Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) are a group of DNA sequences found in bacteria that form the basis for a powerful genome editing tool that enables scientists to edit genes and manipulate gene expression by making specific changes to DNA sequences. Jennifer Doudna and Emmanuelle Charpentier were awarded the Nobel Prize in 2020 for their work on CRISPR technology. Using CRISPR, genes can be turned on/off or substituted with a desired gene. Essentially, it empowers researchers to rewrite the genetic code.

 

CRISPR technology draws upon a naturally occurring anti-viral response that was discovered in bacteria. When infected with viruses, bacteria capture snippets of DNA from the bacteriophage and insert them into their own DNA to create a segment called CRISPR. CRISPR basically consists of short identical palindromic repeats of DNA that are interspaced with spacer DNA. The spacer DNA has sequences that match the bacteriophage DNA. When the bacteria get invaded by the same virus, the CRISPR region of bacteria produces RNA segments with sequences complimentary to the DNA of virus. Associated with CRISPR are the Cas (CRISPR associated) genes that make Cas protein/enzyme. The Cas complex (Cas protein and RNA) then binds to the viral DNA. The Cas 9 enzyme in bacteria is an endonuclease that cuts the viral DNA apart thereby destroying the invading virus.

 

Scientists have utilized this immune defense system to edit DNA. A guide RNA is designed which consists of crRNA (CRISPR RNA) and tracrRNA (trans-activating CRISPR RNA). The crRNA has sequences complimentary to the target DNA. Guide RNA then guides the Cas 9 enzyme to the target site on the DNA that has a complementary sequence. The Cas 9 enzyme will not cleave the target DNA unless there is an adjacent PAM (Protospacer Adjacent Motif) sequence. The Cas enzyme acts as a pair of molecular scissors thereby creating a double stranded break in the DNA. Once the DNA is cut, scientists harness the cell’s own DNA repair mechanism to add/delete or substitute the existing sequence. DNA repair can occur by Homology Directed Repair (HDR) or by Non-Homologous End Joining (NHEJ).

CRISPR is a novel technology with immense potential for genome editing with many applications in basic research, development of new therapies, and in human disease diagnostics. It is already being used to prevent and treat various diseases, including cancer, sickle cell anemia, Huntington’s disease, cystic fibrosis, Alzheimer’s disease, and hemophilia. In addition, CRISPR is widely used in diagnosis and detection of infectious diseases and cancer. CRISPR can be used to edit multiple genes simultaneously and is cheaper, faster, and more efficient than other gene editing technologies.