- Identified hundreds of genes that promote or inhibit reactivation of latent HIV.
- Showed that inhibiting mTOR, a regulatory protein complex, can prevent reactivation of latent HIV.
- Suggested that medications to inhibit mTOR might help people with HIV achieve and maintain undetectable HIV viral loads.
Current treatment of human immunodeficiency virus (HIV) infection centers on the use of multiple medications that disrupt the virus’ lifecycle at several stages. The approach, called highly active antiretroviral therapy (HAART), halts progression of HIV disease, extends patients’ life expectancy and enables many to live relatively free of symptoms. However, HAART does not cure the disease, because it only targets virus that is actively replicating. People infected with HIV also carry a reservoir of inactive, or latent, virus that, under the right circumstances, can emerge from latency and reestablish disease progression.
The Verdin Laboratory at the Gladstone Institute of Virology and Immunology in San Francisco, working with an international team of researchers, applied cell- and molecular-biology approaches to broaden understanding of factors that affect reactivation of latent HIV. Such factors might provide targets for medications to permanently prevent disease progression, either by reactivating latent virus so that it becomes susceptible to HAART or by locking it in its latent state.
A Whole-Genome Screen
Dr. Eric Verdin and his team assessed how suppressing each gene in the human genome would affect reactivation of latent HIV. To accomplish this, they introduced a library of small molecules, called short hairpin RNA (shRNA), into engineered cells that were latently infected with the virus (see Figure 1). Each shRNA binds and inhibits one gene, and, collectively, the shRNA library inhibited every gene in the human genome. The researchers then exposed the cells to a pair of antibodies that stimulated reactivation of latent HIV in some, but not all, of the cells.
Fluorescent markers on the HIV DNA and shRNA enabled the researchers to compare gene expression in cells with shRNA-suppressed genes that did or did not show HIV reactivation. The comparison identified 335 genes that promote viral reactivation; cells in which these genes were suppressed exhibited less viral reactivation than cells in which these genes were active. Another 330 genes were found to support viral latency; cells in which these genes were suppressed showed increased reactivation.
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From Gene to Protein Complex
Several of the genes identified by the genetic screen encode proteins that were already known to affect HIV latency and reactivation. The researchers took particular note of a set of genes that encode proteins for an important regulatory complex not previously implicated in HIV latency and reactivation.
The complex, called mTOR (mammalian target of rapamycin), helps coordinate cellular growth in response to environmental or cellular influences. Two variants, mTORC1 and mTORC2, regulate biological processes ranging from lipid metabolism to cell proliferation. In addition to the current study, some of what is known about mTOR suggests that the complex might promote HIV reactivation. For example, mTOR interacts with other regulatory pathways that have been shown to affect HIV latency and reactivation, including transforming growth factor beta (TGF-β) signaling and actin remodeling.
A Potential Treatment?
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The Verdin Lab researchers reasoned that if mTOR plays a key role in reactivating latent HIV, inhibiting the complex might lock the virus in its latent state. In a new experiment using a different engineered cell line, the researchers demonstrated that, indeed, debilitating the complex (by knocking down its subunit proteins) halted HIV reactivation. Importantly, they duplicated this finding in CD4 cells from patients infected with HIV (see Figure 2).
“The identification of the mTOR pathway as a key regulator of latency and reactivation opens up new therapeutic approaches for preventing HIV from reactivating,” says Dr. Verdin. He notes that several mTOR inhibitors are under development and being tested as cancer therapeutics. Future research will evaluate the effect of these mTOR inhibitors on HIV reservoir suppression and identify candidate targets in the mTOR signaling pathway.
Dr. Amy Lossie, a Program Officer in the Genetics, Epigenetics, and Developmental Neuroscience Branch at NIDA, says that certain patient groups in particular could benefit if mTOR blockade to prevent HIV reactivation proves clinically viable. Dr. Lossie explains, “Some patients who are on HAART continue to have detectable virus in their blood, for example, due to low medication adherence. Adding mTOR inhibitors to HAART could help these patients achieve undetectable viral loads.”
Preventing HIV reactivation is an alternative to another approach that researchers are pursuing as a potential answer to the problem of latent HIV. That approach, called “shock-and-kill,” reactivates latent virus, rendering it susceptible to HAART medications. In theory, if it were possible to shock-and-kill all the latent virus in a patient’s body, patients could be cured of HIV disease. However, Dr. Lossie says, “There are hidden reservoirs of HIV, such as in glia cells in the brain, that are particularly hard to kill with this approach.” Although the alternative approach will not cure the disease, medications that successfully lock HIV in its reservoirs would enhance the efficacy of HAART and might even be sufficient therapy in themselves for patients who have achieved complete viral suppression.
This study was supported by NIH grant DA041742.
Besnard, E.; Hakre, S.; Kampmann, M.; et al. The mTOR complex controls HIV latency. Cell Host Microbe 20(9):785-797, 2016.
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NIDA. (2017, September 22). Blocking Protein Complex May Lock Down HIV Reservoirs. Retrieved from https://www.drugabuse.gov/news-events/nida-notes/2017/09/blocking-protein-complex-may-lock-down-hiv-reservoirs