Improving stem cell gene therapy for severe Mucopolysaccharidosis II Research update – October 2016
Dr. Brian Bigger and Ms. Hélène Gleitz

Mucopolysaccharidosis type II is a paediatric neurological lysosomal storage disease caused by genetic deficiencies in the X-linked iduronate-2-sulphatase (IDS) gene whose normal role is to catalyse the degradation of both heparan sulphate and dermatan sulphate. Although enzyme replacement therapy is used to treat attenuated patients, the inability of the enzyme to cross the blood-brain barrier limits its efficacy in the severe form of the disease (Jones et al., 2009). Our goal is to increase the amount of enzyme produced by stem cells after transplant using gene therapy mediated by lentiviral vectors in a mouse model of MPSII. The aim of this study was to develop novel lentiviral vectors that produce IDS enzyme to specifically target the brain and cross the blood-brain barrier (BBB) more effectively.

We developed and cloned 6 novel enzyme-peptide fusion IDS genes into our most effective and clinically relevant lentiviral vector containing the myeloid-specific promoter CD11b (Figure 1). This plasmid aims to overexpress the IDS-peptide fusion enzyme required for the degradation of heparan sulphate and dermatan sulphate.

Figure 1. The pCCL lentiviral vector containing the human codon-optimised IDS gene, a codon-optimised linker sequence and a fused peptide sequence.

In vitro studies were performed to determine whether adding a linker and peptide fusion would impact enzyme activity and secretion when compared to the regular IDS vector that contains no fusion peptide. Activity assays shows that IDS enzyme is highly secreted out of cells into surrounding media with all our novel fusion peptides, and that there is no decrease in enzyme production or secretion in microglia cells after transfection (Figure 2). Most importantly, this supports the concept of cross- correction, where enzymes can be secreted into surrounding media and taken up by other cells. Cross-correction is crucial for haematopoietic stem cell gene therapy success because only a subgroup of cells in the brain would be transduced and would act as enzyme reservoirs for other brain cells.....