- Raghav Sehgal, Scientific Venture consultant LTF & Jyothi Devakumar, CSO LTF
Silver Tsunami is here and real - people over 60 years of age are becoming an increasingly large percentage of the total population which is creating an imbalance in healthcare spending. Neurodegenerative disorders, cancer, metabolic and inflammatory diseases are some of the most prevalent age-related pathologies affecting this growing population. The most imperative and critical requirement to understand the pathological mechanisms of the diseases is understanding the functions of a gene or multiple genes in primary human cells and targeting them.
In this context CRISPR has come out as a potential therapy that could be used for modulating and curing these diseases. In this article we will discuss What CRISPR is? How is it being used to target age-associated disorders? Can it be used to target aging itself? And what are the limitations to this approach?
CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. Repetitive DNA sequences, called CRISPR, were observed in bacteria with “spacer” DNA sequences in between the repeats that exactly match viral sequences. It was subsequently discovered that bacteria transcribe these DNA elements to RNA upon viral infection. The RNA guides a nuclease (a protein that cleaves DNA) to the viral DNA to cut it, providing protection against the virus. The nucleases are named “Cas,” for “CRISPR-associated.” This is one of the ways bacteria protect and defend themselves from viruses.
In 2012, researchers demonstrated that RNAs could be constructed to guide a Cas nuclease (Cas9 was the first used) to any DNA sequence. The so-called guide RNA can also be made so that it will be specific to only that one sequence, improving the chances that the DNA will be cut at that site and nowhere else in the genome. Further testing revealed that the system works quite well in all types of cells, including human cells.
With CRISPR/Cas, it’s easy to disrupt a targeted gene, or, if a DNA template is added to the mix, insert a new sequence at the precise spot desired. The method has profoundly changed biomedical research, as it greatly reduces the time and expense of developing animal models with specific genomic changes. Scientists now routinely use the CRISPR/Cas system for this purpose in mice. And for human diseases with a known mutation, such as cystic fibrosis, it’s theoretically possible to insert DNA that corrects the mutation. There are clinical applications in human trials now, including for engineering T cells outside of the body for CAR-T cancer therapy and for editing retinal cells for Leber’s congenital amaurosis 10, an inherited form of blindness.
There are a multitude of diseases that are being targeted by CRISPR. Of note many are age-related diseases are neurological diseases, Cancers, and auto-immune diseases. Yet, l CRISPR has not been used to target some of the most prevalent age-related diseases such as CHD, Diabetes and others. Part of this is to do with the multifactorial nature of these diseases which prevents them from having single gene targets. That being said, we have curated a list of age-related diseases that are being targeted by CRISPR and go into detail about their target mechanism of action. Unfortunately, the details of this might be a lot and hence we will have an additional blog talking about these
Answering this question begins with answering another question which is “Is There an Anti-Aging Gene?”. The answer may be yes, however, it is only true in fruit flies with genetic mutations in a gene called methuselah, named for a Biblical entity who lived to be 969, live significantly longer than normal flies. Methuselah is thought to help prevent oxidative damage, and its association with aging proves there is a genetic component to life span and has been shown to be greater than 22%.
Researchers have found several genes in mammals that prolong lifespan, again linking genetics to aging-related damage. If a gene is involved, it can be edited by CRISPR. However, editing a single gene in every cell type in the body is hard to do, if not impossible, in adults. Therefore, we need to determine which cells could be edited, and which oxidative damage genes should be targeted. A great example of this is the now infamous human experiment by He Jiankui, who edited human embryos created by IVF to have crippled CCR5 gene (protects from HIV infection) but unfortunately led to a mosaic of cells with crippled and uncrippled CCR5 gene, offering the babies no protection from HIV. In this experiment, the parents were HIV positive.
Similarly, instability of mitochondrial and genomic DNA are large contributors to age-related damage. This takes the form of small mutations in our DNA that occur and accumulate all over the body as cell types replenish and renew. We are all walking mosaics of millions of little distinct genomes, each with different damage as time goes on.
The mutation rate in mitochondria is thought to be up to 20 times faster than that of nuclear DNA, causing increasing defects in energy production as we age. In theory, these small age-related mutations could be targeted by CRISPR if only we knew where and what they were; a massive problem for future generations to tackle.
There are many limitations to using CRISPR as a life extending technology in its present form. To start with, aging is multifactorial and will we find a single gene that is solely responsible for making us immortal? Rather it would be a slew of genes that would bring about this change. CRISPR presently has been optimised to target specific genes and until we do not figure out the comprehensive list of genes that affect human aging, it might be challenging to use CRISPR for clinical purposes of longevity.
Secondly, CRISPR might solve the problem of slowing aging, but chances are it will not reverse it. At the end of the day it cannot clear out the damage accumulated just slow the accumulation of damage itself. Rather this is the biggest critique of the most recent publication in Nature where it was shown that CRISPR gene editing a progeria causing mutation in mice allowed them to live 2.5X longer! Researchers believe that such an approach could open doors for extending human lifespan as well but it is important to note the mosaic nature of human aging and the fact that CRISPR cannot reverse damage accumulation but only slow it/
Additionally, given the sub-par accuracy of CRISPR, we might actually end up causing more off-target effects that will cause additional damage in our genome, making its usage even riskier. And of course there is the huge cost of actually doing a CRISPR experiment, including quality control which would be another limiting factor.
Then again, there are other issues such as telomere shortening, that CRISPR as a technique could not address in its present form. All in all, we believe CRIPR will not be a magic wand but instead potentially a tool in a sleuth of tools that will help us slow and eventually reverse aging.
The CRISPR-Cas9 system has revolutionised the studies of gene-function and is making a huge impact on genetic therapy in human health. The role of CRISPR-Cas9 seems promising in targeting age-related diseases. That being said, the technology is still far from targeting the multifactorial nature of aging itself and we might still be a few decades away from translatable CRISPR-aging therapies.