Cellular Antivirus: How Foreign DNA is Hushed

The cells in human body are always searching for things that do not suit our body, which is very much similar to computer antivirus you use to protect your system from harmful viruses. The first line of defense in our bodies are the immune cells that circulate throughout the bloodstream and fight against anything they perceive as foreign or different. However, the question is, what happens if anything different manages to dodge the immune system and proceeds to invade one of our cells?

This often occurs with retroviruses. Just like the human immunodeficiency virus (HIV) that significantly invades our cells and goes on to make more copies of themselves. These viruses completely change their RNA genetic information and make it into DNA, and then integrate into our cell’s own DNA. This is how the virus stops the cell to make more copies of itself. However, this integration of the virus isn’t very smooth and perfect. Most of the times, some pieces of viral DNA do not fully integrate in the cell nucleus. Interestingly, these unintegrated retroviral DNA is silenced instead of being expressed by the cell. 

This silencing has posed challenges for scientists who looked to use the non-integrating retroviral DNA for gene therapy. This is the process where new genetic information is provided to patients who do not have a particular gene or have defected copy. High quality DNA extraction kits are to be used for these studies.

A recent study has described cellular machinery that controls the silencing of unintegrated retroviral DNA. Many believe this discovery will be beneficial in enhancing the gene therapy and will provide the much-needed knowledge of our cell’s viral defense system. 

The researchers of the study made use of a Murine leukemia virus (MLV) that cannot integrate its DNA in the cells it is invading. It also contains the genetic information for green fluorescent protein (GFP). Since the DNA does not completely integrate in the cell nucleus, it remains silent. That said, if the silencing of this unintegrated DNA was relieved, the GFP gene cells can glow green because the gene will be expressed. 

Genome-wide Crispr-Cas9 screen was used to find which genes were needed to silence unintegrated DNA. It was found out that the genes NP220; part of the HUSH complex: MPP8, TASOR, and PPHLN1, SETDB1, and histone methyltransferase, all of these demonstrated increased green fluorescence when lost. This is why they are important components of the silencing machinery. 

To further study the function of these proteins, the researchers examined if they are interacting with each other in the cell and if they bind unintegrated DNA. It was found out that NP220 easily and directly integrated with the HUSH complex proteins TASOR and MPP8. These identified proteins are able to bind to unintegrated DNA. NP220 goes on to bind to the DNA with the help of its DNA-binding domain, and when its lost, HUSH and SETDB1 are not able to bind DNA. This indicated that NP220 is the most significant player in gathering all the silencing components to the unintegrated DNA. 

In order to further understand the silencing procedure, researchers showed that when SETDB1, MPP8, or NP220 were lost, there was a heavy decrease in the repressive epigenetic mark H3K9me3 on unintegrated DNA. This showed that these proteins are very crucial for maintaining the repressive mark. It was also discovered that if the histone deacetylases HDAC1 and HDAC4 were lost, histone H3 acetylation of unintegrated DNA increases along with its expression. 

To find how NP220 was bound to unintegrated DNA, researchers found that when mutation occurs or when the cytidine clusters are removed on retroviral DNA, NP220 cannot bind. Intriguingly, it was also made clear that in cells where NP220 wasn’t able to bind with the unintegrated DNA, the virus makes copies at a faster rate. This suggests that NP220 is a very important mediator in the protection of our cells against MLC retroviral gene expression. (DeMarco 2019)