Protein misfolding as an underlying molecular defect in mucopolysaccharidosis III type C

Structural and mechanistic insights into a lysosomal membrane enzyme HGSNAT involved in Sanfilippo syndrome

Heparan sulfate (HS) is degraded in lysosome by a series of glycosidases. Before the glycosidases can act, the terminal glucosamine of HS must be acetylated by the integral lysosomal membrane enzyme heparan-α-glucosaminide N-acetyltransferase (HGSNAT). Mutations of HGSNAT cause HS accumulation and consequently mucopolysaccharidosis IIIC, a devastating lysosomal storage disease characterized by progressive neurological deterioration and early death where no treatment is available. HGSNAT catalyzes a unique transmembrane acetylation reaction where the acetyl group of cytosolic acetyl-CoA is transported across the lysosomal membrane and attached to HS in one reaction. However, the reaction mechanism remains elusive. Here we report six cryo-EM structures of HGSNAT along the reaction pathway. These structures reveal a dimer arrangement and a unique structural fold, which enables the elucidation of the reaction mechanism. We find that a central pore within each monomer traverses the membrane and controls access of cytosolic acetyl-CoA to the active site at its luminal mouth where glucosamine binds. A histidine-aspartic acid catalytic dyad catalyzes the transfer reaction via a ternary complex mechanism. Furthermore, the structures allow the mapping of disease-causing variants and reveal their potential impact on the function, thus creating a framework to guide structure-based drug discovery efforts.

Structure of the human heparan-α-glucosaminide N -acetyltransferase (HGSNAT)

Degradation of heparan sulfate (HS), a glycosaminoglycan (GAG) comprised of repeating units of N -acetylglucosamine and glucuronic acid, begins in the cytosol and is completed in the lysosomes. Acetylation of the terminal non-reducing amino group of α-D-glucosamine of HS is essential for its complete breakdown into monosaccharides and free sulfate. Heparan-α-glucosaminide N -acetyltransferase (HGSNAT), a resident of the lysosomal membrane, catalyzes this essential acetylation reaction by accepting and transferring the acetyl group from cytosolic acetyl-CoA to terminal α-D-glucosamine of HS in the lysosomal lumen. Mutation-induced dysfunction in HGSNAT causes abnormal accumulation of HS within the lysosomes and leads to an autosomal recessive neurodegenerative lysosomal storage disorder called mucopolysaccharidosis IIIC (MPS IIIC). There are no approved drugs or treatment strategies to cure or manage the symptoms of, MPS IIIC. Here, we use cryo-electron microscopy (cryo-EM) to determine a high-resolution structure of the HGSNAT-acetyl-CoA complex, the first step in HGSNAT catalyzed acetyltransferase reaction. In addition, we map the known MPS IIIC mutations onto the structure and elucidate the molecular basis for mutation-induced HGSNAT dysfunction.

Structure and mechanism of lysosome transmembrane acetylation by HGSNAT

Lysosomal transmembrane acetylation of heparan sulfates (HS) is catalyzed by HS acetyl-CoA:α-glucosaminide N-acetyltransferase (HGSNAT), whose dysfunction leads to lysosomal storage diseases. The mechanism by which HGSNAT, the sole non-hydrolase enzyme in HS degradation, brings cytosolic acetyl-coenzyme A (Ac-CoA) and lysosomal HS together for N-acyltransferase reactions remains unclear. Here, we present cryogenic-electron microscopy structures of HGSNAT alone, complexed with Ac-CoA and with acetylated products. These structures explain that Ac-CoA binding from the cytosolic side causes dimeric HGSNAT to form a transmembrane tunnel. Within this tunnel, catalytic histidine and asparagine approach the lumen and instigate the transfer of the acetyl group from Ac-CoA to the glucosamine group of HS. Our study unveils a transmembrane acetylation mechanism that may help advance therapeutic strategies targeting lysosomal storage diseases.

Identification and characterization of novel genetic variants in the first Chinese family of mucopolysaccharidosis IIIC (Sanfilippo C syndrome)

Mucopolysaccharidosis type IIIC (MPS IIIC) is one of inherited lysosomal storage disorders, caused by deficiencies in lysosomal hydrolases degrading acidic mucopolysaccharides. The gene responsible for MPS IIIC is HGSNAT, which encodes an enzyme that catalyses the acetylation of the terminal glucosamine residues of heparan sulfate. So far, few studies have focused on the genetic landscape of MPS IIIC in China, where IIIA and IIIB were the major subtypes. In this study, we utilized whole-exome sequencing (WES) to identify novel compound heterozygous variants in the HGSNAT gene from a Chinese patient with typical MPS IIIC symptoms: c.743G>A; p.Gly248Glu and c.1030C>T; p.Arg344Cys. We performed in silico analysis and experimental validation, which confirmed the deleterious pathogenic nature of both variants, as evidenced by the loss of HGSNAT activity and failure of lysosomal localization. To the best of our knowledge, the MPS IIIC is first confirmed by clinical, biochemical and molecular genetic findings in China. Our study thus expands the spectrum of MPS IIIC pathogenic variants, which is of importance to dissect the pathogenesis and to carry out clinical diagnosis of MPS IIIC. Moreover, this study helps to depict the natural history of Chinese MPS IIIC populations.

Expanding the phenotypic and genotypic spectrum of patients with HGSNAT-related retinopathy

BACKGROUND

Variants in HGSNAT have historically been associated with syndromic mucopolysaccharidosis type IIIC (MPSIIIC) but more recent studies demonstrate cases of HGSNAT-related non-syndromic retinitis pigmentosa. We describe and expand the genotypic and phenotypic spectrum of this disease.

MATERIALS AND METHODS

This is a retrospective, observational, case series of 11 patients with pericentral retinitis pigmentosa due to variants in HGSNAT gene without a syndromic diagnosis of MPSIIIC. We reviewed ophthalmologic data extracted from medical records, genetic testing, color fundus photos, fundus autofluorescence (FAF), and optical coherence tomography (OCT).

RESULTS

Of the 11 patients, the mean age was 52 years (range: 26-78). The mean age of ophthalmologic symptoms onset was 45 years (range: 15-72). The visual acuity varied from 20/20 to 20/80 (mean 20/30 median 20/20). We described five novel variants in HGSNAT: c.715del (p.Arg239Alafs *37), c.118 G>A (p.Asp40Asn), c.1218_1220delinsTAT, c.1297A>G (p.Asn433Asp), and c.1726 G>T (p.Gly576*).

CONCLUSIONS

HGSNAT has high phenotypic heterogeneity. Data from our cohort showed that all patients who had at least one variant of c.1843 G>A (p.Ala615Thr) presented with the onset of ocular symptoms after the fourth decade of life. The two patients with onset of ocular symptoms before the fourth decade did not carry this variant. This may suggest that c.1843 G>A variant is associated with a later onset of retinopathy.

Current Concepts in the Management of Sanfilippo Syndrome (MPS III): A Narrative Review

Sanfilippo syndrome is a childhood-onset (1-4 years) autosomal recessive lysosomal storage disease that presents as a neurodegenerative disease by targeting the brain and spinal cord. It is also known as mucopolysaccharidosis III. Mucopolysaccharidosis III is divided into four subtypes (A, B, C, or D). It can cause delayed speech, behavior problems, and features of autism spectrum disorder. Sanfilippo syndrome is of a higher prevalence within consanguineous families that carry its gene alteration. If both parents have a nonfunctional copy of a gene linked to this condition, their children will have a 25% (1 in 4) chance of developing the disease. In Saudi Arabia, the incidence rate is estimated at 2 per 100,000 live births. Recent research focused on promising treatment approaches, such as gene therapy, modified enzyme replacement therapy, and stem cells. These approaches work by exogenous administration of the proper version of the mutant enzyme (enzyme replacement therapy), cleaning the defective enzyme in individuals with glycolipid storage disorders (substrate reduction therapy), or using a pharmacological chaperone to target improperly folded proteins. However, there is currently no approved curative medication for Sanfilippo syndrome that can effectively halt or reverse the disorder.

Lysosomal Dysfunction: Connecting the Dots in the Landscape of Human Diseases

Lysosomes are the main organelles responsible for the degradation of macromolecules in eukaryotic cells. Beyond their fundamental role in degradation, lysosomes are involved in different physiological processes such as autophagy, nutrient sensing, and intracellular signaling. In some circumstances, lysosomal abnormalities underlie several human pathologies with different etiologies known as known as lysosomal storage disorders (LSDs). These disorders can result from deficiencies in primary lysosomal enzymes, dysfunction of lysosomal enzyme activators, alterations in modifiers that impact lysosomal function, or changes in membrane-associated proteins, among other factors. The clinical phenotype observed in affected patients hinges on the type and location of the accumulating substrate, influenced by genetic mutations and residual enzyme activity. In this context, the scientific community is dedicated to exploring potential therapeutic approaches, striving not only to extend lifespan but also to enhance the overall quality of life for individuals afflicted with LSDs. This review provides insights into lysosomal dysfunction from a molecular perspective, particularly in the context of human diseases, and highlights recent advancements and breakthroughs in this field.

NONSYNDROMIC RETINITIS PIGMENTOSA WITH BILATERAL RETINAL NEOVASCULARIZATION DUE TO HGSNAT MUTATION

PURPOSE

To describe a case of nonsyndromic retinitis pigmentosa caused by presumed compound heterozygous A615T and T522M mutations in HGSNAT, characterized by bilateral cystoid macular edema and retinal neovascularization.

METHODS

Case report. The patient underwent clinical evaluation, multimodal imaging, and next-generation panel sequencing. In silico analysis was performed with PolyPhen-2, SIFT, and MutationTaster. Segregation analysis was not available.

RESULTS

A 35-year-old hypertensive man presented with nyctalopia, photopsia, and difficulty reading for six months. He had no family history of visual deficits. The best-corrected visual acuity was 20/25 in the right eye and 20/20 in the left eye. Examination revealed midperipheral bone spicules and macular neovascularization in both eyes. Multimodal imaging demonstrated cystoid macular edema, ellipsoid band loss outside the central macula, and leakage from the neovascularization in both eyes. Sequencing detected four mutations in three genes, including two heterozygous mutations in HGSNAT (c.1843G>A, p.A615T and c.1565C>T, p.T522M). A615T is a pathogenic, hypomorphic mutation. T522M has not been previously phenotypically described. It is predicted damaging by in silico analysis and occurs at a conserved position near the eighth transmembrane domain, adjacent to residues in which missense mutations result in protein misfolding.

CONCLUSION

This is, to the best of our knowledge, the first reported case of retinal neovascularization in a case of nonsyndromic retinitis pigmentosa due to HGSNAT mutation. The T522M variant likely functions as a severe mutation alongside the hypomorphic A615T mutation. These findings expand the genotypic and phenotypic spectrum of nonsyndromic retinitis pigmentosa.

Decreased Levels of Chaperones in Mucopolysaccharidoses and Their Elevation as a Putative Auxiliary Therapeutic Approach

Mucopolysaccharidoses (MPS) are rare genetic disorders belonging to the lysosomal storage diseases. They are caused by mutations in genes encoding lysosomal enzymes responsible for degrading glycosaminoglycans (GAGs). As a result, GAGs accumulate in lysosomes, leading to impairment of cells, organs and, consequently, the entire body. Many of the therapies proposed thus far require the participation of chaperone proteins, regardless of whether they are therapies in common use (enzyme replacement therapy) or remain in the experimental phase (gene therapy, STOP-codon-readthrough therapy). Chaperones, which include heat shock proteins, are responsible for the correct folding of other proteins to the most energetically favorable conformation. Without their appropriate levels and activities, the correct folding of the lysosomal enzyme, whether supplied from outside or synthesized in the cell, would be impossible. However, the baseline level of nonspecific chaperone proteins in MPS has never been studied. Therefore, the purpose of this work was to determine the basal levels of nonspecific chaperone proteins of the Hsp family in MPS cells and to study the effect of normalizing GAG concentrations on these levels. Results of experiments with fibroblasts taken from patients with MPS types I, II, IIIA, IIIB, IIIC, IID, IVA, IVB, VI, VII, and IX, as well as from the brains of MPS I mice (Idua^-/-), indicated significantly reduced levels of the two chaperones, Hsp70 and Hsp40. Interestingly, the reduction in GAG levels in the aforementioned cells did not lead to normalization of the levels of these chaperones but caused only a slight increase in the levels of Hsp40. An additional transcriptomic analysis of MPS cells indicated that the expression of other genes involved in protein folding processes and the cell response to endoplasmic reticulum stress, resulting from the appearance of abnormally folded proteins, was also modulated. To summarize, reduced levels of chaperones may be an additional cause of the low activity or inactivity of lysosomal enzymes in MPS. Moreover, this may point to causes of treatment failure where the correct structure of the enzyme supplied or synthesized in the cell is crucial to lower GAG levels.

Lysosomal positioning diseases: beyond substrate storage

Lysosomal storage diseases (LSDs) comprise a group of inherited monogenic disorders characterized by lysosomal dysfunctions due to undegraded substrate accumulation. They are caused by a deficiency in specific lysosomal hydrolases involved in cellular catabolism, or non-enzymatic proteins essential for normal lysosomal functions. In LSDs, the lack of degradation of the accumulated substrate and its lysosomal storage impairs lysosome functions resulting in the perturbation of cellular homeostasis and, in turn, the damage of multiple organ systems. A substantial number of studies on the pathogenesis of LSDs has highlighted how the accumulation of lysosomal substrates is only the first event of a cascade of processes including the accumulation of secondary metabolites and the impairment of cellular trafficking, cell signalling, autophagic flux, mitochondria functionality and calcium homeostasis, that significantly contribute to the onset and progression of these diseases. Emerging studies on lysosomal biology have described the fundamental roles of these organelles in a variety of physiological functions and pathological conditions beyond their canonical activity in cellular waste clearance. Here, we discuss recent advances in the knowledge of cellular and molecular mechanisms linking lysosomal positioning and trafficking to LSDs.

Glucosamine amends CNS pathology in mucopolysaccharidosis IIIC mouse expressing misfolded HGSNAT

The majority of mucopolysaccharidosis IIIC (MPS IIIC) patients have missense variants causing misfolding of heparan sulfate acetyl-CoA:α-glucosaminide N-acetyltransferase (HGSNAT), which are potentially treatable with pharmacological chaperones. To test this approach, we generated a novel HgsnatP304L mouse model expressing misfolded HGSNAT Pro304Leu variant. HgsnatP304L mice present deficits in short-term and working/spatial memory 2-4 mo earlier than previously described constitutive knockout Hgsnat-Geo mice. HgsnatP304L mice also show augmented severity of neuroimmune response, synaptic deficits, and neuronal storage of misfolded proteins and gangliosides compared with Hgsnat-Geo mice. Expression of misfolded human Pro311Leu HGSNAT protein in cultured hippocampal Hgsnat-Geo neurons further reduced levels of synaptic proteins. Memory deficits and majority of brain pathology were rescued in mice receiving HGSNAT chaperone, glucosamine. Our data for the first time demonstrate dominant-negative effects of misfolded HGSNAT Pro304Leu variant and show that they are treatable by oral administration of glucosamine. This suggests that patients affected with mutations preventing normal folding of the enzyme can benefit from chaperone therapy.

Complex Diagnostics of Non-Specific Intellectual Developmental Disorder

Intellectual development disorder (IDD) is characterized by a general deficit in intellectual and adaptive functioning. In recent years, there has been a growing interest in studying the genetic structure of IDD. Of particular difficulty are patients with non-specific IDD, for whom it is impossible to establish a clinical diagnosis without complex genetic diagnostics. We examined 198 patients with non-specific IDD from 171 families using whole-exome sequencing and chromosome microarray analysis. Hereditary forms of IDD account for at least 35.7% of non-specific IDD, of which 26.9% are monogenic forms. Variants in the genes associated with the BAF (SWI/SNF) complex were the most frequently identified. We were unable to identify phenotypic features that would allow differential diagnosis of monogenic and microstructural chromosomal rearrangements in non-specific IDD at the stage of clinical examination, but due to its higher efficiency, exome sequencing should be the diagnostic method of the highest priority study after the standard examination of patients with NIDD in Russia.

Hurdles in treating Hurler disease: potential routes to achieve a "real" cure

Mucopolysaccharidoses (MPSs) are multiorgan devastating diseases for which hematopoietic cell transplantation (HCT) and, to a lesser extent, enzyme replacement therapy have substantially altered the course of the disease. Furthermore, they have resulted in increased overall survival, especially for Hurler disease (MPS-1). However, despite the identification of clinical predictors and harmonized transplantation protocols, disease progression still poses a significant burden to patients, although at a slower pace. To design better therapies, we need to understand why and where current therapies fail. In this review, we discuss important aspects of the underlying disease and the disease progression. We note that the majority of progressive symptoms that occur in "hard-to-treat" tissues are actually tissues that are difficult to reach, such as avascular connective tissue or tissues isolated from the circulation by a specific barrier (eg, blood-brain barrier, blood-retina barrier). Although easily reached tissues are effectively cured by HCT, disease progression is observed in these "hard-to-reach" tissues. We used these insights to critically appraise ongoing experimental endeavors with regard to their potential to overcome the encountered hurdles and improve long-term clinical outcomes in MPS patients treated with HCT.

The natural history of neurocognition in MPS disorders: A review

MPS disorders are associated with a wide spectrum of neurocognitive effects, from mild problems with attention and executive functions to progressive and degenerative neuronopathic disease. Studies of the natural history of neurocognition are necessary to determine the profile of abnormality and the rates of change, which are crucial to select endpoints for clinical trials of brain treatments and to make clinical recommendations for interventions to improve patients' quality of life. The goal of this paper is to review neurocognitive natural history studies to determine the current state of knowledge and assist in directing future research in all MPS disorders. There are seven different types of MPS diseases, each resulting from a specific enzyme deficiency and each having a separate natural history. MPS IX, will not be discussed as there are only 4 cases reported in the literature without cognitive abnormality. For MPS IH, hematopoietic cell transplant (HCT) is standard of care and many studies have documented the relationship between age at treatment and neurocognitive outcome, and to a lesser extent, neurocognitive status at baseline. However, the mortality and morbidity associated with the transplant process and residual long-term problems after transplant, have led to renewed efforts to find better treatments. Rather than natural history, new trials will likely need to use the developmental trajectories of the patients with HCT as a comparators. The literature has extensive data regarding developmental trajectories post-HCT. For attenuated MPS I, significant neurocognitive deficits have been documented, but more longitudinal data are needed in order to support a treatment directed at their attention and executive function abnormalities. The neuronopathic form of MPS II has been a challenge due to the variability of the trajectory of the disease with differences in timing of slowing of development and decline. Finding predictors of the course of the disease has only been partially successful, using mutation type and family history. Because of lack of systematic data and clinical trials that precede a thorough understanding of the disease, there is need for a major effort to gather natural history data on the entire spectrum of MPS II. Even in the attenuated disease, attention and executive function abnormalities need documentation. Lengthy detailed longitudinal studies are needed to encompass the wide variability in MPS II. In MPS IIIA, the existence of three good natural history studies allowed a quasi-meta-analysis. In patients with a rapid form of the disease, neurocognitive development slowed up until 42 to 47 months, halted up to about 54 months, then declined rapidly thereafter, with a leveling off at an extremely low age equivalent score below 22 months starting at about chronological age of 6. Those with slower or attenuated forms have been more variable and difficult to characterize. Because of the plethora of studies in IIIA, it has been recommended that data be combined from natural history studies to minimize the burden on parents and patients. Sufficient data exists to understand the natural history of cognition in MPS IIIA. MPS IIIB is quite similar to IIIA, but more attenuated patients in that phenotype have been reported. MPS IIIC and D, because they are so rare, have little documentation of natural history despite the prospects of treatments. MPS IV and VI are the least well documented of the MPS disorders with respect to their neurocognitive natural history. Because, like attenuated MPS I and II, they do not show progression of neurocognitive abnormality and most patients function in the range of normality, their behavioral, attentional, and executive function abnormalities have been ignored to the detriment of their quality of life. A peripheral treatment for MPS VII, extremely rare even among MPS types, has recently been approved with a post-approval monitoring system to provide neurocognitive natural history data in the future. More natural history studies in the MPS forms with milder cognitive deficits (MPS I, II, IV, and VI) are recommended with the goal of improving these patients' quality of life with and without new brain treatments, beyond the benefits of available peripheral enzyme replacement therapy. Recommendations are offered at-a-glance with respect to what areas most urgently need attention to clarify neurocognitive function in all MPS types.

Novel therapies for mucopolysaccharidosis type III

Mucopolysaccharidosis type III (MPS III) or Sanfilippo disease is an orphan inherited lysosomal storage disease and one of the most common MPS subtypes. The classical presentation is an infantile-onset neurodegenerative disease characterised by intellectual regression, behavioural and sleep disturbances, loss of ambulation, and early death. Unlike other MPS, no disease-modifying therapy has yet been approved. Here, we review the numerous approaches of curative therapy developed for MPS III from historical ineffective haematopoietic stem cell transplantation and substrate reduction therapy to the promising ongoing clinical trials based on enzyme replacement therapy or adeno-associated or lentiviral vectors mediated gene therapy. Preclinical studies are presented alongside the most recent translational first-in-man trials. In addition, we present experimental research with preclinical mRNA and gene editing strategies. Lessons from animal studies and clinical trials have highlighted the importance of an early therapy before extensive neuronal loss. A disease-modifying therapy for MPS III will undoubtedly mandate development of new strategies for early diagnosis.

Sanfilippo Syndrome: Molecular Basis, Disease Models and Therapeutic Approaches

Sanfilippo syndrome or mucopolysaccharidosis III is a lysosomal storage disorder caused by mutations in genes responsible for the degradation of heparan sulfate, a glycosaminoglycan located in the extracellular membrane. Undegraded heparan sulfate molecules accumulate within lysosomes leading to cellular dysfunction and pathology in several organs, with severe central nervous system degeneration as the main phenotypical feature. The exact molecular and cellular mechanisms by which impaired degradation and storage lead to cellular dysfunction and neuronal degeneration are still not fully understood. Here, we compile the knowledge on this issue and review all available animal and cellular models that can be used to contribute to increase our understanding of Sanfilippo syndrome disease mechanisms. Moreover, we provide an update in advances regarding the different and most successful therapeutic approaches that are currently under study to treat Sanfilippo syndrome patients and discuss the potential of new tools such as induced pluripotent stem cells to be used for disease modeling and therapy development.

A genetic and clinical study of individuals with nonsyndromic retinopathy consequent upon sequence variants in HGSNAT, the gene associated with Sanfilippo C mucopolysaccharidosis

Pathogenic variants in the gene HGSNAT (heparan-α-glucosaminide N-acetyltransferase) have been reported to underlie two distinct recessive conditions, depending on the specific genotype, mucopolysaccharidosis type IIIC (MPSIIIC)-a severe childhood-onset lysosomal storage disorder, and adult-onset nonsyndromic retinitis pigmentosa (RP). Here we describe the largest cohort to-date of HGSNAT-associated nonsyndromic RP patients, and describe their retinal phenotype, leukocyte enzymatic activity, and likely pathogenic genotypes. We identified biallelic HGSNAT variants in 17 individuals (15 families) as the likely cause of their RP. None showed any other symptoms of MPSIIIC. All had a mild but significant reduction of HGSNAT enzyme activity in leukocytes. The retinal condition was generally of late-onset, showing progressive degeneration of a concentric area of paramacular retina, with preservation but reduced electroretinogram responses. Symptoms, electrophysiology, and imaging suggest the rod photoreceptor to be the cell initially compromised. HGSNAT enzymatic testing was useful in resolving diagnostic dilemmas in compatible patients. We identified seven novel sequence variants [p.(Arg239Cys); p.(Ser296Leu); p.(Phe428Cys); p.(Gly248Ala); p.(Gly418Arg), c.1543-2A>C; c.1708delA], three of which were considered to be retina-disease-specific alleles. The most prevalent retina-disease-specific allele p.(Ala615Thr) was observed heterozygously or homozygously in 8 and 5 individuals respectively (7 and 4 families). Two siblings in one family, while identical for the HGSNAT locus, but discordant for retinal disease, suggest the influence of trans-acting genetic or environmental modifying factors.

Inherited disorders of lysosomal membrane transporters

Disorders caused by defects in lysosomal membrane transporters form a distinct subgroup of lysosomal storage disorders (LSDs). To date, defects in only 10 lysosomal membrane transporters have been associated with inherited disorders. The clinical presentations of these diseases resemble the phenotypes of other LSDs; they are heterogeneous and often present in children with neurodegenerative manifestations. However, for pathomechanistic and therapeutic studies, lysosomal membrane transport defects should be distinguished from LSDs caused by defective hydrolytic enzymes. The involved proteins differ in function, localization, and lysosomal targeting, and the diseases themselves differ in their stored material and therapeutic approaches. We provide an overview of the small group of disorders of lysosomal membrane transporters, emphasizing discovery, pathomechanism, clinical features, diagnostic methods and therapeutic aspects. We discuss common aspects of lysosomal membrane transporter defects that can provide the basis for preclinical research into these disorders.

Neuronal and Astrocytic Differentiation from Sanfilippo C Syndrome iPSCs for Disease Modeling and Drug Development

Sanfilippo syndrome type C (mucopolysaccharidosis IIIC) is an early-onset neurodegenerative lysosomal storage disorder, which is currently untreatable. The vast majority of studies focusing on disease mechanisms of Sanfilippo syndrome were performed on non-neural cells or mouse models, which present obvious limitations. Induced pluripotent stem cells (iPSCs) are an efficient way to model human diseases in vitro. Recently developed transcription factor-based differentiation protocols allow fast and efficient conversion of iPSCs into the cell type of interest. By applying these protocols, we have generated new neuronal and astrocytic models of Sanfilippo syndrome using our previously established disease iPSC lines. Moreover, our neuronal model exhibits disease-specific molecular phenotypes, such as increase in lysosomes and heparan sulfate. Lastly, we tested an experimental, siRNA-based treatment previously shown to be successful in patients' fibroblasts and demonstrated its lack of efficacy in neurons. Our findings highlight the need to use relevant human cellular models to test therapeutic interventions and shows the applicability of our neuronal and astrocytic models of Sanfilippo syndrome for future studies on disease mechanisms and drug development.

Molecular characterization of a large group of Mucopolysaccharidosis type IIIC patients reveals the evolutionary history of the disease

Mucopolysaccharidosis type IIIC (MPSIIIC) is a severe, rare autosomal recessive disorder caused by variants in the heparan-α-glucosaminide N-acetyltransferase (HGSNAT) gene which result in lysosomal accumulation of heparan sulfate. We analyzed clinical presentation, molecular defects and their haplotype context in 78 (27 novel) MPSIIIC cases from 22 countries, the largest group studied so far. We describe for the first time disease-causing variants in the patients from Brazil, Algeria, Azerbaijan, and Iran, and extend their spectrum within Canada, Colombia, Turkey, and the USA. Six variants are novel: two missense, c.773A>T/p.N258I and c.1267G>T/p.G423W, a nonsense c.164T>A/p.L55*, a splice-site mutation c.494-1G>A/p.[P165_L187delinsQSCYVTQAGVRWHHLGSLQALPPGFTPFSYLSLLSSWNC,P165fs], a deletion c.1348delG/p.(D450fs) and an insertion c.1479dupA/p.(Leu494fs). The missense HGSNAT variants lacked lysosomal targeting, enzymatic activity, and likely the correct folding. The haplotype analysis identified founder mutations, p.N258I, c.525dupT, and p.L55* in the Brazilian state of Paraiba, c.493+1G>A in Eastern Canada/Quebec, p.A489E in the USA, p.R384* in Poland, p.R344C and p.S518F in the Netherlands and suggested that variants c.525dupT, c.372-2G>A, and c.234+1G>A present in cis with c.564-98T>C and c.710C>A rare single-nucleotide polymorphisms, have been introduced by Portuguese settlers in Brazil. Altogether, our results provide insights into the origin, migration roots and founder effects of HGSNAT disease-causing variants, and reveal the evolutionary history of MPSIIIC.

Lysosomal N-acetyltransferase interacts with ALIX and is detected in extracellular vesicles

Heparan acetyl CoA: α-glucosaminide N-acetyltransferase (HGSNAT) is a lysosomal multi-pass transmembrane protein whose deficiency may lead to an accumulation of heparan sulphate and the neurodegenerative lysosomal storage disorder mucopolysaccharidosis (MPS) IIIC. In this study, HGSNAT activity was detected in extracellular vesicles isolated from both human urine and culture medium conditioned with HEK 293T cells. We also demonstrate that HGSNAT co-immunoprecipitates with antibodies to ALIX, which is associated with the endosomal sorting complexes required for transport (ESCRT) proteins, and is implicated in the targeting of proteins to intraluminal vesicles of multivesicular bodies, the origin of exosomes. Furthermore, mutation of a putative LYPXnL-based binding site within HGSNAT for the V-domain of ALIX ablated association of HGSNAT with ALIX, post-translational maturation, and transport through the endo-lysosomal network. Unexpectedly, however, a mutation within the V-domain of ALIX demonstrated enhanced HGSNAT association, perhaps due to the actual involvement of other binding sites in this interaction. Indeed, HGSNAT still co-immunoprecipitates with truncations of ALIX lacking the V-domain. Interestingly, CRISPR/Cas9 mediated knock-down of ALIX did not inhibit HGSNAT trafficking through the endo-lysosomal network, suggesting that there is an alternative pathway for trafficking HGSNAT that does not require ALIX. Nonetheless, the targeting of HGSNAT to extracellular vesicles may provide a mechanism to subsequently transfer this enzyme extracellularly to provide a foundation for a therapy for MPS IIIC patients.

Treatment of brain disease in the mucopolysaccharidoses

The mucopolysaccharidosis (MPS) disorders are a group of lysosomal storage diseases caused by lysosomal enzyme deficits that lead to glycosaminoglycan accumulation, affecting various tissues throughout the body based on the specific enzyme deficiency. These disorders are characterized by their progressive nature and a variety of somatic manifestations and neurological symptoms. There are established treatments for some MPS disorders, but these mostly alleviate somatic and non-neurological symptoms and do not cure the disease. Patients with MPS I, II, III, and VII can present with neurological manifestations such as neurocognitive decline and behavioral problems. Treatment of these neurological manifestations remains challenging due to the blood-brain barrier (BBB) that limits delivery of therapeutic agents to the central nervous system (CNS). New therapies that circumvent this barrier and target brain disease in MPS are currently under development. They primarily focus on facilitating penetration of drugs through the BBB, delivery of recombinant enzyme to the brain by gene therapy, or direct CNS administration. This review summarizes existing and potential future treatment approaches that target brain disease in MPS. The information in this review is based on current literature and presentations and discussions during a closed meeting by an international group of experts with extensive experience in managing and treating MPS.

The Genetic Basis of Pericentral Retinitis Pigmentosa-A Form of Mild Retinitis Pigmentosa

Pericentral retinitis pigmentosa (RP) is an atypical form of RP that affects the near-peripheral retina first and tends to spare the far periphery. This study was performed to further define the genetic basis of this phenotype. We identified a cohort of 43 probands with pericentral RP based on a comprehensive analysis of their retinal phenotype. Genetic analyses of DNA samples from these patients were performed using panel-based next-generation sequencing, copy number variations, and whole exome sequencing (WES). Mutations provisionally responsible for disease were found in 19 of the 43 families (44%) analyzed. These include mutations in RHO (five patients), USH2A (four patients), and PDE6B (two patients). Of 28 putatively pathogenic alleles, 15 (54%) have been previously identified in patients with more common forms of typical RP, while the remaining 13 mutations (46%) were novel. Burden testing of WES data successfully identified HGSNAT as a cause of pericentral RP in at least two patients, suggesting it is also a relatively common cause of pericentral RP. While additional sequencing might uncover new genes specifically associated with pericentral RP, the current results suggest that genetically pericentral RP is not a separate clinical entity, but rather is part of the spectrum of mild RP phenotypes.

High-Throughput Screen Fails to Identify Compounds That Enhance Residual Enzyme Activity of Mutant N-Acetyl-α-Glucosaminidase in Mucopolysaccharidosis Type IIIB

BACKGROUND

In the severe neurodegenerative disorder mucopolysaccharidosis type IIIB (MPSIIIB or Sanfilippo disease type B), deficiency of the lysosomal enzyme N-acetyl-α-glucosaminidase (NAGLU) results in accumulation of heparan sulfate. Patients present with a severe, rapidly progressing phenotype (RP) or a more attenuated, slowly progressing phenotype (SP). In a previous study, residual NAGLU activity in fibroblasts of SP patients could be increased by culturing at 30°C, probably as a result of improved protein folding and lysosomal targeting under these conditions. Chaperones are molecules which influence protein folding and could therefore have therapeutic potential in SP MPSIIIB patients. Here we studied the effects of 1,302 different compounds on residual NAGLU activity in SP MPSIIIB patient fibroblasts including 1,280 approved compounds from the Prestwick Chemical Library.

METHODS

Skin fibroblasts of healthy controls, an SP MPSIIIB patient (homozygous for the temperature sensitive mutation p.S612G) and an RP MPSIIIB patient (homozygous for the p.R297* mutation and non-temperature sensitive), were used. A high-throughput assay for measurement of NAGLU activity was developed and validated, after which 1,302 different molecules were tested for their potential to increase NAGLU activity.

RESULTS

None of the compounds tested were able to enhance NAGLU activity.

CONCLUSIONS

This high-throughput screen failed to identify compounds that could enhance residual activity of mutant NAGLU in fibroblasts of SP MPSIIIB patients with temperature sensitive mutations. To therapeutically simulate the positive effect of lower temperatures on residual NAGLU activity, first more insight is needed into the mechanisms underlying this temperature dependent increase.

A selective screening program for the early detection of mucopolysaccharidosis: Results of the FIND project - a 2-year follow-up study

The mucopolysaccharidoses (MPSs) are underdiagnosed but they are evaluated in few newborn screening programs, probably due to the many challenges remaining, such as the identification of late-onset phenotypes. Systematic screening at the onset of clinical symptoms could help to early identify patients who may benefit from specific treatments. The aim of this prospective study was to assess a novel selective screening program, the FIND project, targeting patients aged 0 to 16 years with clinical manifestations of MPS. The project was designed to increase awareness of these diseases among pediatricians and allow early diagnosis.From July 2014 to June 2016, glycosaminoglycan (GAG) levels normalized to creatinine levels were determined in urine-impregnated analytical paper submitted by pediatricians who had patients with clinical signs and/or symptoms compatible with MPS. When high GAG concentrations were detected, a new liquid urine sample was requested to confirm and identify the GAG present. When a specific form of MPS was suspected, enzyme activity was analyzed using blood-impregnated paper to determine MPS type (I, IIIB, IIIC, IVA, IVB, VI, or VII). Age-specific reference values for GAG were previously established using 145 urine samples from healthy children.GAG levels were normal in 147 (81.7%) of the 180 initial samples received. A liquid sample was requested for the other 33 cases (18.3%); GAG levels were normal in 13 of these and slightly elevated in 12, although the electrophoresis study showed no evidence of MPS. Elevated levels with corresponding low enzymatic activity were confirmed in 8 cases. The mean time from onset of clinical symptoms to detection of MPS was 22 months, and just 2 cases were detected at the beginning of the project were detected with 35 and 71 months of evolution of clinical symptoms. Our screening strategy for MPS had a sensitivity of 100%, a specificity of 85%, and a positive predictive value of 24%.The FIND project is a useful and cost-effective screening method for increasing awareness of MPS among pediatricians and enabling the detection of MPS at onset of clinical symptoms.

Glycomimetic-based pharmacological chaperones for lysosomal storage disorders: lessons from Gaucher, GM1-gangliosidosis and Fabry diseases

Lysosomal storage disorders (LSDs) are often caused by mutations that destabilize native folding and impair the trafficking of enzymes, leading to premature endoplasmic reticulum (ER)-associated degradation, deficiencies of specific hydrolytic functions and aberrant storage of metabolites in the lysosomes. Enzyme replacement therapy (ERT) and substrate reduction therapy (SRT) are available for a few of these conditions, but most remain orphan. A main difficulty is that virtually all LSDs involve neurological decline and neither proteins nor the current SRT drugs can cross the blood-brain barrier. Twenty years ago a new therapeutic paradigm better suited for neuropathic LSDs was launched, namely pharmacological chaperone (PC) therapy. PCs are small molecules capable of binding to the mutant protein at the ER, inducing proper folding, restoring trafficking and increasing enzyme activity and substrate processing in the lysosome. In many LSDs the mutated protein is a glycosidase and the accumulated substrate is an oligo- or polysaccharide or a glycoconjugate, e.g. a glycosphingolipid. Although it might appear counterintuitive, substrate analogues (glycomimetics) behaving as competitive glycosidase inhibitors are good candidates to perform PC tasks. The advancements in the knowledge of the molecular basis of LSDs, including enzyme structures, binding modes, trafficking pathways and substrate processing mechanisms, have been put forward to optimize PC selectivity and efficacy. Moreover, the chemical versatility of glycomimetics and the variety of structures at hand allow simultaneous optimization of chaperone and pharmacokinetic properties. In this Feature Article we review the advancements made in this field in the last few years and the future outlook through the lessons taught by three archetypical LSDs: Gaucher disease, GM1-gangliosidosis and Fabry disease.

Carbohydrate-Processing Enzymes of the Lysosome: Diseases Caused by Misfolded Mutants and Sugar Mimetics as Correcting Pharmacological Chaperones

Lysosomal storage diseases are hereditary disorders caused by mutations on genes encoding for one of the more than fifty lysosomal enzymes involved in the highly ordered degradation cascades of glycans, glycoconjugates, and other complex biomolecules in the lysosome. Several of these metabolic disorders are associated with the absence or the lack of activity of carbohydrate-processing enzymes in this cell compartment. In a recently introduced therapy concept, for susceptible mutants, small substrate-related molecules (so-called pharmacological chaperones), such as reversible inhibitors of these enzymes, may serve as templates for the correct folding and transport of the respective protein mutant, thus improving its concentration and, consequently, its enzymatic activity in the lysosome. Carbohydrate-processing enzymes in the lysosome, related lysosomal diseases, and the scope and limitations of reported reversible inhibitors as pharmacological chaperones are discussed with a view to possibly extending and improving research efforts in this area of orphan diseases.

arrEYE: a customized platform for high-resolution copy number analysis of coding and noncoding regions of known and candidate retinal dystrophy genes and retinal noncoding RNAs

Purpose:

Our goal was to design a customized microarray, arrEYE, for high-resolution copy number variant (CNV) analysis of known and candidate genes for inherited retinal dystrophy (iRD) and retina-expressed noncoding RNAs (ncRNAs).

Methods:

arrEYE contains probes for the full genomic region of 106 known iRD genes, including those implicated in retinitis pigmentosa (RP) (the most frequent iRD), cone–rod dystrophies, macular dystrophies, and an additional 60 candidate iRD genes and 196 ncRNAs. Eight CNVs in iRD genes identified by other techniques were used as positive controls. The test cohort consisted of 57 patients with autosomal dominant, X-linked, or simplex RP.

Results:

In an RP patient, a novel heterozygous deletion of exons 7 and 8 of the HGSNAT gene was identified: c.634-408_820+338delinsAGAATATG, p.(Glu212Glyfs*2). A known variant was found on the second allele: c.1843G>A, p.(Ala615Thr). Furthermore, we expanded the allelic spectrum of USH2A and RCBTB1 with novel CNVs.

Conclusion:

The arrEYE platform revealed subtle single-exon to larger CNVs in iRD genes that could be characterized at the nucleotide level, facilitated by the high resolution of the platform. We report the first CNV in HGSNAT that, combined with another mutation, leads to RP, further supporting its recently identified role in nonsyndromic iRD.

Genet Med19 4, 457–466.

Progressive neurologic and somatic disease in a novel mouse model of human mucopolysaccharidosis type IIIC

Mucopolysaccharidosis type IIIC (MPSIIIC) is a severe lysosomal storage disease caused by deficiency in activity of the transmembrane enzyme heparan-α-glucosaminide N-acetyltransferase (HGSNAT) that catalyses the N-acetylation of α-glucosamine residues of heparan sulfate. Enzyme deficiency causes abnormal substrate accumulation in lysosomes, leading to progressive and severe neurodegeneration, somatic pathology and early death. There is no cure for MPSIIIC, and development of new therapies is challenging because of the unfeasibility of cross-correction. In this study, we generated a new mouse model of MPSIIIC by targeted disruption of the Hgsnat gene. Successful targeting left LacZ expression under control of the Hgsnat promoter, allowing investigation into sites of endogenous expression, which was particularly prominent in the CNS, but was also detectable in peripheral organs. Signs of CNS storage pathology, including glycosaminoglycan accumulation, lysosomal distension, lysosomal dysfunction and neuroinflammation were detected in 2-month-old animals and progressed with age. Glycosaminoglycan accumulation and ultrastructural changes were also observed in most somatic organs, but lysosomal pathology seemed most severe in liver. Furthermore, HGSNAT-deficient mice had altered locomotor and exploratory activity and shortened lifespan. Hence, this animal model recapitulates human MPSIIIC and provides a useful tool for the study of disease physiopathology and the development of new therapeutic approaches.

Less Is More: Substrate Reduction Therapy for Lysosomal Storage Disorders

Lysosomal storage diseases (LSDs) are a group of rare, life-threatening genetic disorders, usually caused by a dysfunction in one of the many enzymes responsible for intralysosomal digestion. Even though no cure is available for any LSD, a few treatment strategies do exist. Traditionally, efforts have been mainly targeting the functional loss of the enzyme, by injection of a recombinant formulation, in a process called enzyme replacement therapy (ERT), with no impact on neuropathology. This ineffectiveness, together with its high cost and lifelong dependence is amongst the main reasons why additional therapeutic approaches are being (and have to be) investigated: chaperone therapy; gene enhancement; gene therapy; and, alternatively, substrate reduction therapy (SRT), whose aim is to prevent storage not by correcting the original enzymatic defect but, instead, by decreasing the levels of biosynthesis of the accumulating substrate(s). Here we review the concept of substrate reduction, highlighting the major breakthroughs in the field and discussing the future of SRT, not only as a monotherapy but also, especially, as complementary approach for LSDs.

Sanfilippo syndrome: causes, consequences, and treatments

Sanfilippo syndrome, or mucopolysaccharidosis (MPS) type III, refers to one of five autosomal recessive, neurodegenerative lysosomal storage disorders (MPS IIIA to MPS IIIE) whose symptoms are caused by the deficiency of enzymes involved exclusively in heparan sulfate degradation. The primary characteristic of MPS III is the degeneration of the central nervous system, resulting in mental retardation and hyperactivity, typically commencing during childhood. The significance of the order of events leading from heparan sulfate accumulation through to downstream changes in the levels of biomolecules within the cell and ultimately the (predominantly neuropathological) clinical symptoms is not well understood. The genes whose deficiencies cause the MPS III subtypes have been identified, and their gene products, as well as a selection of disease-causing mutations, have been characterized to varying degrees with respect to both frequency and direct biochemical consequences. A number of genetic and biochemical diagnostic methods have been developed and adopted by diagnostic laboratories. However, there is no effective therapy available for any form of MPS III, with treatment currently limited to clinical management of neurological symptoms. The availability of animal models for all forms of MPS III, whether spontaneous or generated via gene targeting, has contributed to improved understanding of the MPS III subtypes, and has provided and will deliver invaluable tools to appraise emerging therapies. Indeed, clinical trials to evaluate intrathecally-delivered enzyme replacement therapy in MPS IIIA patients, and gene therapy for MPS IIIA and MPS IIIB patients are planned or underway.

Novel Direct Assay for Acetyl-CoA:α-Glucosaminide N-Acetyltransferase Using BODIPY-Glucosamine as a Substrate

Heparan sulfate acetyl-CoA:α-glucosaminide N-acetyltransferase (HGSNAT) catalyzes the transmembrane acetylation of heparan sulfate in lysosomes required for its further catabolism. Inherited deficiency of HGSNAT in humans results in lysosomal storage of heparan sulfate and causes severe neurodegenerative disease, mucopolysaccharidosis III type C (MPS IIIC). MPS IIIC patients can potentially benefit from a therapeutic approach based on active site-specific inhibitors of HGSNAT used as pharmacological chaperons to modify the folding of the mutant protein in the patient's cells. This research however was hampered by the absence of the assay suitable for high-throughput screening of drug libraries for HGSNAT inhibitors. The existing method utilizing 4-methylumbelliferyl-β-D-glucosaminide (MU-βGlcN) requires the sequential action of two enzymes, HGSNAT and β-hexosaminidase, whereas the radioactive assay with [C14]-AcCoA is complicated and expensive. We describe a novel direct method to assay HGSNAT enzymatic activity using fluorescent BODIPY-glucosamine as a substrate. The specificity of the assay was tested using cultured fibroblasts of MPS IIIC patients, which showed a profound deficiency of HGSNAT activity as compared to normal controls as well as to MPS IIIA and D patients known to have normal HGSNAT activity. Known competitive HGSNAT inhibitor, glucosamine, had similar inhibition constants for MU-βGlcN and BODIPY-glucosamine acetylation reactions. Altogether our data show that novel HGSNAT assay is specific and potentially applicable for the biochemical diagnosis of MPS IIIC and high-throughput screening for HGSNAT inhibitors.

Stop-codon and C-terminal nonsense mutations are associated with a lower risk of cardiac events in patients with long QT syndrome type 1

BACKGROUND

In long QT syndrome type 1 (LQT1), the location and type of mutations have been shown to affect the clinical outcome. Although haploinsufficiency, including stop-codon and frameshift mutations, has been associated with a lower risk of cardiac events in patients with LQT1, nonsense mutations have been presumed functionally equivalent.

OBJECTIVE

The purpose of this study was to evaluate clinical differences between patients with nonsense mutations.

METHODS

The study sample comprised 1090 patients with genetically confirmed mutations. Patients were categorized into 5 groups, depending on mutation type and location: missense not located in the high-risk cytoplasmic loop (c-loop) (n = 698), which is used as reference; missense c-loop (n = 192); stop-codon (n = 67); frameshift (n = 39); and others (n = 94). The primary outcome was a composite end point of syncope, aborted cardiac arrest, and long QT syndrome-related death (cardiac events). Outcomes were evaluated using the multivariate Cox proportional hazards regression analysis. Standard patch clamp techniques were used.

RESULTS

Compared to patients with missense non-c-loop mutations, the risk of cardiac events was reduced significantly in patients with stop-codon mutations (hazard ratio [HR] 0.57; 95% confidence interval [CI] 0.34-0.96; P = .035), but not in patients with frameshift mutations (HR 1.01; 95% CI 0.58-1.77; P = .97). Our data suggest that currents of the most common stop-codon mutant channel (Q530X) were larger than those of haploinsufficient channels (wild type: 42 ± 6 pA/pF, n = 20; Q530X+wild type: 79 ± 14 pA/pF, n = 20; P < .05) and voltage dependence of activation was altered.

CONCLUSION

Stop-codon mutations are associated with a lower risk of cardiac events in patients with LQT1, while frameshift mutations are associated with the same risk as the majority of the missense mutations. Our data indicate functional differences between these previously considered equivalent mutation subtypes.

Crosstalk between 2 organelles: Lysosomal storage of heparan sulfate causes mitochondrial defects and neuronal death in mucopolysaccharidosis III type C

More than 30% of all lysosomal diseases are mucopolysaccharidoses, disorders affecting the enzymes needed for the stepwise degradation of glycosaminoglycans (mucopolysaccharides). Mucopolysaccharidosis type IIIC (MPS IIIC) is a severe neurologic disease caused by genetic deficiency of heparan sulfate acetyl-CoA: α-glucosaminide N-acetyltransferase (HGSNAT). Through our studies, we have cloned the gene, identified molecular defects in MPS IIIC patients and most recently completed phenotypic characterization of the first animal model of the disease, a mouse with a germline inactivation of the Hgsnat gene.¹ The obtained data have led us to propose that Hgsnat deficiency and lysosomal accumulation of heparan sulfate in microglial cells followed by their activation and cytokine release result in mitochondrial dysfunction in the neurons causing their death which explains why MPS IIIC manifests primarily as a neurodegenerative disease. The goal of this addendum is to summarize data yielding new insights into the mechanism of MPS IIIC and promising novel therapeutic solutions for this and similar disorders.

Pharmacological Chaperone Therapy: Preclinical Development, Clinical Translation, and Prospects for the Treatment of Lysosomal Storage Disorders

Lysosomal storage disorders (LSDs) are a group of inborn metabolic diseases caused by mutations in genes that encode proteins involved in different lysosomal functions, in most instances acidic hydrolases. Different therapeutic approaches have been developed to treat these disorders. Pharmacological chaperone therapy (PCT) is an emerging approach based on small-molecule ligands that selectively bind and stabilize mutant enzymes, increase their cellular levels, and improve lysosomal trafficking and activity. Compared to other approaches, PCT shows advantages, particularly in terms of oral administration, broad biodistribution, and positive impact on patients' quality of life. After preclinical in vitro and in vivo studies, PCT is now being translated in the first clinical trials, either as monotherapy or in combination with enzyme replacement therapy, for some of the most prevalent LSDs. For some LSDs, the results of the first clinical trials are encouraging and warrant further development. Future research in the field of PCT will be directed toward the identification of novel chaperones, including new allosteric drugs, and the exploitation of synergies between chaperone treatment and other therapeutic approaches.

Non-syndromic retinitis pigmentosa due to mutations in the mucopolysaccharidosis type IIIC gene, heparan-alpha-glucosaminide N-acetyltransferase (HGSNAT)

Retinitis pigmentosa (RP), the most common form of inherited retinal degeneration, is clinically and genetically heterogeneous and can appear as syndromic or non-syndromic. Mucopolysaccharidosis type IIIC (MPS IIIC) is a lethal disorder, caused by mutations in the heparan-alpha-glucosaminide N-acetyltransferase (HGSNAT) gene and characterized by progressive neurological deterioration, with retinal degeneration as a prominent feature. We identified HGSNAT mutations in six patients with non-syndromic RP. Whole exome sequencing (WES) in an Ashkenazi Jewish Israeli RP patient revealed a novel homozygous HGSNAT variant, c.370A>T, which leads to partial skipping of exon 3. Screening of 66 Ashkenazi RP index cases revealed an additional family with two siblings homozygous for c.370A>T. WES in three Dutch siblings with RP revealed a complex HGSNAT variant, c.[398G>C; 1843G>A] on one allele, and c.1843G>A on the other allele. HGSNAT activity levels in blood leukocytes of patients were reduced compared with healthy controls, but usually higher than those in MPS IIIC patients. All patients were diagnosed with non-syndromic RP and did not exhibit neurological deterioration, or any phenotypic features consistent with MPS IIIC. Furthermore, four of the patients were over 60 years old, exceeding by far the life expectancy of MPS IIIC patients. HGSNAT is highly expressed in the mouse retina, and we hypothesize that the retina requires higher HGSNAT activity to maintain proper function, compared with other tissues associated with MPS IIIC, such as the brain. This report broadens the spectrum of phenotypes associated with HGSNAT mutations and highlights the critical function of HGSNAT in the human retina.

Pharmacological chaperone therapy for lysosomal storage diseases

Pharmacological chaperone therapy is an emerging approach to treat lysosomal storage diseases. Small-molecule chaperones interact with mutant enzymes, favor their correct conformation and enhance their stability. This approach shows significant advantages when compared with existing therapies, particularly in terms of the bioavailability of drugs, oral administration and positive impact on the quality of patients' lives. On the other hand, future research in this field must confront important challenges. The identification of novel chaperones is indispensable to expanding the number of patients amenable to this treatment and to optimize therapeutic efficacy. It is important to develop new allosteric drugs, to address the risk of inhibiting target enzymes. Future research must also be directed towards the exploitation of synergies between chaperone treatment and other therapeutic approaches.

A multiparametric computational algorithm for comprehensive assessment of genetic mutations in mucopolysaccharidosis type IIIA (Sanfilippo syndrome)

Mucopolysaccharidosis type IIIA (MPS-IIIA, Sanfilippo syndrome) is a Lysosomal Storage Disease caused by cellular deficiency of N-sulfoglucosamine sulfohydrolase (SGSH). Given the large heterogeneity of genetic mutations responsible for the disease, a comprehensive understanding of the mechanisms by which these mutations affect enzyme function is needed to guide effective therapies. We developed a multiparametric computational algorithm to assess how patient genetic mutations in SGSH affect overall enzyme biogenesis, stability, and function. 107 patient mutations for the SGSH gene were obtained from the Human Gene Mutation Database representing all of the clinical mutations documented for Sanfilippo syndrome. We assessed each mutation individually using ten distinct parameters to give a comprehensive predictive score of the stability and misfolding capacity of the SGSH enzyme resulting from each of these mutations. The predictive score generated by our multiparametric algorithm yielded a standardized quantitative assessment of the severity of a given SGSH genetic mutation toward overall enzyme activity. Application of our algorithm has identified SGSH mutations in which enzymatic malfunction of the gene product is specifically due to impairments in protein folding. These scores provide an assessment of the degree to which a particular mutation could be treated using approaches such as chaperone therapies. Our multiparametric protein biogenesis algorithm advances a key understanding in the overall biochemical mechanism underlying Sanfilippo syndrome. Importantly, the design of our multiparametric algorithm can be tailored to many other diseases of genetic heterogeneity for which protein misfolding phenotypes may constitute a major component of disease manifestation.

Neuroinflammation, mitochondrial defects and neurodegeneration in mucopolysaccharidosis III type C mouse model

Severe progressive neurological paediatric disease mucopolysaccharidosis III type C is caused by mutations in the HGSNAT gene leading to deficiency of acetyl-CoA: α-glucosaminide N-acetyltransferase involved in the lysosomal catabolism of heparan sulphate. To understand the pathophysiology of the disease we generated a mouse model of mucopolysaccharidosis III type C by germline inactivation of the Hgsnat gene. At 6-8 months mice showed hyperactivity, and reduced anxiety. Cognitive memory decline was detected at 10 months and at 12-13 months mice showed signs of unbalanced hesitant walk and urinary retention. Lysosomal accumulation of heparan sulphate was observed in hepatocytes, splenic sinus endothelium, cerebral microglia, liver Kupffer cells, fibroblasts and pericytes. Starting from 5 months, brain neurons showed enlarged, structurally abnormal mitochondria, impaired mitochondrial energy metabolism, and storage of densely packed autofluorescent material, gangliosides, lysozyme, phosphorylated tau, and amyloid-β. Taken together, our data demonstrate for the first time that deficiency of acetyl-CoA: α-glucosaminide N-acetyltransferase causes lysosomal accumulation of heparan sulphate in microglial cells followed by their activation and cytokine release. They also show mitochondrial dysfunction in the neurons and neuronal loss explaining why mucopolysaccharidosis III type C manifests primarily as a neurodegenerative disease.

A phenotypic high throughput screening assay for the identification of pharmacoperones for the gonadotropin releasing hormone receptor

We describe a phenotypic high throughput screening (HTS) calcium flux assay designed to identify pharmacoperones for the gonadotropin releasing hormone receptor (GnRHR). Pharmacoperones are target-specific, small molecules that diffuse into cells, rescue misfolded protein mutants, and restore them to function. Rescue is based on correcting the trafficking of mutants that would otherwise be retained in the endoplasmic reticulum and unable to function correctly. This approach identifies drugs with a significant degree of novelty, relying on cellular mechanisms that are not currently exploited. Development of such assays is important, since the extensive use of agonist/antagonist screens alone means that useful chemical structures may be present in existing libraries but have not been previously identified using existing methods. Our assay utilizes cell lines stably expressing a GnRHR mutant under the control of a tetracycline (OFF) transactivator. This allows us to quantitate the level of functional and properly trafficked G protein coupled receptors present in each test well. Furthermore, since we are able to turn receptor expression on and off, we can rapidly eliminate the majority of false positives from our screening results. Our data show that this approach is likely to be successful in identifying hits from large chemical libraries.

Therapeutic strategies based on modified U1 snRNAs and chaperones for Sanfilippo C splicing mutations

BACKGROUND

Mutations affecting RNA splicing represent more than 20% of the mutant alleles in Sanfilippo syndrome type C, a rare lysosomal storage disorder that causes severe neurodegeneration. Many of these mutations are localized in the conserved donor or acceptor splice sites, while few are found in the nearby nucleotides.

METHODS

In this study we tested several therapeutic approaches specifically designed for different splicing mutations depending on how the mutations affect mRNA processing. For three mutations that affect the donor site (c.234 + 1G > A, c.633 + 1G > A and c.1542 + 4dupA), different modified U1 snRNAs recognizing the mutated donor sites, have been developed in an attempt to rescue the normal splicing process. For another mutation that affects an acceptor splice site (c.372-2A > G) and gives rise to a protein lacking four amino acids, a competitive inhibitor of the HGSNAT protein, glucosamine, was tested as a pharmacological chaperone to correct the aberrant folding and to restore the normal trafficking of the protein to the lysosome.

RESULTS

Partial correction of c.234 + 1G > A mutation was achieved with a modified U1 snRNA that completely matches the splice donor site suggesting that these molecules may have a therapeutic potential for some splicing mutations. Furthermore, the importance of the splice site sequence context is highlighted as a key factor in the success of this type of therapy. Additionally, glucosamine treatment resulted in an increase in the enzymatic activity, indicating a partial recovery of the correct folding.

CONCLUSIONS

We have assayed two therapeutic strategies for different splicing mutations with promising results for the future applications.

Collaboration for rare disease drug discovery research

Rare disease research has reached a tipping point, with the confluence of scientific and technologic developments that if appropriately harnessed, could lead to key breakthroughs and treatments for this set of devastating disorders. Industry-wide trends have revealed that the traditional drug discovery research and development (R&D) model is no longer viable, and drug companies are evolving their approach. Rather than only pursue blockbuster therapeutics for heterogeneous, common diseases, drug companies have increasingly begun to shift their focus to rare diseases. In academia, advances in genetics analyses and disease mechanisms have allowed scientific understanding to mature, but the lack of funding and translational capability severely limits the rare disease research that leads to clinical trials. Simultaneously, there is a movement towards increased research collaboration, more data sharing, and heightened engagement and active involvement by patients, advocates, and foundations. The growth in networks and social networking tools presents an opportunity to help reach other patients but also find researchers and build collaborations. The growth of collaborative software that can enable researchers to share their data could also enable rare disease patients and foundations to manage their portfolio of funded projects for developing new therapeutics and suggest drug repurposing opportunities. Still there are many thousands of diseases without treatments and with only fragmented research efforts. We will describe some recent progress in several rare diseases used as examples and propose how collaborations could be facilitated. We propose that the development of a center of excellence that integrates and shares informatics resources for rare diseases sponsored by all of the stakeholders would help foster these initiatives.

Chaperoning G protein-coupled receptors: from cell biology to therapeutics

G protein-coupled receptors (GPCRs) are membrane proteins that traverse the plasma membrane seven times (hence, are also called 7TM receptors). The polytopic structure of GPCRs makes the folding of GPCRs difficult and complex. Indeed, many wild-type GPCRs are not folded optimally, and defects in folding are the most common cause of genetic diseases due to GPCR mutations. Both general and receptor-specific molecular chaperones aid the folding of GPCRs. Chemical chaperones have been shown to be able to correct the misfolding in mutant GPCRs, proving to be important tools for studying the structure-function relationship of GPCRs. However, their potential therapeutic value is very limited. Pharmacological chaperones (pharmacoperones) are potentially important novel therapeutics for treating genetic diseases caused by mutations in GPCR genes that resulted in misfolded mutant proteins. Pharmacoperones also increase cell surface expression of wild-type GPCRs; therefore, they could be used to treat diseases that do not harbor mutations in GPCRs. Recent studies have shown that indeed pharmacoperones work in both experimental animals and patients. High-throughput assays have been developed to identify new pharmacoperones that could be used as therapeutics for a number of endocrine and other genetic diseases.