Huang Q, Chan KY, Lou S, Keyes C, Wu J, Botticello-Romero NR, Zheng Q, Johnston J, Mills A, Brauer PP, Clouse G, Pacouret S, Harvey JW, Beddow T, Hurley JK, Tobey IG, Powell M, Chen AT, Barry AJ, Eid FE, Chan YA, Deverman BE. An AAV capsid reprogrammed to bind human Transferrin Receptor mediates brain-wide gene delivery. bioRxiv. 2023 Dec 22; PubMed.
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Gene transfer vectors derived from Adeno-Associated Virus (AAV) are the tools of choice for gene therapy (and gene editing) in the central nervous system. This paper is exciting because it shows, through a series of impeccably executed studies, that some of the current limitations of the technology can be solved. It considerably broadens the scope of potential applications, including in Alzheimer’s disease.
A critically needed improvement for CNS gene therapy is the characterization of vectors able to efficiently cross the blood-brain barrier after administration in the bloodstream. Several candidates now exist, but the Deverman group has introduced a new—and successful—twist in this quest.
AAV vectors enter their target cells through interactions between their capsid protein and cell surface proteins and sugars. The effective delivery of the therapeutic payload then involves multiple cellular proteins. Naturally occurring AAVs have evolved their capsids to adapt to the molecular partners found in their host cell of choice and thus, different AAV subtypes may display a particular cellular host range. Understanding this has led to the idea that the AAV capsid can be further evolved through genetic engineering towards a highly precise, and possibly programmable, host-cell specificity.
To do this, highly complex libraries of AAV variants are generated by randomizing surface loops on the AAV capsids. Rounds of selection in vivo then take place, eventually leading to the identification of candidates with an optimized fit for a tissue, cell type, or route of administration. However, it has become quickly evident that many candidates obtained in one species do not perform well in another, which creates a problem of predictability that seriously impairs drug development.
To mitigate this, the Deverman group first screened the AAV library directly against a cellular partner of interest through in vitro panning. The partner they have chosen is, not too surprisingly, the apical domain of the human Transferrin receptor, which is already used to ferry antibodies and enzymes across the vascular endothelial cells forming the blood-brain barrier. The approach yields very potent candidates with high affinity for the BBB cells and a 10- to 50-fold increase in their ability to deliver therapeutic genes into the brain. These are significant improvements over AAV9, the parent capsid from which the library was constructed. AAV9 is used in Zolgensma, an approved drug for gene therapy of spinal muscular atrophy.
This work is an important step forward but certainly does not stop here. We now need to take care of the other hurdles that limit access of AAV vectors to the CNS, finding capsids that do not accumulate in the liver and are less effectively eliminated by the immune system are required. In this respect the direct panning of libraries against relevant proteins deployed in the paper may be relevant.
Others are exploring a different avenue to target AAV capsids to the CNS by directly coupling an otherwise inactive capsid to antibodies that will drive it to the target. This approach is more modular and could have advantages for drug development, including the possibility of studying drug candidates in multiple species.
View all comments by Olivier DanosDenali Therapeutics
Broadening the utilization of AAVs to treat CNS diseases is an exciting area of therapeutic development, but one that faces many challenges for clinical translation. This paper makes significant progress toward one such challenge in this field, which is translatability of successful brain-penetrating capsids from preclinical species to humans. It is a clever approach to combine decades of research and mechanistic understanding of TfR-enabled brain delivery with application to AAV gene delivery.
The enhanced brain tropism compared to other peripheral tissues is another major advance compared to other common AAVs. It is promising to see successful delivery of GBA1 to the brain using BI-hTfR1 as a proof of concept, and at significantly lower doses of virus than typically used for delivery to the CNS. Although many of the major cell types of the brain were successfully transduced, it will be of great value to continue to evolve the capsid for microglia targeting as well, given the role these cells play in a number of genetically linked neurodegenerative diseases. Longer-term studies, and in higher species, will be important to better understand whether TfR-targeting AAVs introduce additional safety and/or immunogenicity challenges.
Nonetheless, this elegant paper represents a significant advance and opens the door for an exciting new approach in the arena of CNS gene delivery.
View all comments by Joy ZucheroUppsala Universitet
Uppsala University
The study by Huang and colleagues describes the evaluation of an AAV vector engineered to actively enter the brain via transferrin receptor (TfR)-mediated transcytosis. This route to enter the brain is well-established for biologics, such as antibodies, with Roche’s brain shuttle (Trontinemab) as an example of an AD-related molecule that is presently evaluated in clinical trials, with promising results. Previously developed brain-targeted AAV vectors have been selected based on efficient brain uptake with an unknown mechanism of action. Here, the authors took the approach to first identify a mechanism (TfR-mediated transcytosis) and then select AAVs from a library with variations in a seven-amino-acid stretch of the capsid surface protein.
The authors convincingly demonstrate that the TfR-AAV enters the brain of a mouse with the apical domain of the human TfR knocked into its genome. The AAV entered the brain parenchyma and, to a large degree, also various brain cells. Importantly, they also show that the AAV can deliver cargo, which in turn can elicit a pharmacological response through increased activity of glucocerebrosidase in both the brain and CSF. However, as the authors point out, the present study did not demonstrate the use of this vector to “correct” a genetic component in a model of disease. It will be interesting to follow future development in this regard and compare it to presently used vectors for gene therapy.
In the field of antibody- or enzyme-based biologics that use TfR-mediated brain entry, the mode of TfR binding has been thoroughly studied and debated. Variables such as affinity, mono- or multivalent binding, or protein architecture have been shown to affect both brain transport and the safety profile of the molecule. An AAV capsid that supposedly carries multiple TfR-binding proteins on its relatively large surface can probably be expected to bind TfR with several binding sites simultaneously, which raises the question of whether this high TfR avidity could induce cell-surface TfR clustering, leading to internalization and degradation rather than transcytosis and recycling of the receptor. Indeed, although similar to the AAV9 control, almost 50 percent of endothelial cells were transduced by the TfR-AAV in the human TfR knock-in mouse, with only 10 to 15 percent transduced in the WT mouse used as a negative control. Another aspect of TfR binding is that it can affect blood pharmacokinetics through interaction with TfR on blood cells and peripheral organs. The present study does not report blood data, making it difficult to judge whether there is interaction with TfR on blood cells. It should be noted that interactions with TfR on reticulocytes in the bloodstream can, under certain circumstances, cause hemolysis.
Finally, off-target effects are often discussed in relation to gene therapy. The authors claim that one advantage of TfR-targeted AAV delivery is the brain endothelial-specific expression of TfR. However, TfR is, in fact, expressed in many other organs, and, although the TfR-AAV was indeed shown to be present at much higher concentrations in the brain compared to the control AAV9, it did not escape uptake in other organs. For example, the biodistribution experiment showed massive amounts of viral genome in the liver. Thus, although impressive in its capacity to deliver cargo to the brain, a certain amount of caution is probably warranted in future studies of this novel virus vector system.
View all comments by Stina SyvänenMake a Comment
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