AAV8-mediated expression of N-acetylglucosamine-1-phosphate transferase attenuates bone loss in a mouse model of mucolipidosis II

Early diagnostic clues of mucolipidosis type II: Significance of radiological findings

Mucolipidosis type-II (ML-II) is an ultra-rare disorder caused by deficiency of N-acetylglucosaminyl-1-phosphotransferase enzyme due to biallelic pathogenic variants in GNPTAB gene. There are a few known about the natural history of ML-II. In this study, we presented the natural course of 24 patients diagnosed with ML-II. Mean age at diagnosis was 9.3 ± 5.7 months. All patients had coarse face, developmental delay, and hypotonia. The mean survival time was 3.01 ± 1.4 years. The oldest patient was 6.5 years old. Twelve patients died due to lung infection and respiratory failure. We observed early and significant radiological findings of ML-II were different from typical dysostosis multiplex such as femoral cloaking, rickets-like changes, and talocalcaneal stippling. These are significant findings observed in the fetal or newborn period which is considered to be highly characteristic of ML-II and disappears in the first year. Cloaking, rickets-like changes, and stippling were not observed in patients older than three months of age and this suggests that these findings disappear within the first year. These radiological features can be used as important clues for diagnosis. We detected eight different pathogenic variants in GNPTAB gene, three of them were novel.

Development of AAV-Mediated Gene Therapy Approaches to Treat Skeletal Diseases

Adeno-associated viral (AAV) vectors have emerged as crucial tools in advancing gene therapy for skeletal diseases, offering the potential for sustained expression with low postinfection immunogenicity and pathogenicity. Preclinical studies support both the therapeutic efficacy and safety of these vectors, illustrating the promise of AAV-mediated gene therapy. Emerging technologies and innovations in AAV-mediated gene therapy strategies, such as gene addition, gene replacement, gene silencing, and gene editing, offer new approaches to clinical application. Recently, the increasing preclinical applications of AAV to rare skeletal diseases, such as fibrodysplasia ossificans progressiva (FOP) and osteogenesis imperfecta (OI), and prevalent bone diseases, such as osteoporosis, bone fracture, critical-sized bone defects, and osteoarthritis, have been reported. Despite existing limitations in clinical use, such as high cost and safety, the AAV-mediated gene transfer platform is a promising approach to deliver therapeutic gene(s) to the skeleton to treat skeletal disorders, including those otherwise intractable by other therapeutic approaches. This review provides a comprehensive overview of the therapeutic advancements, challenges, limitations, and solutions within AAV-based gene therapy for prevalent and rare skeletal diseases.

CNS Manifestations in Mucolipidosis Type II-A Retrospective Analysis of Longitudinal Data on Neurocognitive Development and Neuroimaging in Eleven Patients

Mucolipidosis type II (MLII), an ultra-rare lysosomal storage disorder, manifests as a fatal multi-systemic disease. Mental inhibition and progressive neurodegeneration are commonly reported disease manifestations. Nevertheless, longitudinal data on neurocognitive testing and neuroimaging lack in current literature. This study aimed to provide details on central nervous system manifestations in MLII. All MLII patients with at least one standardized developmental assessment performed between 2005 and 2022 were included by retrospective chart review. A multiple mixed linear regression model was applied. Eleven patients with a median age of 34.0 months (range 1.6-159.6) underwent 32 neurocognitive and 28 adaptive behaviour assessments as well as 14 brain magnetic resonance imagings. The scales used were mainly BSID-III (42%) and VABS-II (47%). Neurocognitive testing (per patient: mean 2.9, standard deviation (SD) 2.0) performed over 0-52.1 months (median 12.1) revealed profound impairment with a mean developmental quotient of 36.7% (SD 20.4) at last assessment. The patients showed sustained development; on average, they gained 0.28 age-equivalent score points per month (confidence interval 0.17-0.38). Apart from common (63%) cervical spinal stenosis, neuroimaging revealed unspecific, non-progressive abnormalities (i.e., mild brain atrophy, white matter lesions). In summary, MLII is associated with profound developmental impairment, but not with neurodegeneration and neurocognitive decline.

Structures of the mannose-6-phosphate pathway enzyme, GlcNAc-1-phosphotransferase

The mannose-6-phosphate (M6P) pathway is responsible for the transport of hydrolytic enzymes to lysosomes. N-acetylglucosamine-1-phosphotransferase (GNPT) catalyzes the first step of tagging these hydrolases with M6P, which when recognized by receptors in the Golgi diverts them to lysosomes. Genetic defects in the GNPT subunits, GNPTAB and GNPTG, cause the lysosomal storage diseases mucolipidosis types II and III. To better understand its function, we determined partial three-dimensional structures of the GNPT complex. The catalytic domain contains a deep cavity for binding of uridine diphosphate-N-acetylglucosamine, and the surrounding residues point to a one-step transfer mechanism. An isolated structure of the gamma subunit of GNPT reveals that it can bind to mannose-containing glycans in different configurations, suggesting that it may play a role in directing glycans into the active site. These findings may facilitate the development of therapies for lysosomal storage diseases.

Current treatment options and novel nanotechnology-driven enzyme replacement strategies for lysosomal storage disorders

Lysosomal storage disorders (LSDs) are a vast group of more than 50 clinically identified metabolic diseases. They are singly rare, but they affect collectively 1 on 5,000 live births. They result in most of the cases from an enzymatic defect within lysosomes, which causes the subsequent augmentation of unwanted substrates. This accumulation process leads to plenty of clinical signs, determined by the specific substrate and accumulation area. The majority of LSDs present a broad organ and tissue engagement. Brain, connective tissues, viscera and bones are usually afflicted. Among them, brain disease is markedly frequent (two-thirds of LSDs). The most clinically employed approach to treat LSDs is enzyme replacement therapy (ERT), which is practiced by administering systemically the missed or defective enzyme. It represents a healthful strategy for 11 LSDs at the moment, but it solves the pathology only in the case of Gaucher disease. This approach, in fact, is not efficacious in the case of LSDs that have an effect on the central nervous system (CNS) due to the existence of the blood-brain barrier (BBB). Additionally, ERT suffers from several other weak points, such as low penetration of the exogenously administered enzyme to poorly vascularized areas, the development of immunogenicity and infusion-associated reactions (IARs), and, last but not least, the very high cost and lifelong needed. To ameliorate these weaknesses lot of efforts have been recently spent around the development of innovative nanotechnology-driven ERT strategies. They may boost the power of ERT and minimize adverse reactions by loading enzymes into biodegradable nanomaterials. Enzyme encapsulation into biocompatible liposomes, micelles, and polymeric nanoparticles, for example, can protect enzymatic activity, eliminating immunologic reactions and premature enzyme degradation. It can also permit a controlled release of the payload, ameliorating pharmacokinetics and pharmacodynamics of the drug. Additionally, the potential to functionalize the surface of the nanocarrier with targeting agents (antibodies or peptides), could promote the passage through biological barriers. In this review we examined the clinically applied ERTs, highlighting limitations that do not allow to completely cure the specific LSD. Later, we critically consider the nanotechnology-based ERT strategies that have beenin-vitroand/orin-vivotested to improve ERT efficacy.

Mucolipidoses Overview: Past, Present, and Future

Mucolipidosis II and III (ML II/III) are caused by a deficiency of uridine-diphosphate N-acetylglucosamine: lysosomal-enzyme-N-acetylglucosamine-1-phosphotransferase (GlcNAc-1-phosphotransferase, EC2.7.8.17), which tags lysosomal enzymes with a mannose 6-phosphate (M6P) marker for transport to the lysosome. The process is performed by a sequential two-step process: first, GlcNAc-1-phosphotransferase catalyzes the transfer of GlcNAc-1-phosphate to the selected mannose residues on lysosomal enzymes in the cis-Golgi network. The second step removes GlcNAc from lysosomal enzymes by N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase (uncovering enzyme) and exposes the mannose 6-phosphate (M6P) residues in the trans-Golgi network, in which the enzymes are targeted to the lysosomes by M6Preceptors. A deficiency of GlcNAc-1-phosphotransferase causes the hypersecretion of lysosomal enzymes out of cells, resulting in a shortage of multiple lysosomal enzymes within lysosomes. Due to a lack of GlcNAc-1-phosphotransferase, the accumulation of cholesterol, phospholipids, glycosaminoglycans (GAGs), and other undegraded substrates occurs in the lysosomes. Clinically, ML II and ML III exhibit quite similar manifestations to mucopolysaccharidoses (MPSs), including specific skeletal deformities known as dysostosis multiplex and gingival hyperplasia. The life expectancy is less than 10 years in the severe type, and there is no definitive treatment for this disease. In this review, we have described the updated diagnosis and therapy on ML II/III.

Pre-clinical Mouse Models of Neurodegenerative Lysosomal Storage Diseases

There are over 50 lysosomal hydrolase deficiencies, many of which cause neurodegeneration, cognitive decline and death. In recent years, a number of broad innovative therapies have been proposed and investigated for lysosomal storage diseases (LSDs), such as enzyme replacement, substrate reduction, pharmacologic chaperones, stem cell transplantation, and various forms of gene therapy. Murine models that accurately reflect the phenotypes observed in human LSDs are critical for the development, assessment and implementation of novel translational therapies. The goal of this review is to summarize the neurodegenerative murine LSD models available that recapitulate human disease, and the pre-clinical studies previously conducted. We also describe some limitations and difficulties in working with mouse models of neurodegenerative LSDs.

Drug delivery systems and novel formulations to improve treatment of rare corneal disease

As the field of ocular drug delivery grows so does the potential for novel drug discovery or reformulation in lesser-known diseases of the eye. In particular, rare corneal diseases are an interesting area of research because drug delivery is limited to the outermost tissue of the eye. This review will highlight the opportunities and challenges of drug reformulation and alternative treatment approaches for rare corneal diseases. The barriers to effective drug delivery and proposed solutions in development will be discussed along with an overview of corneal rare disease resources, their current treatments and ophthalmic drug delivery systems that could benefit such cases. The regulatory considerations for effective translation of orphan-designated products will also be discussed.

GNPTAB c.2404C > T nonsense mutation in a patient with mucolipidosis III alpha/beta: a case report

Background

Mucolipidosis alpha/beta is an inborn error of metabolism characterized by deficiency of GlcNAc-1-phosphotransferase, in which essential alpha/beta subunits are encoded by the GNPTAB gene. The autosomal recessive condition is due to disruptions of hydrolase mannose 6-phosphate marker generation, defective lysosomal targeting and subsequent intracellular accumulation of non-degraded material. Clinical severity depends on residual GlcNAc-1-phosphotransferase activity, which distinguishes between the milder type III disease and the severe, neonatal onset type II disease.

Case presentation

We report the clinical, biochemical and genetic diagnosis of mucolipidosis III alpha/beta in a two-year-old Chinese boy who initially presented with poor weight gain, microcephaly and increased tone. He was confirmed to harbor the common splice site mutation c.2715 + 1G > A and the nonsense variant c.2404C > T (p.Q802*). Clinically, the patient had multiple phenotypic features typical of mucopolysaccharidosis including joint contractures, coarse facial features, kypho-lordosis, pectus carinatum and umbilical hernia. However, the relatively mild developmental delay compared to severe type I and type II mucopolysaccharidosis and the absence of macrocephaly raised the possibility of the less commonly diagnosed mucolipidosis alpha/beta. Critical roles of lysosomal enzyme activity assay, which showed elevated α-iduronidase, iduronate sulfatase, galactose-6-sulphate sulphatase, arylsulfatase B and α-hexosaminidase activities; and genetic study, which confirmed the parental origin of both mutations, were highlighted.

Conclusions

The recently reported nonsense variant c.2404C > T in the GNPTAB gene is further recognized and this contributes to the genotype-phenotype spectrum of mucolipidosis alpha/beta.

Electronic supplementary material

The online version of this article (10.1186/s12881-018-0679-5) contains supplementary material, which is available to authorized users.