Gene disruption of Mfsd8 in mice provides the first animal model for CLN7 disease

Intragenic MFSD8 duplication and histopathological findings in a rabbit with neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses (NCL) are among the most prevalent neurodegenerative disorders of early life in humans. Disease-causing variants have been described for 13 different NCL genes. In this study, a refined pathological characterization of a female rabbit with progressive neurological signs reminiscent of NCL was performed. Cytoplasmic pigment present in neurons was weakly positive with Sudan black B and autofluorescent. Immunohistology revealed astrogliosis, microgliosis and axonal degeneration. During the subsequent genetic investigation, the genome of the affected rabbit was sequenced and examined for private variants in NCL candidate genes. The analysis revealed a homozygous ~10.7 kb genomic duplication on chromosome 15 comprising parts of the MFSD8 gene, NC_013683.1:g.103,727,963_103,738,667dup. The duplication harbors two internal protein coding exons and is predicted to introduce a premature stop codon into the transcript, truncating ~50% of the wild-type MFSD8 open reading frame encoding the major facilitator superfamily domain containing protein 8, XP_002717309.2:p.(Glu235Leufs*23). Biallelic loss-of-function variants in MFSD8 have been described to cause NCL7 in human patients, dogs and a single cat. The available clinical and pathological data, together with current knowledge about MFSD8 variants and their functional impact in other species, point to the MFSD8 duplication as a likely causative defect for the observed phenotype in the affected rabbit.

The involvement of Purkinje cells in progressive myoclonic epilepsy: Focus on neuronal ceroid lipofuscinosis

The progressive myoclonic epilepsies (PMEs) are a group of rare neurodegenerative diseases characterized by myoclonus, epileptic seizures, and progressive neurological deterioration with cerebellar involvement. They include storage diseases like Gaucher disease, Lafora disease, and forms of neuronal ceroid lipofuscinosis (NCL). To date, 13 NCLs have been reported (CLN1-CLN8, CLN10-CLN14), associated with mutations in different genes. These forms, which affect both children and adults, are characterized by seizures, cognitive and motor impairments, and in most cases visual loss. In NCLs, as in other PMEs, central nervous system (CNS) neurodegeneration is widespread and involves different subpopulations of neurons. One of the most affected regions is the cerebellar cortex, where motor and non-motor information is processed and transmitted to deep cerebellar nuclei through the axons of Purkinje cells (PCs). PCs, being GABAergic, have an inhibitory effect on their target neurons, and provide the only inhibitory output of the cerebellum. Degeneration of PCs has been linked to motor impairments and epileptic seizures. Seizures occur when some insult upsets the normal balance in the CNS between excitatory and inhibitory impulses, causing hyperexcitability. Here we review the role of PCs in epilepsy onset and progression following their PME-related loss. In particular, we focus on the involvement of PCs in seizure phenotype in NCLs, highlighting findings from case reports and studies of animal models in which epilepsy can be linked to PC loss.

Systematic in silico discovery of novel solute carrier-like proteins from proteomes

Solute carrier (SLC) proteins represent the largest superfamily of transmembrane transporters. While many of them play key biological roles, their systematic analysis has been hampered by their functional and structural heterogeneity. Based on available nomenclature systems, we hypothesized that many as yet unidentified SLC transporters exist in the human genome, which await further systematic analysis. Here, we present criteria for defining "SLC-likeness" to curate a set of "SLC-like" protein families from the Transporter Classification Database (TCDB) and Protein families (Pfam) databases. Computational sequence similarity searches surprisingly identified ~120 more proteins in human with potential SLC-like properties compared to previous annotations. Interestingly, several of these have documented transport activity in the scientific literature. To complete the overview of the "SLC-ome", we present an algorithm to classify SLC-like proteins into protein families, investigating their known functions and evolutionary relationships to similar proteins from 6 other clinically relevant experimental organisms, and pinpoint structural orphans. We envision that our work will serve as a stepping stone for future studies of the biological function and the identification of the natural substrates of the many under-explored SLC transporters, as well as for the development of new therapeutic applications, including strategies for personalized medicine and drug delivery.

Mfsd8 Modulates Growth and the Early Stages of Multicellular Development in Dictyostelium discoideum

MFSD8 is a transmembrane protein that has been reported to transport chloride ions across the lysosomal membrane. Mutations in MFSD8 are associated with a subtype of Batten disease called CLN7 disease. Batten disease encompasses a family of 13 inherited neurodegenerative lysosomal storage diseases collectively referred to as the neuronal ceroid lipofuscinoses (NCLs). Previous work identified an ortholog of human MFSD8 in the social amoeba D. discoideum (gene: mfsd8, protein: Mfsd8), reported its localization to endocytic compartments, and demonstrated its involvement in protein secretion. In this study, we further characterized the effects of mfsd8 loss during D. discoideum growth and early stages of multicellular development. During growth, mfsd8⁻ cells displayed increased rates of proliferation, pinocytosis, and expansion on bacterial lawns. Loss of mfsd8 also increased cell size, inhibited cytokinesis, affected the intracellular and extracellular levels of the quorum-sensing protein autocrine proliferation repressor A, and altered lysosomal enzyme activity. During the early stages of development, loss of mfsd8 delayed aggregation, which we determined was at least partly due to impaired cell-substrate adhesion, defects in protein secretion, and alterations in lysosomal enzyme activity. Overall, these results show that Mfsd8 plays an important role in modulating a variety of processes during the growth and early development of D. discoideum.

Glial Dysfunction and Its Contribution to the Pathogenesis of the Neuronal Ceroid Lipofuscinoses

While significant efforts have been made in developing pre-clinical treatments for the neuronal ceroid lipofuscinoses (NCLs), many challenges still remain to bring children with NCLs a cure. Devising effective therapeutic strategies for the NCLs will require a better understanding of pathophysiology, but little is known about the mechanisms by which loss of lysosomal proteins causes such devastating neurodegeneration. Research into glial cells including astrocytes, microglia, and oligodendrocytes have revealed many of their critical functions in brain homeostasis and potential contributions to neurodegenerative diseases. Genetically modified mouse models have served as a useful platform to define the disease progression in the central nervous system across NCL subtypes, revealing a wide range of glial responses to disease. The emerging evidence of glial dysfunction questions the traditional “neuron-centric” view of NCLs, and would suggest that directly targeting glia in addition to neurons could lead to better therapeutic outcomes. This review summarizes the most up-to-date understanding of glial pathologies and their contribution to the pathogenesis of NCLs, and highlights some of the associated challenges that require further research.

CLN7 gene therapy: hope for an ultra-rare condition

CLN7 Batten disease, also known as variant late infantile neuronal ceroid lipofuscinosis type 7 (vLINCL7), is an ultra-rare form of Batten disease that presents early in life with severe neurological symptoms, including visual deficits, motor problems, and frequent seizures. There is high unmet need for disease-modifying therapies, as no existing treatment can halt progression or prevent premature death. In this issue of the JCI, Chen et al. present an AAV gene therapy for CLN7 that shows marked benefit in a mouse model of CLN7 Batten disease, paving the way for a phase I trial. The candidate gene therapy shows benefit for histopathology, behavioral abnormalities, and survival in mice and offers an acceptable safety profile in both mice and rats. Questions remain regarding dose, scaling, and timing of administration for patients, but this work is a substantial step forward for a very challenging disease.

AAV9/MFSD8 gene therapy is effective in preclinical models of neuronal ceroid lipofuscinosis type 7 disease

Neuronal ceroid lipofuscinosis type 7 (CLN7) disease is a lysosomal storage disease caused by mutations in the facilitator superfamily domain containing 8 (MFSD8) gene, which encodes a membrane-bound lysosomal protein, MFSD8. To test the effectiveness and safety of adeno-associated viral (AAV) gene therapy, an in vitro study demonstrated that AAV2/MFSD8 dose dependently rescued lysosomal function in fibroblasts from a CLN7 patient. An in vivo efficacy study using intrathecal administration of AAV9/MFSD8 to Mfsd8- /- mice at P7-P10 or P120 with high or low dose led to clear age- and dose-dependent effects. A high dose of AAV9/MFSD8 at P7-P10 resulted in widespread MFSD8 mRNA expression, tendency of amelioration of subunit c of mitochondrial ATP synthase accumulation and glial fibrillary acidic protein immunoreactivity, normalization of impaired behaviors, doubled median life span, and extended normal body weight gain. In vivo safety studies in rodents concluded that intrathecal administration of AAV9/MFSD8 was safe and well tolerated. In summary, these results demonstrated that the AAV9/MFSD8 vector is both effective and safe in preclinical models.

A Novel, Apparently Silent Variant in MFSD8 Causes Neuronal Ceroid Lipofuscinosis with Marked Intrafamilial Variability

Variants in MFSD8 can cause neuronal ceroid lipofuscinoses (NCLs) as well as nonsyndromic retinopathy. The mutation spectrum includes mainly missense and stop variants, but splice sites and frameshift variants have also been reported. To date, apparently synonymous substitutions have not been shown to cause MFSD8-associated diseases. We report two closely related subjects from a consanguineous Turkish family who presented classical features of NCLs but demonstrated marked intrafamilial variability in age at the onset and severity of symptoms. In fact, the difference in the onset of first neurologic symptoms was 15 years and that of ophthalmologic symptoms was 12 years. One subject presented an intellectual disability and a considerable cerebellar ataxia syndrome, while the other subject showed no intellectual disability and only a mild atactic syndrome. The diagnostic genetic testing of both subjects based on genome sequencing prioritized a novel, apparently synonymous variant in MFSD8, which was found in homozygosity in both subjects. The variant was not located within an integral part of the splice site consensus sequences. However, the bioinformatic analyses suggested that the mutant allele is more likely to cause exon skipping due to an altered ratio of exonic splice enhancer and silencer motifs. Exon skipping was confirmed in vitro by minigene assays and in vivo by RNA analysis from patient lymphocytes. The mutant transcript is predicted to result in a frameshift and, if translated, in a truncated protein. Synonymous variants are often given a low priority in genetic diagnostics because of their expected lack of functional impact. This study highlights the importance of investigating the impact of synonymous variants on splicing.

Case Report: Novel MFSD8 Variants in a Chinese Family With Neuronal Ceroid Lipofuscinoses 7

Neuronal ceroid lipofuscinoses (NCLs) are among the most common progressive encephalopathies of childhood. Neuronal ceroid lipofuscinosis 7 (CLN7), one of the late infantile-onset NCLs, is an autosomal recessive disorder caused by mutations in the MFSD8 gene on chromosome 4q28. Almost all reported mutations of MFSD8 in CLN7 patients were SNVs. However, we report a 4-year-old boy with CLN7 harboring compound heterozygous mutations in the MFSD8 gene, including one novel two-nucleotide deletion c.136_137delAT (p. M46Vfs*22) and one whole gene deletion of MFSD8 confirmed by Sanger sequencing, genomic quantitative PCR and CNV-seq. Therefore, for nonconsanguineous CLN7 patients with homozygous mutations in the MFSD8 gene, genetic counseling staff should focus on the possibility of whole gene deletion. This is one case report describing a whole gene deletion in a Chinese patient with CLN7, suggesting the diagnosis of CLN7 should be based on clinical suspicion and genetic testing.

CLN7 is an organellar chloride channel regulating lysosomal function

Neuronal ceroid lipofuscinoses (NCLs) are a group of autosomal recessive lysosomal storage diseases. One variant form of late-infantile NCL (vLINCL) is caused by mutations of a lysosomal membrane protein CLN7, the function of which has remained unknown. Here, we identified CLN7 as a novel endolysosomal chloride channel. Overexpression of CLN7 increases endolysosomal chloride currents and enlarges endolysosomes through a Ca²⁺/calmodulin-dependent way. Human CLN7 and its yeast homolog exhibit characteristics of chloride channels and are sensitive to chloride channel blockers. Moreover, CLN7 regulates lysosomal chloride conductance, luminal pH, and lysosomal membrane potential and promotes the release of lysosomal Ca²⁺ through transient receptor potential mucolipin 1 (TRPML1). Knocking out CLN7 causes pathological features that are similar to those of patients with vLINCL, including retinal degeneration and autofluorescent lipofuscin. The pathogenic mutations in CLN7 lead to a decrease in chloride permeability, suggesting that reconstitution of lysosomal Cl⁻ homeostasis may be an effective strategy for the treatment of vLINCL.

Altered protein secretion in Batten disease

The neuronal ceroid lipofuscinoses (NCLs), collectively known as Batten disease, are a group of neurological diseases that affect all ages and ethnicities worldwide. There are 13 different subtypes of NCL, each caused by a mutation in a distinct gene. The NCLs are characterized by the accumulation of undigestible lipids and proteins in various cell types. This leads to progressive neurodegeneration and clinical symptoms including vision loss, progressive motor and cognitive decline, seizures, and premature death. These diseases have commonly been characterized by lysosomal defects leading to the accumulation of undigestible material but further research on the NCLs suggests that altered protein secretion may also play an important role. This has been strengthened by recent work in biomedical model organisms, including Dictyostelium discoideum, mice, and sheep. Research in D. discoideum has reported the extracellular localization of some NCL-related proteins and the effects of NCL-related gene loss on protein secretion during unicellular growth and multicellular development. Aberrant protein secretion has also been observed in mammalian models of NCL, which has allowed examination of patient-derived cerebrospinal fluid and urine for potential diagnostic and prognostic biomarkers. Accumulated evidence links seven of the 13 known NCL-related genes to protein secretion, suggesting that altered secretion is a common hallmark of multiple NCL subtypes. This Review highlights the impact of altered protein secretion in the NCLs, identifies potential biomarkers of interest and suggests that future work in this area can provide new therapeutic insight.

Summary: This Review discusses work in different model systems and humans, examining the impact of altered protein secretion in the neuronal ceroid lipofuscinoses group of diseases to provide novel therapeutic insights.

Repurposing of tamoxifen ameliorates CLN3 and CLN7 disease phenotype

Batten diseases (BDs) are a group of lysosomal storage disorders characterized by seizure, visual loss, and cognitive and motor deterioration. We discovered increased levels of globotriaosylceramide (Gb3) in cellular and murine models of CLN3 and CLN7 diseases and used fluorescent-conjugated bacterial toxins to label Gb3 to develop a cell-based high content imaging (HCI) screening assay for the repurposing of FDA-approved compounds able to reduce this accumulation within BD cells. We found that tamoxifen reduced the lysosomal accumulation of Gb3 in CLN3 and CLN7 cell models, including neuronal progenitor cells (NPCs) from CLN7 patient-derived induced pluripotent stem cells (iPSC). Here, tamoxifen exerts its action through a mechanism that involves activation of the transcription factor EB (TFEB), a master gene of lysosomal function and autophagy. In vivo administration of tamoxifen to the CLN7Δex2 mouse model reduced the accumulation of Gb3 and SCMAS, decreased neuroinflammation, and improved motor coordination. These data strongly suggest that tamoxifen may be a suitable drug to treat some types of Batten disease.

Ocular Manifestations of Neuronal Ceroid Lipofuscinoses

Neuronal ceroid lipofuscinoses (NCLs) are a group of rare neurodegenerative storage disorders associated with devastating visual prognosis, with an incidence of 1/1,000,000 in the United States and comparatively higher incidence in European countries. The pathophysiological mechanisms causing NCLs occur due to enzymatic or transmembrane defects in various sub-cellular organelles including lysosomes, endoplasmic reticulum, and cytoplasmic vesicles. NCLs are categorized into different types depending upon the underlying cause i.e., soluble lysosomal enzyme deficiencies or non-enzymatic deficiencies (functions of identified proteins), which are sub-divided based on an axial classification system. In this review, we have evaluated the current evidence in the literature and reported the incidence rates, underlying mechanisms and currently available management protocols for these rare set of neuroophthalmological disorders. Additionally, we also highlighted the potential therapies under development that can expand the treatment of these rare disorders beyond symptomatic relief.

[Neuroinflammation in neuronal ceroid lipofuscinosis]

BACKGROUND

Retinal degeneration and neuroinflammation are often early hallmarks of different subtypes of neuronal ceroid lipofuscinosis (NCL) in patients and genetic animal models.

OBJECTIVE

This article gives a summary of recently published research articles and novel concepts in the field of NCL-related neuroinflammation.

MATERIAL AND METHODS

A search was carried out in PubMed for relevant publications and the results as well as own NCL-related research are discussed.

RESULTS

Microglia and other glial cells are chronically activated and show various dysfunctions in the central nervous system (CNS) and retina of NCL patients and animal models. This is accompanied by significant changes in the transcriptome and proteome. In NCL there is also involvement of the adaptive immune response, as demonstrated by the influx of autoantibodies and activated T cells.

CONCLUSION

A deeper understanding of the molecular processes that contribute to neuroinflammation and ultimately lead to neuronal cell death is an important basis for the discovery of possible biomarkers and the development of immunotherapies in NCL.

A tailored Cln3Q352X mouse model for testing therapeutic interventions in CLN3 Batten disease

CLN3 Batten disease (CLN3 disease) is a pediatric lysosomal storage disorder that presents with progressive blindness, motor and cognitive decline, seizures, and premature death. CLN3 disease results from mutations in CLN3 with the most prevalent mutation, a 966 bp deletion spanning exons 7-8, affecting ~ 75% of patients. Mouse models with complete Cln3 deletion or Cln3Δex7/8 mutation have been invaluable for learning about both the basic biology of CLN3 and the underlying pathological changes associated with CLN3 disease. These models, however, vary in their disease presentation and are limited in their utility for studying the role of nonsense mediated decay, and as a consequence, in testing nonsense suppression therapies and read-through compounds. In order to develop a model containing a disease-causing nonsense point mutation, here we describe a first-of-its-kind Cln3Q352X mouse model containing a c.1054C > T (p.Gln352Ter) point mutation. Similar to previously characterized Cln3 mutant mouse lines, this novel model shows pathological deficits throughout the CNS including accumulation of lysosomal storage material and glial activation, and has limited perturbation in behavioral measures. Thus, at the molecular and cellular level, this mouse line provides a valuable tool for testing nonsense suppression therapies or read through compounds in CLN3 disease in the future.

Functional characterization of novel MFSD8 pathogenic variants anticipates neurological involvement in juvenile isolated maculopathy

Biallelic MFSD8 variants are an established cause of severe late-infantile subtype of neuronal ceroid lipofuscinosis (v-LINCL), a severe lysosomal storage disorder, but have also been associated with nonsyndromic adult-onset maculopathy. Here, we functionally characterized two novel MFSD8 variants found in a child with juvenile isolated maculopathy, in order to establish a refined prognosis. ABCA4 locus resequencing was followed by the analysis of other inherited retinal disease genes by whole exome sequencing (WES). Minigene assays and cDNA sequencing were used to assess the effect of a novel MFSD8 splice variant. MFSD8 expression was quantified with qPCR and overexpression studies were analyzed by immunoblotting. Transmission electron microscopy (TEM) was performed on a skin biopsy and ophthalmological and neurological re-examinations were conducted. WES revealed two novel MFSD8 variants: c.[590del];[439+3A>C] p.[Gly197Valfs*2];[Ile67Glufs*3]. Characterization of the c.439+3A>C variant via splice assays showed exon-skipping (p.Ile67Glufs*3), while overexpression studies of the corresponding protein indicated expression of a truncated polypeptide. In addition, a significantly reduced MFSD8 RNA expression was noted in patient's lymphocytes. TEM of a skin biopsy revealed typical v-LINCL lipopigment inclusions while neurological imaging of the proband displayed subtle cerebellar atrophy. Functional characterization demonstrated the pathogenicity of two novel MFSD8 variants, found in a child with an initial diagnosis of juvenile isolated maculopathy but likely evolving to v-LINCL with a protracted disease course. Our study allowed a refined neurological prognosis in the proband and expands the natural history of MFSD8-associated disease.

Probable role for major facilitator superfamily domain containing 6 (MFSD6) in the brain during variable energy consumption

Purpose: The major facilitator superfamily (MFS) is known as the largest and most diverse superfamily containing human transporters, and these transporters are essential as they sustain the homeostasis within cellular compartments by moving substances over lipid membranes.Methods: We have identified a novel MFS protein, named Major facilitator superfamily domain containing 6 (MFSD6), and confirmed that it is phylogenetically related to the human Solute Carrier (SLC) transporter family. A homology model of MFSD6 revealed 12 predicted transmembrane segments (TMS) with the classical MFS fold between TMS 6 and 7.Results: Immunohistological analyses showed specific MFSD6 staining in neurons of wildtype mouse brain tissue, but no expression in astrocytes. Furthermore, we explored expression and probable function(s) of MFSD6 in relation to its phylogenetically related proteins, major facilitator superfamily domain containing 8 (MFSD8) and 10 (MFSD10), which is of interest as both these proteins are involved in diseases.Conclusions: We showed that expression levels of Mfsd6 and Mfsd10 were decreased with elevated or depleted energy consumption, while that of Mfsd8 remained unaffected.

Cardiac pathology in neuronal ceroid lipofuscinoses (NCL): More than a mere co-morbidity

The neuronal ceroid lipofuscinoses (NCLs) are mostly seen as diseases affecting the central nervous system, but there is accumulating evidence that they have co-morbidities outside the brain. One of these co-morbidities is a decline in cardiac function. This is becoming increasingly recognised in teenagers and adolescents with juvenile CLN3, but it may also occur in individuals with other NCLs. The purpose of this review is to summarise the current knowledge of the structural and functional changes found in the hearts of animal models and people diagnosed with NCL. In addition, we present evidence of structural changes that were observed in a systematic comparison of the cardiomyocytes from CLN3^Δex7/8 mice.

Advances in the Treatment of Neuronal Ceroid Lipofuscinosis

Neuronal ceroid lipofuscinoses (NCL) represent a class of neurodegenerative disorders involving defective lysosomal processing enzymes or receptors, leading to lysosomal storage disorders, typically characterized by observation of cognitive and visual impairments, epileptic seizures, ataxia, and deterioration of motor skills. Recent success of a biologic (Brineura®) for the treatment of neurologic manifestations of the central nervous system (CNS) has led to renewed interest in therapeutics for NCL, with the goal of ablating or reversing the impact of these devastating disorders. Despite complex challenges associated with CNS therapy, many treatment modalities have been evaluated, including enzyme replacement therapy, gene therapy, stem cell therapy, and small molecule pharmacotherapy. Because the clinical endpoints for the evaluation of candidate therapies are complex and often reliant on subjective clinical scales, the development of quantitative biomarkers for NCLs has become an apparent necessity for the validation of potential treatments. We will discuss the latest findings in the search for relevant biomarkers for assessing disease progression. For this review, we will focus primarily on recent pre-clinical and clinical developments for treatments to halt or cure these NCL diseases. Continued development of current therapies and discovery of newer modalities will be essential for successful therapeutics for NCL.

AREAS COVERED

The reader will be introduced to the NCL subtypes, natural histories, experimental animal models, and biomarkers for NCL progression; challenges and different therapeutic approaches, and the latest pre-clinical and clinical research for therapeutic development for the various NCLs. This review corresponds to the literatures covering the years from 1968 to mid-2019, but primarily addresses pre-clinical and clinical developments for the treatment of NCL disease in the last decade and as a follow-up to our 2013 review of the same topic in this journal.

EXPERT OPINION

Much progress has been made in the treatment of neurologic diseases, such as the NCLs, including better animal models and improved therapeutics with better survival outcomes. Encouraging results are being reported at symposiums and in the literature, with multiple therapeutics reaching the clinical trial stage for the NCLs. The potential for a cure could be at hand after many years of trial and error in the preclinical studies. The clinical development of enzyme replacement therapy (Brineura® for CLN2), immunosuppression (CellCept® for CLN3), and gene therapy vectors (for CLN1, CLN2, CLN3, and CLN6) are providing encouragement to families that have a child afflicted with NCL. We believe that successful therapies in the future may involve the combination of two or more therapeutic modalities to provide therapeutic benefit especially as the patients grow older.

The contribution of multicellular model organisms to neuronal ceroid lipofuscinosis research

The NCLs (neuronal ceroid lipofuscinosis) are forms of neurodegenerative disease that affect people of all ages and ethnicities but are most prevalent in children. Commonly known as Batten disease, this debilitating neurological disorder is comprised of 13 different subtypes that are categorized based on the particular gene that is mutated (CLN1-8, CLN10-14). The pathological mechanisms underlying the NCLs are not well understood due to our poor understanding of the functions of NCL proteins. Only one specific treatment (enzyme replacement therapy) is approved, which is for the treating the brain in CLN2 disease. Hence there remains a desperate need for further research into disease-modifying treatments. In this review, we present and evaluate the genes, proteins and studies performed in the social amoeba, nematode, fruit fly, zebrafish, mouse and large animals pertinent to NCL. In particular, we highlight the use of multicellular model organisms to study NCL protein function, pathology and pathomechanisms. Their use in testing novel therapeutic approaches is also presented. With this information, we highlight how future research in these systems may be able to provide new insight into NCL protein functions in human cells and aid in the development of new therapies.

A mixed breed dog with neuronal ceroid lipofuscinosis is homozygous for a CLN5 nonsense mutation previously identified in Border Collies and Australian Cattle Dogs

The neuronal ceroid lipofuscinoses (NCLs) are a group of inherited neurodegenerative disorders characterized by progressive declines in neurological functions following normal development. The NCLs are distinguished from similar disorders by the accumulation of autofluorescent lysosomal storage bodies in neurons and many other cell types, and are classified as lysosomal storage diseases. At least 13 genes contain pathogenic sequence variants that underlie different forms of NCL. Naturally occurring canine NCLs can serve as models to develop better understanding of the disease pathologies and for preclinical evaluation of therapeutic interventions for these disorders. To date 14 sequence variants in 8 canine orthologs of human NCL genes have been found to cause progressive neurological disorders similar to human NCLs in 12 different dog breeds. A mixed breed dog with parents of uncertain breed background developed progressive neurological signs consistent with NCL starting at approximately 11 to 12 months of age, and when evaluated with magnetic resonance imaging at 21 months of age exhibited diffuse brain atrophy. Due to the severity of neurological decline the dog was euthanized at 23 months of age. Cerebellar and cerebral cortical neurons contained massive accumulations of autofluorescent storage bodies the contents of which had the appearance of tightly packed membranes. A whole genome sequence, generated with DNA from the affected dog contained a homozygous C-to-T transition at position 30,574,637 on chromosome 22 which is reflected in the mature CLN5 transcript (CLN5: c.619C > T) and converts a glutamine codon to a termination codon (p.Gln207Ter). The identical nonsense mutation has been previously associated with NCL in Border Collies, Australian Cattle Dogs, and a German Shepherd-Australian Cattle Dog mix. The current whole genome sequence and a previously generated whole genome sequence for an Australian Cattle Dog with NCL share a rare homozygous haplotype that extends for 87 kb surrounding 22: 30, 574, 637 and includes 21 polymorphic sites. When genotyped at 7 of these polymorphic sites, DNA samples from the German Shepherd-Australian Cattle Dog mix and from 5 Border Collies with NCL that were homozygous for the CLN5: c.619 T allele also shared this homozygous haplotype, suggesting that the NCL in all of these dogs stems from the same founding mutation event that may have predated the establishment of the modern dog breeds. If so, the CLN5 nonsence allele is probably segregating in other, as yet unidentified, breeds. Thus, dogs exhibiting similar NCL-like signs should be screened for this CLN5 nonsense allele regardless of breed.

A newly generated neuronal cell model of CLN7 disease reveals aberrant lysosome motility and impaired cell survival

Mutations in the CLN7/MFSD8 gene encoding the lysosomal membrane protein CLN7 are causative of CLN7 disease, an inherited neurodegenerative disorder that typically affects children. To gain insight into the pathomechanisms of CLN7 disease, we established an immortalized cell line based on cerebellar (Cb) granule neuron precursors isolated from Cln7^-/- mice. Here, we demonstrate that Cln7-deficient neuron-derived Cb cells display an abnormal phenotype that includes increased size and defective outward movement of late endosomes and lysosomes as well as impaired lysosomal exocytosis. Whereas Cln7^-/- Cb cells appeared to be autophagy-competent, loss of Cln7 resulted in enhanced cell death under prolonged nutrient deprivation. Furthermore, reduced cell survival of Cln7-deficient cells was accompanied by a significantly impaired protein kinase B/Akt phosphorylation at Ser473 during long-term starvation. In summary, our data demonstrate for the first time that the putative lysosomal transporter CLN7 is relevant for lysosome motility and plays an important role for neuronal cell survival under conditions of starvation.

Discovery of a CLN7 model of Batten disease in non-human primates

We have identified a natural Japanese macaque model of the childhood neurodegenerative disorder neuronal ceroid lipofuscinosis, commonly known as Batten Disease, caused by a homozygous frameshift mutation in the CLN7 gene (CLN7^−/−). Affected macaques display progressive neurological deficits including visual impairment, tremor, incoordination, ataxia and impaired balance. Imaging, functional and pathological studies revealed that CLN7^−/− macaques have reduced retinal thickness and retinal function early in disease, followed by profound cerebral and cerebellar atrophy that progresses over a five to six-year disease course. Histological analyses showed an accumulation of cerebral, cerebellar and cardiac storage material as well as degeneration of neurons, white matter fragmentation and reactive gliosis throughout the brain of affected animals. This novel CLN7^−/− macaque model recapitulates key behavioral and neuropathological features of human Batten Disease and provides novel insights into the pathophysiology linked to CLN7 mutations. These animals will be invaluable for evaluating promising therapeutic strategies for this devastating disease.

Loss of CLN7 results in depletion of soluble lysosomal proteins and impaired mTOR reactivation

Defects in the MFSD8 gene encoding the lysosomal membrane protein CLN7 lead to CLN7 disease, a neurodegenerative lysosomal storage disorder belonging to the group of neuronal ceroid lipofuscinoses. Here, we have performed a SILAC-based quantitative analysis of the lysosomal proteome using Cln7-deficient mouse embryonic fibroblasts (MEFs) from a Cln7 knockout (ko) mouse model. From 3335 different proteins identified, we detected 56 soluble lysosomal proteins and 29 highly abundant lysosomal membrane proteins. Quantification revealed that the amounts of 12 different soluble lysosomal proteins were significantly reduced in Cln7 ko MEFs compared with wild-type controls. One of the most significantly depleted lysosomal proteins was Cln5 protein that underlies another distinct neuronal ceroid lipofuscinosis disorder. Expression analyses showed that the mRNA expression, biosynthesis, intracellular sorting and proteolytic processing of Cln5 were not affected, whereas the depletion of mature Cln5 protein was due to increased proteolytic degradation by cysteine proteases in Cln7 ko lysosomes. Considering the similar phenotypes of CLN5 and CLN7 patients, our data suggest that depletion of CLN5 may play an important part in the pathogenesis of CLN7 disease. In addition, we found a defect in the ability of Cln7 ko MEFs to adapt to starvation conditions as shown by impaired mammalian target of rapamycin complex 1 reactivation, reduced autolysosome tubulation and increased perinuclear accumulation of autolysosomes compared with controls. In summary, depletion of multiple soluble lysosomal proteins suggest a critical role of CLN7 for lysosomal function, which may contribute to the pathogenesis and progression of CLN7 disease.

Retinal Degeneration In A Mouse Model Of CLN5 Disease Is Associated With Compromised Autophagy

The Finnish variant of late infantile neuronal ceroid lipofuscinosis (CLN5 disease) belongs to a family of neuronal ceroid lipofuscinosis (NCLs) diseases. Vision loss is among the first clinical signs in childhood forms of NCLs. Mutations in CLN5 underlie CLN5 disease. The aim of this study was to characterize how the lack of normal functionality of the CLN5 protein affects the mouse retina. Scotopic electroretinography (ERG) showed a diminished c-wave amplitude in the CLN5 deficient mice already at 1 month of age, indicative of pathological events in the retinal pigmented epithelium. A- and b-waves showed progressive impairment later from 2 and 3 months of age onwards, respectively. Structural and immunohistochemical (IHC) analyses showed preferential damage of photoreceptors, accumulation of autofluorescent storage material, apoptosis of photoreceptors, and strong inflammation in the CLN5 deficient mice retinas. Increased levels of autophagy-associated proteins Beclin-1 and P62, and increased LC3b-II/LC3b-I ratio, were detected by Western blotting from whole retinal extracts. Photopic ERG, visual evoked potentials, IHC and cell counting indicated relatively long surviving cone photoreceptors compared to rods. In conclusion, CLN5 deficient mice develop early vision loss that reflects the condition reported in clinical childhood forms of NCLs. The vision loss in CLN5 deficient mice is primarily caused by photoreceptor degeneration.

The Novel Membrane-Bound Proteins MFSD1 and MFSD3 are Putative SLC Transporters Affected by Altered Nutrient Intake

Membrane-bound solute carriers (SLCs) are essential as they maintain several physiological functions, such as nutrient uptake, ion transport and waste removal. The SLC family comprise about 400 transporters, and we have identified two new putative family members, major facilitator superfamily domain containing 1 (MFSD1) and 3 (MFSD3). They cluster phylogenetically with SLCs of MFS type, and both proteins are conserved in chordates, while MFSD1 is also found in fruit fly. Based on homology modelling, we predict 12 transmembrane regions, a common feature for MFS transporters. The genes are expressed in abundance in mice, with specific protein staining along the plasma membrane in neurons. Depriving mouse embryonic primary cortex cells of amino acids resulted in upregulation of Mfsd1, whereas Mfsd3 is unaltered. Furthermore, in vivo, Mfsd1 and Mfsd3 are downregulated in anterior brain sections in mice subjected to starvation, while upregulated specifically in brainstem. Mfsd3 is also attenuated in cerebellum after starvation. In mice raised on high-fat diet, Mfsd1 was specifically downregulated in brainstem and hypothalamus, while Mfsd3 was reduced consistently throughout the brain.

Lysosomal dysfunction and impaired autophagy in a novel mouse model deficient for the lysosomal membrane protein Cln7

CLN7 disease is an autosomal recessive, childhood-onset neurodegenerative lysosomal storage disorder caused by the defective lysosomal membrane protein CLN7. We have disrupted the Cln7/Mfsd8 gene in mice by targeted deletion of exon 2 generating a novel knockout (KO) mouse model for CLN7 disease, which recapitulates key features of human CLN7 disease pathology. Cln7 KO mice showed increased mortality and a neurological phenotype including hind limb clasping and myoclonus. Lysosomal dysfunction in the brain of mutant mice was shown by the storage of autofluorescent lipofuscin-like lipopigments, subunit c of mitochondrial ATP synthase and saposin D and increased expression of lysosomal cathepsins B, D and Z. By immunohistochemical co-stainings, increased cathepsin Z expression restricted to Cln7-deficient microglia and neurons was found. Ultrastructural analyses revealed large storage bodies in Purkinje cells of Cln7 KO mice containing inclusions composed of irregular, curvilinear and rectilinear profiles as well as fingerprint profiles. Generalized astrogliosis and microgliosis in the brain preceded neurodegeneration in the olfactory bulb, cerebral cortex and cerebellum in Cln7 KO mice. Increased levels of LC3-II and the presence of neuronal p62- and ubiquitin-positive protein aggregates suggested that impaired autophagy represents a major pathomechanism in the brain of Cln7 KO mice. The data suggest that loss of the putative lysosomal transporter Cln7 in the brain leads to lysosomal dysfunction, impaired constitutive autophagy and neurodegeneration late in disease.

Towards a new understanding of NCL pathogenesis

The Neuronal Ceroid Lipofuscinoses (NCLs, Batten disease) are a group of inherited neurodegenerative disorders that have been traditionally grouped together on the basis of certain shared clinical and pathological features. However, as the number of genes that appear to cause new forms of NCL continues to grow, it is timely to reassess our understanding of the pathogenesis of these disorders and what groups them together. The various NCL subtypes do indeed share features of a build-up of autofluorescent storage material, progressive neuron loss and activation of the innate immune system. The characterisation of animal models has highlighted the selective nature of neuron loss and its intimate relationship with glial activation, rather than the generalised build-up of storage material. More recent data provide evidence for the pathway-dependent nature of pathology, the contribution of glial dysfunction, and the involvement of new brain regions previously thought to be unaffected, and it is becoming apparent that pathology extends beyond the brain. These data have important implications, not just for therapy, but also for our understanding of these disorders. However, looking beneath these broadly similar pathological themes evidence emerges for marked differences in the nature and extent of these events in different forms of NCL. Indeed, given the widely different nature of the mutated gene products it is perhaps more surprising that these disorders resemble each other as much as they do. Such data raise the question whether we should rethink the collective grouping of these gene deficiencies together, or whether it would be better to consider them as separate entities. This article is part of a Special Issue entitled: Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease).

The neuronal ceroid lipofuscinoses: Opportunities from model systems

The neuronal ceroid lipofuscinoses are a group of severe and progressive neurodegenerative disorders, generally with childhood onset. Despite the fact that these diseases remain fatal, significant breakthroughs have been made in our understanding of the genetics that underpin these conditions. This understanding has allowed the development of a broad range of models to study disease processes, and to develop new therapeutic approaches. Such models have contributed significantly to our knowledge of these conditions. In this review we will focus on the advantages of each individual model, describe some of the contributions the models have made to our understanding of the broader disease biology and highlight new techniques and approaches relevant to the study and potential treatment of the neuronal ceroid lipofuscinoses. This article is part of a Special Issue entitled: "Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease)".

A rare homozygous MFSD8 single-base-pair deletion and frameshift in the whole genome sequence of a Chinese Crested dog with neuronal ceroid lipofuscinosis

Background

The neuronal ceroid lipofuscinoses are heritable lysosomal storage diseases characterized by progressive neurological impairment and the accumulation of autofluorescent storage granules in neurons and other cell types. Various forms of human neuronal ceroid lipofuscinosis have been attributed to mutations in at least 13 different genes. So far, mutations in the canine orthologs of 7 of these genes have been identified in DNA from dogs with neuronal ceroid lipofuscinosis. The identification of new causal mutations could lead to the establishment of canine models to investigate the pathogenesis of the corresponding human neuronal ceroid lipofuscinoses and to evaluate and optimize therapeutic interventions for these fatal human diseases.

Case presentation

We obtained blood and formalin-fixed paraffin-embedded brain sections from a rescue dog that was reported to be a young adult Chinese Crested. The dog was euthanized at approximately 19 months of age as a consequence of progressive neurological decline that included blindness, anxiety, and cognitive impairment. A diagnosis of neuronal ceroid lipofuscinosis was made based on neurological signs, magnetic resonance imaging of the brain, and fluorescence microscopic and electron microscopic examination of brain sections. We isolated DNA from the blood and used it to generate a whole genome sequence with 33-fold average coverage. Among the 7.2 million potential sequence variants revealed by aligning the sequence reads to the canine genome reference sequence was a homozygous single base pair deletion in the canine ortholog of one of 13 known human NCL genes: MFSD8:c.843delT. MFSD8:c.843delT is predicted to cause a frame shift and premature stop codon resulting in a truncated protein, MFSD8:p.F282Lfs13*, missing its 239 C-terminal amino acids. The MFSD8:c.843delT allele is absent from the whole genome sequences of 101 healthy canids or dogs with other diseases. The genotyping of archived DNA from 1478 Chinese Cresteds did not identify any additional MFSD8:c.843delT homozygotes and found only one heterozygote.

Conclusion

We conclude that the neurodegenerative disease of the Chinese Crested rescue dog was neuronal ceroid lipofuscinosis and that homozygosity for the MFSD8:c.843delT sequence variant was very likely to be the molecular-genetic cause of the disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-014-0181-z) contains supplementary material, which is available to authorized users.

Mutations in MFSD8, encoding a lysosomal membrane protein, are associated with nonsyndromic autosomal recessive macular dystrophy

PURPOSE

This study aimed to identify the genetic defects in 2 families with autosomal recessive macular dystrophy with central cone involvement.

DESIGN

Case series.

PARTICIPANTS

Two families and a cohort of 244 individuals with various inherited maculopathies and cone disorders.

METHODS

Genome-wide linkage analysis and exome sequencing were performed in 1 large family with 5 affected individuals. In addition, exome sequencing was performed in the proband of a second family. Subsequent analysis of the identified mutations in 244 patients was performed by Sanger sequencing or restriction enzyme digestion. The medical history of individuals carrying the MFSD8 variants was reviewed and additional ophthalmic examinations were performed, including electroretinography (ERG), multifocal ERG (mfERG), perimetry, optical coherence tomography (OCT), fundus autofluorescence, and fundus photography.

MAIN OUTCOME MEASURES

MFSD8 variants, age at diagnosis, visual acuity, fundus appearance, color vision defects, visual field, ERG, mfERG, fundus autofluorescence, and OCT findings.

RESULTS

Compound heterozygous variants in MFSD8, a gene encoding a lysosomal transmembrane protein, were identified in 2 families with macular dystrophy with a normal or subnormal ERG, but reduced mfERG. In both families, a heterozygous missense variant p.Glu336Gln was identified, which was predicted to have a mild effect on the protein. In the first family, a protein-truncating variant (p.Glu381*) was identified on the other allele, and in the second family, a variant (c.1102G>C) was identified that results in a splicing defect leading to skipping of exon 11 (p.Lys333Lysfs*3). The p.Glu336Gln allele was found to be significantly enriched in patients with maculopathies and cone disorders (6/488) compared with ethnically matched controls (35/18 682; P < 0.0001), suggesting that it may act as a genetic modifier.

CONCLUSIONS

In this study, we identified variants in MFSD8 as a novel cause of nonsyndromic autosomal recessive macular dystrophy with central cone involvement. Affected individuals showed no neurologic features typical for variant late-infantile neuronal ceroid lipofuscinosis (vLINCL), a severe and devastating multisystem lysosomal storage disease previously associated with mutations in MFSD8. We propose a genotype-phenotype model in which a combination of a severe and a mild variant cause nonsyndromic macular dystrophy with central cone involvement, and 2 severe mutations cause vLINCL.