CRISPR-Cas9 genome editing in
human cells occurs via the Fanconi Anemia pathway
The genome editing
tool CRISPR-Cas9 has the potential
to treat of range of genetic diseases.
Scientists have learned how Cas9
locates and cuts a specific sequence of
DNA but that's only half the equation.
Once the cut is made we need to
understand how the DNA break is repaired
in order to effectively add, subtract, or
change the genetic letters.
Now Jacob Corns' lab at UC Berkeley shines a light
into this black box and reveals how a
little-known DNA repair pathway could play a big role in the success of genome editing therapeutics.
After Cas9 makes a cut in the cell's DNA, the call can repair
the break in a variety of ways. One
repair method involves patching in
double-stranded DNA that's been added by
the experimenter. This DNA is designed to
have long ends that match the sequences
at the cut site telling the cell where it
needs to go. A different pathway called
single stranded template repair or SSTR
allows the cell to insert
single-stranded DNA with shorter
matching ends. Experiments show that
cells can insert single-stranded DNA at
cut sites more efficiently than
double-stranded DNA. But how?
To answer these questions a scientist in
the Corn lab named Chris Richardson
turned off specific genes across the
human genome to see if any of them were
essential for single strand insertion.
Richardson observed that when he turned
off genes involved in the Fanconi
anemia pathway the cell was unable to
repair the Cas9 brake using single-stranded DNA. Further experimentation
reveals that FA proteins migrate to the
site of the Cas9 brake and act like a
traffic signal that directs repair
pathways like SSTR. Understanding the
role of the FA pathway in repairing
Cas9 brakes give scientists an upper
hand in developing genome editing
therapies. By controlling the traffic
signal, scientists may be able to direct
DNA repair towards the insertion of
healthy DNA with SSTR instead of random
mutation a choice that is currently
difficult to control but essential for
one day curing genetic diseases.
The black box of DNA repair mechanisms just
got a bit brighter.
