical evaluation was conducted at a medical genetics center. Family data provided the basis for constructing the pedigree, and biological samples (blood or oral swabs) were collected from both affected and unaffected family members. Following informed consent from one patient, Whole Exome Sequencing (WES) was carried out, encompassing exome sequencing, assembly, genotyping, and annotation. A potentially deleterious variant was then singled out for further segregation analysis through Sanger Sequencing, involving both the proband and select family members.

RESULTS AND CONCLUSION

These individuals exhibit severe neurodevelopmental delays, encompassing symptoms such as spastic paraplegia, neuropathy, intellectual impairments, and language challenges. Through next-generation sequencing (NGS) techniques, a previously unreported homozygous variant within the ERLIN2 gene linked to spastic paraplegia 18 (SPG18) was identified across all four patients. Also, all patients displayed childhood cataract, expanding the known clinical spectrum of SPG18.

A novel autosomal dominant ERLIN2 variant activates endoplasmic reticulum stress in a Chinese HSP family

OBJECTIVE

Hereditary spastic paraplegia (HSP) has been reported rarely because of a monoallelic variant in ERLIN2. The present study aimed at describing a novel autosomal dominant ERLIN2 pedigree in a Chinese family and exploring the possible mechanism of HSP caused by ERLIN2 variants.

METHODS

The proband and his family underwent a comprehensive medical history inquiry and neurological examinations. Whole-exome sequencing was performed on the proband, and Sanger sequencing was performed on some family members. HeLa cell lines and mouse primary cortical neurons were used for immunofluorescence (IF) and reverse transcription-PCR (RT-PCR).

RESULTS

Seven patients were clinically diagnosed with pure spastic paraplegia in four consecutive generations with the autosomal dominant inheritance model. All patients presented juvenile-adolescent onset and gradually worsening pure HSP phenotype. Whole-exome sequencing of the proband and Sanger sequencing of all available family members identified a novel heterozygous c.212 T>C (p.V71A) variant in exon 8 of the ERLIN2 gene. The c.212 T>C demonstrated a high pathogenic effect score through functional prediction. RT-PCR and IF analysis of overexpressed V71A revealed an altered ER morphology and increased XBP-1S mRNA levels, suggesting the activation of ER stress. Overexpression of V71A in primary cultured cortical neurons promoted axon growth.

INTERPRETATION

The novel c.212 T>C heterozygous variant in human ERLIN2 caused pure HSP. Moreover, c.212 T>C heterozygous variant in ERLIN2 increased ER stress and affected axonal development.

AMFR dysfunction causes autosomal recessive spastic paraplegia in human that is amenable to statin treatment in a preclinical model

Hereditary spastic paraplegias (HSP) are rare, inherited neurodegenerative or neurodevelopmental disorders that mainly present with lower limb spasticity and muscle weakness due to motor neuron dysfunction. Whole genome sequencing identified bi-allelic truncating variants in AMFR, encoding a RING-H2 finger E3 ubiquitin ligase anchored at the membrane of the endoplasmic reticulum (ER), in two previously genetically unexplained HSP-affected siblings. Subsequently, international collaboration recognized additional HSP-affected individuals with similar bi-allelic truncating AMFR variants, resulting in a cohort of 20 individuals from 8 unrelated, consanguineous families. Variants segregated with a phenotype of mainly pure but also complex HSP consisting of global developmental delay, mild intellectual disability, motor dysfunction, and progressive spasticity. Patient-derived fibroblasts, neural stem cells (NSCs), and in vivo zebrafish modeling were used to investigate pathomechanisms, including initial preclinical therapy assessment. The absence of AMFR disturbs lipid homeostasis, causing lipid droplet accumulation in NSCs and patient-derived fibroblasts which is rescued upon AMFR re-expression. Electron microscopy indicates ER morphology alterations in the absence of AMFR. Similar findings are seen in amfra-/- zebrafish larvae, in addition to altered touch-evoked escape response and defects in motor neuron branching, phenocopying the HSP observed in patients. Interestingly, administration of FDA-approved statins improves touch-evoked escape response and motor neuron branching defects in amfra-/- zebrafish larvae, suggesting potential therapeutic implications. Our genetic and functional studies identify bi-allelic truncating variants in AMFR as a cause of a novel autosomal recessive HSP by altering lipid metabolism, which may potentially be therapeutically modulated using precision medicine with statins.

Hereditary spastic paraplegias proteome: common pathways and pathogenetic mechanisms

INTRODUCTION

Hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative disorders characterized by progressive spasticity and weakness of the lower limbs. These conditions are caused by lesions in the neuronal pyramidal tract and exhibit clinical and genetic variability. Ongoing research focuses on understanding the underlying mechanisms of HSP onset, which ultimately lead to neuronal degeneration. Key molecular mechanisms involved include axonal transport, cytoskeleton dynamics, myelination abnormalities, membrane trafficking, organelle morphogenesis, ER homeostasis, mitochondrial dysfunction, and autophagy deregulation.

AREAS COVERED

This review aims to provide an overview of the shared pathogenetic mechanisms in various forms of HSPs. By examining disease-causing gene products and their associated functional pathways, this understanding could lead to the discovery of new therapeutic targets and the development of treatments to modify the progression of the disease.

EXPERT OPINION

Investigating gene functionality is crucial for identifying shared pathogenetic pathways underlying different HSP subtypes. Categorizing protein function and identifying pathways aids in finding biomarkers, predicting early onset, and guiding treatment for a better quality of life. Targeting shared mechanisms enables efficient and cost-effective therapies. Prospects involve identifying new disease-causing genes, refining molecular processes, and implementing findings in diagnosis, key for advancing HSP understanding and developing effective treatments.

Disruption of Intracellular Calcium Homeostasis Leads to ERLIN2-Linked Hereditary Spastic Paraplegia in Patient-Derived Stem Cell Models

Hereditary spastic paraplegia (HSP) is a category of neurodegenerative illnesses with significant clinical and genetic heterogeneity. Homozygous truncated variants of the ERLIN2 gene lead to HSP18 (MIM #611225). However, it is still unclear whether there is an autosomal dominant pathogenic pattern. The specific molecular mechanism needs to be investigated. We generated patient-derived iPSC models to study the mechanism of ERLIN2 heterogeneous variants leading to HSP. We identified a heterozygous missense variant p.Val71Ala of ERLIN2 in an HSP family. Based on IP-mass spectrometry, we found that the ERLIN2 heterozygous missense variant protein recruited the ubiquitin E3 ligase RNF213 to degrade IP3R1. The degradation of IP3R1 leads to the reduction of intracellular free calcium, which triggered endoplasmic reticulum (ER) stress-mediated apoptosis. Calcium homeostasis imbalance inhibited the MAPK signaling pathway that contributed to decreased cell proliferation. In summary, these results suggest that the autosomal dominant inheritance of heterozygous missense variants in ERLIN2 is a novel pathogenic mode of HSP. Furthermore, the disruption of intracellular calcium homeostasis is the pathological mechanism.

The hereditary spastic paraplegias

The hereditary spastic paraplegias (HSPs) are a group of more than 90 genetic disorders in which lower extremity spasticity and weakness are either the primary neurologic impairments ("uncomplicated HSP") or when accompanied by other neurologic deficits ("complicated HSP"), important features of the clinical syndrome. Various genetic types of HSP are inherited such as autosomal dominant, autosomal recessive, X-linked, and maternal (mitochondrial) traits. Symptoms that begin in early childhood may be nonprogressive and resemble spastic diplegic cerebral palsy. Symptoms that begin later, typically progress insidiously over a number of years. Genetic testing is able to confirm the diagnosis for many subjects. Insights from gene discovery indicate that abnormalities in diverse molecular processes underlie various forms of HSP, including disturbance in axon transport, endoplasmic reticulum morphogenesis, vesicle transport, lipid metabolism, and mitochondrial function. Pathologic studies in "uncomplicated" HSP have shown axon degeneration particularly involving the distal ends of corticospinal tracts and dorsal column fibers. Treatment is limited to symptom reduction including amelioration of spasticity, reducing urinary urgency, proactive physical therapy including strengthening, stretching, balance, and agility exercise.

The role of autophagy-lysosomal pathway in motor neuron diseases

Motor neuron diseases (MNDs) include a broad group of diseases in which neurodegeneration mainly affects upper and/or lower motor neurons (MNs). Although the involvement of specific MNs, symptoms, age of onset, and progression differ in MNDs, the main pathogenic mechanism common to most MNDs is represented by proteostasis alteration and proteotoxicity. This pathomechanism may be directly related to mutations in genes encoding proteins involved in the protein quality control system, particularly the autophagy-lysosomal pathway (ALP). Alternatively, proteostasis alteration can be caused by aberrant proteins that tend to misfold and to aggregate, two related processes that, over time, cannot be properly handled by the ALP. Here, we summarize the main ALP features, focusing on different routes utilized to deliver substrates to the lysosome and how the various ALP pathways intersect with the intracellular trafficking of membranes and vesicles. Next, we provide an overview of the mutated genes that have been found associated with MNDs, how these gene products are involved in different steps of ALP and related processes. Finally, we discuss how autophagy can be considered a valid therapeutic target for MNDs treatment focusing on traditional autophagy modulators and on emerging approaches to overcome their limitations.

A novel homozygous mutation in ERLIN1 gene causing spastic paraplegia 62 and literature review

Hereditary spastic paraplegia (HSP) is a group of genetic neurodegenerative disorders, which is characterized by the presence of progressive spasticity and weakness in bilateral lower limbs. Spastic paraplegia 62 (SPG62) caused by the endoplasmic reticulum lipid raft associated 1 (ERLIN1) gene mutation is a rare subtype of HSP. Herein, we report the case of the first Chinese SPG62 patient, explore the potential pathogenic mechanism and review ERLIN1-related HSP patients. A 23-year-old man had progressive difficulty in walking and gait abnormalities for more than 11 years. Physical examination showed slightly reduced muscle strength (5^-/5) and elevated muscle tone in the lower limbs and hyperreflexia in four limbs. Genetic analysis identified a novel splicing site mutation in ERLIN1 gene (c.504+1G > A), which was predicted to disturb the normal splicing process of mRNA by bioinformatic tools. Minigene experiment further confirmed the mutation c.504+1G > A could cause erroneous deletion of Exon 7 in the mRNA, which may change the conserved prohibitin (PHB) domain of erlin-1 and affect the function of erlin1/2 complex. Thus, we identified a pathogenic mutation of ERLIN1 splicing site causing delayed-onset pure HSP. This study widened the genetic and phenotypic spectrum of SPG62.

Hereditary spastic paraplegia: Genetic heterogeneity and common pathways

Hereditary Spastic Paraplegias (HSPs) are a heterogeneous group of disease, mainly characterized by progressive spasticity and weakness of the lower limbs resulting from distal degeneration of corticospinal tract axons. Although HSPs represent rare or ultra-rare conditions, with reported cases of mutated genes found in single families, overall, with 87 forms described, they are an important health and economic problem for society and patients. In fact, they are chronic and life-hindering conditions, still lacking a specific therapy. Notwithstanding the number of forms described, and 73 causative genes identified, overall, the molecular diagnostic rate varies among 29% to 61.8%, based on recent published analysis, suggesting that more genes are involved in HSP and/or that different molecular diagnostic approaches are necessary. The accumulating data in this field highlight several peculiar features of HSPs, such as genetic heterogeneity, the discovery that different mutations in a single gene can be transmitted in dominant and recessive trait in families and allelic heterogeneity, resulting in the involvement of HSP-genes in other conditions. Based on the observation of protein functions, the activity of many different proteins encoded by HSP-related genes converges into some distinct pathophysiological mechanisms. This suggests that common pathways could be a potential target for a therapy, possibly addressing several forms at once. Furthermore, the overlap of HSP genes with other neurological conditions can further expand this concept.

More autosomal dominant SPG18 cases than recessive? The first AD-SPG18 pedigree in Chinese and literature review

OBJECTIVE

Hereditary spastic paraplegia (HSP) due to ERLIN2 gene mutations was designated as spastic paraplegia 18 (SPG18). To date, SPG18 families/cases are still rarely reported. All early reported cases shared the autosomal recessive (AR) inheritance pattern. Over the past 3 years, autosomal dominant (AD) or sporadic SPG18 cases had been continuously reported. Here, we reported the clinical and genetic features of the first autosomal dominant SPG18 pedigree in Chinese.

METHODS

We conducted detailed medical history inquiry, neurological examinations of the proband and his family members, and charted the family tree. The proband underwent brain and cervical magnetic resonance imaging (MRI), electromyography (EMG), and whole exome sequencing. Sanger sequencing was performed to verify the genetic variation in the proband and some family members. A literature review of all reported SPG18 families/cases was carried out to summarize the clinical-genetic characteristics of SPG18 under different inheritance patterns.

RESULTS

Four patients were clinically diagnosed as chronic spastic paraplegia in three consecutive generations with the autosomal dominant inheritance model. All the patients presented juvenile-adolescent onset and gradually worsening pure HSP phenotype. Clinical phenotypes were consistent within the family. Whole exome sequencing in the proband identified a previously reported heterozygous c.502G > A (p.V168M) mutation in exon 8 of ERLIN2 gene. This mutation was cosegregated with the phenotype in the family and was classified as likely pathogenic according to American College of Medical Genetics and Genomics (ACMG) guidelines. To date, eight AR-SPG18 families, five AD-SPG18 families, and three sporadic cases had been reported. Clinical phenotype of AD-SPG18 was juvenile-adolescent onset pure HSP, while the phenotype of AR-SPG18 was mostly complicated HSP with earlier onset and more severe conditions. In rare cases, the initial spastic paraplegia could evolve to rapidly progressive amyotrophic lateral sclerosis (ALS).

CONCLUSIONS

We reported the first autosomal dominant SPG18 pedigree in Chinese Han population, which added more pathogenic evidence for V168M mutation. As more SPG18 cases reported, the essentials of SPG18 need to be updated in clinical practice. Special attentions should be given in gene test for upper motor neuron disorders in case of missing heterozygous mutations in ERLIN2.

Juvenile Amyotrophic Lateral Sclerosis: A Review

Juvenile amyotrophic lateral sclerosis (JALS) is a rare group of motor neuron disorders with gene association in 40% of cases. JALS is defined as onset before age 25. We conducted a literature review of JALS and gene mutations associated with JALS. Results of the literature review show that the most common gene mutations associated with JALS are FUS, SETX, and ALS2. In familial cases, the gene mutations are mostly inherited in an autosomal recessive pattern and mutations in SETX are inherited in an autosomal dominant fashion. Disease prognosis varies from rapidly progressive to an indolent course. Distinct clinical features may emerge with specific gene mutations in addition to the clinical finding of combined upper and lower motor neuron degeneration. In conclusion, patients presenting with combined upper and lower motor neuron disorders before age 25 should be carefully examined for genetic mutations. Hereditary patterns and coexisting features may be useful in determining prognosis.

Insights into Clinical, Genetic, and Pathological Aspects of Hereditary Spastic Paraplegias: A Comprehensive Overview

Hereditary spastic paraplegias (HSP) are a heterogeneous group of motor neurodegenerative disorders that have the core clinical presentation of pyramidal syndrome which starts typically in the lower limbs. They can present as pure or complex forms with all classical modes of monogenic inheritance reported. To date, there are more than 100 loci/88 spastic paraplegia genes (SPG) involved in the pathogenesis of HSP. New patterns of inheritance are being increasingly identified in this era of huge advances in genetic and functional studies. A wide range of clinical symptoms and signs are now reported to complicate HSP with increasing overall complexity of the clinical presentations considered as HSP. This is especially true with the emergence of multiple HSP phenotypes that are situated in the borderline zone with other neurogenetic disorders. The genetic diagnostic approaches and the utilized techniques leave a diagnostic gap of 25% in the best studies. In this review, we summarize the known types of HSP with special focus on those in which spasticity is the principal clinical phenotype ("SPGn" designation). We discuss their modes of inheritance, clinical phenotypes, underlying genetics, and molecular pathways, providing some observations about therapeutic opportunities gained from animal models and functional studies. This review may pave the way for more analytic approaches that take into consideration the overall picture of HSP. It will shed light on subtle associations that can explain the occurrence of the disease and allow a better understanding of its observed variations. This should help in the identification of future biomarkers, predictors of disease onset and progression, and treatments for both better functional outcomes and quality of life.

Role of ERLINs in the Control of Cell Fate through Lipid Rafts

ER lipid raft-associated protein 1 (ERLIN1) and 2 (ERLIN2) are 40 kDa transmembrane glycoproteins belonging to the family of prohibitins, containing a PHB domain. They are generally localized in the endoplasmic reticulum (ER), where ERLIN1 forms a heteroligomeric complex with its closely related ERLIN2. Well-defined functions of ERLINS are promotion of ER-associated protein degradation, mediation of inositol 1,4,5-trisphosphate (IP₃) receptors, processing and regulation of lipid metabolism. Until now, ERLINs have been exclusively considered protein markers of ER lipid raft-like microdomains. However, under pathophysiological conditions, they have been described within mitochondria-associated endoplasmic reticulum membranes (MAMs), tethering sites between ER and mitochondria, characterized by the presence of specialized raft-like subdomains enriched in cholesterol and gangliosides, which play a key role in the membrane scrambling and function. In this context, it is emerging that ER lipid raft-like microdomains proteins, i.e., ERLINs, may drive mitochondria-ER crosstalk under both physiological and pathological conditions by association with MAMs, regulating the two main processes underlined, survival and death. In this review, we describe the role of ERLINs in determining cell fate by controlling the “interchange” between apoptosis and autophagy pathways, considering that their alteration has a significant impact on the pathogenesis of several human diseases.

Importance of lipids for upper motor neuron health and disease

Building evidence reveals the importance of maintaining lipid homeostasis for the health and function of neurons, and upper motor neurons (UMNs) are no exception. UMNs are critically important for the initiation and modulation of voluntary movement as they are responsible for conveying cerebral cortex' input to spinal cord targets. To maintain their unique cytoarchitecture with a prominent apical dendrite and a very long axon, UMNs require a stable cell membrane, a lipid bilayer. Lipids can act as building blocks for many biomolecules, and they also contribute to the production of energy. Therefore, UMNs require sustained control over the production, utilization and homeostasis of lipids. Perturbations of lipid homeostasis lead to UMN vulnerability and progressive degeneration in diseases such as hereditary spastic paraplegia (HSP) and primary lateral sclerosis (PLS). Here, we discuss the importance of lipids, especially for UMNs.

RNF170-Related Hereditary Spastic Paraplegia: Confirmation by a Novel Mutation

BACKGROUND

Spastic paraparesis and biallelic variants functionally characterized as deleterious in the RNF170 gene have recently been reported by Wagner et al. 2019, strongly supporting the involvement of this gene in hereditary spastic paraplegia.

METHODS

Exome sequencing was performed on 6 hereditary spastic paraplegia families previously tested on an hereditary spastic paraplegia-specific panel.

RESULTS

We describe here a novel hereditary spastic paraplegia family with 4 affected members carrying a homozygous p.(Tyr114*) stop gain variant in RNF170.

CONCLUSIONS

We confirm the involvement of biallelic truncating variants in RNF170 in a novel form of hereditary spastic paraplegia. © 2020 International Parkinson and Movement Disorder Society.

Amplification of 8p11.23 in cancers and the role of amplicon genes

Copy number alterations are widespread in cancer genomes and are part of the genomic instability underlying the pathogenesis of neoplastic diseases. Recurrent copy number alterations of specific chromosomal loci may result in gains of oncogenes or losses of tumor suppressor genes and become entrenched in the genomic framework of certain types of cancers. The locus at chromosome 8p11.23 presents recurrent amplifications most commonly in squamous lung carcinomas, breast cancers, squamous esophageal carcinomas, and urothelial carcinomas. Amplification is rare in other cancers. The amplified segment involves several described oncogenes that may promote cancer cell survival and proliferation, as well as less well characterized genes that could also contribute to neoplastic processes. Genes proposed to be "drivers" in 8p11.23 amplifications include ZNF703, FGFR1 and PLPP5. Additional genes in the locus that could be functionally important in neoplastic networks include co-chaperone BAG4, lysine methyltransferase NSD3, ASH2L, a member of another methyltransferase complex, MLL and the mRNA processing and translation regulators LSM1 and EIF4EBP1. In this paper, genes located in the amplified segment of 8p11.23 will be examined for their role in cancer and data arguing for their importance for cancers with the amplification will be presented.

Lipid metabolic pathways converge in motor neuron degenerative diseases

Motor neuron diseases (MNDs) encompass an extensive and heterogeneous group of upper and/or lower motor neuron degenerative disorders, in which the particular clinical outcomes stem from the specific neuronal component involved in each condition. While mutations in a large number of molecules associated with lipid metabolism are known to be implicated in MNDs, there remains a lack of clarity regarding the key functional pathways involved, and their inter-relationships. This review highlights evidence that defines defects within two specific lipid (cholesterol/oxysterol and phosphatidylethanolamine) biosynthetic cascades as being centrally involved in MND, particularly hereditary spastic paraplegia. We also identify how other MND-associated molecules may impact these cascades, in particular through impaired organellar interfacing, to propose ‘subcellular lipidome imbalance’ as a likely common pathomolecular theme in MND. Further exploration of this mechanism has the potential to identify new therapeutic targets and management strategies for modulation of disease progression in hereditary spastic paraplegias and other MNDs.

Complex Analysis of Retroposed Genes' Contribution to Human Genome, Proteome and Transcriptome

Gene duplication is a major driver of organismal evolution. One of the main mechanisms of gene duplications is retroposition, a process in which mRNA is first transcribed into DNA and then reintegrated into the genome. Most gene retrocopies are depleted of the regulatory regions. Nevertheless, examples of functional retrogenes are rapidly increasing. These functions come from the gain of new spatio-temporal expression patterns, imposed by the content of the genomic sequence surrounding inserted cDNA and/or by selectively advantageous mutations, which may lead to the switch from protein coding to regulatory RNA. As recent studies have shown, these genes may lead to new protein domain formation through fusion with other genes, new regulatory RNAs or other regulatory elements. We utilized existing data from high-throughput technologies to create a complex description of retrogenes functionality. Our analysis led to the identification of human retroposed genes that substantially contributed to transcriptome and proteome. These retrocopies demonstrated the potential to encode proteins or short peptides, act as cis- and trans- Natural Antisense Transcripts (NATs), regulate their progenitors’ expression by competing for the same microRNAs, and provide a sequence to lncRNA and novel exons to existing protein-coding genes. Our study also revealed that retrocopies, similarly to retrotransposons, may act as recombination hot spots. To our best knowledge this is the first complex analysis of these functions of retrocopies.

Expansion of the genetic landscape of ERLIN2-related disorders

ERLIN2‐related disorders are rare conditions of the motor system and clinical details are limited to a small number of prior descriptions. We here presented clinical and genetic details in five individuals (four different families) where three subjects carried a common homozygous p.Asn292ArgfsX26, associated also with sensorineural hearing loss in one child. One further subject had a de novo p.Gln63Lys and one harbors the homozygous p.Val136Gly because of maternal isodisomy of chromosome 8. Overall, we expanded the clinical and genetic spectrum of ERLIN2‐related disorders and we reiterate that autosomal‐dominant transmission is a potential mode of inheritance. Future research will elucidate disease mechanisms.

Lipids in the Physiopathology of Hereditary Spastic Paraplegias

Hereditary spastic paraplegias (HSP) are a group of neurodegenerative diseases sharing spasticity in lower limbs as common symptom. There is a large clinical variability in the presentation of patients, partly underlined by the large genetic heterogeneity, with more than 60 genes responsible for HSP. Despite this large heterogeneity, the proteins with known function are supposed to be involved in a limited number of cellular compartments such as shaping of the endoplasmic reticulum or endolysosomal function. Yet, it is difficult to understand why alteration of such different cellular compartments can lead to degeneration of the axons of cortical motor neurons. A common feature that has emerged over the last decade is the alteration of lipid metabolism in this group of pathologies. This was first revealed by the identification of mutations in genes encoding proteins that have or are supposed to have enzymatic activities on lipid substrates. However, it also appears that mutations in genes affecting endoplasmic reticulum, mitochondria, or endolysosome function can lead to changes in lipid distribution or metabolism. The aim of this review is to discuss the role of lipid metabolism alterations in the physiopathology of HSP, to evaluate how such alterations contribute to neurodegenerative phenotypes, and to understand how this knowledge can help develop therapeutic strategy for HSP.

An autosomal dominant ERLIN2 mutation leads to a pure HSP phenotype distinct from the autosomal recessive ERLIN2 mutations (SPG18)

Hereditary spastic paraplegia (HSP) is a heterogeneous inherited disorder that manifests with lower extremity weakness and spasticity. HSP can be inherited by autosomal dominant, autosomal recessive, and X-linked inheritance patterns. Recent studies have shown that, although rare, mutations in a single gene can lead to multiple patterns of inheritance of HSP. We enrolled the HSP family showing autosomal dominant inheritance and performed genetic study to find the cause of phenotype in this family. We recruited five members of a Korean family as study participants. Four of the five family members had pure HSP. Part of the family members underwent whole-exome sequencing (WES) to identify the causative mutation. As the result of WES and Sanger sequencing analysis, a novel missense mutation (c.452 C > T, p.Ala151Val) of ERLIN2 gene was identified as the cause of the autosomal dominant HSP in the family. Our study suggests that the ERLIN2 gene leads to both autosomal recessive and autosomal dominant patterns of inheritance in HSP. Moreover, autosomal dominant HSP caused by ERLIN2 appears to cause pure HSP in contrast to autosomal recessive ERLIN2 related complicated HSP (SPG18).

Spastic paraplegia due to recessive or dominant mutations in ERLIN2 can convert to ALS

Objective

The aim of this study was to evaluate whether mutations in ERLIN2, known to cause SPG18, a recessive hereditary spastic paraplegia (SP) responsible for the degeneration of the upper motor neurons leading to weakness and spasticity restricted to the lower limbs, could contribute to amyotrophic lateral sclerosis (ALS), a distinct and more severe motor neuron disease (MND), in which the lower motor neurons also profusely degenerates, leading to tetraplegia, bulbar palsy, respiratory insufficiency, and ultimately the death of the patients.

Methods

Whole-exome sequencing was performed in a large cohort of 200 familial ALS and 60 sporadic ALS after a systematic screening for C9orf72 hexanucleotide repeat expansion. ERLIN2 variants identified by exome analysis were validated using Sanger analysis. Segregation of the identified variant with the disease was checked for all family members with available DNA.

Results

Here, we report the identification of ERLIN2 mutations in patients with a primarily SP evolving to rapid progressive ALS, leading to the death of the patients. These mutations segregated with the disease in a dominant (V168M) or recessive (D300V) manner in these families or were found in apparently sporadic cases (N125S).

Conclusions

Inheritance of ERLIN2 mutations appears to be, within the MND spectrum, more complex that previously reported. These results expand the clinical phenotype of ERLIN2 mutations to a severe outcome of MND and should be considered before delivering a genetic counseling to ERLIN2-linked families.

Bi-allelic variants in RNF170 are associated with hereditary spastic paraplegia

Alterations of Ca2+ homeostasis have been implicated in a wide range of neurodegenerative diseases. Ca2+ efflux from the endoplasmic reticulum into the cytoplasm is controlled by binding of inositol 1,4,5-trisphosphate to its receptor. Activated inositol 1,4,5-trisphosphate receptors are then rapidly degraded by the endoplasmic reticulum-associated degradation pathway. Mutations in genes encoding the neuronal isoform of the inositol 1,4,5-trisphosphate receptor (ITPR1) and genes involved in inositol 1,4,5-trisphosphate receptor degradation (ERLIN1, ERLIN2) are known to cause hereditary spastic paraplegia (HSP) and cerebellar ataxia. We provide evidence that mutations in the ubiquitin E3 ligase gene RNF170, which targets inositol 1,4,5-trisphosphate receptors for degradation, are the likely cause of autosomal recessive HSP in four unrelated families and functionally evaluate the consequences of mutations in patient fibroblasts, mutant SH-SY5Y cells and by gene knockdown in zebrafish. Our findings highlight inositol 1,4,5-trisphosphate signaling as a candidate key pathway for hereditary spastic paraplegias and cerebellar ataxias and thus prioritize this pathway for therapeutic interventions.

Hereditary Myelopathies

PURPOSE OF REVIEW

Hereditary myelopathies are very diverse genetic disorders, and many of them represent a widespread neurodegenerative process rather than isolated spinal cord dysfunction. This article reviews various types of inherited myelopathies, with emphasis on hereditary spastic paraplegias and spastic ataxias.

RECENT FINDINGS

The ever-growing number of myelopathy-causing genes and broadening of phenotype-genotype correlations makes the molecular diagnosis of inherited myelopathies a daunting task. This article emphasizes the main phenotypic clusters among inherited myelopathies that can facilitate the diagnostic process. This article focuses on newly identified genetic causes and the most important identifying clinical features that can aid the diagnosis, including the presence of a characteristic age of onset and additional neurologic signs such as leukodystrophy, thin corpus callosum, or amyotrophy.

SUMMARY

The exclusion of potentially treatable causes of myelopathy remains the most important diagnostic step. Syndromic diagnosis can be supported by molecular diagnosis, but the genetic diagnosis at present does not change the management. Moreover, a negative genetic test does not exclude the diagnosis of a hereditary myelopathy because comprehensive molecular testing is not yet available, and many disease-causing genes remain unknown.

The erlin2 T65I mutation inhibits erlin1/2 complex-mediated inositol 1,4,5-trisphosphate receptor ubiquitination and phosphatidylinositol 3-phosphate binding

The erlin1/2 complex is a ∼2-MDa endoplasmic reticulum membrane-located ensemble of the ∼40-kDa type II membrane proteins erlin1 and erlin2. The best defined function of this complex is to mediate the ubiquitination of activated inositol 1,4,5-trisphosphate receptors (IP₃Rs) and their subsequent degradation. However, it remains unclear how mutations of the erlin1/2 complex affect its cellular function and cause cellular dysfunction and diseases such as hereditary spastic paraplegia. Here, we used gene editing to ablate erlin1 or erlin2 expression to better define their individual roles in the cell and examined the functional effects of a spastic paraplegia-linked mutation to erlin2 (threonine to isoleucine at position 65; T65I). Our results revealed that erlin2 is the dominant player in mediating the interaction between the erlin1/2 complex and IP₃Rs and that the T65I mutation dramatically inhibits this interaction and the ability of the erlin1/2 complex to promote IP₃R ubiquitination and degradation. Remarkably, we also discovered that the erlin1/2 complex specifically binds to phosphatidylinositol 3-phosphate, that erlin2 binds this phospholipid much more strongly than does erlin1, that the binding is inhibited by T65I mutation of erlin2, and that multiple determinants within the erlin2 polypeptide comprise the phosphatidylinositol 3-phosphate-binding site. Overall, these results indicate that erlin2 is the primary mediator of the cellular roles of the erlin1/2 complex and that disease-linked mutations of erlin2 can affect both IP₃R processing and lipid binding.

The Making and Breaking of Inositol 1,4,5-Trisphosphate Receptor Tetramers

Mammalian cells express three highly conserved inositol 1,4,5-trisphosphate (IP₃) receptor types (IP₃R1, IP₃R2 and IP₃R3), which have broadly similar characteristics, but markedly different distributions, and form homo- or heterotetrameric Ca²⁺ channels in endoplasmic reticulum (ER) membranes. A vast array of published work details how mature, ER membrane-located IP₃ receptor tetramers are regulated, but much less attention has been paid to the intriguing questions of how the tetramers are assembled and destroyed as part of their natural life cycle. Are they assembled at the ER membrane from nascent, or completely translated polypeptides? How are they disassembled and degraded? These questions and other recently defined modes of IP₃ receptor processing will be briefly reviewed.

Emerging Links between Lipid Droplets and Motor Neuron Diseases

Lipid droplets (LDs) are ubiquitous fat storage organelles and play key roles in lipid metabolism and energy homeostasis; in addition, they contribute to protein storage, folding, and degradation. However, a role for LDs in the nervous system remains largely unexplored. We discuss evidence supporting an intimate functional connection between LDs and motor neuron disease (MND) pathophysiology, examining how LD functions in systemic energy homeostasis, in neuron-glia metabolic coupling, and in protein folding and clearance may affect or contribute to disease pathology. An integrated understanding of LD biology and neurodegeneration may open the way for new therapeutic interventions.

Massive sequencing of 70 genes reveals a myriad of missing genes or mechanisms to be uncovered in hereditary spastic paraplegias

Hereditary spastic paraplegias (HSP) are neurodegenerative disorders characterized by lower limb spasticity and weakness that can be complicated by other neurological or non-neurological signs. Despite a high genetic heterogeneity (>60 causative genes), 40–70% of the families remain without a molecular diagnosis. Analysis of one of the pioneer cohorts of 193 HSP families generated in the early 1990s in Portugal highlighted that SPAST and SPG11 are the most frequent diagnoses. We have now explored 98 unsolved families from this series using custom next generation sequencing panels analyzing up to 70 candidate HSP genes. We identified the likely disease-causing variant in 20 of the 98 families with KIF5A being the most frequently mutated gene. We also found 52 variants of unknown significance (VUS) in 38% of the cases. These new diagnoses resulted in 42% of solved cases in the full Portuguese cohort (81/193). Segregation of the variants was not always compatible with the presumed inheritance, indicating that the analysis of all HSP genes regardless of the inheritance mode can help to explain some cases. Our results show that there is still a large set of unknown genes responsible for HSP and most likely novel mechanisms or inheritance modes leading to the disease to be uncovered, but this will require international collaborative efforts, particularly for the analysis of VUS.

Associating transcription factors and conserved RNA structures with gene regulation in the human brain

Anatomical subdivisions of the human brain can be associated with different neuronal functions. This functional diversification is reflected by differences in gene expression. By analyzing post-mortem gene expression data from the Allen Brain Atlas, we investigated the impact of transcription factors (TF) and RNA secondary structures on the regulation of gene expression in the human brain. First, we modeled the expression of a gene as a linear combination of the expression of TFs. We devised an approach to select robust TF-gene interactions and to determine localized contributions to gene expression of TFs. Among the TFs with the most localized contributions, we identified EZH2 in the cerebellum, NR3C1 in the cerebral cortex and SRF in the basal forebrain. Our results suggest that EZH2 is involved in regulating ZIC2 and SHANK1 which have been linked to neurological diseases such as autism spectrum disorder. Second, we associated enriched regulatory elements inside differentially expressed mRNAs with RNA secondary structure motifs. We found a group of purine-uracil repeat RNA secondary structure motifs plus other motifs in neuron related genes such as ACSL4 and ERLIN2.

A mutation of EPT1 (SELENOI) underlies a new disorder of Kennedy pathway phospholipid biosynthesis

Mutations in genes involved in lipid metabolism have increasingly been associated with various subtypes of hereditary spastic paraplegia, a highly heterogeneous group of neurodegenerative motor neuron disorders characterized by spastic paraparesis. Here, we report an unusual autosomal recessive neurodegenerative condition, best classified as a complicated form of hereditary spastic paraplegia, associated with mutation in the ethanolaminephosphotransferase 1 (EPT1) gene (now known as SELENOI), responsible for the final step in Kennedy pathway forming phosphatidylethanolamine from CDP-ethanolamine. Phosphatidylethanolamine is a glycerophospholipid that, together with phosphatidylcholine, constitutes more than half of the total phospholipids in eukaryotic cell membranes. We determined that the mutation defined dramatically reduces the enzymatic activity of EPT1, thereby hindering the final step in phosphatidylethanolamine synthesis. Additionally, due to central nervous system inaccessibility we undertook quantification of phosphatidylethanolamine levels and species in patient and control blood samples as an indication of liver phosphatidylethanolamine biosynthesis. Although this revealed alteration to levels of specific phosphatidylethanolamine fatty acyl species in patients, overall phosphatidylethanolamine levels were broadly unaffected indicating that in blood EPT1 inactivity may be compensated for, in part, via alternate biochemical pathways. These studies define the first human disorder arising due to defective CDP-ethanolamine biosynthesis and provide new insight into the role of Kennedy pathway components in human neurological function.

Chapter 4 - Inositol 1,4,5-Trisphosphate Receptor Ubiquitination

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are large (∼300kDa) proteins that associate into tetrameric ion channels in the endoplasmic reticulum (ER) membrane. Activation and opening of the channel upon binding of IP3 and Ca(2+) allows the flow of Ca(2+) ions from stores within the ER lumen to the cytosol, thereby promoting a number of Ca(2+)-dependent cellular events, such as secretion, neurotransmitter release, and cell division. Intriguingly, it appears that the same conformational change that IP3Rs undergo during activation makes them a target for degradation by the ubiquitin-proteasome pathway and that this mode of processing allows the cell to tune its internal Ca(2+) response to extracellular signals. Here, we review recent studies showing that activated IP3Rs interact with an array of proteins that mediate their degradation, that IP3Rs are modified by a complex array of ubiquitin conjugates, that this ubiquitination and degradation functions to regulate IP3-mediated Ca(2+) responses in the cell, and that mutations to different proteins involved in IP3R degradation result in a set of similar diseases.

A novel ER-microtubule-binding protein, ERLIN2, stabilizes Cyclin B1 and regulates cell cycle progression

The gene encoding endoplasmic reticulum (ER) lipid raft-associated protein 2 (ERLIN2) is amplified in human breast cancers. ERLIN2 gene mutations were also found to be associated with human childhood progressive motor neuron diseases. Yet, an understanding of the physiological function and mechanism for ERLIN2 remains elusive. In this study, we reveal that ERLIN2 is a spatially and temporally regulated ER–microtubule-binding protein that has an important role in cell cycle progression by interacting with and stabilizing the mitosis-promoting factors. Whereas ERLIN2 is highly expressed in aggressive human breast cancers, during normal development ERLIN2 is expressed at the postnatal stage and becomes undetectable in adulthood. ERLIN2 interacts with the microtubule component α-tubulin, and this interaction is maximal during the cell cycle G2/M phase where ERLIN2 simultaneously interacts with the mitosis-promoting complex Cyclin B1/Cdk1. ERLIN2 facilitates K63-linked ubiquitination and stabilization of Cyclin B1 protein in G2/M phase. Downregulation of ERLIN2 results in cell cycle arrest, represses breast cancer proliferation and malignancy and increases sensitivity of breast cancer cells to anticancer drugs. In summary, our study revealed a novel ER–microtubule-binding protein, ERLIN2, which interacts with and stabilizes mitosis-promoting factors to regulate cell cycle progression associated with human breast cancer malignancy.

Clinical and genetic heterogeneity in hereditary spastic paraplegias: from SPG1 to SPG72 and still counting

Hereditary spastic paraplegias (HSPs) are genetically determined neurodegenerative disorders characterized by progressive weakness and spasticity of lower limbs, and are among the most clinically and genetically heterogeneous human diseases. All modes of inheritance have been described, and the recent technological revolution in molecular genetics has led to the identification of 76 different spastic gait disease-loci with 59 corresponding spastic paraplegia genes. Autosomal recessive HSP are usually associated with diverse additional features (referred to as complicated forms), contrary to autosomal dominant HSP, which are mostly pure. However, the identification of additional mutations and families has considerably enlarged the clinical spectra, and has revealed a huge clinical variability for almost all HSP; complicated forms have also been described for primary pure HSP subtypes, adding further complexity to the genotype-phenotype correlations. In addition, the introduction of next generation sequencing in clinical practice has revealed a genetic and phenotypic overlap with other neurodegenerative disorders (amyotrophic lateral sclerosis, neuropathies, cerebellar ataxias, etc.) and neurodevelopmental disorders, including intellectual disability. This review aims to describe the most recent advances in the field and to provide genotype-phenotype correlations that could help clinical diagnoses of this heterogeneous group of disorders.

The critical role of membralin in postnatal motor neuron survival and disease

Hitherto, membralin has been a protein of unknown function. Here, we show that membralin mutant mice manifest a severe and early-onset motor neuron disease in an autosomal recessive manner, dying by postnatal day 5–6. Selective death of lower motor neurons, including those innervating the limbs, intercostal muscles, and diaphragm, is predominantly responsible for this fatal phenotype. Neural expression of a membralin transgene completely rescues membralin mutant mice. Mechanistically, we show that membralin interacts with Erlin2, an endoplasmic reticulum (ER) membrane protein that is located in lipid rafts and known to be important in ER-associated protein degradation (ERAD). Accordingly, the degradation rate of ERAD substrates is attenuated in cells lacking membralin. Membralin mutations or deficiency in mouse models induces ER stress, rendering neurons more vulnerable to cell death. Our study reveals a critical role of membralin in motor neuron survival and suggests a novel mechanism for early-onset motor neuron disease.

DOI:http://dx.doi.org/10.7554/eLife.06500.001

As new proteins are built inside a cell, many will pass into a structure called the endoplasmic reticulum for processing. There, the proteins are folded into the specific three-dimensional shapes that allow them to carry out their respective jobs. Sometimes the folding process goes awry, leading to a build-up of unfolded proteins that stress the endoplasmic reticulum and can kill the cell. Brain cells are particularly vulnerable to death from endoplasmic reticulum stress. To combat a deadly build-up of unfolded proteins, each cell has systems that respond when the endoplasmic reticulum is under stress.

Unchecked stress on the endoplasmic reticulum has been linked to diseases like amyotrophic lateral sclerosis (called ALS for short). In diseases like ALS, the nerve cells that control muscle movements gradually die off, causing a loss of muscle control and eventually death. Scientists suspect that these nerve cells (called motor neurons) are particularly sensitive to endoplasmic reticulum stress because they are highly active. Drugs that help counteract stress on the endoplasmic reticulum extend the lives of mice with motor neuron disease, suggesting this may be a useful strategy for treating such diseases in humans.

Now, Yang, Qu et al. identify a new protein that appears necessary for a healthy endoplasmic reticulum. Mice that lack the gene for a protein called membralin die within five or six days after birth because their motor neurons die off. Further experiments showed that re-introducing membralin in their nervous system can rescue these membralin-deficient mice.

Yang, Qu et al. found that membralin interacts with another protein that helps eliminate poorly folded or unfolded proteins in the endoplasmic reticulum, and thus relieves stress on the cell. Mutations in this endoplasmic reticulum stress response protein have previously been linked to motor neuron diseases. The motor neurons in membralin-deficient mice show signs of endoplasmic reticulum stress and are extra vulnerable to chemicals that induce protein misfolding. Together, the experiments show membralin plays an important role in mitigating stress on the endoplasmic reticulum. More studies of mice lacking membralin may help explain why the endoplasmic reticulum stress increases in motor neuron diseases and may point to possible treatments.

DOI:http://dx.doi.org/10.7554/eLife.06500.002

A Point Mutation in the Ubiquitin Ligase RNF170 That Causes Autosomal Dominant Sensory Ataxia Destabilizes the Protein and Impairs Inositol 1,4,5-Trisphosphate Receptor-mediated Ca2+ Signaling

RNF170 is an endoplasmic reticulum membrane ubiquitin ligase that contributes to the ubiquitination of activated inositol 1,4,5-trisphosphate (IP3) receptors, and also, when point mutated (arginine to cysteine at position 199), causes autosomal dominant sensory ataxia (ADSA), a disease characterized by neurodegeneration in the posterior columns of the spinal cord. Here we demonstrate that this point mutation inhibits RNF170 expression and signaling via IP3 receptors. Inhibited expression of mutant RNF170 was seen in cells expressing exogenous RNF170 constructs and in ADSA lymphoblasts, and appears to result from enhanced RNF170 autoubiquitination and proteasomal degradation. The basis for these effects was probed via additional point mutations, revealing that ionic interactions between charged residues in the transmembrane domains of RNF170 are required for protein stability. In ADSA lymphoblasts, platelet-activating factor-induced Ca(2+) mobilization was significantly impaired, whereas neither Ca(2+) store content, IP3 receptor levels, nor IP3 production were altered, indicative of a functional defect at the IP3 receptor locus, which may be the cause of neurodegeneration. CRISPR/Cas9-mediated genetic deletion of RNF170 showed that RNF170 mediates the addition of all of the ubiquitin conjugates known to become attached to activated IP3 receptors (monoubiquitin and Lys(48)- and Lys(63)-linked ubiquitin chains), and that wild-type and mutant RNF170 have apparently identical ubiquitin ligase activities toward IP3 receptors. Thus, the Ca(2+) mobilization defect seen in ADSA lymphoblasts is apparently not due to aberrant IP3 receptor ubiquitination. Rather, the defect likely reflects abnormal ubiquitination of other substrates, or adaptation to the chronic reduction in RNF170 levels.

Delving into the complexity of hereditary spastic paraplegias: how unexpected phenotypes and inheritance modes are revolutionizing their nosology

Hereditary spastic paraplegias (HSP) are rare neurodegenerative diseases sharing the degeneration of the corticospinal tracts as the main pathological characteristic. They are considered one of the most heterogeneous neurological disorders. All modes of inheritance have been described for the 84 different loci and 67 known causative genes implicated up to now. Recent advances in molecular genetics have revealed clinico-genetic heterogeneity of these disorders including their clinical and genetic overlap with other diseases of the nervous system. The systematic analysis of a large set of genes, including exome sequencing, is unmasking unusual phenotypes or inheritance modes associated with mutations in HSP genes and related genes involved in various neurological diseases. A new nosology may emerge after integration and understanding of these new data to replace the current classification. Collectively, functions of the known genes implicate the disturbance of intracellular membrane dynamics and trafficking as the consequence of alterations of cytoskeletal dynamics, lipid metabolism and organelle structures, which represent in fact a relatively small number of cellular processes that could help to find common curative approaches, which are still lacking.