Miranda Smith
Making the cut: CRISPR gene editing and HIV
CRISPR/cas9 is a gene-editing strategy acting like a targeted pair of scissors. The technology has generated a lot of excitement, especially in the field of HIV. This is because it can target specific genes, and potentially remove HIV from infected cells. A recent study has been widely reported this month as bringing us one step closer to an HIV cure (see for example here and here). In this article, we look at what the study shows and what it means for people living with HIV.
What is all the fuss about?
Researchers from the Temple University Lewis Katz School of Medicine and University of Pittsburgh have published a paper in the journal Molecular Therapy. Associate Professor Wenhui Hu, Professor Kamel Khalili and Assistant Professor Won-Bin Young led the new work. It builds on previous studies using CRISPR/Cas9 to get rid of HIV in test tube models which we covered last year. This time, the group have used a modified CRISPR/Cas9 system to tackle HIV in three different mouse models of infection. This is not the first time the strategy has been used in mice. The same group published a proof-of-concept study last year showing that CRISPR/Cas9 could be used in both mice and rats. The new study explores a new delivery system to get the CRISPR/Cas9 into cells. It also provides evidence for the delivery of four separate gene targets in the one package.
What did the study involve?
The recent study uses CRISPR/Cas9 in three different mouse models of HIV infection. The first is the same model containing harmless HIV the team used last year (Tg26 transgenic mice, with each cell containing multiple copies of a crippled HIV). The other two types of mice involve infectious HIV. One of these models (NCr strain of nude mice infected with an HIV reporter virus) mimics the early phase of HIV infection. The other model (humanized bone marrow/liver/thymus (BLT) mice infected with a slightly different HIV reporter virus) imitates chronic HIV infection with latency. The treatment of all three types of mice resulted in reduced integrated HIV. This means that CRISPR/Cas9 converted some HIV positive cells into HIV negative cells. The researchers looked for HIV in different body parts, including the liver, lymph nodes, lungs and blood. These results show the promise of this approach for getting rid of HIV from infected cells. Aiming for more than one sequence at a time may make it harder for resistance to develop. Last year, we explained how easily resistance can develop and undermine efforts to get rid of the virus.
What does this mean for people with HIV?
This work provides some important evidence for treating HIV with CRISPR/Cas9. The results will inform proof-of-concept work in other animal models and, eventually, in humans. A lot of questions remain, however:
- Mouse models of disease are highly artificial, so the same approach in humans may have a different result. Would the same delivery system work as well in humans?
- HIV is hard to find in people on treatment. One big question is if researchers could use this strategy to reach the few HIV infected cells in people on effective treatment.
- Can this strategy reduce HIV enough to be useful? It doesn’t take long for undetectable HIV to rebound to high levels if treatment stops. This means that even tiny amounts of HIV can have a big impact.
- What are the side effects of this strategy? Are there any unintended effects on other genes and processes?
Click here for a great review of the study.
Click here for the original press release.
What is CRISPR/cas9?
CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. This basically means a bunch of short, repeated gene sequences. Researchers first found CRISPR in the genomes of bacteria. Initially, the sequences were a puzzle. People then worked out that the sequences in between the repeats matched viruses that infected those bacteria. The gene snippets were basically a catalogue of viruses stockpiled for later reference.
The second component of this mechanism are the CRISPR-associated proteins (Cas). Cas are enzymes that cut DNA at a precise spot. Cas enzymes make copies of the virus sequences spread within the CRISPR region. The Cas enzyme carries one of these virus copies and patrols the cell for a match. When a match occurs, the virus copy binds to the actual virus and the Cas enzyme activates and snips the virus in half.
This simple system can be harnessed by scientists to snip any sequence of their choice. By giving the Cas enzymes a sequence for a specific gene, the Cas can target that gene and cut it.
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