0
We're unable to sign you in at this time. Please try again in a few minutes.
Retry
We were able to sign you in, but your subscription(s) could not be found. Please try again in a few minutes.
Retry
There may be a problem with your account. Please contact the AMA Service Center to resolve this issue.
Contact the AMA Service Center:
Telephone: 1 (800) 262-2350 or 1 (312) 670-7827  *   Email: subscriptions@jamanetwork.com
Error Message ......
Medical News & Perspectives |

Gene Researchers Work to Engineer HIV-Resistant Cells FREE

Tracy Hampton, PhD
JAMA. 2014;312(4):323-325. doi:10.1001/jama.2014.8659.
Text Size: A A A
Published online

Ever since the evolution of the HIV/AIDS pandemic, researchers have long sought effective strategies to prevent HIV infection or help the body keep the virus in check after infection has occurred. In addition to a decades-long effort to develop an effective vaccine and continuing research to find less toxic drugs, some researchers are focusing on another strategy—modifying the genes of host cells to make them resistant to infection.

“To date, vaccines and current drug therapies have shown a limited ability to achieve functional control of HIV infection, as defined by a state where patients no longer are required to take medication on a daily basis,” said Carl June, MD, a professor of immunology at the University of Pennsylvania’s Perelman School of Medicine.

Place holder to copy figure label and caption

Graphic Jump LocationImage not available.

HIV can invade host cells by binding to the CD4 receptor on the host cell surface and to a coreceptor, CCR5. Preventing expression of CCR5 may help patients with HIV control the infection.

He and others hope that gene editing strategies may someday help patients attain such control. Various strategies that target certain receptors for HIV that are expressed on immune cells are now being developed in the laboratory and in some cases are already being tested in early clinical trials.

When HIV-1 invades its main target, the immune system’s CD4 T cells, the virus must bind to at least 2 receptors on the cell surface, first to the CD4 glycoprotein and then to either of 2 coreceptors, CCR5 or CXCR4. About 1% of people of European descent are resistant to HIV because they carry 2 copies of a mutation in the CCR5 gene. The mutation, called CCR5-Δ-32, results in a smaller protein that is not expressed on the cell surface. Several years ago, researchers discovered that when a patient in Berlin who had both AIDS and leukemia underwent a bone marrow transplant from a donor with the CCR5-Δ-32 mutation, the procedure effectively treated both diseases (Hütter G et al. N Engl J Med. 2009;360[7]:692-698).

Because compatible donors with CCR5-Δ-32 are difficult to find—and the use of bone marrow transplantation would not be a feasible way to treat HIV on a large scale—researchers have been working to develop treatments based on the same principle. For example, inhibitory RNAs designed to block production of CCR5 can be delivered via viral vectors or other methods (Burke BP et al. Viruses. 2014;6[1]:54-68). The idea of therapeutically creating the equivalent of the natural CCR5-Δ-32 mutation by using gene ablation or suppressing gene expression to eliminate the CCR5 HIV coreceptor on CD4 cells is an old one, said Oregon Health and Science University’s Louis Picker, MD, whose research team has developed a vaccine that eradicates simian immunodeficiency virus in monkeys (Hansen SG et al. Nature. 2011;473[7348]:523-527). For example, researchers attempted to ablate or suppress CCR5 using intrabodies (intracellular antibodies that target a specific intracellular antigen) or small inhibitory RNAs. “The problem before was that the means to achieve CCR5 ablation or suppression—such as through the use of intrabodies or small inhibitory RNAs—didn’t work very well,” said Picker.

Recent strategies that are generating promising results involve the use of engineered nucleases that create double-stranded breaks within specified genes. When the cell attempts to repair the induced breaks, mutations are created, and the proteins that are normally made are truncated or not expressed. Paula Cannon, PhD, an associate professor at the University of Southern California, and her team have been using one such nuclease—called a zinc-finger nuclease—to target the CCR5 gene in human hematopoietic stem cells. After they transplanted the altered cells into a “humanized” mouse model (mice engineered to develop a functional human immune system), the human cells retained the ability to differentiate into multiple blood cell lineages that also maintained high rates of CCR5 disruption. When the researchers challenged the transplanted mice with HIV-1, viral replication was blocked and normal levels of human T cells were preserved (Holt N et al. Nat Biotechnol. 2010;28[8]:839-847).

“An advantage of this approach is that the stem cells constantly make new CD4 T cells, the natural host cells for HIV, that are effectively resistant to the virus,” said Cannon. “Over time, these cells survive and come to dominate, which allows the host to control the virus.” Cannon and her team are now working to launch a clinical trial.

June and his colleagues have recently completed a clinical trial that uses the same zinc-finger nuclease in a similar strategy in autologous CD4 cells rather than hematopoietic stem cells (Tebas P et al. N Engl J Med. 2014;370[10]:901-910). During the study, 12 patients with chronic HIV infection received 10 billion of their own CD4 cells (11%-28% of which were modified at the CCR5 gene) in addition to antiretroviral therapy. One serious adverse event was noted and was attributed to a transfusion reaction. The median CD4 cell count was 1517/μL at week 1, more than 3 times the preinfusion count of 448/μL. An estimated 8.8% of circulating peripheral-blood mononuclear cells and 13.9% of circulating CD4 cells were CCR5-modified after 1 week, and the modified cells had an average half-life of 48 weeks. Six of the patients underwent an interruption in antiretroviral treatment 4 weeks after the infusion, and during interruption, the decline in circulating CCR5-modified cells was significantly less than the decline in unmodified cells. HIV RNA even became undetectable in 1 of 4 patients whose RNA could be evaluated, and blood levels of HIV DNA decreased in most patients.

“This is the first example of targeted gene editing in humans, and the work highlights the potential to create an immune system that is resistant to infection by the HIV-1 virus,” said June. The researchers are working to extend their findings to larger numbers of patients and to determine the optimal dose of CCR5-modified T cells.

Like zinc-finger nucleases, other types of engineered nucleases can also facilitate targeted genome editing in human cells with high specificity and low cytotoxicity. For example, programmable, sequence-specific DNA-binding enzymes called transcription activator-like effector nucleases, or TALENs, have been engineered to recognize and destroy the CCR5 gene in laboratory studies (Mussolino C et al. Nucleic Acids Res. 2014;42[10]:6762-6773).

Another new approach to targeting CCR5 features a method of genome editing that involves a tool called CRISPR, which stands for the clustered regulatory interspaced short palindromic repeat–Cas9 system. This genome-editing method is based on an ancient bacterial process that takes fragments of DNA from invading viruses and splices them into the bacterial cell’s own DNA as a way to recognize viruses that should be attacked in the future.

Investigators led by Yuet Kan, MD, of the University of California, San Francisco (UCSF), recently used this system to introduce 2 copies of the protective CCR5 gene variant into induced pluripotent stem cells (iPSCs). Monocytes and macrophages that were differentiated from these cells were resistant to HIV infection in laboratory experiments (Ye L et al. Proc Natl Acad Sci U S A. 2014 111[26]:9591-9596).

“Theoretically, if the gene-edited stem cells could be reconstituted in the patient, they would produce white blood cells and T cells resistant to HIV and ultimately lead to the patient being functionally cured,” said lead author Lin Ye, PhD, an assistant professor at the UCSF School of Medicine. She noted that such a strategy would lack the toxicity of antiretroviral drugs and would not require an initially healthy immune system, as vaccines do. “The challenges that remain include how to efficiently differentiate iPSCs to be engraftable hematopoietic stem cells to produce cells resistant to HIV, as well as safety issues of using iPSCs,” said Ye. Such issues include potential genetic abnormalities and tumorigenicity linked with iPSCs.

Experts worry that using nucleases to target the CCR5 gene could also have potential “off-target” effects that might influence the expression of other genes. “All engineered nucleases exhibit some degree of off-target gene disruption, typically at highly homologous DNA sequences,” said Cannon. “But since multiple different nucleases can be tested for any given target gene, there’s no reason why high-performing reagents can’t be identified that are also clinically very safe.”

Picker noted that a cure could theoretically be achieved with transplantation of a patient’s own stem cells that express altered CCR5, although there is the possibility that the Berlin patient’s cure derived in part from the graft vs host response that occurs in transplantation of cells from a donor. “From a practical perspective, though, this approach is a bit suspect, because stem cell transplantation is quite risky,” he said, adding that the potential for problems is likely higher than what patients taking modern antiretroviral therapy might face over their lifetime. “Will this approach be tried in people? I both expect and hope it will, as we will learn an enormous amount,” he said. “Will this approach solve the HIV/AIDS problem? Probably not.”

Researchers are also interested in targeting the CXCR4 gene as an anti-HIV strategy. Although blocking or inactivating either CCR5 or CXCR4 with drugs or genetic manipulation protects cells against infection by viruses that exclusively use the targeted coreceptor, targeting only 1 could favor evolution of a population of HIV that relies on the other coreceptor. Therefore, June and his colleagues have used zinc-finger nucleases to simultaneously modify CCR5 and CXCR4 in human CD4 cells. When these modified cells were administered to humanized mice, they were protected from infection with CCR5- and CXCR4-using HIV-1 strains (Didigu CA et al. Blood. 2014;123[1]:61-69).

While targeting both receptors may indeed be a better strategy than targeting CCR5 alone, researchers previously noted that a minority of the HIV-1 viruses in the Berlin patient’s body before he underwent transplantation was predicted to use CXCR4, but this viral population did not thrive after the patient stopped taking antiretroviral therapy. Investigators recently demonstrated that such minority viruses were unable to rebound because they relied on CCR5 for replication (Symons J et al. Clin Infect Dis. doi: 10.1093/cid/ciu284 [published online: April 23, 2014]). Therefore, CCR5 may be more than simply a door that opens to allow HIV entrance into a cell.

Despite CCR5’s importance for the survival of HIV within hosts, CCR5 mutations are not the whole story regarding immunity to infection. Because some individuals who are able to overcome and clear HIV infection have 2 perfectly normal copies of CCR5, other genes also play key roles, and scientists are busy working to identify them (O’Brien SJ, Hendrickson SL. Genome Biol. 2013;14[1]:201).

“Understanding the genetic differences that lead to different outcomes when people are exposed to or infected by HIV-1 is an incredibly exciting area of research,” said Cannon. “And just as CCR5 zinc-finger nucleases have been developed to recreate the genetic advantage of the Berlin patient’s donor, gene therapists will continue to try to turn any such discoveries into new treatments.”

Figures

Place holder to copy figure label and caption

Graphic Jump LocationImage not available.

HIV can invade host cells by binding to the CD4 receptor on the host cell surface and to a coreceptor, CCR5. Preventing expression of CCR5 may help patients with HIV control the infection.

Tables

References

Letters

CME
Meets CME requirements for:
Browse CME for all U.S. States
Accreditation Information
The American Medical Association is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians. The AMA designates this journal-based CME activity for a maximum of 1 AMA PRA Category 1 CreditTM per course. Physicians should claim only the credit commensurate with the extent of their participation in the activity. Physicians who complete the CME course and score at least 80% correct on the quiz are eligible for AMA PRA Category 1 CreditTM.
Note: You must get at least of the answers correct to pass this quiz.
You have not filled in all the answers to complete this quiz
The following questions were not answered:
Sorry, you have unsuccessfully completed this CME quiz with a score of
The following questions were not answered correctly:
Commitment to Change (optional):
Indicate what change(s) you will implement in your practice, if any, based on this CME course.
Your quiz results:
The filled radio buttons indicate your responses. The preferred responses are highlighted
For CME Course: A Proposed Model for Initial Assessment and Management of Acute Heart Failure Syndromes
Indicate what changes(s) you will implement in your practice, if any, based on this CME course.

Multimedia

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging & repositioning the boxes below.

Articles Related By Topic
Related Collections
JAMAevidence.com