Efficacy Evaluation of Liver-Directed Gene Therapy in Fabry Mice

Introduction: Fabry disease (FD) is an X-linked lysosomal storage disorder resulting from mutations in the gene encoding α-galactosidase A (GLA). It is characterised by the abnormal accumulation of neutral glycosphingolipids (GSL), predominantly Globotriaosylceramide (Gb3), in the lysosomes of multiple cell types including vascular endothelial cells. It is associated with early-onset stroke, cardiomyopathy, and progression to end-stage renal failure. Current standard of care for FD is enzyme replacement therapy but alternatives including allogeneic stem cell transplantation have also been explored. Fabry disease is a good model for liver-targeted gene therapy, as relatively low levels of enzyme correction may suffice to reduce storage. Additionally, a broad therapeutic window exists, and mouse models of FD enabling preclinical proof-of-concept studies are readily available. Here we report the preclinical results of adeno-associated virus (AAV) mediated, liver-targeted gene transfer of a self-complementary (sc) vector encoding a codon-optimised GLA transgene under transcriptional control of a hepatocyte-specific promoter pseudotyped with AAV8 capsid (scAAV2/8-LP1-GLAco).

Methods: In this study a single dose of scAAV2/8-LP1-GLAco vectors were administered into the tail vein of homozygote GLA knockout Fabry mice at either 1 month or 3 months of age. Two doses were tested for each age group; 2x1012 vg/kg and 2x1013 vg/kg, and an additional age-matched group left as untreated to serve as an experimental control for the effects of treatment. Mice were followed in the treatment for up to 11 months before sacrificing at 11 or 14 months of age.

To assess the kinetics and durability of transgene expression, plasma GLA levels were measured at various time intervals post injection.

Results: Long-term follow-up of Fabry mice injected with scAAV2/8-LP1-GLAco vector demonstrated sustained, durable levels of plasma GLA throughout the typical lifespan of a mouse. Plasma GLA activity levels at 10 or 11-months post vector administration to 1 and 3-month-old mice respectively, demonstrated a dose response from 2 x 1012 to 2 x 1013 vg/kg of >1 log increase in expression. These GLA activity levels represent supra-physiological expression (x 4000 of normal levels) which resulted in uptake of GLA from the bloodstream into kidney and cardiac tissues leading to dose-dependent clearance of lipid deposits in these tissues as analysed by electron microscopy.

Most importantly, sustained expression of GLA resulted in complete normalisation of Gb3 and lyso-Gb3 levels in plasma and liver tissues as quantified by mass spectrometry. Interestingly, these preclinical studies in Fabry mice at the age of 1 and 3 months demonstrated not only prevention but also reversal of lysosomal storage deposits through liver-specific expression of GLA.

Finally, in these in vivo mouse studies no adverse clinical signs were observed, despite the increased plasma exposure to GLA.

Conclusions: Collectively, these data provide strong evidence that our liver-directed AAV-mediated gene therapy approach holds considerable therapeutic potential for the treatment of Fabry disease. We anticipate that a single dose IV procedure will pose minimal burden to Fabry patients and will be a viable alternative to biweekly enzyme infusions, potentially reducing treatment-related morbidity whislt improving patient quality of life and potentially providing them with a functional long-term cure.