You may have heard about CRISPR. It
probably brings up thoughts of GMOs and
DNA editing. And, well yes, in a simplified
explanation, CRISPR edits DNA. But how
does it work?
CRISPR stands for clusters of regularly
interspaced short palindromic repeats.
This refers to a series of DNA sequences
left behind from bacteriophage
infections. When faced with the pathogen
again, the bacteria used CRISPR
associated (Cas) enzymes to recognize
and destroy these viral DNA sequences.
These enzymes are able to do this with
CRISPR RNA (crRNA). This is the viral DNA that
was left behind from the infection that
has been transcribed into
single-stranded RNA. It acts as a
template for the Cas enzymes to
recognize foreign DNA so that they can
destroy it.
One such enzyme, known as Cas9, has been
re-engineered by scientists to work in
human cells.
CRISPR-Cas9 can be reprogrammed to
remove any piece of DNA by changing the
sequence of the crRNA that Cas9
binds to. Using this method, only two
components are needed to edit the genes:
the Cas9 enzyme and a guide RNA. The
main flaw with Cas9 is that once the
DNA is cut, the cell's natural repair
mechanisms will try to heal the broken
DNA. This can lead to mutations and other
changes within the genome. There are two
ways that the cell tries to heal the DNA.
The first is by gluing the two cuts back
together,
also known as "non-homologous end joining."
This method tends to be error prone
because nucleotides can be accidentally
inserted or deleted, which can disrupt
the gene. The other method that the cell
uses is filling the gap with a series
of nucleotides using a short strand of
template DNA. Scientists can provide the
DNA for the template, and thus rewrite
any gene and fix any mutation. Another
problem with Cas9 is that it can only
cut DNA and is unable to edit
single-stranded RNA. This is an issue
because most viruses are made of RNA and
very few produce DNA versions. However
another Cas enzyme is able to cut
strands of RNA. Cas13... Ahem, really?
You don't have to be such a show-off.
Cas13 functions similarly to Cas9
in that it uses a strand of guide RNA
to target a specific sequence of
nucleotides. However,
Cas13 has a few advantages. As
mentioned previously, it targets RNA
rather than DNA. In fact, it can use
multiple strands of guide RNA to target
a virus, making it hard for any part of
the virus to escape detection. However,
there still remains the problem of how
to inject Cas13 into a living human
being and get it to target a virus. Also,
viruses will evolve and eventually
develop resistance. But Cas13 has
another advantage here. When Cas9 cuts a
strand of DNA, the cell will try to
repair it, which may lead to mutations
that cause the virus to be more
resilient. But when Cas13 cuts RNA, the cell
does not have the ability to heal it.
Even if viruses do evolve, it is very
easy to reprogram Cas13 to attack the
mutated virus.
