- HIV infection/AIDS is a major health concern worldwide
- Currently available anti-retroviral therapy (ART) inhibits viral replication and has improved survival of infected patients. However, increasing drug resistance is also a major cause for concern.
- Presently, there is no cure for the disease as the virus is capable of inserting itself into the host cell DNA.
- New CRISPR editing tool has identified human or ‘host’ genes that promote HIV infection, but are not essential for cell survival.
- These genes can be potential therapeutic targets to develop newer drugs for HIV therapy. The chances of drug resistance also remain low in this approach.
Recent rapid advances in the field of genetic engineering have thrown open new and exciting possibilities to explore in HIV treatment, and the still elusive cure.
Novel Study Identifies Target Human Genes For Therapeutic Approach in HIV Infection
CRISPR-Cas9 gene-editing technology was employed to identify three promising new genetic targets for treatment of HIV infection by scientists from Whitehead Institute, the Ragon Institute of MGH, MIT and Harvard and the Broad Institute of MIT and Harvard.
"The beauty of the CRISPR-based genetic screens is the clear and robust results they yield," notes Sabatini, who is also member of the Broad Institute and Professor of Biology at Massachusetts.
The research team wished to identify and inactivate human genes or 'host' genes essential for viral replication and survival but not critical for host cell survival. Such genes could be used as targets for development of novel therapies against HIV without harming the patient, and also less chances of developing resistance to treatment.
"Current anti-HIV medications overwhelmingly target viral proteins," says Ryan J. Park of the Ragon Institute and the Broad Institute, co-lead author of the report. "Because HIV mutates so rapidly, drug-resistant strains frequently emerge, particularly when patients miss doses of their medication. Developing new drugs to target human genes required for HIV infection is a promising approach to HIV therapy, with potentially fewer opportunities for the development of resistance."
Another aspect of their study was to concentrate on T cells which happen to be the primary targets of HIV, and to identify host genes that are most critical to T cell infection by the virus.
Identifying Human Genes that are Critical to Viral Infection of T-cells
Previous research has identified a host protein termed C-C chemokine receptor type 5 (CCR5) critical to HIV binding to the T cell. Mutation of the CCR5 gene has been shown to confer protection against HIV. However, though CCR5 inhibitors have been developed and are being used, they cause serious side effects.
- Using CRISPR, the authors screened a cell line derived from HIV-susceptible CD4 T cells and detected five genes, which when inactivated, protected cells from HIV infection without affecting cellular survival.
- The screen also identified genes for two enzymes -- TPST2 (Tyrosylprotein Sulfotransferase 2) and SLC35B2 (Solute Carrier Family 35 Member B2) -- that alter the CCR5 molecule in a manner making HIV binding easier.
- An additional gene identified through CRISPR screen was ALCAM, which is essential for cell-to-cell adhesion. When CD4 T cells are exposed to low amounts of virus, as might occur in natural transmission, loss of ALCAM was associated with a marked protection from HIV infection.
Park explains, "ALCAM is necessary for cell-to-cell adhesion in our cell line, allowing more efficient viral transfer from one cell to the next. In fact, we found that artificially inducing the aggregation of cells lacking ALCAM restored the cell-to-cell transmission of HIV. Further studies are needed to investigate whether targeting these genes would be toxic to humans. However, even if systemic inhibition has toxic effects, gene therapy approaches that selectively target these genes only in CD4 T cells or their precursors may avoid these toxicities, although it's important to note that gene therapy remains a challenging and potentially costly therapeutic approach."
What is CRISPR System?
A recently discovered novel mechanism is the clustered regulatory interspaced short palindromic repeat(CRISPR)-associated 9 (Cas9) system, which is powerful, specific, and highly effective and versatile tool for genome editing and inactivating specific target genes in the genome.
Earlier methods that used RNA interference (RNAi) to identify host dependency factors in HIV infection yielded lots of false positive results and additionally did not completely block the expression of host genes leading to sufficient production of host protein to permit HIV infection.
How the CRISPR/Cas9 System Operates to Target Specific Regions in the Gene
Cas9 enzyme works like 'molecular scissors' and cleaves two strands of DNA at specific locations in the genome so that pieces of DNA can then be added or removed.
The ease with which this tool has been employed is evident by the numerous applications it has been put to in the last couple of years.
CRISPR/Cas9 system has emerged as a precise, inexpensive and quick method to edit portions of the human genome, overcoming limitations of traditional methods.
Though research is still in the nascent stages, the CRISPR/Cas9 system has the potential to be developed as a therapeutic and prophylactic option in HIV infection management. In fact, this approach might also find use in the development of treatment options for other viral infections.
Further research in animal models and humans with HIV/AIDS setting needs to be undertaken in the future to evaluate its efficacy and safety.
- What is CRISPR? - (http://www.yourgenome.org/facts/what-is-crispr-cas9)
- The CRISPR/Cas9 genome editing methodology as a weapon against human viruses - (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4445958/)