Competitors to Daybue, page-22

Tbh, I retain a healthy scepticism particularly in regard to AAV-delivered transgene gene therapy.

In addition to the immunogenicity issues, there are significant question marks around the durability of expression of the introduced transgenes. AAV-derived DNA enters the cell nucleus and exists there extra-chromosomally. This results in an initial burst of transgene expression but, over time, it is known this can be progressively silenced by epigenetic and other mechanisms that we have evolved over millions of years to protect us against naturally-occurring viruses.

It is also known that AAV can insert itself into our chromosomes at a low frequency, potentially resulting in more stable expression. Sounds good, but this comes with a number of caveats, one of them particularly serious:

1. Integration only happens in some cells and expression from integrated copies tends to be low. Is this enough for a therapeutic effect (eg normal MECP2 - the subject of Neurogene's gene therapy - is known to be highly expressed in neurons).

2. Integrated DNA can also be silenced by mechanisms such as epigenetic silencing or chromatin remodelling.

3. Here's the serious lurker. AAV is known to have a preference for integration into transcriptionally active areas - that means genes. This raises the spectre of genotoxicity caused by disruption of important genes. The most serious would be disruption of a gene that protects us from cancer and, indeed, hepatocellular carcinoma has been observed in animal models of liver gene therapy.

I will temper the third point by saying significant genotoxicity has not been observed in human trials to date, but sufficient time has not passed in the gene therapy field to provide definitive answers to the caveats above. In particular, mapping of AAV chromosomal insertion sites in gene therapy patients is currently an area of intense interest and surveillance.

Here's a couple of interesting recent papers if anyone's sufficiently interested!

https://www.nature.com/articles/s41587-023-01974-7
https://www.liebertpub.com/doi/10.1089/hum.2023.134