The discovery of CRISPR/Cas9 is one of the biggest scientific breakthroughs in recent memory. This technology, first discovered in microbes, enables the cleaving of DNA in order to remove or replace existing genes. For a crash course into the history of CRISPR/Cas9 discovery and development with primary sources please refer to the footnote*. This technology will enable us cure many genetic diseases. Work is currently being done to harness viruses that infect specific tissues to provide specificity for where the genetic editing will take place.

There are many questions surrounding the development of this new tool. How often is a site cut that wasn’t meant to be? How do we deliver CRISPR/Cas9 most effectively to ensure it gets to where we want it? How will the body react to CRISPR/Cas9 being present in our cells? This last question was tackled recently by a group from Stanford University. They discovered humans have preexisting immunity to the proteins most commonly used to cleave DNA by CRISPR/Cas9. Charlesworth et al. collected patient derived samples, and checked to see how the samples responded to two different Cas9 proteins. The team found that 79% and 65% of healthy patients already had antibodies against the two different proteins. Moreover, one of the Cas9 proteins was able to elicit a response that could lead to the degradation of host tissues.

Humans have such a high prevalence of preexisting immunity because of how common infections are from the bacteria harboring the Cas9 proteins most widely used. Studies ( Colque-Navarro et al. 2010, Mortenson et al. 2015) have shown that 100% of patient samples were positive for antibodies against these bacteria. Together, these studies bring to light some of the difficulties in using naturally occurring proteins for gene therapy. Reengineering Cas9 to make it less detectable by the immune system, or using Cas9 proteins from an organism not known to infect humans, could help ameliorate this current predicament. The CRIPSR/Cas9 story will be one to watch. If we can get it right, this system could be one of the greatest breakthroughs in medical history.

Managing Correspondent: Aaron Aker

Original article: Identification of Pre-Existing Adaptive Immunity to Cas9 Proteins in Humans

News coverage: You May Already Be Immune to CRISPR
How the immune system could stymie some CRISPR gene therapies
Immunity May Make CRISPR-Based Therapies Ineffective

* From the identification of DNA arrays within microbes and mitochondria that shared much of the same sequence(Mojica et al. 2000) to figuring out spacers between these shared sequences are mostly from foreign DNA(Bolotin et al. 2005) and can provide immunity to viral infection(Barrangou et al. 2007). Next, the function of the protein Cas9 was determined and could be used to cut DNA specifically with engineered RNAs, this was ground breaking(Jinek et al. 2012). In 2013, it was shown that Cas9 could cleave DNA in human cells specifically( Cong et al. 2013, Mali et al. 2013), opening the doors to using this microbe derived system for targeted genome engineering in humans. Many others contributed to this technology but these are some of the most formative works that figured out that it could be used for gene therapy. CRISPR/Cas9 gene therapy has already been shown to be effective in mice( Long et al. 2014) and the first clinical trial in humans was announced for sickle cell and β-thalassemia that will start this year, Vertex and CRISPR Therapeutics announcement.