Targeting Ageing With Gene Therapy – Part 1: The Toolbox

Getting your Trinity Audio player ready...

We already know of many genes in humans that are strongly associated with the risk of age related disease, such as the ApoE gene that influences Alzheimer’s risk, or the BRCA1 and 2 genes that affect breast and ovarian cancer risk. Animal studies have also demonstrated that editing certain genes that are also present in humans can significantly extend lifespan.

We have the technology to precisely target specific genes, switching them on or off or even introducing copies of new genes or editing existing ones. Gene therapy is already providing revolutionary solutions to all kinds of diseases, from correcting disease-causing genetic mutations to the development of vaccines with unprecedented speed. How long will it be before we can use this technology to prevent age-related diseases and extend human lifespan?

Ageing is complex, made up of multiple interlinked processes, and our chances of stumbling across a handful of miracle drugs that will hit them all are very slim. A targeted approach using gene therapy to address each component of biological ageing seems more plausible, such as by editing DNA to repair critically damaged genes, restoring shortened telomeres and reprogramming cells to a younger state by partially reversing epigenetic alterations.

So, what are we waiting for? This recent review provides a summary of where gene therapies targeting ageing currently stand. What are the main approaches being looked at, and why are they hard to implement in practice? In part 1 of this breakdown, we’re first going to build a baseline of what is already possible with current gene therapy technology.

What’s In The Toolbox?

The review outlines four main strategies that fall under the definition of gene therapy. Not all gene therapy involves making permanent and potentially risky edits to the genetic code. Different gene therapy approaches each have their advantages as well as their limitations in terms of what can be achieved, but together they give us a great deal of potential control over biological pathways involved in ageing and age-related diseases.

Gene editing

The most radical form of gene therapy, gene editing, allows for precise edits to be made within the target genome. Using technologies like CRISPR-Cas9 or its more recent modified versions, a specific section of a gene can be deleted, replaced, or new genetic material can be inserted.

Advantages: This is an incredibly powerful technique that can be used to correct genetic mutations. For example, CRISPR-Cas9 gene editing is being used to correct the genetic mutation in the haemoglobin subunit gene that is responsible for sickle cell disease. In a similar way, it might be possible to alter gene variants known to be associated with age related diseases, such as changing the ApoE gene (variants of which are strong determinants of Alzheimer’s risk) to the low-risk ApoE2 variant. Later down the line, it may even be possible to invent entirely new variants of genes implicated in the ageing process.

Drawbacks: Gene editing, though powerful, also poses significant risks that need to be taken into account. Since gene editing introduces breaks into the DNA molecule when making edits, there is a potential for unintended mutations to occur. Gene editing may also erroneously edit parts of the genome that are similar to the target sequence. This makes safety with gene editing a more significant concern than with other strategies.

Transcriptional regulation

This includes a variety of approaches used to increase or decrease the expression of a gene – that is to say, the extent to which an existing gene is read and used to make a corresponding protein.

Advantages: Transcriptional regulation has the potential to alter the expression of ageing-related genes without causing any permanent changes to the DNA. There is evidence that some genes that are active in youth get suppressed in old age and vice versa, potentially contributing to age-related degeneration. There are also some known lifespan-associated genes that humans posses, but that are inactivated in the majority of our cells. These could be targeted by transcriptional regulation as a means to delay ageing.

Disadvantages: Though not permanent, changes to gene expression aren’t without risk. Gene expression levels might be set the way they are for a reason, and altering this might have unintended consequences. Transcriptional regulation can only work with genes we already possess – it cannot correct faulty genes or nor can it introduce new ones, so there is a limit to what we can achieve with this technique.

Introduction of new genes

A fairly self-explanatory form of gene therapy. A new gene might be permanently integrated into the recipient’s genome to replace a less desirable or defective variant. Sometimes replacing existing genes isn’t necessary, and it is sufficient simply to introduce copies of a beneficial gene. Such a gene does not necessarily need to be permanently integrated into the genome.

Advantages: Again, this form of gene therapy does not need to be permanent, which enhances the safety of this approach. It has also been validated in a very large human population, since it forms the basis for some of the COVID-19 vaccines, in which DNA encoding surface proteins of the virus was introduced without integration in to recipients’ genomes. Unlike transcriptional regulation, this approach allows entirely new genes to be introduced.

Disadvantages: Methods for delivering new genes carry the risk of immune reactions. Lack of permanence can also be more of a drawback than an advantage if the new gene doesn’t stay around for very long.

Gene silencing:

This is when an existing gene is ‘switched off’, often using small interfering RNA (siRNA), a molecule that binds to a corresponding sequence in the DNA molecule and prevents it from being read.

Advantages: Gene silencing is in some ways the opposite of gene introduction – it’s a non-permanent way to prevent a gene from being expressed. This allows us to prevent disease-causing genes from being used to make proteins without permanently removing that gene from the genome.

Disadvantages: Gene silencing also faces a challenge in that the gene suppression might not last very long.

When combined together, there is little that could not be achieved with these techniques, at least in theory. In practice, the application of these technologies to humans is limited because editing the genome is the second half of the battle – actually delivering these technologies to the right cells is the first half, and also the most problematic step.

The Delivery Problem

Unlike conventional drugs, which generally work by binding to receptors on the surface of cells, gene therapy needs to get into the nucleus where the DNA is located. This means loading it into some kind of delivery system that can take it there. Since viruses have already evolved to deliver their own genetic material into our cell nuclei as part of their life cycle, loading gene therapies into a non-pathogenic virus (most commonly an adeno-associated virus (AAV) or lentivirus (LV)) is the most widespread approach. A less common but emerging technique is to load gene therapy into microscopic membrane packages called exosomes or the larger synthetic lipid nanoparticles (LNPs). It is also possible to deliver gene therapies physically, by using electric pulses to punch holes in the membranes of cells (electroporation) or via microscopic needles. These approaches are very limited with regard to the tissues to which the therapy can be delivered.

Unfortunately, each of these delivery strategies have significant weaknesses. Some of them have small cargo capacities, which limits the size of the genetic material that can be delivered. Others are limited by tropism (they are preferentially absorbed by certain organs) which limits the amount of gene therapy that reaches the desired targets. Some also elicit an immune response, which is problematic not only for safety reasons, but because it may lead to the gene therapy being cleared by the immune system before it has a chance to reach its target.

Now that we have a baseline understanding of the available tools, it’s time for the fun part. In part 2, we’ll see how these gene therapy technologies could actually be used (and are being used in animals) to target the ageing process.