The CRISPR-Cas system: the latest trend in gene modification

Article written by Rosa Martínez Corral.

In less than a decade, the field of genome engineering has been revolutionised by a series of techniques that now allow to accurately, efficiently and economically modify virtually any point in the genome of any organism. It all started in the early 1990s, when a pattern of repetitive sequences was observed in the DNA of certain bacteria. It was subsequently seen that many other types of bacteria also possess these patterns and, after years of research, these random observations have led to a revolution with strong implications both for research and therapy.

These repetitive sequences were part of a bacterial genomic region now known as CRISPR, or Clustered Regularly Interspaced Short Palindromic Repeats. They consist of repetitive DNA sequences that flank DNA fragments from past viral infections. Faced with a new infection by the same virus, the viral DNA fragment generates a complementary RNA molecule. This RNA guides its associated Cas proteins towards the viral genome, and these degrade the DNA of the virus. Therefore, this is a bacterial immune system that, like ours, retains a memory of past infections.

Left: The endogenous CRISPR-Cas system of bacteria. The schema shows how a bacterial genome acquires a fragment of viral DNA. Upon reinfection, it can generate a complementary RNA molecule that specifically guides a Cas protein towards the viral DNA, to degrade it and thus eliminate the infection. Right: The CRISPR-Cas system is used in the laboratory to modify specific points in the genome of organisms.
Left: The endogenous CRISPR-Cas system of bacteria. The schema shows how a bacterial genome acquires a fragment of viral DNA. Upon reinfection, it can generate a complementary RNA molecule that specifically guides a Cas protein towards the viral DNA, to degrade it and thus eliminate the infection.
Right: The CRISPR-Cas system is used in the laboratory to modify specific points in the genome of organisms.

 

But not only that, the CRISPR-Cas system has also proved a useful tool in the laboratory, leading to a series of powerful techniques to modify the DNA of organisms.  In the most widely-used version, an RNA molecule complementary to the region of the genome to be modified is designed in such a way that it guides a Cas protein to make a cut in that specific location. The cell then repairs itself, and depending on the aim and thus how the technique is applied, a mutation can either be repaired or introduced. The first is desirable, for instance, in the case where a genetic defect exists. On the other hand, in order to understand what the function of a genomic region is, scientists often make deleterious mutations to inactivate it and see what the consequence is.

The CRISPR-Cas system is, therefore, a simple and easily scalable tool that allows specific changes to be made to multiple parts of the genome, needing little time and few resources, something unprecedented in the field of genome engineering. For example, to inactivate a gene in a model organism, it was often necessary to cause random mutations and then select those of interest, involving a long and costly process. But with the CRISPR-Cas system this is no longer necessary. It has even been used to make genetic modifications in human embryos, opening new doors to the study of human development and the therapy of inherited genetic diseases.

But there are still problems to be solved. The most important of these is the fact that the system is not perfect and it can make cuts in unintended regions of the genome. It is also necessary to improve the methods that allow the Cas protein and RNA guide to be effective only in specific tissues, particularly if it is to be used as a therapeutic tool.

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