Gene editing gets CRISPR – In the news

 4th September, 2015

Many techniques used in molecular biology and gene technology are based on naturally occurring processes using normal functioning enzymes. You may already be familiar with the way that restriction enzymes, made by bacteria to cut viral DNA, are employed in recombinant (cut and paste) DNA technology. Well, here is a hot new method, modelled on another bacterial protection mechanism, which is being applied as a gene editing method. This method relies on principles you might be familiar with: complementary base pairing, nuclease enzymes and RNA.

What does it mean to edit a gene?

Gene editing means to add, replace, repair or disrupt a gene sequence. Gene editing might be used to replace mutant genes that cause inherited fatal disorders like Tay Sachs disease or haemophilia, to disable faulty gene regulators, or to remove viral DNA that has been inserted into the genome. Gene editing might also provide an alternative to making genetically modified organisms (GMOs) for agriculture.

A crisp new method

This gene editing method is called the CRISPR/Cas9 method. It is based on a naturally occurring bacterial protection system. CRISPR stands for clustered regularly interspaced short palindromic repeatswhat a mouthful! You don’t need to remember it. CRISPR regions provide a nucleic acid sequence to target a particular gene. Cas9 is a nuclease enzyme that teams up and works with CRISPR regions. Nucleases cut DNA.

How do bacteria naturally use the system?

CRISPR regions of the bacterial genome contain DNA that originated from an invading bacteriophage (bacterial virus). In bacteria, CRISPR DNA is transcribed into RNA which is complementary to the bacteriophage DNA. When this RNA teams up with Cas9, it acts as a guide to recognise the virus when it invades again. When the RNA-enzyme complex finds its target bacteriophage DNA, it binds by complementary base pairing and the nuclease enzyme cuts the DNA, preventing the virus from replicating (Figure 1). Neat.

Figure 1 A simplified view of the way bacteria use the CRISPR method with guide RNA and Cas9 nuclease enzyme to protect against bacteriophage invasion

Figure 1 A simplified view of the way bacteria use the CRISPR method with guide RNA and Cas9 nuclease enzyme to protect against bacteriophage invasion

How do scientists use the system?

Scientists can create guide RNA of whatever precise sequence they want to target a particular gene, like the mutations that cause Tay Sachs disease, haemophilia and many other inherited disorders. So, with a custom made CRISPR-style guide RNA teamed up with Cas9, it is possible to target a gene for removal, disruption or replacement with another gene introduced at the same time (Figure 2).

For example, it may be possible to cure people of HIV infection. The human immunodeficiency virus (HIV), a retrovirus, inserts its DNA into the genome of infected people; it can continually reactivate causing long term infection leading to AIDS (acquired immune deficiency syndrome). Scientists are working with CRISPR/Cas9 to cut HIV DNA sequences out of human cells in the laboratory.  See the video clip at http://www.umassmed.edu/news/news-archives/2015/04/editing-hiv-out-of-our-genome-with-crispr/ . Perhaps, one day this can be used in the clinic.

Figure 2 A specific guide RNA is added to cells. It directs a nuclease enzyme to the target gene for editing

Figure 2 A specific guide RNA is added to cells. It directs a nuclease enzyme to the target gene for editing

Did you read our other news item about a method for preventing transmission of mitochondrial disease? Scientist are aiming to use modifications of gene editing techniques like CRISPR/Cas9 to replace faulty genes; they would add the correct gene into the cell at the same time as the guide RNA/Cas9 team is cutting out the defective one. This may provide another way to counter inherited mitochondrial diseases. If it works there would be no need to find egg donors.

The best thing since ...

Some scientists have described the CRISPR/Cas9 method as the most important new method in molecular biology since the advent of PCR, the polymerase chain reaction, in the 1980’s which revolutionised DNA research and technology. That is partly because it can rapidly edit genes in a cell or organism without changing the genome or creating genetically modified organisms. A challenge with all potential genetic therapies is how to get the correct gene into the relevant cells in the body. Treating embryos is one way, but that carries ethical concerns. There have already been claims that a research group has used CRISPR technology to edit genes in human embryos. Groups of scientists have gathered together to urge colleagues not to edit human genes before the method is fully tested for safety and ethical issues have been fully considered and regulations are in place. Concerns include unpredictable effects on future generations if the method is applied to embryos or reproductive cells

Author - Fran Maher

 

Further reading and links

The Scientist gives a guide: www.the-scientist.com/?articles.view/articleNo/39239/title/A-CRISPR-Fore-Cas-t/

Nature gives a detailed guide: www.nature.com/news/crispr-1.17547

Australian Popular Science: For The First Time, Researchers Edit The Genes Of Human Embryos:www.popsci.com.au/science/medicine/for-the-first-time-researchers-edit-the-genes-of-human-embryos,403156  24/04/15

Editing HIV out of our genome with CRISPR, Jim Fessenden and Bryan Goodchild, University of Massachusetts: www.umassmed.edu/news/news-archives/2015/04/editing-hiv-out-of-our-genome-with-crispr/

Ethics & Regulation: www.nature.com/news/don-t-edit-the-human-germ-line-1.17111?con&dom=pscau&src=syndication