Evaluating stem cell gene therapy for treating the brain in Mucopolysaccharidosis II

Abstract

Mucopolysaccharidosis type II (MPS II) is a rare X-linked lysosomal storage disorder caused by a deficiency in the iduronate-2-sulphatase (IDS) enzyme that normally degrades heparan sulphate and dermatan sulphate in the lysosomal compartment. Most MPS II patients display progressive cognitive and skeletal impairment that results in death in teenage years. Enzyme replacement therapy is available but remains inefficacious to treat the neurological symptoms in MPS II due to its size and the presence of the blood-brain barrier (BBB). The assessment of novel therapeutics to treat the brain in MPS II mice is primarily evaluated through biochemical and behavioural examinations. Previous literature highlighted behaviour differences using a variety of behavioural test paradigms, with important discrepancies in testing ages. We therefore performed a longitudinal study of behavioural differences in the MPS II mouse to identify the most appropriate age to measure therapeutic efficacy. In MPS II mice, a reduction in spontaneous alternation in the Y-maze and anxiolytic behaviour in the open-field were detected at 8 months of age, concomitantly to decreased coordination and balance, and skeletal thickening of the long bones and zygomatic arches. Although bone marrow transplantation is recommended to treat the cognitive symptoms in MPS I Hurler, it remains ineffectual in MPS II, likely due to the low levels of enzyme secreted in the brain by bone-marrow derived microglial cells. Hence, we hypothesised that increasing IDS enzyme expression in haematopoietic stem cells and immune cells would significantly improve MPS II phenotype. We compared standard bone marrow transplantation (WT-HSCT) to lentiviral-mediated overexpression of IDS (LV.IDS) or IDS.ApoEII (LV.IDS.ApoEII) enzyme in MPS II haematopoietic stem cells. The IDS.ApoEII enzyme was modified to include a short peptide residue designed to transcytose from the bloodstream into the central nervous system. Cognitive behaviour, neuro-inflammation, a complete reduction in heparan sulphate and correction of neuropathology were obtained with LV.IDS.ApoEII, with only partial correction using LV.IDS and no improvements using WT-HSCT. Interestingly, similar levels of brain IDS enzyme were obtained with LV.IDS and LV.IDS.ApoEII. We hypothesised that full correction in the brain using LV.IDS.ApoEII was mediated by improved enzyme stability and uptake into cells, and increased cellular targeting, leading to full correction. We subsequently designed five novel lentiviral vectors containing short peptide residues previously described as able to cross the BBB. Hydrodynamic injections of plasmid DNA in WT and MPS II animals, and the use of a basic in vitro BBB did not highlight any superior enzyme fusions. Subsequent experiments will focus on a primary porcine BBB model and long-term stem cell gene therapy experiments to determine increased transcytosis potential in vivo. Overall, this thesis provides a strong proof-of-principle study for the use of lentiviral-mediated stem cell gene therapy using LV.IDS.ApoEII to treat the neurological phenotype in MPS II patients.

Date of Award	1 Aug 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Rebecca Holley (Supervisor) & Brian Bigger (Supervisor)