As reported in
Nature, scientists have generated the first monkeys harboring specific mutations, through the
use of CRISPR-mediated gene editing.
This breakthrough promises to produce better models for studying human disease. CRISPR/Cas9 technology for precise gene editing has already proven successful in other species including in mice, C. elegans, Xenopus tropicalis, and plants.
Most traditional methods for gene targeting require the introduction of a transgene cassette via homologous recombination. Recombination occurs rarely, so these methods typically use many individual animals – which is challenging to accomplish with non-human primates for both ethical and logistical/budgetary reasons. Successful recombination must be followed by selective breeding, which is very slow in primates compared to other model organisms; monkeys take years to reach reproductive age – compared to only weeks in mice or days in flies.
Viral vectors allow highly efficient transgene delivery, and have been used successfully in primates – including human patients receiving gene therapy! However, it is difficult with viral vectors to control the number of copies inserted into the genome and the location of those gene insertions – which may be okay when you just want to express a therapeutic gene in a specific cell type in an adult individual, but creates challenges when you want to make a heritable change to the genome in order to propagate a transgenic strain of laboratory animals for reproducible long-term experimental use. Without precise control over the insertion site, virally-mediated gene delivery can have unintended consequences by altering the expression of off-target genes whose coding or regulatory sequences get interrupted, and copy-number variation can create major challenges in interpreting experimental results.
CRISPR allows precise targeting of gene mutations – including deletions, alterations, or insertions of novel DNA sequences and is much faster and simpler than earlier techniques such as TALEN, Zinc-Finger nuclease, and Cre-Lox-mediated recombination. Because they use synthetic guide RNA complementary to the targeted sequence, CRISPR-associated nucleases like Cas9 have a much lower rate of off-target effects than viral vectors.
The CRISPR method relies upon customized gene constructs that encode a codon-optimized Cas9 nuclease and a synthetic guide RNA for precise targeting.
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