What is the science behind CRISPR-Cas9?

CRISPR-Cas9, a gene-editing technique that can target and modify DNA with groundbreaking accuracy, is both the newest darling and the newest villain of genetics research. Invented in 2012 by scientists at the University of California, Berkeley, CRISPR-Cas9 has received a lot of attention this year. Not only are scientists publishing reports on the technique at breakneck speed, but it also seems that each piece of news that comes out about CRISPR-Cas9 is grander and juicier than the last.

Recently scientists have announced that they have used the new technology to inhibit hepatitis C in human cells and to defy Mendel’s laws of inheritance, which have governed the field of genetics for over a century. Other highly-publicized attempts to eradicate a disease-causing gene in human embryos were met with limited success. The experiment, which constituted the first time scientists have reported trying to genetically engineer humans at the reproductive level, triggered concerns from an international community of scientists and ethicists.

Now, CRISPR-Cas9 research has reached the point where scientists are considering how the technology will fit into the future of mankind.

What is

Scientists in Japan were the first to discover CRISPR in the DNA of bacteria in 1987. In their attempts to study a particular protein-encoding gene in E.Coli, the researchers noticed a pattern of short, repeating, palindromic DNA sequences separated by short, non-repeating, “spacer” DNA sequences.

Over the next five years, researchers realized that these repeats were present in many bacteria and other single-celled organisms. In 2012, scientists coined the term CRISPR, short for “clustered regularly interspaced short palindromic repeats,” to describe the pattern.

For another decade, scientists hammered out the details of CRISPR. They figured out that the repeating DNA patterns, along with a family of “Cas” (CRISPR-associated) proteins and specialized RNA molecules, play a role in bacterial immune systems. They deemed the entire complex of DNA repeats, Cas proteins, and RNA molecules as the CRISPR-Cas system.

Here’s how CRISPR-Cas works in bacteria: When bacteria encounter an invading source of DNA, such as from a virus, they can copy and incorporate segments of the foreign DNA into their genome as “spacers” between the short DNA repeats in CRISPR.

These spacers enhance the bacteria’s immune response by providing a template for RNA molecules to quickly identify and target the same DNA sequence in the event of future viral infections. If the RNA molecules recognize an incoming sequence of foreign DNA, they guide the CRISPR complex to that sequence. There, the bacteria’s Cas proteins, which are specialized for cutting DNA, splice and disable the invading gene.

In the fall of 2012, a team of researchers led by UC Berkeley scientists Jennifer Doudna and Emmanuelle Charpentier announced that they had hijacked the bacteria’s CRISPR-Cas immune system to create a new gene-editing tool. Their CRISPR-Cas9 system involved CRISPR, a Cas protein called Cas9, and hybrid RNA that could be programmed to identify, cut, and even replace any gene sequence. By the start of 2013, research applying CRISPR-Cas9 to genetic engineering was underway.