The central nervous system (CNS) is an important target for gene therapy for a range of diseases including Freidrich’s Ataxia, spinal muscular atrophy, lysosomal storage diseases, Parkinson’s Disease and Alzheimer’s Disease. However, a key challenge has been identifying vectors able to traverse the blood brain barrier (BBB), a tight endothelial barrier which blocks transport of macromolecules and particles into the brain (Ballabh et al., 2004).
Adeno-associated virus (AAV) vectors, a family of viruses in distinct clades organized through sequence identity and structural characteristics and including many individual serotypes and variants, have emerged as one of the most powerful and flexible vector systems for gene delivery in a wide range of diseases. AAV vectors are also demonstrating immense clinical potential for gene therapy in diseases such as haemophilia A and B.
For CNS gene delivery AAVrh.10 has been reported to cross the BBB and transduce the CNS (Yang et al., 2014), however AAVrh.10 also retains the ability to efficiently transduce other tissues following intravascular (iv) delivery, including the heart and liver. This has implications for the scale up and translation of this vector for efficient and safe CNS gene delivery without toxicity.
In contrast to AAVrh.10, AAV serotype 1 (AAV1) is unable to cross the BBB, but instead transduces brain endothelial cells when administered iv, despite a high conservation of capsid similarity between AAVrh.10 and AAV1.
In the study reported by Albright et al., (2018), the authors generated a library of chimeric AAV vectors consisting of swapping different capsid domains from AAV1 into the AAVrh.10 capsid and assessed gene delivery from individual capsid variants following iv delivery. Using this approach the authors identified two variants, termed AAV1R6 and AAV1R7, which retained the ability to cross the BBB and transduce the CNS, while showing significantly reduced hepatic and vascular endothelial transduction.
Neuronal transduction was preferential and was achieved in all regions of the brain with varying efficiencies, including the cortex, hippocampus, thalamus and hypothalamus, striatum and amygdala. In the liver transduction was negligible from both novel AAV variants while AAV1 produced detectable transduction of hepatocytes and AAVrh.10 mediated high hepatic transduction. Interestingly, both AAV variants were able to transduce the heart with equivalent efficiency to the parental serotypes.
Structural analysis of the capsids revealed that both novel variants were 97-98% related to AAV1 with only 18 (AAV1R6) or 22 (AAV1R7) amino acid residues from AAVrh.10. Further analysis of AAV1R6 revealed that 8 of the amino acids gained from AAVrh.10 were in the tropism determining region of the AAV1 capsid, while other residues were not exposed on the surface of the capsid.
Using this structural information the authors used rational design to engineer a new variant (AAV1RX) which only included the 8 surface exposed residues from AAVrh.10. Following iv delivery AAV1RX retained the ability to cross the BBB and transduce neurons, retained cardiac transduction, but with negligible hepatic transduction.
For future development of these novel AAV capsid variants it will be important to assess a further range of higher doses of the vectors to obtain a fuller profile of their relative tissue tropism and to assess whether reduced hepatic transduction also reduces potential hepatic toxicity, e.g. transient raised liver enzyme levels.
Understanding the exact mechanism by which the vectors traverse the BBB would also be helpful for future refinement of the vectors. However, overall these new AAV vectors have exciting potential for a wide range of disorders of the CNS and add to the flexibility of AAV and the expanding range of different AAV capsids available for optimal and selective gene delivery to target tissues.
Albright BH, Storey CM, Murlidharan G, Castellanos Rivera RM, Berry GE, Madigan VJ, Asokan A. (2018). Mapping the Structural Determinants Required for AAVrh.10 Transport across the Blood-Brain Barrier. Mol Ther. 26:510-523.
Ballabh P, Braun A, Nedergaard M (2004). The blood-brain barrier: an overview: structure, regulation, and clinical implications. Neurobiol. Dis. 16:1–13.
Yang B, Li S, Wang H, Guo Y, Gessler DJ, Cao C, Su Q, Kramer J, Zhong, L, Ahmed SS, Zhang H, He R, Descrosiers RC, Brown R, Xu Z, Gao G. (2014). Global CNS transduction of adult mice by intravenouslydelivered rAAVrh.8 and rAAVrh.10 and nonhuman primates by rAAVrh.10. Mol Ther. 22:1299–1309.