Currently there is no specific treatment for Sanfilippo Type C (MPS IIIC), and because the Central Nervous System is the major affected organ and the deficient HGSNAT gene is a multi-spanning transmembrane protein, it is unlikely that enzyme replacement or bone marrow transplant (BMT) can be effective for treating this disorder. Thus, chaperone therapy (where active, site-specific inhibitors restore mutant enzyme conformation and activity) and substrate reduction therapy are the only potentially suitable approaches.
MPS IIIC is a good candidate for chaperone therapy because it is estimated that a threshold activity of approximately 10% is sufficient to prevent storage. As such, even a minor increase in enzyme activity induced by chaperone therapy is likely to positively impact the disease’s pathology and be beneficial for patients. A recent study (Feldhammer, M., S. Durand, et al. (2009). “Protein misfolding as an underlying molecular defect in mucopolysaccharidosis III type C.” PLoS One 4(10): e7434.) demonstrated that for 5 missense mutations (N273K, R344C, R344H, S518F and S541L), the mutant enzyme could be rescued by treating patients’ cells with the competitive inhibitor of HGSNAT, glucosamine.
These “responding” mutations are among the most frequent. The majority of known MPS IIIC patients are affected with at least one of them and could benefit from a small molecule chaperone to induce proper folding and stabilization of the mutant protein.
Efforts to identify chaperones include high throughput screening and computational screening.
Curative approaches for LSDs rely mainly on direct delivery of a gene therapy vector to the brain. This has shown good efficacy in mice (Cressant, A., N. et al., Improved behavior and neuropathology in the mouse model of Sanfilippo type IIIB disease after adeno-associated virus-mediated gene transfer in the striatum J Neurosci, 2004. 24(45): p. 10229-39.) and dogs with MPSIIIB (Ciron, C., N. et al., Gene therapy of the brain in the dog model of Hurler’s syndrome Ann Neurol, 2006. 60(2): p. 204-13.).
Direct brain delivery of a gene therapy vector can result in immune responses and scale-up issues, which is why it is important to assess these factors when designing any protocol for this form of gene therapy. Approaches to improving the distribution of brain delivery include the investigation of novel injection sites and the delivery of vector into the ventricular spaces within the brain to take advantage of diffusion by the cerebrospinal fluid.
Substrate Reduction Therapies (SRT)
These aim to reduce substrate by blocking its production. However, there are problems with long-term toxicity of drugs and the ability of the drug to cross the blood brain barrier is crucial to its action in LSDs. Recent work has shown that the broad spectrum protein tyrosine kinase inhibitor genistein aglycone hinders synthesis of glycosaminoglycans (GAGs) in cultures of fibroblasts from MPS patients (MPSI, II, IIIA, IIIB and IIIC) (Piotrowska, E., J. et al.,, Genistein-mediated inhibition of glycosaminoglycan synthesis as a basis for gene expression-targeted isoflavone therapy for mucopolysaccharidoses Eur J Hum Genet, 2006. 14(7): p. 846-52) and normalizes cells.
HANDS Pre-Clinical Research
Treating Neuropathology in Lysosomal Storage Disease
Primary Investigator: Alessandro Fraldi Telethon Institute of Genetics and Medicine (TIGEM)
A research line in Fraldi’s lab is aimed to design and test new low-invasive therapeutic approaches to treat neuropathology in lysosomal storage diseases (LSDs).
Several LSDs are caused by deficiency of soluble proteins (hydrolases), which may be secreted with different efficiency rates. One of the approaches we are developing in our lab is based on the modification of these proteins in order to both enhance their secretion from the liver and allow their brain targeting upon intravascular administration of either the recombinant modified protein or a viral vector containing the gene encoding the engineered protein. We are using different mucopolysaccharidoses (MPSs) mouse models to test the therapeutic feasibility and efficacy of this therapeutic strategy.
A subset of LSDs is caused by deficits of specific lysosomal membrane proteins. The non-soluble nature of these proteins makes the therapy of these disorders challenging or even unfeasible. In our lab we are also working on the development of novel nanoparticles vectors capable of delivering a membrane protein to lysosomes of the diseased brain.
Using Animal Models to understand and Cure Sanfilippo Disease
Primary Investigator: Alexey Pshezhetsky University of Montreal
Sanfilippo disease belongs to a group of devastating inherited diseases of children affecting the lysosome, a cellular compartment responsible for degradation and recycling of big biological molecules. It causes rapid brain damage in infants and children, accompanied by variable problems of the eyes, skin and bones. Most patients become demented and die before adulthood.
Previously, we identified defects in the patient’s genes that cause Sanfilippo disease. Now we want to produce and study colonies of mice that will have the same defects in their genes. This will allow us to understand how exactly the disease affects human patients. We also want to use these mice to verify different drugs that can potentially cure or partially cure the disease in some patients. Such drugs (called chaperones) bind to the defective protein that causes Sanfilippo disease and change the way that it is folded and shaped in the cells. In the result the mutant protein becomes normal and this cures or ameliorates the disease in the patients.
Defects in Communication between Brain Cells that Cause Dementia in Sanfilippo Disease
Primary Investigator: Alexey Pshezhetsky, University of Montreal
The major aim of this project is to understand the reason for mental decline and dementia in children affected with Sanfilippo disease type C and to develop new strategies for its therapy. Through our previous studies we developed a mouse model for the disease and studied pathological changes in these mice. Our studies have led us to propose that the mechanism underlying the hyperactivity, memory loss and other changes in the behavior of patients is related to defects in communications between the brain cells called neurons. To test this hypothesis we will study these cells in the brains of the mouse model of Sanfilippo disease.
To achieve this goal we will first analyze brain structure by a special type of microscopy. Second, we will generate Sanfilippo mouse strain that have green fluorescent protein produced by the brain cells, which will allow us to study the shape of these cells and see if they are able to communicate with each other. We recently obtained results that strongly support our hypothesis that Sanfilippo diseased brain cells cannot produce small vesicles that communicate the signal from one cell to another. We therefore hope that drugs normalizing production of such vesicles could be a suitable approach to treat Sanfilippo patients.
Assessment of the Efficacy of Long-Term Intracerebral AAV-hHGSNAT Delivery in MPSIIIC Mice
Primary Investigator: Brian Bigger, Manchester University
MPSIIIC is one of four clinically indistinguishable diseases known as Sanfilippo. Each is caused by deficiencies in enzymes breaking down long chain sugars. Children with the disease suffer progressive behavioral and neurodegenerative impairment and there are currently no treatments. MPSIIIC is caused by defects in the HGSNAT gene coding for a lysosomal enzyme responsible for breaking down the long chain sugar heparan sulphate. This results in cellular storage of Heparan Sulphate, neuroinflammation and ultimately neurodegeneration. In particular, the enzyme missing in MPSIIIC is not secreted as it is present in the lysosomal membrane. This means that enzyme replacement strategies that are effective in other lysosomal diseases do not work in MPSIIIC, as affected cells are unable to take up replacement enzyme.
We are developing a gene therapy strategy to deliver the missing HGSNAT enzyme directly into affected cells in the brain where it is needed. This uses the well characterized Adeno Associated Virus (AAV) vector via direct intracerebral (brain) injection into the mouse model of MPSIIIC. This approach is currently in a phase I/II clinical trial for the related Sanfilippo disease MPSIIIA in Paris. We have developed an optimized vector configuration for this approach to maximize delivery and gene expression in affected cells. Our aim is to prove preclinical efficacy in this mouse model in preparation for a phase I/II clinical trial.