How can CRISPR/Cas9 be used to edit genomes?

Crisprs | Tebu Bio

Genes are key components of our lives. With many other factors, they contribute to shaping us, making people and any other living organism what they are. Genomes, whether human or not, are able to evolve through the generations.

Sometimes, these changes can lead to the appearance of malfunctions or genetic diseases. Some living organisms have created and set up cell mechanisms to protect and repair their DNA or RNA when being attacked.

DNA is the material support of the genome, except for some viruses, where the genome is in the RNA. For years, scientists have studied these editing mechanisms and developed several genome editing methods.

But one of them has recently generated hope within the scientific community, regarding the development of gene therapy when fighting severe diseases.

An exciting new gene editing method

Throughout History, Men have always tried to influence and modify the genetic material of plants and/or animals; the purpose being to acquire favourable characteristics for mankind. For instance, sheep’s genetic material has been influenced to make them able to grow more wool.

This used to be done by cross-breeding two closely related species or by orienting their reproduction. Gene editing technologies have been developed across the last decades.

Today, genome editing gives scientists the ability to modify an organism’s DNA. Using these technologies, genetic material can be added, deleted, or altered in any particular location in the genome.

The very recent CRISPR/Cas9 method, short for Clustered Regularly Interspaced Short Palindromic Repeats, associated with Cas9 (an enzyme of bacterial origin), has been the source of a huge excitement within the field of Science.

This CRISPR/Cas9 system is indeed revolutionary: it is able to execute DNA repair and mutation of any living species as many times as required. It represents a cheaper, faster, more accurate and more efficient way to edit genomes than any other method.

The reason why scientists are so excited about this new method, is that genetic engineering could become much easier to achieve. This genome editing method is extremely interesting for the scientific community, in the prevention and treatment of human diseases.

Researchers are currently conducting studies regarding a variety of illnesses like haemophilia, as well as more complex diseases including cancer, heart disease, mental illness and human immunodeficiency virus (HIV) infection.

How does CRISPR/Cas9 work?

Genetic mutations happen every day, our cells consistently have to adapt themselves to new conditions. Most of the time, cells do not recognize that their DNA has changed.

The solution is to determine the location of the DNA sequence that needs to be manipulated and to revert it to the original and normal sequence. This is where CRISPR/Cas9 comes into action and adds, removes or modifies genetic material in the sequence.

This system is composed of two key molecules able to introduce the change. The first one is a protein called Cas9, an endo-nuclease enzyme which is going to act as a pair of scissors meant to cut the two strands of DNA where it needs to be changed.

The second one is a piece of RNA called guide RNA which is meant to locate and bind to the targeted DNA sequence. This guide RNA cell uses base pairs that are complementary to the specific DNA sequence.

The Cas9 enzyme follows the guide RNA to the location and cuts both strands of DNA. The cell instantly recognizes that the DNA has been damaged and tries to repair it. Scientists are then able to use this DNA repair machinery to introduce changes to one or more genes in the genome of a cell of interest.

With techniques like CRISPR/Cas9, scientists can study the gene’s function and how it can be manipulated, repaired or modified, in order to treat human diseases.

A powerful tool for gene therapy

Gene therapy uses genetic editing to provide therapeutic benefits while modifying genes via disruption, correction or replacement. But gene therapy is a very complex field of Science, and it has known both early successes and tragic failures in its different clinical trials.

But CRISPR/Cas9 represents a new opportunity for gene therapy to prove its efficiency. Indeed, scientists consider this method has a lot of potential as a tool for treating a wide range of medical conditions that are always the results of genetic mutations, such as cancer or high cholesterol.

For the moment, gene editing technologies, including CRISPR/Cas9 have been applied to somatic cells (non-reproductive cells) exclusively. But many debates have emerged regarding the potential to edit germ line cells (cells meant to be transmitted to the descendants). It implies that any changes made will be passed on to the next generations, and this is an important ethical concern.

Editing germ line cells is currently illegal in most countries, mostly as regards of the ethical issues the method raises. On the other hand, editing somatic cells’ genome has already proven its efficiency in treating human diseases on a very small number of life-threatening cases.

It may take many years before CRISPR/Cas9 is used on humans on a regular basis.

The future of CRISPR/Cas9

Genetic engineering brings up ethical issues regarding CRISPR/Cas9 being used to alter human genomes. The controversy essentially relies on the fact that editing germ line cells (human eggs or human embryos) create a permanent change to the genome that can be passed down for generations.

Indeed, Eugenics is the main ethical issue raised CRISPR/Cas9. Eugenics represents the possibility to modify people genomes, not for medical reasons which are justified and laudable reasons, but for more trivial reasons which will probably lead to important abuse, such as the selection of eye colour, or skin colour, height, etc.

The changes are irreversible.

CRISPR/Cas9 system also needs to be submitted to thorough studies and researches, especially on solving the off-target effects.

In most cases, guide RNA are composed of a specific 20-bases sequence, complementary to the target DNA sequence in the gene that needs to be edited.

But the guide RNA doesn’t need all 20 bases to match and bind to the targeted site. The problem with this, is that a sequence composed of, for example, 19 of the 20 complementary bases might exist somewhere else in the genome. So the guide RNA might miss the target sequence and bind to a completely different one.

Meaning that the Cas9 enzyme might snip out another sequence and introduce a mutation in the wrong location. This could potentially affect a crucial gene or an important part of the genome.

In other words, scientists still have to figure out how to ensure the CRISPR/Cas9 locates, binds and cuts the correct DNA sequences, before thinking about the development of gene therapies meant to treat severe medical conditions.

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