Genome Editing using CRISPR/Cas
What is a genome?
A genome is the complete set of genetic information needed to make and maintain all living things. It is made of DNA (deoxyribonucleic acid) which contains four different chemicals represented by the letters A,T,C and G. These letters are arranged in a particular order to provide instructions for producing the building blocks (proteins) that make up our bodies. These instructions are known as genes, and the order of the letters is known as the DNA sequence. The DNA sequence is unique to every person and makes us who we are.
What is genome editing?
Genome editing is when the order of the four chemical letters that make up DNA, or the DNA sequence, is changed. This can involve adding, replacing or removing a single letter or a section of DNA.
How is genome editing done?
Genome editing can be done using a number of different techniques that have been developed by scientists. A recent technique, known as CRISPR/Cas, is the most powerful approach to date. It is fast, accurate, simple to use and relatively cheap compared to other methods.
How does CRISPR/Cas work?
The technique uses two key components that work together to identify and cut the DNA at a precise location in the genome. The first part is a small piece of RNA (very similar to DNA) which determines where the DNA sequence will be cut. The RNA does this by finding and attaching itself to the corresponding DNA sequence. The second part is an enzyme called ‘Cas9’ which is guided by the attached RNA to the same location in the DNA sequence. Here it acts like a pair of molecular scissors to cut the DNA. The DNA is then repaired by the cell, allowing the DNA sequence to be changed or replaced.
What could it be used for?
CRISPR/Cas could be used to prevent or treat many genetic diseases in the future. This would involve replacing a faulty gene that causes a disease with a healthy gene, or changing a gene so that it behaves differently. This could be done in human somatic (non-reproductive) cells or in germline cells (eggs and sperm). This has ethical implications as any changes that are made in germline cells will be passed from generation to generation.
What research is being done?
CRISPR/Cas is being used in research studies to find out what genes do and how they might be involved in disease. Understanding more about this process is important as it could help in the development of new treatments.
Many research groups around the world are also using genome editing techniques including CRISPR/Cas to try and correct genetic defects associated with a number of different medical conditions. For example, US scientists recently published results of a research study showing that CRISPR/Cas can be used in early human embryos to correct a faulty gene that causes a certain kind of heart disease.
Is it being used to treat patients?
CRISPR/Cas and other genome editing techniques have been used in somatic cells to treat patients in a small number of exceptional cases. It is likely to be many years before it is routinely used as a treatment.
It has not been used as a treatment in germline cells as this is currently illegal in the UK (and most other countries).
Are there any risks?
There are potential risks with the use of CRISPR/Cas9. One of these is the risk of ‘off target’ effects, where changes are made to genes other than those intended to be changed. This is one reason why genome editing in germline cells is currently illegal in the UK. The use of the technique in humans will not be routinely allowed until it is shown to be safe for the individuals being treated, as well as future generations.
Could it be used to treat Mitochondrial Diseases?
CRISPR/Cas has the potential to prevent or treat Mitochondrial Diseases caused by genetic defects (mutations) in the nuclear DNA.
It is unclear whether CRISPR/Cas could be used for Mitochondrial Diseases caused by mutations in mitochondrial DNA (mtDNA). This is because it is not yet known whether both components needed for CRISPR/Cas to work can readily cross the membranes surrounding the mitochondria to reach the mtDNA. Further research is needed to address this.