CRISPR/Cas9 genome editing is the future of medicine

Julia Eve Napolitano Jan 30, 2017

DNA is an incredible thing, carrying precious information in a small space to make you who you are — genetically, at least.

This double-helix polymer carries all the information required for growth, development, functioning, and reproduction for all living things. DNA is made up of four nucleotide bases that pair up with each other – adenine (A) and thymine (T), and guanine (G) and cytosine (C). In a hierarchical structure, the base pairs form DNA, and the complete set of DNA forms the genome.

In humans, the genome manifests itself in the form of 23 chromosomes, consisting of 3 billion base pairs. When your cells need to create proteins, the genetic information flows from DNA to RNA through transcription and then from RNA to protein through translation.

With such an important molecule, you may think that the information is set in stone and untouchable, but in the 1990s, scientists began looking into methods to purposefully affect the genome. When DNA is manipulated by inserting, deleting, or replacing segments in the genome, it is called genome editing.

In order to edit a genome, researchers have engineered nucleases, which are enzymes that can cut between the nucleotide base pairs to make smaller segments of DNA. There are currently four types of nucleases: zinc finger nucleases (ZFNs), transcription activator-like effective-based nucleases (TALEN), meganucleases, and the CRISPR-Cas9 system.

CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeat, and consist of short sequences of RNA (called CRISPR RNA or crRNA) that can guide a system to a specific, matching DNA strand.

The CRISPR-Cas9 method consists of Cas9, which is the enzyme that cuts the DNA into smaller sequences, and guide RNA (gRNA), a combination of crRNA and tracrRNA that helps guide Cas9 to the part of the genome that needs editing so it can cut in the right place.

Like DNA, RNA is also made up of four nucleotide bases that pair off — adenine (A) and uracil (U), and guanine (G) and cytosine (C). In order to locate a specific place in the DNA sequence of the genome, the gRNA will have complementary bases to the target DNA, thus only binding to the target sequence. The Cas9 enzyme then follows the gRNA to the target DNA and cuts it in two.

Of the four methods, CRISPR-Cas9 is the simplest, most versatile, and precise method, and was heralded as Science's 2015 Breakthrough of the Year. It can be used to target multiple genes at the same time, which sets this method apart from the rest.

CRISPR-Cas9 (and genome editing, in general) has a lot of potential to help treat medical conditions that are caused by genetics. These can include various types of cancers, hepatitis B, and even cholesterol, to name a few. In April 2015, A team from China attempted to alter the DNA of non-viable human embryos, which sparked wide-spread debate regarding the ethics of testing gene-editing technology on humans. Their study concluded that CRISPR was unfit for use in reproductive medicine. Last year, in October, CRISPR gene-editing was testing for the first time on a human subject, by scientists at Sichuan University in Chengdu, China. It was an attempt to treat a patient with aggressive lung cancer, according to an article in the science journal, Nature.

However, as with any emerging biological technology, there are ethical concerns that accompany this method. There are two types of cells in the body – somatic cells, which are non-reproductive cells, and germline cells, which are. DNA from germline cells is passed from generation to generation. While gene editing in somatic cells is currently relatively uncontroversial, gene editing in germline cells would be passed to future generations and is currently illegal in many countries. Regardless, scientists continue to make CRISPR-Cas9 even more accurate.