CRISPR-Cas9 System

CRISPR-Cas9 System

Article: HIV can develop resistance to CRISPR/Cas9

How can they overcome this limitation of viral resistance?

Researchers that used this defense system to mutate HIV-1 in cellular DNA found that some mutations caused by this complex did prevent viral replication, but there was also mutations that led to resistance.

When HIV RNA enters the cell, it converts into DNA and inserts itself into the cellular DNA. CRISPR/Cas9 is programmed to target the DNA sequence and attach to the viral DNA, cleaving it off. However, HIV thrives with mutations, while most viruses are eventually killed, the viruses that survive the CRISPR/Cas9 system develop their surviving mechanisms to prevent themselves from being recognized again. These mutations in the viruses don't cause harm to the virus itself but replicates similar resistant viruses. The researchers committing this study, a collaboration between McGill University, University of Montreal, Chinese Academy of Medical Sciences and Peking Union Medical College, stated that their study should serve as a cautionary example to be careful in using CRISPR/Cas9 as an antiviral.

The senior study author Chen Liang believes there is strategies in overcoming these limitations, for example, targeting multiple sites in the viral DNA with the CRISPR/Cas9 complex or using different enzymes that are not Cas9. Once this limitation is lifted, they would have to develop different methods to deliver the treatment to patients. But the most concerning issue, in my opinion, would this treatment be affordable to the worldwide treatment of HIV that affects both first world and developing countries.

What is CRISPR?

Harnesssing CRISPR's potential

Benefits and Disadvantages of CRISPR

The CRISPR-Cas9 system consists of two key molecules that cause a DNA mutation change:

-an enzyme called Cas9 that acts as the "molecular scissors" cutting off specific DNA sequences that other sequences can be added in this gap.

-a RNA sequnce attached to Cas9 called guide RNA (gRNA) that ensures that the Cas9 enzyme cuts the right point in the genome by matching its complementary strand.

It is a unique gene editing tool that allows geneticists edit parts of the genome by removing, adding or modifying specific sections of the DNA sequence in living organisms.

There's many subtypes of CRISPR/Cas complexes that fulfill the same function that uses subtly different Cas enyzmes and PAM sequences. For genome engineering, scientists use the Type II CRISPR/Cas system but there is a diversity of this defense system that provides powerful alternatives of defense against viruses or altering mutations in the genome that causes genetic diseases.

The most obvious benefit of this CRISPR/Cas9 system is that it would eradicate some viruses and bacteria and allows us to modify genes in humans that cause genetic diseases. The process of getting to that point is very difficult, because along with the great benefit of this system, there comes the disadvantage of these viruses creating their own surviving mechanisms that could protect them from the CRISPR/Cas9 system. If these researchers want to eradicate the contraction of HIV in humans, I believe that instead of making the CRISPR/Cas9 attack the virus itself, they should use this system to encode a certain gene mutation in the human genome that causes an immunity to the virus. So that the human genome can create their own immune system responders instead of relying on the CRISPR to destroy the virus itself.

Where did it come from?

The CRISPR-Cas9 system was found in E. coli bacterial cells that functioned as a defense system against viruses by recognizing viral DNA that attached to their own DNA sequence. By cutting off this sequence, it deprived viruses from using bacterial transcription machinery to create more viruses. Not only does it destroy these viruses, but the cas genes stores genetic information from those viruses to destroy the phage upon re-exposure of the virus.

How does it work?

To be more specific, particular Cas enzymes cuts foreign DNA into smaller fragments and sticks these sequences into the CRISPR sequence to serve as a recognition signal if it appears again. Seperate Cas enyzmes (CAS II) uses the modified CRISPR sequence to generate RNA (crRNA) that guides a Cas nuclease to particular viral genetic material and species-specific protospacer adjacent motif (PAM). This CRISPR-Cas enzyme complex binds to the viral DNA and inactivates the viral DNA.