Expansion of the genetic landscape of ERLIN2-related disorders

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.

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.

A case report of concurrent occurrence of two inherited axonopathies within a family: the benefit of whole-exome sequencing

Mutations in ERLIN2 and MFN2 lead to the development of spastic paraplegia-18 (SPG18) and Charcot-Marie-Tooth type-2A (CMT2A), respectively. These disorders are unified by the fact that both can be termed inherited axonopathies. With whole-exome sequencing (WES), more patients of neurological disorders with clinical overlaps receive a genetic result than ever before. This study describes an Iranian family who harbor mutations in ERLIN2 and MFN2, simultaneously. The proband was a 73-year old man who has experienced weakness and spasticity of lower limbs since late childhood. He was diagnosed with hereditary spastic paraplegia (HSP). His WES identified a novel homozygous variant in ERLIN2 as well as a known heterozygous variant in MFN2. These variants were cosegregated with the phenotypes among the family members. His sister with a similar phenotype just carried the homozygous ERLIN2 variant, whereas, his asymptomatic brother and daughter carried the heterozygous variant of MFN2. Re-evaluation of the MFN2 variant carriers by nerve conduction study revealed that only the proband's daughter has peripheral neuropathy. Herein, using WES two distinct disease-causing variants with different modes of inheritance in ERLIN2 and MFN2 were detected in the proband. As expected, individuals with a defined MFN2 variant, p.Arg468His, were asymptomatic or had a mild phenotype. The co-occurrence of such diseases, SPG18 and CMT2A, may result in the milder phenotype to be overlooked or its features considered as a part of the symptoms of other disease. Certainly, providing genetic counseling in such cases can be challenging. These cases reveal the importance of WES.

Childhood-onset hereditary spastic paraplegia and its treatable mimics

Early-onset forms of hereditary spastic paraplegia and inborn errors of metabolism that present with spastic diplegia are among the most common "mimics" of cerebral palsy. Early detection of these heterogenous genetic disorders can inform genetic counseling, anticipatory guidance, and improve outcomes, particularly where specific treatments exist. The diagnosis relies on clinical pattern recognition, biochemical testing, neuroimaging, and increasingly next-generation sequencing-based molecular testing. In this short review, we summarize the clinical and molecular understanding of: 1) childhood-onset and complex forms of hereditary spastic paraplegia (SPG5, SPG7, SPG11, SPG15, SPG35, SPG47, SPG48, SPG50, SPG51, SPG52) and, 2) the most common inborn errors of metabolism that present with phenotypes that resemble hereditary spastic paraplegia.

Clinical and genetic spectra of 1550 index patients with hereditary spastic paraplegia

Hereditary spastic paraplegia refers to rare genetic neurodevelopmental and/or neurodegenerative disorders in which spasticity due to length-dependent damage to the upper motor neuron is a core sign. Their high clinical and genetic heterogeneity makes their diagnosis challenging. Multigene panels allow a high-throughput targeted analysis of the increasing number of genes involved using next-generation sequencing. We report here the clinical and genetic results of 1550 index cases tested for variants in a panel of hereditary spastic paraplegia related genes analyzed in routine diagnosis. A causative variant was found in 475 patients (30.7%) in 35/65 screened genes. SPAST and SPG7 were the most frequently mutated genes, representing 142 (9.2%) and 75 (4.8%) index cases of the whole series, respectively. KIF1A, ATL1, SPG11, KIF5A and REEP1 represented more than 1% (> 17 cases) each. There were 661 causative variants (382 different ones) and 30 of them were structural variants. This large cohort allowed us obtaining an overview of the clinical and genetic spectrum of hereditary spastic paraplegia in clinical practice. Because of the wide phenotypic variability, there was no very specific sign that could predict the causative gene but there were some constellations of symptoms that were found often related to specific subtypes. Finally, we confirmed the diagnostic effectiveness of a targeted sequencing panel as a first-line genetic test in hereditary spastic paraplegia. This is a pertinent strategy because of the relative frequency of several known genes (i.e.: SPAST, KIF1A) and it allows identifying variants in the rarest involved genes and to detect structural rearrangements via coverage analysis, which is less efficient in exome data sets. It is crucial because these structural variants represent a significant proportion of the pathogenic hereditary spastic paraplegia variants (∼6% of patients), notably for SPAST and REEP1. In a subset of 42 index cases negative for the targeted multigene panel, subsequent whole exome sequencing allowed to reach a theoretical diagnosis yield of ∼50%. We then propose a two-step strategy combining the use of a panel of genes followed by whole exome sequencing in negative cases.

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.

Evolution and diversification of the nuclear envelope

Eukaryotic cells arose ~1.5 billion years ago, with the endomembrane system a central feature, facilitating evolution of intracellular compartments. Endomembranes include the nuclear envelope (NE) dividing the cytoplasm and nucleoplasm. The NE possesses universal features: a double lipid bilayer membrane, nuclear pore complexes (NPCs), and continuity with the endoplasmic reticulum, indicating common evolutionary origin. However, levels of specialization between lineages remains unclear, despite distinct mechanisms underpinning various nuclear activities. Several distinct modes of molecular evolution facilitate organellar diversification and  to understand which apply to the NE, we exploited proteomic datasets of purified nuclear envelopes from model systems for comparative analysis. We find enrichment of core nuclear functions amongst the widely conserved proteins to be less numerous than lineage-specific cohorts, but enriched in core nuclear functions. This, together with consideration of additional evidence, suggests that, despite a common origin, the NE has evolved as a highly diverse organelle with significant lineage-specific functionality.