Pure Hereditary Spastic Paraplegia Research Articles

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

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. 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. 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. Despite the commonp.V168M mutation identified inERLIN2,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. 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.

KCNJ3 is a novel candidate gene for autosomal dominant pure hereditary spastic paraplegia identified using whole genome sequencing.

Hereditary spastic paraplegia (HSP) is a group of familial diseases characterized by progressive corticospinal tract degeneration. Clinically, patients present with lower-limb spasticity and weakness. To date, more than 80 genetic HSP types have been identified. Despite advances in molecular genetics, novel HSP gene discoveries are ongoing, with a low genetic diagnostic yield. In this study, we aimed to determine pathogenic variants in a family with HSP, which was not diagnosed through conventional genetic testing. We clinically characterized a large family and conducted whole genome sequencing (WGS) analysis of four affected and three unaffected individuals in the family to identify the genetic cause of HSP. This family had autosomal dominant pure (uncomplicated) late childhood-onset HSP. The patients' symptoms accelerated between the ages of 20 and 30. Brain magnetic resonance images typically showed white matter changes, a thin corpus callosum, and cerebellar atrophy. We identified a heterozygous missense variant, KCNJ3 c.1297T>G (p.Leu433Val), through WGS and family genetic analysis, confirmed by Sanger sequencing. We suggest that the identification of KCNJ3 c.1297T>G (p.Leu433Val) constitutes the discovery of a potential novel gene responsible for HSP in this family. This is the first study to report the possible role of a KCNJ3 variant in HSP pathogenesis. Our findings further expand the phenotypic and genotypic spectrum of HSP.

Cohort analysis of novel SPAST variants in SPG4 patients and implementation of in vitro and in vivo studies to identify the pathogenic mechanism caused by splicing mutations.

Pure hereditary spastic paraplegia (SPG) type 4 (SPG4) is caused by mutations of SPAST gene. This study aimed to analyze SPAST variants in SPG4 patients to highlight the occurrence of splicing mutations and combine functional studies to assess the relevance of these variants in the molecular mechanisms of the disease. We performed an NGS panel in 105 patients, in silico analysis for splicing mutations, and in vitro minigene assay. The NGS panel was applied to screen 105 patients carrying a clinical phenotype corresponding to upper motor neuron syndrome (UMNS), selectively affecting motor control of lower limbs. Pathogenic mutations in SPAST were identified in 12 patients (11.42%), 5 missense, 3 frameshift, and 4 splicing variants. Then, we focused on the patients carrying splicing variants using a combined approach of in silico and in vitro analysis through minigene assay and RNA, if available. For two splicing variants (i.e., c.1245+1G>A and c.1414-2A>T), functional assays confirm the types of molecular alterations suggested by the in silico analysis (loss of exon 9 and exon 12). In contrast, the splicing variant c.1005-1delG differed from what was predicted (skipping exon 7), and the functional study indicates the loss of frame and formation of a premature stop codon. The present study evidenced the high splice variants in SPG4 patients and indicated the relevance of functional assays added to in silico analysis to decipher the pathogenic mechanism.

Biallelic COQ4 Variants in Hereditary Spastic Paraplegia: Clinical and Molecular Characterization.

Hereditary spastic paraplegias (HSP) are neurologic disorders characterized by progressive lower-extremity spasticity. Despite the identification of several HSP-related genes, many patients lack a genetic diagnosis. The aims were to confirm the pathogenic role of biallelic COQ4 mutations in HSP and elucidate the clinical, genetic, and functional molecular features of COQ4-associated HSP. Whole exome sequences of 310 index patients with HSP of unknown cause from three distinct populations were analyzed to identify potential HSP causal genes. Clinical data obtained from patients harboring candidate causal mutations were examined. Functional characterization of COQ4 variants was performed using bioinformatic tools, single-cell RNA sequencing, biochemical assays in cell lines, primary fibroblasts, induced pluripotent stem cell-derived pyramidal neurons, and zebrafish. Compound heterozygous variants in COQ4, which cosegregated with HSP in pedigrees, were identified in 7 patients from six unrelated families. Patients from four of the six families presented with pure HSP, whereas probands of the other two families exhibited complicated HSP with epilepsy or with cerebellar ataxia. In patient-derived fibroblasts and COQ4 knockout complementation lines, stable expression of these missense variants exerted loss-of-function effects, including mitochondrial reactive oxygen species accumulation, decreased mitochondrial membrane potential, and lower ubiquinone biosynthesis. Whereas differentiated pyramidal neurons expressed high COQ4 levels, coq4 knockdown zebrafish displayed severe motor dysfunction, reflecting motor neuron dysregulation. Our study confirms that loss-of-function, compound heterozygous, pathogenic COQ4 variants are causal for autosomal recessive pure and complicated HSP. Moreover, reduced COQ4 levels attributable to variants correspond with decreased ubiquinone biosynthesis, impaired mitochondrial function, and higher phenotypic disease severity. © 2023 International Parkinson and Movement Disorder Society.

Biallelic variants in the COQ4 gene caused hereditary spastic paraplegia predominant phenotype.

Hereditary spastic paraplegias (HSPs) comprise a group of neurodegenerative disorders characterized by progressive degeneration of upper motor neurons. Homozygous or compound heterozygous variants in COQ4 have been reported to cause primary CoQ10 deficiency-7 (COQ10D7), which is a mitochondrial disease. We aimed to screened COQ4 variants in a cohort of HSP patients. A total of 87 genetically unidentified HSP index patients and their available family members were recruited. Whole exome sequencing (WES) was performed in all probands. Functional studies were performed to identify the pathogenicity of those uncertain significance variants. In this study, five different COQ4 variants were identified in three Chinese HSP pedigrees and two variants were novel, c.87dupT (p.Arg30*), c.304C>T (p.Arg102Cys). More importantly, we firstly described two early-onset pure HSP caused by COQ4 variants. Functional studies in patient-derived fibroblast lines revealed a reduction cellular CoQ10 levels and the abnormal mitochondrial structure. Our findings revealed that bilateral variants in the COQ4 gene caused HSP predominant phenotype, expanding the phenotypic spectrum of the COQ4-related disorders.

Novel TFG mutation causes autosomal-dominant spastic paraplegia and defects in autophagy

BackgroundMutations in the tropomyosin receptor kinase fused (TFG) gene are associated with various neurological disorders, including autosomal recessive hereditary spastic paraplegia (HSP), autosomal dominant hereditary motor and sensory neuropathy with...

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

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. 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). 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. 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.

Alu Retrotransposition Event in SPAST Gene as a Novel Cause of Hereditary Spastic Paraplegia.

To diagnose the molecular cause of hereditary spastic paraplegia (HSP) observed in a four-generation family with autosomal dominant inheritance. Multiplex ligation-dependent probe amplification (MLPA), whole-exome sequencing (WES), and RNA sequencing (RNA-seq) of peripheral blood leukocytes were performed. Reverse transcription polymerase chain reaction (RT-PCR) and Sanger sequencing were used to characterize target regions of SPAST. A 121-bp AluYb9 insertion with a 30-bp poly-A tail flanked by 15-bp direct repeats on both sides was identified in the edge of intron 16 in SPAST that segregated with the disease phenotype. We identified an intronic AluYb9 insertion inducing splicing alteration in SPAST causing pure HSP phenotype that was not detected by routine WES analysis. Our findings suggest RNA-seq is a recommended implementation for undiagnosed cases by first-line diagnostic approaches. © 2023 International Parkinson and Movement Disorder Society.

Novel mutation of SPG4 gene in a Chinese family with hereditary spastic paraplegia: A case report

Hereditary spastic paraplegia (HSP) is a group of neurogenetic diseases of the corticospinal tract, accompanied by distinct spasticity and weakness of the lower extremities. Mutations in the spastic paraplegia type 4 (SPG4) gene, encoding the spastin protein, are the major cause of the disease. This study reported a Chinese family with HSP caused by a novel mutation of the SPG4 gene. A 44-year-old male was admitted to our hospital for long-term right lower limb weakness, leg stiffness, and unstable walking. His symptoms gradually worsened, while no obvious muscle atrophy in the lower limbs was found. Neurological examinations revealed that the muscle strength of the lower limbs was normal, and knee reflex hyperreflexia and bilateral positive Babinski signs were detected. Members of his family also had the same symptoms. Using mutation analysis, a novel heterozygous duplication mutation, c.1053dupA, p. (Gln352Thrfs*15), was identified in the SPG4 gene in this family. A Chinese family with HSP had a novel mutation of the SPG4 gene, which is autosomal dominant and inherited as pure HSP. The age of onset, sex distribution, and clinical manifestations of all existing living patients in this family were analyzed. The findings may extend the current knowledge on the existing mutations in the SPG4 gene.

Genetic characterization of primary lateral sclerosis

Background and objectivesPrimary lateral sclerosis (PLS) is a motor neuron disease characterised by loss of the upper motor neurons. Most patients present with slowly progressive spasticity of the legs, which may also spread to the arms or bulbar regions. It is challenging to distinguish between PLS, early-stage amyotrophic lateral sclerosis (ALS) and hereditary spastic paraplegia (HSP). The current diagnostic criteria advise against extensive genetic testing. This recommendation is, however, based on limited data.MethodsWe aim to genetically characterize a PLS cohort using whole exome sequencing (WES) for genes associated with ALS, HSP, ataxia and movement disorders (364 genes) and C9orf72 repeat expansions. Patients fulfilling the definite PLS criteria by Turner et al. and with available DNA samples of sufficient quality were recruited from an on-going, population-based epidemiological study. Genetic variants were classified according to the ACMG criteria and assigned to groups based on disease association.ResultsWES was performed in 139 patients and the presence of repeat expansions in C9orf72 was analysed separately in 129 patients. This resulted in 31 variants of which 11 were (likely) pathogenic. (Likely) pathogenic variants resulted in 3 groups based on disease association: ALS-FTD (C9orf72, TBK1), pure HSP (SPAST, SPG7), “ALS-HSP-CMT overlap” (FIG4, NEFL, SPG11).DiscussionIn a cohort of 139 PLS patients, genetic analyses resulted in 31 variants (22%) of which 10 (7%) (likely) pathogenic associated with different diseases (predominantly ALS and HSP). Based on these results and the literature, we advise to consider genetic analyses in the diagnostic work-up for PLS.

NMNAT1 and hereditary spastic paraplegia (HSP): expanding the phenotypic spectrum of NMNAT1 variants

In the NAD biosynthetic network, the nicotinamide mononucleotide adenylyltransferase (NMNAT) enzyme fuels NAD as a co-substrate for a group of enzymes. Mutations in the nuclear-specific isoform, NMNAT1, have been extensively reported as the cause of Leber congenital amaurosis-type 9 (LCA9). However, there are no reports of NMNAT1 mutations causing neurological disorders by disrupting the maintenance of physiological NAD homeostasis in other types of neurons. In this study, for the first time, the potential association between a NMNAT1 variant and hereditary spastic paraplegia (HSP) is described. Whole-exome sequencing was performed for two affected siblings diagnosed with HSP. Runs of homozygosity (ROH) were detected. The shared variants of the siblings located in the homozygosity blocks were selected. The candidate variant was amplified and Sanger sequenced in the proband and other family members. Homozygous variant c.769G>A:p.(Glu257Lys) in NMNAT1, the most common variant of NMNAT1 in LCA9 patients, located in the ROH of chromosome 1, was detected as a probable disease-causing variant. After detection of the variant in NMNAT1, as a LCA9-causative gene, ophthalmological and neurological re-evaluations were performed. No ophthalmological abnormality was detected and the clinical manifestations of these patients were completely consistent with pure HSP. No NMNAT1 variant had ever been previously reported in HSP patients. However, NMNAT1 variants have been reported in a syndromic form of LCA which is associated with ataxia. In conclusion, our patients expand the clinical spectrum of NMNAT1 variants and represent the first evidence of the probable correlation between NMNAT1 variants and HSP.

Clinical and genetic characterization of NIPA1 mutations in a Taiwanese cohort with hereditary spastic paraplegia.

NIPA1 mutations have been implicated in hereditary spastic paraplegia (HSP) as the cause of spastic paraplegia type 6 (SPG6). The aim of this study was to investigate the clinical and genetic features of SPG6 in a Taiwanese HSP cohort. We screened 242 unrelated Taiwanese patients with HSP for NIPA1 mutations. The clinical features of patients with a NIPA1 mutation were analyzed. Minigene-based splicing assay, RT-PCR analysis on the patients' RNA, and cell-based protein expression study were utilized to assess the effects of the mutations on splicing and protein expression. Two patients were identified to carry a different heterozygous NIPA1 mutation. The two mutations, c.316G>A and c.316G>C, are located in the 3' end of NIPA1 exon 3 near the exon-intron boundary and putatively lead to the same amino acid substitution, p.G106R. The patient harboring NIPA1 c.316G>A manifested spastic paraplegia, epilepsy and schizophrenia since age 17 years, whereas the individual carrying NIPA1 c.316G>C had pure HSP since age 12 years. We reviewed literature and found that epilepsy was present in multiple individuals with NIPA1 c.316G>A but none with NIPA1 c.316G>C. Functional studies demonstrated that both mutations did not affect splicing, but only the c.316G>A mutation was associated with a significantly reduced NIPA1 protein expression. SPG6 accounted for 0.8% of HSP cases in the Taiwanese cohort. The NIPA1 c.316G>A and c.316G>C mutations are associated with adolescent-onset complex and pure form HSP, respectively. The different effects on protein expression of the two mutations may be associated with their phenotypic discrepancy.

Gait-Adaptability Training in People With Hereditary Spastic Paraplegia: A Randomized Clinical Trial

Background and objectives In people with hereditary spastic paraplegia (HSP), reduced gait adaptability is common and disabling. Gait impairments result from lower extremity spasticity, muscle weakness, and impaired proprioception. The aim of this study was to assess the efficacy of a 5-week gait-adaptability training in people with pure HSP. Method We conducted a randomized clinical trial with a cross-over design for the control group, and a 15-week follow-up period after training. Thirty-six people with pure HSP were randomized to 5 weeks of (i) gait-adaptability training (10 hours of C-Mill training—a treadmill equipped with augmented reality) or (ii) a waiting-list control period followed by 5 weeks gait-adaptability training. Both groups continued to receive usual care. The primary outcome was the obstacle subtask of the Emory Functional Ambulation Profile. Secondary outcome measures consisted of clinical balance and gait assessments, fall rates, and spatiotemporal gait parameters assessed via 3D motion analysis. Results The gait-adaptability training group (n = 18) did not significantly decrease the time required to perform the obstacle subtask compared to the waiting-list control group (n = 18) after adjusting for baseline differences (mean: −0.33 seconds, 95% CI: −1.3, 0.6). Similar, non-significant results were found for most secondary outcomes. After merging both groups (n = 36), the required time to perform the obstacle subtask significantly decreased by 1.3 seconds (95% CI: −2.1, −0.4) directly following 5 weeks of gait-adaptability training, and this effect was retained at the 15-week follow-up. Conclusions We found insufficient evidence to conclude that 5 weeks of gait-adaptability training leads to greater improvement of gait adaptability in people with pure HSP.

A novel truncating variant of SPAST associated with hereditary spastic paraplegia indicates a haploinsufficiency pathogenic mechanism.

Hereditary spastic paraplegias (HSPs) are genetic neurodegenerative diseases. The most common form of pure HSP that is inherited in an autosomal dominant manner is spastic paraplegia type 4 (SPG4), which is caused by mutations in the SPAST gene. Different theories have been proposed as the mechanism underlying SPAST-HSP for different types of genetic mutations, including gain- and loss-of-function mechanisms. To better understand the mutation mechanisms, we performed genetic analysis and investigated a truncating SPAST variant that segregated with disease in one family. We described a pure HSP pedigree with family members across four generations. We performed genetic analysis and investigated a novel frameshift pathogenic variant (c.862_863dupAC, p. H289Lfs*27) in this family. We performed reverse transcription-polymerase chain reaction (RT-PCR), Sanger sequencing, and quantitative RT-PCR using total RNA from an Epstein-Barr virus-induced lymphoblastoid cell line produced from the proband. We also performed Western blotting on cell lysates to investigate if the protein expression of spastin is affected by this variant. This variant (c.862_863dupAC, p. H289Lfs*27) co-segregated with pure HSP in this family and is not registered in any public database. Measurement of SPAST transcripts in lymphoblasts from the proband demonstrated a reduction of SPAST transcript levels through likely nonsense-mediated mRNA decay. Immunoblot analyses demonstrated a reduction of spastin protein expression levels in lymphoblasts. We report an SPG4 family with a novel heterozygous frameshift variant p.H289Lfs*27 in SPAST. Our study implies haploinsufficiency as the pathogenic mechanism for this variant and expands the known mutation spectrum of SPAST.

Clinical and genetic spectrum of hereditary spastic paraplegia in Chinese children.

To explore the clinical and genetic spectrum of hereditary spastic paraplegia (HSP) in Chinese children. This retrospective study was conducted between January 2014 and October 2021 in children clinically diagnosed with either pure HSP (pHSP) or complex HSP (cHSP). We investigated 45 children (32 males, 13 females; mean age [SD] at symptom onset 4 years [7 months]). clinically diagnosed with HSP and identified genetic causes in 35 patients. Most patients with autosomal dominant HSP had pHSP (16/18), whereas most patients with autosomal recessive HSP tended to have cHSP (14/16). SPG11 was the most common autosomal recessive subtype, followed by FA2H/SPG35, whereas SPAST/SPG4 was the most frequent cause of autosomal dominant HSP. Two patients with CPT1C mutations presented with a complex phenotype. Meanwhile, 10 patients were found to have likely pathogenic variants/variants of uncertain clinical significance in six genes related to HSP. SPG11 and SPG4 were the most frequent subtypes in Chinese children with autosomal recessive HSP and autosomal dominant HSP. However, the prevalence of SPG4 was much lower than that in adults, which might be explained by the late onset of the disease. On the other hand, FA2H/SPG35 was common in our cohort, while it contributed to only a small proportion of adult cases, which might be explained by its rapid progression and early death in some patients. We also expanded the genetic and clinical spectra of SPG73.

Early-Onset and Severe Complex Hereditary Spastic Paraplegia Caused by De Novo Variants in SPAST.

Familial hereditary spastic paraplegia (HSP)-SPAST (SPG4) typically presents with a pure HSP phenotype. The aim of this study was to delineate the genotypic and phenotypic spectrum of children with de novo HSP-SPAST. This study used a systematic cross-sectional analysis of clinical and molecular features. We report the clinical and molecular spectrum of 40 patients with heterozygous pathogenic de novo variants in SPAST (age range: 2.2-27.7 years). We identified 19 unique variants (16/40 carried the same recurrent variant, p.Arg499His). Symptom onset was in early childhood (median: 11.0 months, interquartile range: 6.0 months) with significant motor and speech delay, followed by progressive ascending spasticity, dystonia, neurogenic bladder dysfunction, gastrointestinal dysmotility, and epilepsy. The mean Spastic Paraplegia Rating Scale score was 32.8 ± 9.7 (standard deviation). These results confirm that de novo variants in SPAST lead to a severe and complex form of HSP that differs from classic familial pure HSP-SPAST. Clinicians should be aware of this syndrome in the differential diagnosis for cerebral palsy. © 2022 International Parkinson and Movement Disorder Society.

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.

Phenotypic and Genetic Heterogeneity of Adult Patients with Hereditary Spastic Paraplegia from Serbia.

Hereditary spastic paraplegia (HSP) is among the most genetically diverse of all monogenic diseases. The aim was to analyze the genetic causes of HSP among adult Serbian patients. The study comprised 74 patients from 65 families clinically diagnosed with HSP during a nine-year prospective period. A panel of thirteen genes was analyzed: L1CAM (SPG1), PLP1 (SPG2), ATL1 (SPG3A), SPAST (SPG4), CYP7B1 (SPG5A), SPG7 (SPG7), KIF5A (SPG10), SPG11 (SPG11), ZYFVE26 (SPG15), REEP1 (SPG31), ATP13A2 (SPG78), DYNC1H1, and BICD2 using a next generation sequencing-based technique. A copy number variation (CNV) test for SPAST, SPG7, and SPG11 was also performed. Twenty-three patients from 19 families (29.2%) had conclusive genetic findings, including 75.0% of families with autosomal dominant and 25.0% with autosomal recessive inheritance, and 15.7% of sporadic cases. Twelve families had mutations in the SPAST gene, usually with a pure HSP phenotype. Three sporadic patients had conclusive findings in the SPG11 gene. Two unrelated patients carried a homozygous pathogenic mutation c.233T>A (p.L78*) in SPG7 that is a founder Roma mutation. One patient had a heterozygous de novo variant in the KIF5A gene, and one had a compound heterozygous mutation in the ZYFVE26 gene. The combined genetic yield of our gene panel and CNV analysis for HSP was around 30%. Our findings broaden the knowledge on the genetic epidemiology of HSP, with implications for molecular diagnostics in this region.

Neuropathy-associated Fars2 deficiency affects neuronal development and potentiates neuronal apoptosis by impairing mitochondrial function

BackgroundNeurodegenerative diseases encompass an extensive and heterogeneous group of nervous system disorders which are characterized by progressive degeneration and death of neurons. Many lines of evidence suggest the participation of mitochondria dysfunction in these diseases. Mitochondrial phenylalanyl-tRNA synthetase, encoded by FARS2, catalyzes the transfer of phenylalanine to its cognate tRNA for protein synthesis. As a member of mt-aaRSs genes, FARS2 missense homozygous mutation c.424G > T (p.D142Y) found in a Chinese consanguineous family first built the relationship between pure hereditary spastic paraplegia (HSP) and FARS2 gene. More FARS2 variations were subsequently found to cause heterogeneous group of neurologic disorders presenting three main phenotypic manifestations: infantile-onset epileptic mitochondrial encephalopathy, later-onset spastic paraplegia and juvenile onset refractory epilepsy. Studies showed that aminoacylation activity is frequently disrupt in cases with FARS2 mutations, indicating a loss-of-function mechanism. However, the underlying pathogenesis of neuropathy-associated Fars2 deficiency is still largely unknown.ResultsEarly gestation lethality of global Fars2 knockout mice was observed prior to neurogenesis. The conditional Fars2 knockout-mouse model delayed lethality to late-gestation, resulting in a thinner cortex and an enlarged ventricle which is consist with the MRI results revealing cortical atrophy and reduced cerebral white matter volume in FARS2-deficient patients. Delayed development of neurite outgrowth followed by neuronal apoptosis was confirmed in Fars2-knockdown mouse primary cultured neurons. Zebrafish, in which fars2 was knocked down, exhibited aberrant motor neuron function including reduced locomotor capacity which well restored the spastic paraplegia phenotype of FARS2-deficient patients. Altered mitochondrial protein synthesis and reduced levels of oxidative phosphorylation complexes were detected in Fars2-deficient samples. And thus, reduced ATP, total NAD levels and mitochondrial membrane potential, together with increased ROS production, revealed mitochondrial dysfunction both in vitro and in vivo. Dctn3 is a potential downstream molecule in responds to Fars2 deficient in neurons, which may provide some evidence for the development of pathogenesis study and therapeutic schedule.ConclusionsThe Fars2 deficiency genetic models developed in this study cover the typical clinical manifestations in FARS2 patients, and help clarify how neuropathy-associated Fars2 deficiency, by damaging the mitochondrial respiratory chain and impairing mitochondrial function, affects neuronal development and potentiates neuronal cell apoptosis.

TMEM63C mutations cause mitochondrial morphology defects and underlie hereditary spastic paraplegia.

The hereditary spastic paraplegias (HSP) are among the most genetically diverse of all Mendelian disorders. They comprise a large group of neurodegenerative diseases that may be divided into ‘pure HSP’ in forms of the disease primarily entailing progressive lower-limb weakness and spasticity, and ‘complex HSP’ when these features are accompanied by other neurological (or non-neurological) clinical signs. Here, we identified biallelic variants in the transmembrane protein 63C (TMEM63C) gene, encoding a predicted osmosensitive calcium-permeable cation channel, in individuals with hereditary spastic paraplegias associated with mild intellectual disability in some, but not all cases. Biochemical and microscopy analyses revealed that TMEM63C is an endoplasmic reticulum-localized protein, which is particularly enriched at mitochondria–endoplasmic reticulum contact sites. Functional in cellula studies indicate a role for TMEM63C in regulating both endoplasmic reticulum and mitochondrial morphologies. Together, these findings identify autosomal recessive TMEM63C variants as a cause of pure and complex HSP and add to the growing evidence of a fundamental pathomolecular role of perturbed mitochondrial-endoplasmic reticulum dynamics in motor neurone degenerative diseases.