Snipping Out HIV

Someday, editing our genome could be as simple as cut-and-paste, writes Tang Yew Chung.

AsianScientist (Apr. 4, 2015) – Imagine being able to edit your genome at will, replacing faulty sections of disease-causing genes and freeing yourself from life-threatening genetic disorders. Cancer-causing oncogene? Snip, it’s gone. Sounds far-fetched, but this may be possible in the not-too-distant future, thanks to gene editing techniques that have taken the genomics world by storm.

Although the concept of using gene therapy to treat genetic disorders has been around for more than 40 years, limitations in gene editing and gene delivery technologies have hampered progress toward its application in the clinic. Most gene therapies work by inserting a functional, or partially functional, copy of a gene into the patient’s cells. The problem persists, however, as the original faulty copy of the gene is still present, preventing the error from being fully corrected.

Newer gene editing techniques solve this problem by completely eliminating the errant gene that causes disease. Zinc finger nucleases (ZFNs), which are enzymes composed of a nuclease, introduce double stranded breaks in the DNA that completely remove the problematic gene. In addition, they work in tandem with DNA binding proteins known as zinc fingers to introduce editing specificity.

Outsmarting HIV

In a Phase II clinical study published in the New England Journal of Medicine in March 2014, researchers used ZFNs to edit white blood cells taken from human immunodeficiency virus (HIV)-positive patients. Here, the CCR5 gene, which produces a binding site on white blood cells that HIV latches onto, was rendered completely dysfunctional. Not only did the technique prove safe and effective, HIV positive patients who received the edited cells also became resistant to the virus.

Although ZFN-based approaches are already undergoing clinical trials, a newer editing technique is on the horizon. Called CRISPR/Cas system, it stands for of two components that make up its name: clustered regularly interspaced short palindromic repeats (CRISPR), which are unusual repeating DNA sequences, and CRISPR-associated genes (Cas). First discovered three years ago in bacteria as a form of adaptive immunity against viruses, CRISPR is much cheaper, more convenient and more flexible than zinc finger techniques.

Instead of using DNA-binding proteins to introduce specificity, CRISPR uses guide RNA. Hence, researchers can use the same enzyme, Cas9, to edit DNA sequences by simply using different RNA partners, rather than developing a new enzyme for each DNA sequence.

In a sign that CRISPR is rapidly catching up to more established genome editing technologies, scientists have used it to remove the entire HIV genome from human cell lines. The study, published in the Proceedings of the National Academy of Sciences in July 2014, also found that the edited human cells were resistant to subsequent HIV infection.

Gene editing in stem cells

If gene editing isn’t cool enough, pair it up with stem cells. Scientists have used CRSIPR to edit induced pluripotent stem (iPS) cells, which are adult cells that have been reprogrammed to behave like stem cells. Conceptually, the idea is simple: extract skin cells from a patient and turn them into patient-specific stem cells that can be edited at will. Once re-introduced into patients, the edited stem cells undergo cell division, producing millions of stem cells with the functioning gene and eliminating further need for gene therapy.

In the laboratory, scientists have used CRISPR to correct the defective gene responsible for beta-thalassemia, a common blood disorder caused by mutations in the hemoglobin gene. In a study published in the journal Genome Research in August 2014, researchers successfully reintroduced a functional hemoglobin gene into iPS cells derived from a patient with beta-thalassemia.

To snip or not to snip

Despite the flurry of research papers on this technology, we are still far from completely eliminating hereditary genetic disorders. In an early preclinical study, CRISPR reversed the symptoms of tyrosinemia (a rare but fatal liver disorder) in mice. Unfortunately, the treatment required a high-pressure injection to deliver the components of CRISPR into the bloodstream, a method that is not applicable clinically.

Limitations aside, CRISPR looks certain to transform the field of biology much like how restriction enzymes did in the 1970s. There are already commercial partners on board. US-based Taconic Biosciences, an international supplier of genetically modified rodents, has already made CRISPR/Cas-expressing mice commercially available in what is possibly one of the fastest research commercialization efforts to date.

Editing the genome may not yet be as easy as editing a word document on your laptop, but recent gene editing technologies have made it easier than ever before. Although major challenges in patient delivery and both ethical and safety concerns remain to be addressed, CRISPR is definitely the genome editing technology to watch.


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Yew Chung is a postdoctoral research fellow at the Duke-NUS Graduate Medical School, Singapore.

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