Loss of function of glucocerebrosidase GBA2 is responsible for motor neuron defects in hereditary spastic paraplegia

The Link between Gaucher Disease and Parkinson's Disease Sheds Light on Old and Novel Disorders of Sphingolipid Metabolism

Sphingolipid metabolism starts with the biosynthesis of ceramide, a bioactive lipid and the backbone for the biosynthesis of complex sphingolipids such as sphingomyelin and glycosphingolipids. These are degraded back to ceramide and then to sphingosine, which enters the ceramide–sphingosine-1-phosphate signaling pathway or is further degraded. Several enzymes with multiple catalytic properties and subcellular localizations are thus involved in such metabolism. Hereditary defects of lysosomal hydrolases have been known for several years to be the cause of lysosomal storage diseases such as gangliosidoses, Gaucher disease, Niemann–Pick disease, Krabbe disease, Fabry disease, and Farber disease. More recently, many other inborn errors of sphingolipid metabolism have been recognized, involving enzymes responsible for the biosynthesis of ceramide, sphingomyelin, and glycosphingolipids. Concurrently, epidemiologic and biochemical evidence has established a link between Gaucher disease and Parkinson’s disease, showing that glucocerebrosidase variants predispose individuals to α-synuclein accumulation and neurodegeneration even in the heterozygous status. This appears to be due not only to lysosomal overload of non-degraded glucosylceramide, but to the derangement of vesicle traffic and autophagy, including mitochondrial autophagy, triggered by both sphingolipid intermediates and misfolded proteins. In this review, old and novel disorders of sphingolipid metabolism, in particular those of ganglioside biosynthesis, are evaluated in light of recent investigations of the link between Gaucher disease and Parkinson’s disease, with the aim of better understanding their pathogenic mechanisms and addressing new potential therapeutic strategies.

Clinical features and genetic spectrum in Chinese patients with recessive hereditary spastic paraplegia

BACKGROUND

Although many causative genes of hereditary spastic paraplegia (HSP) have been uncovered in recent years, there are still approximately 50% of HSP patients without genetically diagnosis, especially in autosomal recessive (AR) HSP patients. Rare studies have been performed to determine the genetic spectrum and clinical profiles of recessive HSP patients in the Chinese population.

METHODS

In this study, we investigated 24 Chinese index AR/sporadic patients by targeted next-generation sequencing (NGS), Sanger sequencing and multiplex ligation-dependent probe amplification (MLPA). Further functional studies were performed to identify pathogenicity of those uncertain significance variants.

RESULTS

We identified 11 mutations in HSP related genes including 7 novel mutations, including two (p.V1979_L1980delinsX, p.F2343 fs) in SPG11, two (p.T55 M, p.S308 T) in AP5Z1, one (p.S242 N) in ALDH18A1, one (p.D597fs) in GBA2, and one (p.Q486X) in ATP13A2 in 8 index patients and their family members. Mutations in ALDH18A1, AP5Z1, CAPN1 and ATP13A2 genes were firstly reported in the Chinese population. Furthermore, the clinical phenotypes of the patients carrying mutations were described in detail. The mutation (p.S242 N) in ALDH18A1 decreased enzyme activity of P5CS and mutations (p.T55 M, p.S308 T) in AP5Z1 induced lysosomal dysfunction.

CONCLUSION

Our results expanded the genetic spectrum and clinical profiles of AR-HSP patients and further demonstrated the efficiency and reliability of targeted NGS diagnosing suspected HSP patients.

Assay of β-glucosidase 2 (GBA2) activity using lithocholic acid β-3-O-glucoside substrate for cultured fibroblasts and glucosylceramide for brain tissue

Beta (β)-glucosidase 2 (GBA2) is deficient in a form of human spastic paraplegia due to defects in GBA2 (SPG46). GBA2 was proposed as a modifier of Gaucher disease, a lysosomal storage disease resulting from deficient β-glucosidase 1; GBA1. Current GBA2 activity assays using artificial substrates incompletely model the activity encountered in vivo. We studied GBA2 activity, using lithocholic acid β-glucoside or glucosylceramide as natural β-glucosidase substrates in murine tissues or cultured patient fibroblasts with the pathologic genotypes: Gba1-/-; Gba2-/-; GBA1-/-; GBA2+/- and found expected and unexpected deviations from normal controls.

Hereditary spastic paraplegias: time for an objective case definition and a new nosology for neurogenetic disorders to facilitate biomarker/therapeutic studies

INTRODUCTION

Hereditary spastic paraplegias (HSPs) are heterogeneous neurodegenerative disorders characterized by progressive lower limb weakness and spasticity as core symptoms of the degeneration of the corticospinal motor neurons. Even after exclusion of infectious and toxic mimickers of these disorders, the definitive diagnosis remains tricky, mainly in sporadic forms, as there is significant overlap with other disorders. Since their first description, various attempts failed to reach an appropriate classification. This was due to the constant expansion of the clinical spectrum of these diseases and the discovery of new genes, a significant number of them was involved in overlapping diseases. Areas covered: In this perspective review, an extensive literature study was conducted on the historical progress of HSP research. We also revised the previous and the current classifications of HSP and the closely related neurogenetic disorders and analyzed the areas of overlap. Expert opinion: There is undeniable need for objective case definition and reclassification of all neurogenetic disorders including HSPs, a prerequisite to improve patient follow-up, biomarker identification and develop therapeutics. The challenge is to understand why mutations can give rise to multiple phenotypic presentations along this spectrum of diseases in which the corticospinal tract is affected.

Lucky, times ten: A career in Texas science

On January 21, 2017, I received an E-mail from Herb Tabor that I had been simultaneously hoping for and dreading for several years: an invitation to write a "Reflections" article for the Journal of Biological Chemistry On the one hand, I was honored to receive an invitation from Herb, a man I have admired for over 40 years, known for 24 years, and worked with as a member of the Editorial Board and Associate Editor of the Journal of Biological Chemistry for 17 years. On the other hand, the invitation marked the waning of my career as an academic scientist. With these conflicting emotions, I wrote this article with the goals of recording my career history and recognizing the many mentors, trainees, and colleagues who have contributed to it and, perhaps with pretension, with the desire that students who are beginning a career in research will find inspiration in the path I have taken and appreciate the importance of luck.

Species-specific differences in nonlysosomal glucosylceramidase GBA2 function underlie locomotor dysfunction arising from loss-of-function mutations

The nonlysosomal glucosylceramidase β2 (GBA2) catalyzes the hydrolysis of glucosylceramide to glucose and ceramide. Mutations in the human GBA2 gene have been associated with hereditary spastic paraplegia (HSP), autosomal-recessive cerebellar ataxia (ARCA), and the Marinesco-Sjögren-like syndrome. However, the underlying molecular mechanisms are ill-defined. Here, using biochemistry, immunohistochemistry, structural modeling, and mouse genetics, we demonstrate that all but one of the spastic gait locus #46 (SPG46)-connected mutations cause a loss of GBA2 activity. We demonstrate that GBA2 proteins form oligomeric complexes and that protein-protein interactions are perturbed by some of these mutations. To study the pathogenesis of GBA2-related HSP and ARCA in vivo, we investigated GBA2-KO mice as a mammalian model system. However, these mice exhibited a high phenotypic variance and did not fully resemble the human phenotype, suggesting that mouse and human GBA2 differ in function. Whereas some GBA2-KO mice displayed a strong locomotor defect, others displayed only mild alterations of the gait pattern and no signs of cerebellar defects. On a cellular level, inhibition of GBA2 activity in isolated cerebellar neurons dramatically affected F-actin dynamics and reduced neurite outgrowth, which has been associated with the development of neurological disorders. Our results shed light on the molecular mechanism underlying the pathogenesis of GBA2-related HSP and ARCA and reveal species-specific differences in GBA2 function in vivo.

Whole-genome resequencing reveals signatures of selection and timing of duck domestication

Background

The genetic basis of animal domestication remains poorly understood, and systems with substantial phenotypic differences between wild and domestic populations are useful for elucidating the genetic basis of adaptation to new environments as well as the genetic basis of rapid phenotypic change. Here, we sequenced the whole genome of 78 individual ducks, from two wild and seven domesticated populations, with an average sequencing depth of 6.42X per individual.

Results

Our population and demographic analyses indicate a complex history of domestication, with early selection for separate meat and egg lineages. Genomic comparison of wild to domesticated populations suggests that genes that affect brain and neuronal development have undergone strong positive selection during domestication. Our F_ST analysis also indicates that the duck white plumage is the result of selection at the melanogenesis-associated transcription factor locus.

Conclusions

Our results advance the understanding of animal domestication and selection for complex phenotypic traits.

Distinguishing the differences in β-glycosylceramidase folds, dynamics, and actions informs therapeutic uses

Glycosyl hydrolases (GHs) are carbohydrate-active enzymes that hydrolyze a specific β-glycosidic bond in glycoconjugate substrates; β-glucosidases degrade glucosylceramide, a ubiquitous glycosphingolipid. GHs are grouped into structurally similar families that themselves can be grouped into clans. GH1, GH5, and GH30 glycosidases belong to clan A hydrolases with a catalytic (β/α)8 TIM barrel domain, whereas GH116 belongs to clan O with a catalytic (α/α)6 domain. In humans, GH abnormalities underlie metabolic diseases. The lysosomal enzyme glucocerebrosidase (family GH30), deficient in Gaucher disease and implicated in Parkinson disease etiology, and the cytosol-facing membrane-bound glucosylceramidase (family GH116) remove the terminal glucose from the ceramide lipid moiety. Here, we compare enzyme differences in fold, action, dynamics, and catalytic domain stabilization by binding site occupancy. We also explore other glycosidases with reported glycosylceramidase activity, including human cytosolic β-glucosidase, intestinal lactase-phlorizin hydrolase, and lysosomal galactosylceramidase. Last, we describe the successful translation of research to practice: recombinant glycosidases and glucosylceramide metabolism modulators are approved drug products (enzyme replacement therapies). Activity-based probes now facilitate the diagnosis of enzyme deficiency and screening for compounds that interact with the catalytic pocket of glycosidases. Future research may deepen the understanding of the functional variety of these enzymes and their therapeutic potential.

Lipids and synaptic functions

Synaptic functions have long been thought to be driven by proteins, especially the SNARE complex, contrasting with a relatively passive role for lipids constituting cell membranes. It is now clear that not only lipids, i.e. glycerophospholipids, sphingolipids and sterols, play a determinant role in the dynamics of synaptic membranes but they also actively contribute to the endocytosis and exocytosis of synaptic vesicles in conjunction with synaptic proteins. On the other hand, a growing number of inborn errors of metabolism affecting the nervous system have been related to defects in the synthesis and remodelling of fatty acids, phospholipids and sphingolipids. Alterations of the metabolism of these lipids would be expected to affect the dynamics of synaptic membranes and synaptic vesicles. Still, only few examples are currently documented. It remains to be determined to which extent the pathophysiology of disorders of complex lipids biosynthesis and remodelling share common pathogenic mechanisms with the more traditional synaptopathies.

Biochemical Characterization of the GBA2 c.1780G>C Missense Mutation in Lymphoblastoid Cells from Patients with Spastic Ataxia

The GBA2 gene encodes the non-lysosomal glucosylceramidase (NLGase), an enzyme that catalyzes the conversion of glucosylceramide (GlcCer) to ceramide and glucose. Mutations in GBA2 have been associated with the development of neurological disorders such as autosomal recessive cerebellar ataxia, hereditary spastic paraplegia, and Marinesco-Sjogren-Like Syndrome. Our group has previously identified the GBA2 c.1780G>C [p.Asp594His] missense mutation, in a Cypriot consanguineous family with spastic ataxia. In this study, we carried out a biochemical characterization of lymphoblastoid cell lines (LCLs) derived from three patients of this family. We found that the mutation strongly reduce NLGase activity both intracellularly and at the plasma membrane level. Additionally, we observed a two-fold increase of GlcCer content in LCLs derived from patients compared to controls, with the C₁₆ lipid being the most abundant GlcCer species. Moreover, we showed that there is an apparent compensatory effect between NLGase and the lysosomal glucosylceramidase (GCase), since we found that the activity of GCase was three-fold higher in LCLs derived from patients compared to controls. We conclude that the c.1780G>C mutation results in NLGase loss of function with abolishment of the enzymatic activity and accumulation of GlcCer accompanied by a compensatory increase in GCase.

Genetics and genomic medicine in Tunisia

Genetics and genomic medicine in Tunisia.

Revealing Clusters of Connected Pathways Through Multisource Data Integration in Huntington's Disease and Spastic Ataxia

The advancement of scientific and medical research over the past years has generated a wealth of experimental data from multiple technologies, including genomics, transcriptomics, proteomics, and other forms of -omics data, which are available for a number of diseases. The integration of such multisource data is a key component toward the success of precision medicine. In this paper, we are investigating a multisource data integration method developed by our group, regarding its ability to drive to clusters of connected pathways under two different approaches: first, a disease-centric approach, where we integrate data around a disease, and second, a gene-centric approach, where we integrate data around a gene. We have used as a paradigm for the first approach Huntington's disease (HD), a disease with a plethora of available data, whereas for the second approach the GBA2, a gene that is related to spastic ataxia (SA), a phenotype with sparse availability of data. Our paper shows that valuable information at the level of disease-related pathway clusters can be obtained for both HD and SA. New pathways that classical pathway analysis methods were unable to reveal, emerged as necessary "connectors" to build connected pathway stories formed as pathway clusters. The capability to integrate multisource molecular data, concluding to something more than the sum of the existing information, empowers precision and personalized medicine approaches.

Diseases of ganglioside biosynthesis: An expanding group of congenital disorders of glycosylation

Among the numerous congenital disorders of glycosylation concerning glycoproteins, only a single mutation in ganglioside biosynthesis had been reported until a few years ago: one in the ST3GAL5 gene, encoding GM3 synthase. More recently, additional mutations in the same gene were reported, together with several distinct mutations in the B4GALNT1 gene, encoding GM2/GD2/GA2 synthase. Patients suffering from ST3GAL5 deficiency present a devastating syndrome characterized by early onset and dramatic neurological and cognitive impairment, sometimes associated with dyspigmentation and an increased blood lactate concentration. On the other hand, B4GALNT1 mutations give rise to a form of complicated hereditary spastic paraplegia (HSP), previously referred to as HSP26. It is characterized by the late onset of lower limb weakness and mild to moderate intellectual impairment, which is usually not progressive. In addition to the most typical signs, some patients present ocular and endocrine signs, pes cavus, and psychiatric illness. Since the nineties, mice lacking genes for single glycosyltransferases involved in ganglioside biosynthesis, including ST3GAL5 and B4GALNT1, were created and studied. The resulting phenotypes were frequently mild or very mild, so double knock-out animals were created to effectively study the function of gangliosides. The main clinical and biochemical features of patients suffering from GM3 synthase or GM2/GD2/GA2 synthase deficiency, compared with the phenotypes described in mice that are null for single or multiple glycosyltransferase genes, provide suggestions to improve the recognition of novel mutations and potentially related disorders.

The genetic nomenclature of recessive cerebellar ataxias

The recessive cerebellar ataxias are a large group of degenerative and metabolic disorders, the diagnostic management of which is difficult because of the enormous clinical and genetic heterogeneity. Because of several limitations, the current classification systems provide insufficient guidance for clinicians and researchers. Here, we propose a new nomenclature for the genetically confirmed recessive cerebellar ataxias according to the principles and criteria laid down by the International Parkinson and Movement Disorder Society Task Force on Classification and Nomenclature of Genetic Movement Disorders. We apply stringent criteria for considering an association between gene and phenotype to be established. The newly proposed list of recessively inherited cerebellar ataxias includes 62 disorders that were assigned an ATX prefix, followed by the gene name, because these typically present with ataxia as a predominant and/or consistent feature. An additional 30 disorders that often combine ataxia with a predominant or consistent other movement disorder received a double prefix (e.g., ATX/HSP). We also identified a group of 89 entities that usually present with complex nonataxia phenotypes, but may occasionally present with cerebellar ataxia. These are listed separately without the ATX prefix. This new, transparent and adaptable nomenclature of the recessive cerebellar ataxias will facilitate the clinical recognition of recessive ataxias, guide diagnostic testing in ataxia patients, and help in interpreting genetic findings. © 2018 International Parkinson and Movement Disorder Society.

Interactome analysis reveals ZNF804A, a schizophrenia risk gene, as a novel component of protein translational machinery critical for embryonic neurodevelopment

Recent genome-wide association studies identified over 100 genetic loci that significantly associate with schizophrenia (SZ). A top candidate gene, ZNF804A, was robustly replicated in different populations. However, its neural functions are largely unknown. Here we show in mouse that ZFP804A, the homolog of ZNF804A, is required for normal progenitor proliferation and neuronal migration. Using a yeast two-hybrid genome-wide screen, we identified novel interacting proteins of ZNF804A. Rather than transcriptional factors, genes involved in mRNA translation are highly represented in our interactome result. ZNF804A co-fractionates with translational machinery and modulates the translational efficiency as well as the mTOR pathway. The ribosomal protein RPSA interacts with ZNF804A and rescues the migration and translational defects caused by ZNF804A knockdown. RNA immunoprecipitation-RNAseq (RIP-Seq) identified transcripts bound to ZFP804A. Consistently, ZFP804A associates with many short transcripts involved in translational and mitochondrial regulation. Moreover, among the transcripts associated with ZFP804A, a SZ risk gene, neurogranin (NRGN), is one of ZFP804A targets. Interestingly, downregulation of ZFP804A decreases NRGN expression and overexpression of NRGN can ameliorate ZFP804A-mediated migration defect. To verify the downstream targets of ZNF804A, a Duolink in situ interaction assay confirmed genes from our RIP-Seq data as the ZNF804A targets. Thus, our work uncovered a novel mechanistic link of a SZ risk gene to neurodevelopment and translational control. The interactome-driven approach here is an effective way for translating genome-wide association findings into novel biological insights of human diseases.

Clinical and Functional Characterization of a Missense ELF2 Variant in a CANVAS Family

Cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome (CANVAS) is a rare disorder with an unknown etiology. We present a British family with presumed autosomal dominant CANVAS with incomplete penetrance and variable expressivity. Exome sequencing identified a rare missense variant in the ELF2 gene at chr4:g.140058846 C > T, c.10G > A, p.A4T which segregated in all affected patients. By using transduced BE (2)-M17 cells, we found that the mutated ELF2 (mt-ELF2) gene increased ATXN2 and reduced ELOVL5 gene expression, the causal genes of type 2 and type 38 spinocerebellar ataxias. Both, western blot and confocal microscopy confirmed an increase of ataxin-2 in BE(2)-M17 cells transduced with lentivirus expressing mt-ELF2 (CEE-mt-ELF2), which was not observed in cells transduced with lentivirus expressing wt-ELF2 (CEE-wt-ELF2). Moreover, we observed a significant decrease in the number and size of lipid droplets in the CEE-mt-ELF2-transduced BE (2)-M17 cells, but not in the CEE-wt-ELF2-transduced BE (2)-M17. Furthermore, changes in the expression of ELOVL5 could be related with the reduction of lipid droplets in BE (2)-M17 cells. This work supports that ELF2 gene regulates the expression of ATXN2 and ELOVL5 genes, and defines new molecular links in the pathophysiology of cerebellar ataxias.

Novel homozygous GBA2 mutation in a patient with complicated spastic paraplegia

Hereditary spastic paraplegias (HSPs) are a heterogeneous group of neurological disorders characterized primarily by a pyramidal syndrome with lower limb spasticity, which can manifest as pure HSP or associated with a number of neurological or non-neurological signs (i.e., complicated HSPs). The clinical variability of HSPs is associated with a wide genetic heterogeneity, with more than eighty causative genes known. Recently, next generation sequencing (NGS) has allowed increasing genetic definition in such a heterogeneous group of disorders. We report on a 56- year-old man affected by sporadic complicated HSP consisting of a pyramidal syndrome, cerebellar ataxia, congenital cataract, pes cavus, axonal sensory-motor peripheral neuropathy and cognitive decline. Brain MRI showed cerebellar atrophy and thin corpus callosum. By NGS we found a novel homozygous biallelic c.452-1G > C mutation in the b-glucosidase 2 gene (GBA2), known to be causative for autosomal recessive hereditary spastic paraplegia type 46 (SPG46). The rarity of this inherited form besides reporting on a novel mutation, expands the genetic and clinical spectrum of SPG46 related HSP.

Endocrine disorders and the cerebellum: from neurodevelopmental injury to late-onset ataxia

Hormonal disorders are a source of cerebellar ataxia in both children and adults. Normal development of the cerebellum is critically dependent on thyroid hormone, which crosses both the blood-brain barrier and the blood-cerebrospinal fluid barrier thanks to specific transporters, including monocarboxylate transporter 8 and the organic anion-transporting polypeptide 1C1. In particular, growth and dendritic arborization of Purkinje neurons, synaptogenesis, and myelination are dependent on thyroid hormone. Disturbances of thyroid hormone may also impact on cerebellar ataxias of other origin, decompensating or aggravating the pre-existing ataxia manifesting with motor ataxia, oculomotor ataxia, and/or Schmahmann syndrome. Parathyroid disorders are associated with a genuine cerebellar syndrome, but symptoms may be subtle. The main conditions combining diabetes and cerebellar ataxia are Friedreich ataxia, ataxia associated with anti-GAD antibodies, autoimmune polyglandular syndromes, aceruloplasminemia, and cerebellar ataxia associated with hypogonadism (especially Holmes ataxia/Boucher-Neuhäuser syndrome). The general workup of cerebellar disorders should include the evaluation of hormonal status, including thyroid-stimulating hormone and free thyroxine levels, and hormonal replacement should be considered depending on the laboratory results. Cerebellar deficits may be reversible in some cases.

The Enigmatic Role of GBA2 in Controlling Locomotor Function

The non-lysosomal glucosylceramidase GBA2 catalyzes the hydrolysis of glucosylceramide to glucose and ceramide. Loss of GBA2 function results in accumulation of glucosylceramide. Mutations in the human GBA2 gene have been associated with hereditary spastic paraplegia (HSP) and autosomal-recessive cerebellar ataxia (ARCA). Patients suffering from these disorders exhibit impaired locomotion and neurological abnormalities. GBA2 mutations found in these patients have been proposed to impair GBA2 function. However, the molecular mechanism underlying the occurrence of mutations in the GBA2 gene and the development of locomotor dysfunction is not well-understood. In this review, we aim to summarize recent findings regarding mutations in the GBA2 gene and their impact on GBA2 function in health and disease.

Lipid Involvement in Neurodegenerative Diseases of the Motor System: Insights from Lysosomal Storage Diseases

Lysosomal storage diseases (LSDs) are a heterogeneous group of rare inherited metabolic diseases that are frequently triggered by the accumulation of lipids inside organelles of the endosomal-autophagic-lysosomal system (EALS). There is now a growing realization that disrupted lysosomal homeostasis (i.e., lysosomal cacostasis) also contributes to more common neurodegenerative disorders such as Parkinson disease (PD). Lipid deposition within the EALS may also participate in the pathogenesis of some additional neurodegenerative diseases of the motor system. Here, I will highlight the lipid abnormalities and clinical manifestations that are common to LSDs and several diseases of the motor system, including amyotrophic lateral sclerosis (ALS), atypical forms of spinal muscular atrophy, Charcot-Marie-Tooth disease (CMT), hereditary spastic paraplegia (HSP), multiple system atrophy (MSA), PD and spinocerebellar ataxia (SCA). Elucidating the underlying basis of intracellular lipid mislocalization as well as its consequences in each of these disorders will likely provide innovative targets for therapeutic research.

A Fluorescence Polarization Activity-Based Protein Profiling Assay in the Discovery of Potent, Selective Inhibitors for Human Nonlysosomal Glucosylceramidase

Human nonlysosomal glucosylceramidase (GBA2) is one of several enzymes that controls levels of glycolipids and whose activity is linked to several human disease states. There is a major need to design or discover selective GBA2 inhibitors both as chemical tools and as potential therapeutic agents. Here, we describe the development of a fluorescence polarization activity-based protein profiling (FluoPol-ABPP) assay for the rapid identification, from a 350+ library of iminosugars, of GBA2 inhibitors. A focused library is generated based on leads from the FluoPol-ABPP screen and assessed on GBA2 selectivity offset against the other glucosylceramide metabolizing enzymes, glucosylceramide synthase (GCS), lysosomal glucosylceramidase (GBA), and the cytosolic retaining β-glucosidase, GBA3. Our work, yielding potent and selective GBA2 inhibitors, also provides a roadmap for the development of high-throughput assays for identifying retaining glycosidase inhibitors by FluoPol-ABPP on cell extracts containing recombinant, overexpressed glycosidase as the easily accessible enzyme source.

Peripheral neuropathy in complex inherited diseases: an approach to diagnosis

Peripheral neuropathy is a common finding in patients with complex inherited neurological diseases and may be subclinical or a major component of the phenotype. This review aims to provide a clinical approach to the diagnosis of this complex group of patients by addressing key questions including the predominant neurological syndrome associated with the neuropathy, for example, spasticity, the type of neuropathy and the other neurological and non-neurological features of the syndrome. Priority is given to the diagnosis of treatable conditions. Using this approach, we associated neuropathy with one of three major syndromic categories: (1) ataxia, (2) spasticity and (3) global neurodevelopmental impairment. Syndromes that do not fall easily into one of these three categories can be grouped according to the predominant system involved in addition to the neuropathy, for example, cardiomyopathy and neuropathy. We also include a separate category of complex inherited relapsing neuropathy syndromes, some of which may mimic Guillain-Barré syndrome, as many will have a metabolic aetiology and be potentially treatable.

Hereditary Spastic Paraplegia: Clinical and Genetic Hallmarks

Hereditary spastic paraplegia comprises a wide and heterogeneous group of inherited neurodegenerative and neurodevelopmental disorders resulting from primary retrograde dysfunction of the long descending fibers of the corticospinal tract. Although spastic paraparesis and urinary dysfunction represent the most common clinical presentation, a complex group of different neurological and systemic compromise has been recognized recently and a growing number of new genetic subtypes were described in the last decade. Clinical characterization of individual and familial history represents the main step during diagnostic workup; however, frequently, few and unspecific data allows a low rate of definite diagnosis based solely in clinical and neuroimaging basis. Likewise, a wide group of neurological acquired and inherited disorders should be included in the differential diagnosis and properly excluded after a complete laboratorial, neuroimaging, and genetic evaluation. The aim of this review article is to provide an extensive overview regarding the main clinical and genetic features of the classical and recently described subtypes of hereditary spastic paraplegia (HSP).

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.

A novel function for glucocerebrosidase as a regulator of sterylglucoside metabolism

BACKGROUND

Sterols are major cell membrane lipids, and in many organisms they are modified with glucose to generate sterylglucosides. Glucosylation dramatically changes the functional properties of sterols. The formation of sterylglucosides from sterols in plants, fungi, and bacteria uses UDP-glucose as a glucose donor. By contrast, sterylglucoside biosynthesis in mammals is catalyzed by the transglucosylation activity of glucocerebrosidases, with glucosylceramide acting as the glucose donor. Recent success in isolation and structural determination of sterylglucosides in the vertebrate central nervous system shows that transglucosylation also occurs in vivo. These analyses also revealed that sterylglucoside aglycons are composed of several cholesterol-related metabolites, including a plant-type sitosteryl.

SCOPE OF REVIEW

In this review, we discuss the biological functions and metabolism of sterylglucosides. We also summarize new findings from studies on the metabolism of vertebrate sterylglucosides and review the circumstances underlying the recent discovery of sterylglucosides in vertebrate brain. Finally, we discuss the role of sterylglucosides in a variety of neurodegenerative disorders such as Gaucher disease and Parkinson's disease.

MAJOR CONCLUSIONS

The biological significance of UDP-glucose-independent sterol glucosylation is still unknown, but it is plausible that glucosylation may provide sterols with novel biological functions. Even though sterol glucosylation is a simple reaction, it can dramatically change the physical properties of sterols.

GENERAL SIGNIFICANCE

Sterylglucosides may play roles in various physiological processes and in the pathogenesis of different diseases. Arriving at a better understanding of them at the organ and cellular level may open up new approaches to developing therapeutics for a variety of diseases. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Overcoming the divide between ataxias and spastic paraplegias: Shared phenotypes, genes, and pathways

Autosomal-dominant spinocerebellar ataxias, autosomal-recessive spinocerebellar ataxias, and hereditary spastic paraplegias have traditionally been designated in separate clinicogenetic disease classifications. This classification system still largely frames clinical thinking and genetic workup in clinical practice. Yet, with the advent of next-generation sequencing, phenotypically unbiased studies have revealed the limitations of this classification system. Various genes (eg, SPG7, SYNE1, PNPLA6) traditionally rooted in either the ataxia or hereditary spastic paraplegia classification system have now been shown to cause ataxia on the one end of the disease continuum and hereditary spastic paraplegia on the other. Other genes such as GBA2 and KIF1C were almost simultaneously published as both a hereditary spastic paraplegia and an ataxia gene. The variability and fluidity of observed phenotypes along the ataxia-spasticity spectrum warrants a rethinking of the traditional classification system. We propose to replace this divisive diagnosis-driven ataxia and hereditary spastic paraplegia classification system by a descriptive, unbiased approach of modular phenotyping. This approach is also open to expansion of the phenotype beyond ataxia and spasticity, which often occur as part of broader multisystem neuronal dysfunction. The concept of a continuous ataxia-spasticity disease spectrum is further supported by ataxias and hereditary spastic paraplegias sharing not only overlapping phenotypes and underlying genes, but also common cellular pathways and disease mechanisms. This suggests a shared vulnerability of cerebellar and corticospinal neurons for common pathophysiological processes. It might be this mechanistic overlap that drives their clinical overlap. A mechanistically inspired classification system will help to pave the way for mechanism-based strategies for drug development. © 2017 International Parkinson and Movement Disorder Society.

GBA2 Mutations Cause a Marinesco-Sjögren-Like Syndrome: Genetic and Biochemical Studies

Background

With the advent new sequencing technologies, we now have the tools to understand the phenotypic diversity and the common occurrence of phenocopies. We used these techniques to investigate two Norwegian families with an autosomal recessive cerebellar ataxia with cataracts and mental retardation.

Methods and Results

Single nucleotide polymorphism (SNP) chip analysis followed by Exome sequencing identified a 2 bp homozygous deletion in GBA2 in both families, c.1528_1529del [p.Met510Valfs*17]. Furthermore, we report the biochemical characterization of GBA2 in these patients. Our studies show that a reduced activity of GBA2 is sufficient to elevate the levels of glucosylceramide to similar levels as seen in Gaucher disease. Furthermore, leucocytes seem to be the proper enzyme source for in vitro analysis of GBA2 activity.

Conclusions

We report GBA2 mutations causing a Marinesco-Sjögren-like syndrome in two Norwegian families. One of the families was originally diagnosed with Marinesco-Sjögren syndrome based on an autosomal recessive cerebellar ataxia with cataracts and mental retardation. Our findings highlight the phenotypic variability associated with GBA2 mutations, and suggest that patients with Marinesco-Sjögren-like syndromes should be tested for mutations in this gene.

Aglycon diversity of brain sterylglucosides: structure determination of cholesteryl- and sitosterylglucoside

To date, sterylglucosides have been reported to be present in various fungi, plants, and animals. In bacteria, such as Helicobacter pylori, proton NMR spectral analysis of isolated 1-O-cholesteryl-β-d-glucopyranoside (GlcChol) demonstrated the presence of an α-glucosidic linkage. By contrast, in animals, no detailed structural analysis of GlcChol has been reported, in part because animal-derived samples contain a high abundance of glucosylceramides (GlcCers)/galactosylceramides, which exhibit highly similar chromatographic behavior to GlcChol. A key step in vertebrate GlcChol biosynthesis is the transglucosylation reaction catalyzed by glucocerebrosidase (GBA)1 or GBA2, utilizing GlcCer as a glucose donor. These steps are expected to produce a β-glucosidic linkage. Impaired GBA1 and GBA2 function is associated with neurological disorders, such as cerebellar ataxia, spastic paraplegia, and Parkinson's disease. Utilizing a novel three-step chromatographic procedure, we prepared highly enriched GlcChol from embryonic chicken brain, allowing complete structural confirmation of the β-glucosidic linkage by ¹H-NMR analysis. Unexpectedly, during purification, two additional sterylglucoside fractions were isolated. NMR and GC/MS analyses confirmed that the plant-type sitosterylglucoside in vertebrate brain is present throughout embryonic development. The aglycon structure of the remaining sterylglucoside (GSX-2) remains elusive due to its low abundance. Together, our results uncovered unexpected aglycon heterogeneity of sterylglucosides in vertebrate brain.

High β-glucosidase (GBA) activity not attributable to GBA1 and GBA2 in live normal and enzyme-deficient fibroblasts may emphasise the role of additional GBAs

Beta-glucosidases (GBA) include GBA1, GBA2 and other β-glucosidases (non-GBA1-2). GBA1 is a lysosomal and GBA2 an extra-lysosomal enzyme. GBA1- and GBA2-deficient genetic conditions, with different phenotypes, are glucosylceramide (GC; the main GBA substrate) accumulating diseases. To study the activity profile of GBA, live fibroblasts were loaded with radioactive GC. The GC metabolism was measured in wild-type, GBA1-deficient (Gaucher disease) and GBA2-deficient (Gba2(-/- )mouse) cells. The differences found allowed the prediction of marked proportions of GBA1, GBA2, and particularly non-GBA1-2 (probably including GBA3, a cytosolic β-glucosidase) activity for wild-type cells. The high proportion of non-GBA1-2 suggests an important role of these enzymes.

Bacterial β-Glucosidase Reveals the Structural and Functional Basis of Genetic Defects in Human Glucocerebrosidase 2 (GBA2)

Human glucosylcerebrosidase 2 (GBA2) of the CAZy family GH116 is responsible for the breakdown of glycosphingolipids on the cytoplasmic face of the endoplasmic reticulum and Golgi apparatus. Genetic defects in GBA2 result in spastic paraplegia and cerebellar ataxia, while cross-talk between GBA2 and GBA1 glucosylceramidases may affect Gaucher disease. Here, we report the first three-dimensional structure for any GH116 enzyme, Thermoanaerobacterium xylanolyticum TxGH116 β-glucosidase, alone and in complex with diverse ligands. These structures allow identification of the glucoside binding and active site residues, which are shown to be conserved with GBA2. Mutagenic analysis of TxGH116 and structural modeling of GBA2 provide a detailed structural and functional rationale for pathogenic missense mutations of GBA2.

Nomenclature of genetic movement disorders: Recommendations of the international Parkinson and movement disorder society task force

The system of assigning locus symbols to specify chromosomal regions that are associated with a familial disorder has a number of problems when used as a reference list of genetically determined disorders,including (I) erroneously assigned loci, (II) duplicated loci, (III) missing symbols or loci, (IV) unconfirmed loci and genes, (V) a combination of causative genes and risk factor genes in the same list, and (VI) discordance between phenotype and list assignment. In this article, we report on the recommendations of the International Parkinson and Movement Disorder Society Task Force for Nomenclature of Genetic Movement Disorders and present a system for naming genetically determined movement disorders that addresses these problems. We demonstrate how the system would be applied to currently known genetically determined parkinsonism, dystonia, dominantly inherited ataxia, spastic paraparesis, chorea, paroxysmal movement disorders, neurodegeneration with brain iron accumulation, and primary familial brain calcifications. This system provides a resource for clinicians and researchers that, unlike the previous system, can be considered an accurate and criterion-based list of confirmed genetically determined movement disorders at the time it was last updated.

Klotho-Related Protein KLrP: Structure and Functions

Klotho (KL) family proteins share one or two glycoside hydrolase (GH) motifs homologous to GH family 1. However, the biological significance of GH motifs in KL family proteins remains elusive. We describe here that KL-related protein (KLrP), which is composed of a single GH motif, is a cytosolic β-glucocerebrosidase (GCase, EC 3.2.1.145). We detected a neutral conduritol B epoxide (CBE)-insensitive glucosylceramide (GlcCer)-degrading activity in the cytosol fractions of human fibroblasts, rat brains, and zebrafish embryos. KL family proteins emerged as a potent candidate for the neutral GCase using a bioinformatics approach. Recombinant human KLrP, but not α-KL, β-KL, or KLPH, exhibited GCase activity with a neutral pH optimum in the presence of CBE. We solved the crystal structures of KLrP and a KLrP mutant (E165Q) in complex with glucose, which indicate that KLrP forms a (β/α)8TIM barrel structure with the double-displacement mechanism of the retaining β-glycosidase. Furthermore, knockdown of endogenous KLrP in CHOP cells using small interfering RNA (siRNA) decreased the CBE-insensitive neutral GCase activity and increased the cellular levels of GlcCer, which suggests that KLrP is involved in a novel GlcCer catabolism pathway. A KLrP D106N mutant was discovered in patients with severe Gaucher disease; however, this mutation did not affect the GCase activity of KLrP.

Transcriptomic analysis of bottlenose dolphin (Tursiops truncatus) skin biopsies to assess the effects of emerging contaminants

Chemicals discovered in water at levels that may be significantly different than expected are referred to as contaminants of emerging concern (CECs) because the risk to environmental health posed by their occurrence/frequency is still unknown. The worldwide distributed compounds perfluorooctanoic acid (PFOA) and bisphenol A (BPA) may fall into this category due to effects on endocrine receptors. We applied an ex vivo assay using small slices of bioptic skin from the bottlenose dolphin, Tursiops truncatus, cultured and treated for 24 h with different PFOA or BPA concentrations to analyze global gene expression. RNA was labeled and hybridized to a species-specific oligomicroarray. The skin transcriptome held information on the contaminant exposure, potentially predictive about long-term effects on health, being the genes affected involved in immunity modulation, response to stress, lipid homeostasis, and development. The transcriptomic signature of dolphin skin could be therefore relevant as classifier for a specific contaminant.

Characterization of the Zebrafish Homolog of β-Glucosidase 2: A Target of the Drug Miglustat

The small-molecular compound miglustat (N-butyldeoxynojirimycin, Zavesca(®)) has been approved for clinical use in type 1 Gaucher disease and Niemann-Pick type C disease, which are disorders caused by dysfunction of the endosomal-autophagic-lysosomal system. Miglustat inhibits a number of enzymes involved in glycoconjugate and glycan metabolism, including β-glucosidase 2 (GBA2), which is exceptionally sensitive to inhibition by miglustat. GBA2 is a glucosylceramide-degrading enzyme that is located on the plasma membrane/endoplasmic reticulum, and is distinct from the lysosomal enzyme glucocerebrosidase (GBA). Various strands of evidence suggest that inhibition of GBA2 contributes to the therapeutic benefits of miglustat. To further explore the pharmacology and biology of GBA2, we investigated whether the zebrafish homolog of GBA2 has similar enzymatic properties and pharmacological sensitivities to its human counterpart. We established that zebrafish has endogenous β-glucosidase activity toward lipid- and water-soluble GBA2 substrates, which can be inhibited by miglustat, N-butyldeoxygalactonojirimycin, and conduritol B epoxide. β-Glucosidase activities with highly similar characteristics were expressed in cells transfected with the zebrafish gba2 cDNA and in cells transfected with the human GBA2 cDNA. These results provide a foundation for the use of zebrafish in screening GBA2-targeting molecules, and for wider studies investigating GBA2 biology.

Loss of function variants in human PNPLA8 encoding calcium-independent phospholipase A2 γ recapitulate the mitochondriopathy of the homologous null mouse

Mitochondriopathies are a group of clinically heterogeneous genetic diseases caused by defects in mitochondrial metabolism, bioenergetic efficiency, and/or signaling functions. The large majority of proteins involved in mitochondrial function are encoded by nuclear genes, with many yet to be associated with human disease. We performed exome sequencing on a young girl with a suspected mitochondrial myopathy that manifested as progressive muscle weakness, hypotonia, seizures, poor weight gain, and lactic acidosis. She was compound heterozygous for two frameshift mutations, p.Asn112HisfsX29 and p.Leu659AlafsX4, in the PNPLA8 gene, which encodes mitochondrial calcium-independent phospholipase A2 γ (iPLA2 γ). Western blot analysis of affected muscle displayed the absence of PNPLA8 protein. iPLA2 s are critical mediators of a variety of cellular processes including growth, metabolism, and lipid second messenger generation, exerting their functions through catalyzing the cleavage of the acyl groups in glycerophospholipids. The clinical presentation, muscle histology and the mitochondrial ultrastructural abnormalities of this proband are highly reminiscent of Pnpla8 null mice. Although other iPLA2 -related diseases have been identified, namely, infantile neuroaxonal dystrophy and neutral lipid storage disease with myopathy, this is the first report of PNPLA8-related disease in a human. We suggest PNPLA8 join the increasing list of human genes involved in lipid metabolism associated with neuromuscular diseases due to mitochondrial dysfunction.

ALS5/SPG11/KIAA1840 mutations cause autosomal recessive axonal Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease is a group of hereditary peripheral neuropathies that share clinical characteristics of progressive distal muscle weakness and atrophy, foot deformities, distal sensory loss, as well as diminished tendon reflexes. Hundreds of causative DNA changes have been found, but much of the genetic basis of the disease is still unexplained. Mutations in the ALS5/SPG11/KIAA1840 gene are a frequent cause of autosomal recessive hereditary spastic paraplegia with thin corpus callosum and peripheral axonal neuropathy, and account for ∼ 40% of autosomal recessive juvenile amyotrophic lateral sclerosis. The overlap of axonal Charcot-Marie-Tooth disease with both diseases, as well as the common autosomal recessive inheritance pattern of thin corpus callosum and axonal Charcot-Marie-Tooth disease in three related patients, prompted us to analyse the ALS5/SPG11/KIAA1840 gene in affected individuals with autosomal recessive axonal Charcot-Marie-Tooth disease. We investigated 28 unrelated families with autosomal recessive axonal Charcot-Marie-Tooth disease defined by clinical, electrophysiological, as well as pathological evaluation. Besides, we screened for all the known genes related to axonal autosomal recessive Charcot-Marie-Tooth disease (CMT2A2/HMSN2A2/MFN2, CMT2B1/LMNA, CMT2B2/MED25, CMT2B5/NEFL, ARCMT2F/dHMN2B/HSPB1, CMT2K/GDAP1, CMT2P/LRSAM1, CMT2R/TRIM2, CMT2S/IGHMBP2, CMT2T/HSJ1, CMTRID/COX6A1, ARAN-NM/HINT and GAN/GAN), for the genes related to autosomal recessive hereditary spastic paraplegia with thin corpus callosum and axonal peripheral neuropathy (SPG7/PGN, SPG15/ZFYVE26, SPG21/ACP33, SPG35/FA2H, SPG46/GBA2, SPG55/C12orf65 and SPG56/CYP2U1), as well as for the causative gene of peripheral neuropathy with or without agenesis of the corpus callosum (SLC12A6). Mitochondrial disorders related to Charcot-Marie-Tooth disease type 2 were also excluded by sequencing POLG and TYMP genes. An additional locus for autosomal recessive Charcot-Marie-Tooth disease type 2H on chromosome 8q13-21.1 was excluded by linkage analysis. Pedigrees originated in Italy, Brazil, Canada, England, Iran, and Japan. Interestingly, we identified 15 ALS5/SPG11/KIAA1840 mutations in 12 families (two sequence variants were never reported before, p.Gln198* and p.Pro2212fs*5). No large deletions/duplications were detected in these patients. The novel mutations seemed to be pathogenic since they co-segregated with the disease in all pedigrees and were absent in 300 unrelated controls. Furthermore, in silico analysis predicted their pathogenic effect. Our results indicate that ALS5/SPG11/KIAA1840 is the causative gene of a wide spectrum of clinical features, including autosomal recessive axonal Charcot-Marie-Tooth disease.

Novel mutations in genes causing hereditary spastic paraplegia and Charcot-Marie-Tooth neuropathy identified by an optimized protocol for homozygosity mapping based on whole-exome sequencing

PURPOSE

Homozygosity mapping is an effective approach for detecting molecular defects in consanguineous families by delineating stretches of genomic DNA that are identical by descent. Constant developments in next-generation sequencing created possibilities to combine whole-exome sequencing (WES) and homozygosity mapping in a single step.

METHODS

Basic optimization of homozygosity mapping parameters was performed in a group of families with autosomal-recessive (AR) mutations for which both single-nucleotide polymorphism (SNP) array and WES data were available. We varied the criteria for SNP extraction and PLINK thresholds to estimate their effect on the accuracy of homozygosity mapping based on WES.

RESULTS

Our protocol showed high specificity and sensitivity for homozygosity detection and facilitated the identification of novel mutations in GAN, GBA2, and ZFYVE26 in four families affected by hereditary spastic paraplegia or Charcot-Marie-Tooth disease. Filtering and mapping with optimized parameters was integrated into the HOMWES (homozygosity mapping based on WES analysis) tool in the GenomeComb package for genomic data analysis.

CONCLUSION

We present recommendations for detection of homozygous regions based on WES data and a bioinformatics tool for their identification, which can be widely applied for studying AR disorders.Genet Med 18 6, 600-607.

Visualization of Active Glucocerebrosidase in Rodent Brain with High Spatial Resolution following In Situ Labeling with Fluorescent Activity Based Probes

Gaucher disease is characterized by lysosomal accumulation of glucosylceramide due to deficient activity of lysosomal glucocerebrosidase (GBA). In cells, glucosylceramide is also degraded outside lysosomes by the enzyme glucosylceramidase 2 (GBA2) of which inherited deficiency is associated with ataxias. The interest in GBA and glucosylceramide metabolism in the brain has grown following the notion that mutations in the GBA gene impose a risk factor for motor disorders such as α-synucleinopathies. We earlier developed a β-glucopyranosyl-configured cyclophellitol-epoxide type activity based probe (ABP) allowing in vivo and in vitro visualization of active molecules of GBA with high spatial resolution. Labeling occurs through covalent linkage of the ABP to the catalytic nucleophile residue in the enzyme pocket. Here, we describe a method to visualize active GBA molecules in rat brain slices using in vivo labeling. Brain areas related to motor control, like the basal ganglia and motor related structures in the brainstem, show a high content of active GBA. We also developed a β-glucopyranosyl cyclophellitol-aziridine ABP allowing in situ labeling of GBA2. Labeled GBA2 in brain areas can be identified and quantified upon gel electrophoresis. The distribution of active GBA2 markedly differs from that of GBA, being highest in the cerebellar cortex. The histological findings with ABP labeling were confirmed by biochemical analysis of isolated brain areas. In conclusion, ABPs offer sensitive tools to visualize active GBA and to study the distribution of GBA2 in the brain and thus may find application to establish the role of these enzymes in neurodegenerative disease conditions such as α-synucleinopathies and cerebellar ataxia.

Overexpression of KLC2 due to a homozygous deletion in the non-coding region causes SPOAN syndrome

SPOAN syndrome is a neurodegenerative disorder mainly characterized by spastic paraplegia, optic atrophy and neuropathy (SPOAN). Affected patients are wheelchair bound after 15 years old, with progressive joint contractures and spine deformities. SPOAN patients also have sub normal vision secondary to apparently non-progressive congenital optic atrophy. A potential causative gene was mapped at 11q13 ten years ago. Here we performed next-generation sequencing in SPOAN-derived samples. While whole-exome sequencing failed to identify the causative mutation, whole-genome sequencing allowed to detect a homozygous 216-bp deletion (chr11.hg19:g.66,024,557_66,024,773del) located at the non-coding upstream region of the KLC2 gene. Expression assays performed with patient's fibroblasts and motor neurons derived from SPOAN patients showed KLC2 overexpression. Luciferase assay in constructs with 216-bp deletion confirmed the overexpression of gene reporter, varying from 48 to 74%, as compared with wild-type. Knockdown and overexpression of klc2 in Danio rerio revealed mild to severe curly-tail phenotype, which is suggestive of a neuromuscular disorder. Overexpression of a gene caused by a small deletion in the non-coding region is a novel mechanism, which to the best of our knowledge, was never reported before in a recessive condition. Although the molecular mechanism of KLC2 up-regulation still remains to be uncovered, such example adds to the importance of non-coding regions in human pathology.

Lack of enzyme activity in GBA2 mutants associated with hereditary spastic paraplegia/cerebellar ataxia (SPG46)

Glucosylceramide is a membrane glycolipid made up of the sphingolipid ceramide and glucose, and has a wide intracellular distribution. Glucosylceramide is degraded to ceramide and glucose by distinct, non-homologous enzymes, including glucocerebrosidase (GBA), localized in the endolysosomal pathway, and β-glucosidase 2 (GBA2), which is associated with the plasma membrane and/or the endoplasmic reticulum. It is well established that mutations in the GBA gene result in endolysosomal glucosylceramide accumulation, which triggers Gaucher disease. In contrast, the biological significance of GBA2 is less well understood. GBA2-deficient mice present with male infertility, but humans carrying mutations in the GBA2 gene are affected with a combination of cerebellar ataxia and spastic paraplegia, as well as with thin corpus callosum and cognitive impairment (SPastic Gait locus #46, SPG46). To improve our understanding of the biochemical consequences of the GBA2 mutations, we have evaluated five nonsense and five missense GBA2 mutants for their enzyme activity. In transfected cells, the mutant forms of GBA2 were present in widely different amounts, ranging from overabundant to very minor, compared to the wild type enzyme. Nevertheless, none of the GBA2 mutant cDNAs raised the enzyme activity in transfected cells, in contrast to the wild-type enzyme. These results suggest that SPG46 patients have a severe deficit in GBA2 activity, because the GBA2 mutants are intrinsically inactive and/or reduced in amount. This assessment of the expression levels and enzyme activities of mutant forms of GBA2 offers a first insight in the biochemical basis of the complex pathologies seen in SPG46.

Glycosphingolipids are modulators of disease pathogenesis in amyotrophic lateral sclerosis

Recent genetic evidence suggests that aberrant glycosphingolipid metabolism plays an important role in several neuromuscular diseases including hereditary spastic paraplegia, hereditary sensory neuropathy type 1, and non-5q spinal muscular atrophy. Here, we investigated whether altered glycosphingolipid metabolism is a modulator of disease course in amyotrophic lateral sclerosis (ALS). Levels of ceramide, glucosylceramide, galactocerebroside, lactosylceramide, globotriaosylceramide, and the gangliosides GM3 and GM1 were significantly elevated in spinal cords of ALS patients. Moreover, enzyme activities (glucocerebrosidase-1, glucocerebrosidase-2, hexosaminidase, galactosylceramidase, α-galactosidase, and β-galactosidase) mediating glycosphingolipid hydrolysis were also elevated up to threefold. Increased ceramide, glucosylceramide, GM3, and hexosaminidase activity were also found in SOD1(G93A) mice, a familial model of ALS. Inhibition of glucosylceramide synthesis accelerated disease course in SOD1(G93A) mice, whereas infusion of exogenous GM3 significantly slowed the onset of paralysis and increased survival. Our results suggest that glycosphingolipids are likely important participants in pathogenesis of ALS and merit further analysis as potential drug targets.

An Update on the Hereditary Spastic Paraplegias: New Genes and New Disease Models

Aims

The hereditary spastic paraplegias (HSPs) are a heterogeneous group of disorders characterized by spasticity in the lower limbs. We provide an overview of HSP with an emphasis on recent developments.

Methods

A PubMed search using the term "hereditary spastic paraplegia" and "hereditary spastic paraparesis" was conducted for a period from January 2012 to January 2015. We discuss and critique the major studies in the field over this 36-month period.

Results

A total of 346 publications were identified, of which 47 were selected for review. We provide an update of the common forms of HSP and include patient videos. We also discuss how next-generation sequencing (NGS) has led to the accelerated discovery of new HSP genes, including B4GALNT1,DDHD1, C19orf12,GBA2,TECPR2,DDHD2, C12orf65,REEP2, and IBA57. Moreover, a single study alone identified 18 previously unknown putative HSP genes and created a model for the protein interactions of HSP, called the "HSPome." Many of the newly reported genes cause rare, complicated, autosomal recessive forms of HSP. NGS also has important clinical applications by facilitating the molecular diagnosis of HSP. Furthermore, common genetic forms of HSP have been studied using new disease models, such as neurons derived from induced pluripotent stem cells. These models have been used to elucidate important disease mechanisms and have served as platforms to screen for candidate drug compounds.

Conclusion

The field of HSP research has been progressing at a rapid pace. The challenge remains in translating these advances into new targeted disease therapies.

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.