Citation: Molecular Therapy — Methods & Clinical Development (2016) 3, 16037; doi:10.1038/mtm.2016.37 Ofcial journal of the American Society of Gene & Cell Therapy www.nature.com/mtm INTRODUCTION The ability of vectors derived from adeno-associated virus (AAV) to stably transduce a broad range of dividing and postmitotic cells with high efciency makes AAV-based vectors attractive for development as central nervous system (CNS) gene therapies. 1 An important ele- ment of a successful gene therapy treatment is the choice of an appro- priate AAV serotype. Although AAV2 is neurotropic, other serotypes (e.g., AAV1, AAV5, AAV7, AAV8, AAV9, and AAVrh10) have a high afn- ity for neurons, astrocytes, and other non-neuronal cells. 2–9 Tropism is important not only for targeting specifc cells in the brain but also for avoiding the transduction of cells that could trigger an immune response to a non-self protein. 10,11 The clinical failure of potentially efective molecular therapeutics is often due not to a lack of potency, but to shortcomings in the method by which they are delivered. Huntington’s disease (HD) is caused by a CAG repeat expansion mutation that encodes an elongated polyglutamine (polyQ) repeat in the mutant huntingtin protein (mHtt). HD is a particularly attrac- tive target for DNA- and RNA-based therapies because it is an auto- somal dominant disease. Recombinant AAV vectors provide an ideal delivery system for nucleic acid therapeutics as they can enable the permanent expression of huntingtin-lowering molecules in the brain. AAV-mediated gene therapies to deliver huntingtin-lowering strategies have shown great promise for HD, as indicated by the robust efcacy data presented in several preclinical studies. 12–16 To achieve maximal clinical efcacy in HD, delivery of a vector to both the striatum and cortex will likely be required. Postmortem analy- sis of HD patient brains revealed extensive medium spiny neuronal loss in the striatum in addition to the loss of pyramidal neurons in the cerebral cortex. 17,18 A recent report showed that genetically reducing mHTT expression in neuronal populations in both the striatum and the cortex of conditional transgenic mouse models of HD provides a signifcantly higher level of therapeutic efcacy than reducing mHTT in either of these regions of the brain alone. 19 This evidence suggests that delivering gene therapy agents to both striatal and cortical regions would be ideal to achieve maximal therapeutic efcacy. Demonstrating that the requirements for ther- apeutic efcacy can be translated from mice to larger animal spe- cies, whose anatomical characteristics are more refective of those in human patients, is imperative. The presence of the blood–brain barrier impedes vector transfer to the CNS after systemic delivery. Recent studies have demon- strated that the AAV9 serotype can cross the blood–brain barrier after systemic delivery. However, this led to predominantly astro- cytic transduction. 20,21 Furthermore, although systemic delivery of AAV9-RNAi to N171-82Q mice signifcantly reduced HTT in multiple brain regions, it was not associated with motor improvements. 22 For almost two decades, the optimal conditions for direct brain delivery of viral vectors via convection-enhanced delivery (CED) have been extensively evaluated. 23–26 A new, refux-free, stepped- design cannula that permits the use of CED with higher fow rates Received 2 December 2015; accepted 13 April 2016 Huntington’s disease (HD) is caused by a toxic gain-of-function associated with the expression of the mutant huntingtin (htt) pro- tein. Therefore, the use of RNA interference to inhibit Htt expression could represent a disease-modifying therapy. The potential of two recombinant adeno-associated viral vectors (AAV), AAV1 and AAV2, to transduce the cortico-striatal tissues that are predomi- nantly afected in HD was explored. Green fuorescent protein was used as a reporter in each vector to show that both serotypes were broadly distributed in medium spiny neurons in the striatum and cortico-striatal neurons after infusion into the putamen and caudate nucleus of nonhuman primates (NHP), with AAV1-directed expression being slightly more robust than AAV2-driven expres- sion. This study suggests that both serotypes are capable of targeting neurons that degenerate in HD, and it sets the stage for the advanced preclinical evaluation of an RNAi-based therapy for this disease. Molecular Therapy — Methods & Clinical Development (2016) 3, 16037; doi:10.1038/mtm.2016.37; published online 29 June 2016 The first two authors contributed equally to this work. 1 Interventional Neuro Center, Department of Neurological Surgery, University of California San Francisco, San Francisco, California, USA; 2 Rare Disease Unit, Neuroscience, Sanofi-Genzyme, Framingham, MA, USA. Correspondence: LS Shihabuddin (Lamya.Shihabuddin@genzyme.com) Or KS Bankiewicz (Krystof.Bankiewicz@ucsf.edu) Widespread AAV1- and AAV2-mediated transgene expression in the nonhuman primate brain: implications for Huntington’s disease Piotr Hadaczek 1 , Lisa Stanek 2 , Agnieszka Ciesielska 1 , Vivek Sudhakar 1 , Lluis Samaranch 1 , Philip Pivirotto 1 , John Bringas 1 , Catherine O’Riordan 2 , Bryan Mastis 2 , Waldy San Sebastian 1 , John Forsayeth 1 , Seng H Cheng 2 , Krystof S Bankiewicz 1 and Lamya S Shihabuddin 2 ARTICLE