What is CRISPR?

 

CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats

Repetitive DNA sequences, called CRISPR, were observed in bacteria with “spacer” DNA sequences in between the repeats that exactly match viral sequences. It was subsequently discovered that bacteria transcribe these DNA elements to RNA upon viral infection. The RNA guides a nuclease (a protein that cleaves DNA) to the viral DNA to cut it, providing protection against the virus. The nucleases are named “Cas,” for “CRISPR-associated.” 

Genome editing

In 2012, researchers demonstrated that RNAs could be constructed to guide a Cas nuclease (Cas9 was the first used) to any DNA sequence. The so-called guide RNA can also be made so that it will be specific to only that one sequence, improving the chances that the DNA will be cut at that site and nowhere else in the genome. Further testing revealed that the system works quite well in all types of cells, including human cells.

Implications

With CRISPR/Cas, it’s easy to disrupt a targeted gene, or, if a DNA template is added to the mix, insert a new sequence at the precise spot desired. The method has profoundly changed biomedical research, as it greatly reduces the time and expense of developing animal models with specific genomic changes. JAX scientists now routinely  Advancing therapeutic gene editingNIH awards $3.5M to JAX under multi-institution Somatic Cell Genome Editing program.use the CRISPR/Cas system for this purpose in mice. And for human diseases with a known mutation, such as cystic fibrosis, it’s theoretically possible to insert DNA that corrects the mutation. There are clinical applications in human trials now, including for engineering T cells outside of the body for CAR-T cancer therapy and for editing retinal cells for leber’s congenital amaurosis 10, an inherited form of blindness.

Limitations

CRISPR/Cas is an extremely powerful tool, but it has important limitations. It is:

  • difficult to deliver the CRISPR/Cas material to mature cells in large numbers, which remains a problem for many clinical applications. Viral vectors are the most common delivery method.
  • not 100% efficient, so even the cells that take in CRISPR/Cas may not have genome editing activity.
  • not 100% accurate, and “off-target” edits, while rare, may have severe consequences, particularly in clinical applications.

Ethical issues

In addition to editing somatic cells (the cells that make up most of the body), it’s possible to edit the genomes of gametes (eggs and sperm) and early embryos, called germline editing. Any such edits in humans would not only affect an individual but also his or her progeny. They could also theoretically be used to enhance desirable traits instead of curing disease. Scientists have therefore called for a moratorium on human germline editing until the serious ethical and societal implications are more fully understood. JAX and its researchers fully support and adhere to the moratorium.