Therapeutic applications of CRISPR-Cas genome editing in viral infections (Part 44- CRISPR in gene editing and beyond)
Welcome to the 44th part of the multi-part series on applications of CRISPR in gene editing and beyond.
CRISPR-Cas9 tool can also act as antiviral therapy in treating viral infections like:
Human Immunodeficiency Virus or HIV
HIV belongs to the retrovirus family of viruses and stores its genetic information using single-stranded RNA. The outer shell of the virus contains ‘glycoproteins’ gp120 and gp41, which allow HIV to lock onto the CD4 receptor and another co-receptor, either CCR5 or CXCR4, present on the surface of CD4 T cells. Following fusion with the cell membrane, HIV genome RNA enters the T cell, where it is reverse transcribed into proviral DNA using the viral reverse transcriptase enzyme. And then, the integrase enzyme of the virus integrates the viral DNA into the DNA of the host cell. The integrated viral DNA provides the instructions to the host transcription machinery to drive transcription of HIV RNA, producing enough progeny virions that bud off from the cell, leading to binding and infecting other CD4 T cells.
Helper CD4 T cells play an essential role in the adaptive immune system by activating and recruiting other immune cells, including cytotoxic T cells, and promoting the formation of antibodies by B cells. Thus, by infecting and killing the helper T cells, HIV prevents the immune system from working properly and can lead to the development of acquired immune deficiency syndrome or AIDS, leaving the host vulnerable to other pathogens and cancerous cells.
Active antiretroviral therapy (ART), a combination of three or more drugs, can keep HIV replication under control but cannot eradicate the integrated HIV DNA in infected CD4+ T cells. An integrated HIV genome will not form viral particles and will not circulate in the bloodstream as long as the ART effect is there. Thus, HIV even with long-term ART, HIV cannot be entirely eliminated from the body. ART can also have various side effects and must be taken for life to keep HIV infection under control.
(i) However CRISPR-Cas9 system can provide a permanent effective antiviral therapy to eliminate HIV infection completely by targeting different regions of the viral genome. In vivo delivery of Cas9 and multiple gRNAs using vectors like Adenovirus-associated virus vector serotype 9 (AAV9) or lentiviral vectors, the gene editing tool can result in complete HIV eradication by excising the entire integrated viral DNA from the host DNA. Thus, impeding the chances of viral gene reactivation and virus replication in HIV-1 infected T cells.
CRISPR-based one-time treatment to cut HIV DNA from human cells is under clinical trials.
(ii) CRISPR-Cas9 tool can also be used to prevent new HIV infections by creating mutations in the genes encoding co-receptors CXCR4 and CXCR5 required for HIV entry into the CD4 T cells. According to studies, mutated co-receptors showed no obvious effects on cell viability but can have a significant effect in preventing helper T cells from HIV-infection when compared to unedited T cells.
Hepatitis B virus
Hepatitis B virus (HBV) is a dsDNA virus that infects hepatocytes causing acute or chronic infection of the liver, including cirrhosis and hepatocellular carcinoma. Upon infection, the partial dsDNA genome of HBV is converted into an episomal stable covalently closed circular DNA (cccDNA) form using the host DNA repair machinery. The cccDNA associates with host transcription factors and viral proteins to enable the viral gene expression and viral replication. The cccDNA molecule pose a major roadblock to HBV cure because current antiviral therapies can suppress HBV replication but fail to disrupt cccDNA and integrated HBV DNA, which leads to the relapse of the disease after the cessation of the treatment.
In vitro and in vivo studies have shown that AAV vectors containing SaCas9 (a Cas9 ortholog) and multiple gRNAs can cleave the HBV cccDNA and integrated HBV DNA inside the infected cells, thus demonstrating the potential cure for HBV infection.
Herpes Virus
Of the more than 100 known species of herpesviruses, 9 are known to infect humans: herpes simplex virus types 1 and 2, which cause oral cold sores and genital herpes, respectively; varicella-zoster virus, which causes chickenpox and shingles; cytomegalovirus, which causes mononucleosis, Epstein-Barr virus which causes infectious mononucleosis and Burkitt lymphoma, human herpesvirus 6 (variants A and B), human herpesvirus 7, and Kaposi’s sarcoma virus or human herpesvirus 8 which causes Kaposi’s sarcoma in AIDS patients and a B virus.
Herpesviruses possess a double-stranded linear DNA genome. All herpesviruses have a latent period in their life cycle, during which the virus is “hidden,” genes expressed during the lytic cycle are suppressed and do not produce virus progeny. The ability to establish a latent infection represents a key challenge in treating herpesvirus infections. Most antiviral therapies suppress the virus replication but are unable to eradicate the latent infection.
Genome editing using CRISPR-Cas9 technology may serve as a promising therapeutic strategy against the latent infection of the herpesvirus. Delivery of Cas9 and multiple gRNAs using appropriate vectors can disrupt the viral genome directly, thus eradicating the latent herpesviruses from the infected cells.
Other viruses
Other studies have also revealed the therapeutic applications of CRISPR-Cas9 in the treatment of numerous other viral diseases, including papillomavirus, various other oncogenic viruses responsible for causing cancer in humans, influenza virus, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), respiratory syncytial virus (RSV), human neurotropic polyomavirus, etc.
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