Viruses are parasites. The only way they can grow is by hijacking their hosts. When they infect a human host, viruses use human proteins to multiply and modify the human cells to sustain the infection. At the same time, the human host activates defense mechanisms to fight the infection.
Most current drugs against viral infections target the virus itself. But scientists are interested in developing therapies that aim for host proteins instead, or the genes that produce them, in part because such therapies are believed less likely to elicit drug resistance. A detailed understanding of virus-host interactions is crucial to the success of this strategy.
A team of scientists have been cataloging host proteins that physically bind to virus proteins. These physical interactions identify human proteins that the virus can use to infect cells and propagate. However, they don't reveal how host proteins work together to facilitate infection.
To address this gap, the researchers have developed a new way to understand how host cells control HIV infection in human cells.Their approach entails disrupting host genes rather than proteins. In a paper published in Molecular Cell, the team describes a map of the genes controlling HIV infection in human cells, which they built by assessing more than 63,000 combinations of human genes associated with HIV infection.
The map, which the team refers to as a viral epistasis map (vE-MAP), is an essential advance for HIV research in several other ways. For one thing, it uncovers a previously unsuspected set of genes required for the growth of the virus in human cells. For another, the vE-MAP can be used to analyze how different HIV mutants affect host cells or to test drugs that disrupt HIV-host interactions.
The vE-MAP is an adaptation of the E-MAP, which the researchers pioneered and refined over the past 15 years to identify genes that control how cells grow. At the core of this approach is the the lab's ability to disrupt a large number of genes, test them in pairs, and analyze the results via sophisticated computational methods. By carrying out these pairwise disruptions across hundreds of genes, scientists can find groups of genes with similar patterns of interactions, a sign that they are likely to take part in the same molecular process.
Among the genes that stood out in the vE-MAP were several members of the CNOT family, whose role in HIV biology had never before been established. The authors demonstrated that the CNOT complex promotes HIV infection by suppressing innate immunity in CD4+ T cells, the type of immune cells that HIV preferentially targets in humans. Innate immunity is a defense mechanism by which host cells can fight infection. This phenotype was rescued by deletion of IRF7, a transcription factor regulating interferon-stimulated genes, revealing a previously unrecognized host signaling pathway involved in HIV infection.
Furthermore, the vE-MAP uncovered genes that had little impact when disrupted individually, but a great effect when tested together.
"These genes would be overlooked in classic, single-gene disruption experiments," said the author. "They confirm the potential of the vE-MAP to uncover new mechanisms by which HIV interacts with human cells."
Combining drugs that target two of these genes at the same time might thus be a promising therapeutic strategy, especially for a virus such as HIV/AIDS, which has evolved multiple ways of tapping its hosts' resources.
The vE-MAP was also able to pick up genes that specifically interact with a known HIV mutant. This observation bodes well for the ability of the vE-MAP to identify distinct host factors affecting the various forms of HIV, or the virus mutants that arise in response to currently available drugs.
Additional testing with a drug known to interfere with HIV-associated human proteins gives the authors confidence that their vE-MAP approach could, in the future, be used to screen for novel anti-HIV drugs and to understand their mode of action.
"Our work is proof-of-principle that the vE-MAP approach is a powerful way to map out the interface between HIV and human cells, and to uncover new therapeutic avenues," said the author. "We now look forward to testing it on other pathogens."
New human genes controlling HIV infection identified!
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