479. ZFN-Mediated In Vivo Genome Editing Results in Supraphysiological Levels of Lysosomal Enzymes Deficient in Hunter and Hurler Syndrome and Gaucher Disease
Authors: Thomas Wechsler, Russel DeKelver, Michelle Rohde, Susan Tom, Scott Sproul, Leo Ou, Kanut Laoharawee, Kelly M. Podetz-Pedersen, Chester B. Whitley, Scott R. McIvor, Philip D. Gregory, Michael C. Holmes
Journal: Molecular Therapy (2015)

Lysosomal storage diseases (LSDs) represent a group of inherited metabolic disorders associated with mutations in genes encoding lysosomal enzymes leading to systemic accumulation of toxic storage materials. Manifestations include organomegaly, skeletal dysplasias, cardiopulmonary obstruction, and severe neurologic impairment, often leading to death by age 10. Current LSD therapies include enzyme replacement therapy, substrate reduction therapy and hematopoietic stem cell transplant (HSCT). However, all of these therapies are expensive, incompletely effective, and HSCT is associated with significant risk of morbidity and mortality. Due to the unmet need genome editing strategies are proposed to permanently modify patient cells by genetically complementing the LSD defect. This is achieved by utilizing engineered zinc finger nucleases (ZFNs) to introduce a DNA cut at the target locus, mediating integration of a therapeutic LSD donor cDNA. To ensure long-term expression of the transgenes in vivo we target the albumin locus as a "safe harbor" in hepatocytes via co-delivery of the albumin ZFNs and LSD donors by adeno-associated virus (AAV). This system exploits the high transcriptional activity of the native albumin enhancer/promoter; uses stably modified hepatocytes to potentially allow long-term expression of the inserted transgene; and utilizes an endogenous promoter obviating this requirement in the AAV payload. We have previously exploited AAV-mediated in vivo targeting of the murine albumin "safe harbor" locus for the synthesis of therapeutic levels of FVIII and FIX to overcome the clotting defect in hemophilic mice. Using the same approach, we co-delivered mouse albumin ZFNs with donor constructs encoding either human iduronate-2-sulfatase (IDS) deficient in Hunter syndrome, α-L-iduronidase (IDUA) for Hurler syndrome, or Glucocerebrosidase (GBA) for Gaucher disease via AAV in WT mice. We show stable integration of the LSD donors at the albumin locus, resulting in liver-specific expression and secretion of these proteins into plasma. This led to a 4-fold (GBA), 10-fold (IDUA) or 100-fold (IDS) increase in enzymatic activity in the plasma, demonstrating that the secreted proteins are functional. Importantly, increased activity was also detected in secondary tissues (spleen) showing efficient uptake and activity in distal tissues. Moreover, preliminary data in MPSI and MPSII mice suggests these levels of IDS and IDUA expression may lead to correction of the enzyme deficiency. IDS, IDUA and GBA expression remained stable throughout the study (up to 2 months), while expression of FIX has been stable for >1 yr suggesting this process results in long-term protein expression. In summary, our data provide proof of concept for ZFN-mediated targeting of the albumin locus in hepatocytes as an in vivo protein replacement platform to express different proteins associated with lysosomal storage diseases.