With viral diseases bearing the largest chunk of public health burden every year, medical scientists and researchers are starting to look into new ways of tackling the issue of viral infections. Since it seems logically impossible to silence every other viral strain out there in the environment, would it be possible to stop ourselves from getting infected? If we can’t stop the viruses from coming, maybe there is something that we can do to ourselves!
One of the possible solutions can come from the widely acclaimed CRISPR (clustered regularly interspaced short palindromic repeats) technology for gene editing. This is a mechanism that evolved in bacteria millions of years ago, which has now emerged as a potential genetic engineering tool. The CRISPR/Cas9 protein can be used to target specific sequences of DNA which it cuts like a pair of scissors where a new DNA sequence can be inserted. A short guide RNA (gRNA) is used to direct this specific degradation of specific nucleic acids, and later on, the break is repaired by the repair enzymes [1]. There are two possible approaches to CRISPR mechanism that can be exploited: somatic cell editing [2] and germline editing. While somatic cell editing is done for a person’s body cells, germline editing involves editing the DNA in sperm, eggs and embryos that result in genetic changes in the descendants thereafter. While somatic cell editing is already in a clinical trial, germline editing is an ethically controversial approach and is restricted in a lot of nations worldwide.
CRISPR/Cas9 can be theoretically used to edit the host cells in a number of ways. Host cell's immune modulation can be overexpressed against specific viral epitopes by using the editing tools. Also, the cells can be modified such that the receptor proteins on the cell membrane of the hosts that the viruses use to infect can be modified, and hence the host cells are made technically unrecognizable.
One of the exciting examples of CRISPR against viral infection comes from the in vitro study in which cancer-inducing Human Papilloma Virus (HPV) was targeted. CRISPR/Cas system using HPV 16-E7 specific gRNA in the HPV-positive SiHa and CaSki cells could successfully disrupt the HPV16-E7 DNA at the specific sites that resulted in apoptosis induction and growth inhibition in these cells. However, HPV negative cells were not induced for apoptosis [3]. Similarly, the Hepatitis B virus (HBV) has also been effectively targeted both in vitro and in vivo in several independent investigations. When Huh-7 hepatocyte-derived cellular carcinoma cells were transfected with an HBV expression vector, the production of HBV core and HB surface Antigen (HBsAg) was significantly reduced when CRISPR/Cas9 system was used with eight different gRNAs [4].
When latently infected epithelial cell lines were edited using CRISPR/Cas-9 using two guide RNAs to delete a 558 base pairs portion in the promoter region of BamHI rightward transcripts (BARTs), it was seen that it significantly reduced BART miRNA expression and hence hindered the possible viral reactivation and subsequent infection [5]. In the case of latently infected cells, a similar technique has been used to inactivate Human Immunodeficiency Virus (HIV-1) as well. For HIV, a more futuristic approach would be to use the CRISPR/Cas9 system to target cellular genes encoding proteins required for HIV-1 infection such as CCR5, a coreceptor for HIV-1 entry. As observed by Wang et al., HIV-1 susceptible human CD4+ cells in which the CCR5 gene was disrupted using CRISPR/Cas9 became resistant to R5-tropic HIV-1 and showed a selective advantage over cells with undisrupted CCR5 during R5-tropic HIV-1 infection [6]. Hence, the examples mentioned above can serve as a primary proof of concept that somatic cell engineering can indeed confer a significant level of resistance in case of particular viral infections.
References:
1. White M.K., Hu W., Khalili K. The CRISPR/Cas9 genome editing methodology as a weapon against human viruses. Discov. Med. 2015;19:255–262.
2. National Academies of Sciences, Engineering, and Medicine; National Academy of Medicine; National Academy of Sciences; Committee on Human Gene Editing: Scientific, Medical, and Ethical Considerations. Human Genome Editing: Science, Ethics, and Governance. Washington (DC): National Academies Press (US); 2017 Feb 14. 4, Somatic Genome Editing.
3. Hu Z, Yu L, Zhu D, Ding W, Wang X, Zhang C, Wang L, Jiang X, Shen H, He D, Li K, Xi L, Ma D, Wang H Disruption of HPV16-E7 by CRISPR/Cas system induces apoptosis and growth inhibition in HPV16 positive human cervical cancer cells. Biomed Res Int. 2014; 2014():612823.
4. Lin SR, Yang HC, Kuo YT, Liu CJ, Yang TY, Sung KC, Lin YY, Wang HY, Wang CC, Shen YC, Wu FY, Kao JH, Chen DS, Chen PJ. The CRISPR/Cas9 System Facilitates Clearance of the Intrahepatic HBV Templates In Vivo. Mol Ther Nucleic Acids. 2014;3:e186
5. Yuen KS, Chan CP, Wong NH, Ho CH, Ho TH, Lei T, Deng W, Tsao SW, Chen H, Kok KH, Jin DY. CRISPR/Cas9-mediated genome editing of Epstein-Barr virus in human cells. J Gen Virol. 2015;96(3):626–636
6. Zhu W, Lei R, Le Duff Y, Li J, Guo F, Wainberg MA, Liang C. The CRISPR/Cas9 system inactivates latent HIV-1 proviral DNA. Retrovirology.
Jamila Jul 27, 2020
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