Have you ever played an out of tune piano?
It sounds awful. It’s not even like the instrument itself is broken. The piano itself is fine but it just takes one cold, humid day to send all your notes sharp. But not all hope is lost because you get it tuned!
The keyboard is like our genome and each individual key is a gene. Sometimes our keys are out of tune and other times they’re just plain faulty.
Similarly, some people are born with faulty genes which cause genetic diseases. However, almost half of Americans have chronic diseases. These are acquired in our lives and mostly caused by over/under expression of specific proteins.
Guess what’s in charge of expression of proteins?
Genes!
So what! Some of us have faulty, sharp or flat keys. It doesn’t mean that all hope is lost.
It just means it’s time we tune our genome.
Gene Therapy
Gene therapy is when you insert the correct genetic information into our cells. However, traditional gene therapy methods don’t swap out genes like other genetic methods (CRISPR!) do. Think of it like this: your milk has gone bad.
Don’t drink that.
CRISPR would remove the milk from the fridge and place in a brand new bag; gene therapy would just add a new bag while today’s medicine would spray febreeze in the fridge to cover the smell. Basically, Gene therapy adds the proper genes; it doesn’t remove or replace the old ones.
Gene therapy also doesn’t touch embryos. Although there is a lot of stigma about designer babies and genetic terrorism with genetics, gene therapy is entirely different. Gene therapy can only target somatic cells: non-reproductive cells.
There are two main gene therapy approaches: in vivo & ex vivo. The ex vivo approach has had more success so far in which targeted cells are removed, genetically modified outside of the body and then put back in. The in vivo approach is preferable because ex vivo can be very inconvenient and expensive. It’s like the ‘in house tutoring’ option; the genes are delivered to you (your target cells). The ex vivo approach also has restrictions, not able to target cells within the body (lungs, heart, and brain).
However, the question still remains, how do we deliver the genes to the target cells?
For that we use something call vectors!
No, no. Not those lovelies with “both direction and magnitude” but the medical ones. Vectors are also carriers genetically engineering to deliver the gene in gene therapy!
In the case of gene therapy, viruses are often used as vectors…
Yes, the same ones.
Viruses
We all know that viruses are bad but do we know why? Viruses look something like this…
Or this…
Basically, they have a head where they store the virus DNA and a tail. When they find a target cell in our body, they will attach to it and inject their DNA in our cells using their tail.
& just like that they’ve corrupted our cell’s DNA, thus changing its function. And every time that cell duplicates, it will have that very same DNA in the nucleus.
Bummer right?
Except for the fact that we can use it to our advantage. We can genetically modify viruses to introduce correct genetic information into faulty or damaged cells. We can rehabilitate these viruses and turn them into good hackers (if those exist).
There are three main types of viruses that are used in gene therapy, each with their own defects and desirables. These are retroviruses, adenoviruses and adeno-associated viruses.
Let’s go through them one by one.
Retroviruses
These viruses are usually used ex vivo (outside of body) and are very efficient. Yes, ex vivo is not preferable but retroviruses are still important because they are being researched for a number of bone marrow cancers.
In terms of using retroviruses for in vivo delivery, it is possible but far less efficient. In attempt to improve this process, scientists tried putting a sort of ‘viral receptor’ within the target cells. This was to prevent the virus from spreading to non-target cells. These are called packaging cell lines. These identify the host cell and were a huge scientific advancement.
But…
This cell lines are not as effective as we may have hoped because viruses still end up spreading to other cells. Scientists are working on improving them.
However…
That’s not the only problem with retroviruses because they also follow a process called proviral integration. The injected DNA doesn’t fully integrate into the target cell until it replicates.
Basically, this process depends on mitosis.
On top of that, retroviruses are easily charged (very unstable). Even the slightest attempt to purify or concentrate can make it less effective.
Adenoviruses
Adenoviruses are the most commonly used vectors for cancer therapies and have in vivo administration potential. Unlike retroviruses, they can inject in non-dividing cells and are stable. They’re also efficient!
Yay!
But it’s not that simple. The reason adenoviruses don’t rely on cell division is because they inject the DNA in an unintegrated form. Quick refresher: when a cell is about duplicate, all the DNA in the nucleus winds into chromosomes. When a gene is added to a cell in an integrated form, it adds it into the chromosomes; hence, it relies on cell duplication (mitosis).
Adenoviruses insert DNA in an unintegrated form, meaning it doesn’t rely on mitosis but the change will not be in the daughter cells. That’s why the challenge with Adenoviruses is duration of the gene expression. Extended expressions are rare.
AAVs (Adeno-Associated Viruses)
These viruses are less efficient and precise!
So why do we use them?
Because, these vectors are special. In fact, they are the most frequently used gene therapy vector. These viruses are most effective when paired with a helper virus. Adenoviruses are common helper viruses used to co-infect the cell. AAVs are also able to infect both dividing and non-dividing cells; they also integrate into the DNA of the cell and thus have longer gene expressions.
What’s really cool about these viruses is that they’re able to do site-specific integration of the viral DNA into the target cell. AAVs inject into Human Chromosome 19 AAVS1. This essentially eliminates the dangers of random site integration such as insertational mutagenesis (issues while inserting DNA).
They’re still slow though. But we’re working on it.
Phew! Moving On.
So, let’s say scientist tweak one of these methods (or come up with a different one all together; yay!) and one finally works. How would we actually go about doing gene therapy?
It’s quite clear how this method can be used to attack genetic diseases. Using a viral vector, you inject the correct gene(s) into the target cells. However, how would we attack acquired diseases?
Each disease is different. For instance, diabetes happens when our body builds immunity to insulin while cancer occurs when our cells don’t undergo apoptosis (cell suicide). Tinkering with the genome of our cells can help us come up with innovative ways to solve these problems and researchers have found a few.
Cancer Therapies
For instance, scientists are looking into a potential genetic modification of tumor cells for cancer treatments. One method is to essentially poison cancerous cells. You inject a series of genes to produce proteins toxic to themselves (tumor cells).
Freaky right?
Another option is to basically put a bag over it’s head. You add a gene which suppresses the tumor’s properties by inhibiting certain gene expressions. You could also make specific cells (white blood cells) immune to chemotherapy or reprogram them to recognize cancerous cells (immunotherapy).
So we’ve got lots of options. Once you can hack, there are many ways to go about it.
Regenerative Medicine
From poor eyesight to dementia, there are a variety of diseases that are (so called) ‘irreversible.’ That is because there was no way to rebuild cells or tissues that had been damaged.
Keyword: was.
If you can program mammal stem cells into building functioning transplantable tissue, then voila, you can reverse the so-called irreversible.
But what if something goes wrong. What if you want it to stop?
Is there a way to control gene expression even after you’ve had the gene therapy procedure?
Controlled Delivery
You can actually have an oral drug control therapeutic gene expression. You make the therapeutic gene responsive to something which is responsive to this oral drug. Basically, if the therapeutic gene is supposed to produce anti-tumor toxins but you want it to reduce less or stop all together, you can take this drug. Suddenly you have a non-invasive way of controlling therapeutic gene expression.
So, why am I still wearing glasses?
Because we’re not exactly there! Gene therapy, or any gene editing for that matter, has a long way to go. The most significant problems involve building better vectors but there are also constraints in our knowledge of the body.
Even though we’ve sequenced the genome, it’s not yet been mapped. Of course, to handle that we have the field of bioinformatics, and companies such as Deep Genomics are using AI to better understand our code.
Gene therapy is cool, it’s new, but it’s not there.
Not yet.
Takeaways
- AAVs are likely the most promising viral vector
- Different vectors have different strengths
- Gene therapy could help us reverse ‘irreversible’ diseases
- Edits can be controlled post delivery with drugs
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