Converging Role for REEP1/SPG31 in Oxidative Stress.

Mutations in the receptor expression-enhancing protein 1 gene (REEP1) are associated with hereditary spastic paraplegia type 31 (SPG31), a neurological disorder characterized by length-dependent degeneration of upper motor neuron axons. Mitochondrial dysfunctions have been observed in patients harboring pathogenic variants in REEP1, suggesting a key role of bioenergetics in disease-related manifestations. Nevertheless, the regulation of mitochondrial function in SPG31 remains unclear. To elucidate the pathophysiology underlying REEP1 deficiency, we analyzed in vitro the impact of two different mutations on mitochondrial metabolism. Together with mitochondrial morphology abnormalities, loss-of-REEP1 expression highlighted a reduced ATP production with increased susceptibility to oxidative stress. Furthermore, to translate these findings from in vitro to preclinical models, we knocked down REEP1 in zebrafish. Zebrafish larvae showed a significant defect in motor axon outgrowth leading to motor impairment, mitochondrial dysfunction, and reactive oxygen species accumulation. Protective antioxidant agents such as resveratrol rescued free radical overproduction and ameliorated the SPG31 phenotype both in vitro and in vivo. Together, our findings offer new opportunities to counteract neurodegeneration in SPG31.

A nonstop variant in REEP1 causes peripheral neuropathy by unmasking a 3'UTR-encoded, aggregation-inducing motif.

Single-nucleotide variants that abolish the stop codon ("nonstop" alterations) are a unique type of substitution in genomic DNA. Whether they confer instability of the mutant mRNA or result in expression of a C-terminally extended protein depends on the absence or presence of a downstream in-frame stop codon, respectively. Of the predicted protein extensions, only few have been functionally characterized. In a family with autosomal dominant Charcot-Marie-Tooth disease type 2, that is, an axonopathy affecting sensory neurons as well as lower motor neurons, we identified a heterozygous nonstop variant in REEP1. Mutations in this gene have classically been associated with the upper motor neuron disorder hereditary spastic paraplegia (HSP). We show that the C-terminal extension resulting from the nonstop variant triggers self-aggregation of REEP1 and of several reporters. Our findings support the recently proposed concept of 3'UTR-encoded "cryptic amyloidogenic elements." Together with a previous report on an aggregation-prone REEP1 deletion variant in distal hereditary motor neuropathy, they also suggest that toxic gain of REEP1 function, rather than loss-of-function as relevant for HSP, specifically affects lower motor neurons. A search for similar correlations between genotype, phenotype, and effect of mutant protein may help to explain the wide clinical spectra also in other genetically determined disorders.

Functional mutation analysis provides evidence for a role of REEP1 in lipid droplet biology.

Hereditary axonopathies are frequently caused by mutations in proteins that reside in the endoplasmic reticulum (ER). Which of the many ER functions are pathologically relevant, however, remains to be determined. REEP1 is an ER protein mutated in hereditary spastic paraplegia (HSP) and hereditary motor neuropathy (HMN). We found that HSP-associated missense variants at the N-terminus of REEP1 abolish ER targeting, whereas two more central variants are either rare benign SNPs or confer pathogenicity via a different mechanism. The mis-targeted variants accumulate at lipid droplets (LDs). N-terminal tagging, deletion of the N-terminus, and expression of a minor REEP1 isoform had the same effect. We also confirmed an increase in LD size upon cooverexpression of atlastins and REEP1. Neither wild-type REEP1, LD-targeted HSP variants, nor a non-LD-targeted HMN variant reproduced this effect when expressed alone. We conclude that the N-terminus of REEP1 is necessary for proper targeting to and/or retention in the ER. The protein's potential to also associate with LDs corroborates a synergistic effect with atlastins on LD size. Interestingly, LD size is also altered upon knockdown of seipin, mutations of which also cause HSP and HMN. Regulation of LDs may thus be an ER function critical for long-term axonal maintenance.

Phenotypic variability in a large kindred with spastic paraplegia associated with a novel REEP1 variant.

Background and objectives: The aim of this study is to provide a comprehensive characterization of a large Estonian family spanning five generations with seventeen individuals affected by spastic paraplegia associated with a novel variant in the receptor expression-enhancing protein-1 (REEP1) gene. Methods: Comprehensive clinical evaluation, neuroimaging, and neurophysiological studies were performed on six patients who provided oral and written consent. Whole-exome sequencing was performed on the index case. Targeted carrier testing was done in all other available affected and at-risk relatives. Results: Four individuals presented with pure spastic paraplegia, with onset from early childhood to adult age. None had bladder or bowel dysfunction. Two subjectively asymptomatic mutation carriers displayed pyramidal signs on examination. Imaging of the neuroaxis was normal in three patients, three had MRI findings interpreted as unrelated. Motor evoked potential (MEP) was abnormal in five; the patient with the longest disease duration had additional somatosensory evoked potential (SSEP) abnormalities. The novel splice-site variant, c.32 + 1G > C in the REEP1 gene, found in the index case, co-segregates with disease in the family. Expressivity in this family is variable. Conclusion: Our findings are in keeping with previous descriptions of the SPG31 spectrum. The phenotype associated with splice variants is not necessarily more severe than other conventional REEP1 variants. As for other forms of familial spastic paraplegias, the factors modulating variable expressivity in SPG31 are still unknown.

REEP1 Preserves Motor Function in SOD1G93A Mice by Improving Mitochondrial Function via Interaction with NDUFA4.

A decline in the activities of oxidative phosphorylation (OXPHOS) complexes has been consistently reported in amyotrophic lateral sclerosis (ALS) patients and animal models of ALS, although the underlying molecular mechanisms are still elusive. Here, we report that receptor expression enhancing protein 1 (REEP1) acts as an important regulator of complex IV assembly, which is pivotal to preserving motor neurons in SOD1G93A mice. We found the expression of REEP1 was greatly reduced in transgenic SOD1G93A mice with ALS. Moreover, forced expression of REEP1 in the spinal cord extended the lifespan, decelerated symptom progression, and improved the motor performance of SOD1G93A mice. The neuromuscular synaptic loss, gliosis, and even motor neuron loss in SOD1G93A mice were alleviated by increased REEP1 through augmentation of mitochondrial function. Mechanistically, REEP1 associates with NDUFA4, and plays an important role in preserving the integrity of mitochondrial complex IV. Our findings offer insights into the pathogenic mechanism of REEP1 deficiency in neurodegenerative diseases and suggest a new therapeutic target for ALS.

Expanding Genotype/Phenotype Correlation in 2p11.2-p12 Microdeletion Syndrome.

Chromosomal abnormalities on the short arm of chromosome 2 in the region p11.2 have been associated with developmental delay, intellectual disability, facial anomalies, abnormal ears, skeletal and genital malformations. Here we describe a patient with a de novo interstitial heterozygous microdeletion on the short arm of chromosome 2 in the region p11.2-p12. He presents with facial dysmorphism characterized by a broad and low root of the nose and low-set protruding ears. Clinical examinations during follow-up visits revealed congenital pendular nystagmus, decreased visual acuity and psychomotor development disorder including intellectual disability. The heterozygous 5 Mb-microdeletion was characterized by an array CGH (Comparative Genomic Hybridization) analysis. In the past two decades, nine patients with microdeletions in this region have been identified by array CGH analysis and were reported in the literature. All these patients show psychomotor development disorder and outer and/or inner ear anomalies. In addition, most of the patients have mild to severe intellectual disability and show facial malformations. We reviewed the literature on PubMed and OMIM using the gene/loci names as search terms in an attempt to identify correlations between genes located within the heterozygous microdeletion and the clinical phenotype of the patient, in order to define a recognizable phenotype for the 2p11.2p12 microdeletion syndrome. We discuss additional symptoms that are not systematically present in all patients and contribute to a heterogeneous clinical presentation of this microdeletion syndrome.

A novel REEP1 splicing mutation with broad clinical variability in a family with hereditary spastic paraplegia.

Hereditary spastic paraplegia (HSP) is a heterogeneous group of genetic disorders characterized by lower-limb spastic paralysis. We report on a family with three generations of autosomal dominant inheritance of HSP caused by a novel heterozygous splice-site mutation (c.303 + 2 T > C) in REEP1 that was confirmed by RFLP analysis. Carriers of the mutation, including one asymptomatic individual, showed a mild HSP phenotype with a wide range of intrafamilial variation. All symptomatic carriers had ankle contractures in addition to other classical clinical symptoms of HSP. Clinicians should suspect REEP1-related HSP in patients who show ankle contractures with other symptoms of HSP and should consider that these patients have asymptomatic carriers within their family.

Naringenin Ameliorates Drosophila ReepA Hereditary Spastic Paraplegia-Linked Phenotypes.

Defects in the endoplasmic reticulum (ER) membrane shaping and interaction with other organelles seem to be a crucial mechanism underlying Hereditary Spastic Paraplegia (HSP) neurodegeneration. REEP1, a transmembrane protein belonging to TB2/HVA22 family, is implicated in SPG31, an autosomal dominant form of HSP, and its interaction with Atlastin/SPG3A and Spastin/SPG4, the other two major HSP linked proteins, has been demonstrated to play a crucial role in modifying ER architecture. In addition, the Drosophila ortholog of REEP1, named ReepA, has been found to regulate the response to ER neuronal stress. Herein we investigated the role of ReepA in ER morphology and stress response. ReepA is upregulated under stress conditions and aging. Our data show that ReepA triggers a selective activation of Ire1 and Atf6 branches of Unfolded Protein Response (UPR) and modifies ER morphology. Drosophila lacking ReepA showed Atf6 and Ire1 activation, expansion of ER sheet-like structures, locomotor dysfunction and shortened lifespan. Furthermore, we found that naringenin, a flavonoid that possesses strong antioxidant and neuroprotective activity, can rescue the cellular phenotypes, the lifespan and locomotor disability associated with ReepA loss of function. Our data highlight the importance of ER homeostasis in nervous system functionality and HSP neurodegenerative mechanisms, opening new opportunities for HSP treatment.

Peripheral neuropathy in hereditary spastic paraplegia caused by REEP1 variants.

SPG31 is a hereditary spastic paraplegia (HSP) caused by pathogenic variants in the REEP1 gene. The phenotype (SPG31) has occasionally been described with peripheral nervous system involvement, in additional to the gradually progressing lower limb spasticity that characterizes HSP. The objective of this study was to characterize patients with pathogenic REEP1 variants and neurophysiologically assess the extent of peripheral nerve involvement in this patient group. Thirty-eight index cases were molecular-genetically tested, yielding two previously reported pathogenic REEP1 variants and a novel missense variant, in a total of four index patients. Three of four probands and five additional family members underwent nerve conduction studies, electromyography, quantitative sensory testing, and examination of the autonomic nervous system. None of the examined patients had completely unremarkable results of peripheral nerve studies. Most showed electrophysiological signs of carpal tunnel syndrome, and one patient demonstrated a multifocal compression neuropathy. Autonomic testing revealed no severe dysfunction, and findings were limited to adrenergic function. HSP caused by pathogenic REEP1 variants may be accompanied by a generally mild and subclinical polyneuropathy with a predisposition to compression neuropathy, and should be considered in such cases.

Spastic paraplegia type 31: A novel REEP1 splice site donor variant and expansion of the phenotype variability.

Mutations in REEP1 have been identified in three types of neurological disorders, autosomal dominant form of Hereditary Spastic Paraplegia type 31 (SPG31), autosomal dominant distal hereditary motor neuronopathy type VB (HMN5B), and autosomal recessive form of congenital axonal neuropathy and diaphragmatic palsy. Previous studies demonstrated different molecular pathogenesis in SPG31, including loss-of-function, gain-of-function and haploinsufficiency. A four-generation family from Japan, including 12 members, was investigated clinically and genetically. Seven affected members displayed pure spastic paraplegia. Impression of genetic anticipation was observed in the family, including tendency of earlier age-at-onset and increasing severity in subsequent generations. Genetic analysis revealed a heterozygous intronic variant, c.303+2T > A, in REEP1, which segregated with disease, and was also identified in one unaffected member. The variant causes exon 4 skipping leading to frame shift and a truncated transcript identified by complementary DNA sequencing of reverse transcription polymerase chain reaction products. Measurement of REEP1 transcripts in lymphocytes demonstrated a reduction through nonsense mediated mRNA decay (NMD). Our study demonstrated further evidence of allelic heterogeneity in SPG31, mutant REEP1 mRNA dosage effects through NMD and intra-familial phenotype variability.

Rare interstitial deletion of chromosome 2p11.2p12. Report of a new patient with developmental delay and unusual clinical features.

De novo interstitial deletions of the short arm of chromosome 2 are rare chromosomal abnormalities. Patients showing these kind of microdeletions have developmental delay/intellectual disability, minor facial anomalies including high forehead, frontal bossing, broad nasal bridge, abnormal ears and congenital defects such as skeletal and genital malformations. We describe the second child of a healthy and non consanguineous couple presenting at birth multiple malformations and minor facial anomalies. Because of the clinical findings, an array CGH analysis was performed using Agilent 60K microarray oligonucleotide. The analysis detected a 9.3 Mb deletion on the short arm of chromosome 2 at band p11.2p12 spanning the bases 77,946,599-87,277,610. The five patients previously described display a minimal common deleted region which explains the clinical features shared by all of them, while their individual characteristics might be explained by the different sizes of the deletion. The common deleted region involves several genes (CTNNA2, LRRTM1, REEP1), highly expressed in the nervous system. The deletion found in this case overlaps with most of those reported in literature but our patient displays extra clinical signs such as bilateral choanal atresia and atrial septal defect. It was impossible to find any direct correlation between the genes involved in the deletion and the choanal atresia and the heart defect. The question remains open as to whether these clinical features are a consequence of the deletion or are due to a second pathogenic event. Our case emphasizes the difficulties to find a close correlation between a large deletion and a well defined clinical picture. As only five patients with 2p11.2p12 deletions, reported in the literature are characterized by array CGH, further reports will be necessary to well define a clinical phenotype related to the 2p11.2p12 microdeletion.

Molecular spectrum of the SPAST, ATL1 and REEP1 gene mutations associated with the most common hereditary spastic paraplegias in a group of Polish patients.

Hereditary spastic paraplegias (HSPs) consist of a heterogeneous group of genetically determined neurodegenerative disorders. Progressive lower extremity weakness and spasticity are the prominent features of HSPs resulting from retrograde axonal degeneration of the corticospinal tracts. Three genetic types, SPG3 (ATL1), SPG4 (SPAST) and SPG31 (REEP1), appear predominantly and may account for up to 50% of autosomal dominant hereditary spastic paraplegias (AD-HSPs). Here, we present the results of genetic testing of the three mentioned SPG genetic types in a group of 216 unrelated Polish patients affected with spastic paraplegia. Molecular evaluation was performed by multiplex ligation-dependent probe amplification (MLPA) and DNA sequencing. Nineteen novel mutations: 13 in SPAST, 4 in ATL1 and 2 in REEP1, were identified among overall 50 different mutations detected in 57 families. Genetic analysis resulted in the identification of molecular defects in 54% of familial and 8.4% of isolated cases. Our research expanded the causative mutations spectrum of the three most common genetic forms of HSPs found in a large cohort of probands originating from the Central Europe.

Mutation analysis of SPAST, ATL1, and REEP1 in Korean Patients with Hereditary Spastic Paraplegia.

BACKGROUND AND PURPOSE: Hereditary spastic paraplegia (HSP) is a genetically heterogeneous group of neurodegenerative disorders that are characterized by progressive spasticity and weakness of the lower limbs. Mutations in the spastin gene (SPAST) are the most common causes of HSP, accounting for 40-67% of autosomal dominant HSP (AD-HSP) and 12-18% of sporadic cases. Mutations in the atlastin-1 gene (ATL1) and receptor expression-enhancing protein 1 gene (REEP1) are the second and third most common causes of AD-HSP, respectively. METHODS: Direct sequence analysis was used to screen mutations in SPAST, ATL1, and REEP1 in 27 unrelated Korean patients with pure and complicated HSP. Multiplex ligation-dependent probe amplification was also performed to detect copy-number variations of the three genes. RESULTS: Ten different SPAST mutations were identified in 11 probands, of which the following 6 were novel: c.760A>T, c.131C>A, c.1351_1353delAGA, c.376_377dupTA, c.1114A>G, and c.1372A>C. Most patients with SPAST mutations had AD-HSP (10/11, 91%), and the frequency of SPAST mutations accounted for 66.7% (10/15) of the AD-HSP patients. No significant correlation was found between the presence of the SPAST mutation and any of the various clinical parameters of pure HSP. No ATL1 and REEP1 mutations were detected. CONCLUSIONS: We conclude that SPAST mutations are responsible for most Korean cases of genetically confirmed AD-HSP. Our observation of the absence of ATL1 and REEP1 mutations needs to be confirmed in larger series.