Medical Wikipedia : The Medical Encyclopedia
It is a genome editing tool. It is currently one of the fastest and most accurate
technique of editing DNA. The CRISPR-Cas9 system is a simplified version of the
antiviral defense system seen in bacteria. It is of great importance in
biotechnology and medicine due to high precision genome editing available for
The CRISPR-Cas9 system consists of two different molecules
CRISPR is “clustered regularly interspaced short palindromic repeats”. It is a
family of DNA sequences found in the genome of prokaryotes. This is a
primitive form of secondary immunity in the bacteria as CRISPR retains
information of previous infections by bacteriophage viruses and detect and
destroy bacteriophage DNA during subsequent infections.
It stands for “CRISPR associated protein 9”. It plays an important role in
immunity of bacteria against plasmids and DNA viruses. It cuts the foreign DNA
that enters the bacterial cell and nullifies its pathogenicity. This endonuclease
action is used to cut the bacterial cell genome at specific points.
The CRISPR-Cas9 system has high fidelity and is simple to make. It is also highly
specific. This specificity is due to the presence of a target sequence and
Protospacer Adjacent Motif (PAM) on the host genome. These two
components allow for binding of Cas9 protein for its action.
For recognition of the target sequence and binding to host DNA, there is a
need of guide RNA. The guide RNA is called CRISPR RNA (crRNA). This crRNA
binds with tracrRNA (trans-acting CRISPR RNA). The tracrRNA, being partially
complementary to crRNA, forms a RNA duplex, which is the active complex
that is involved in guiding the Cas9 protein to the site in the DNA where editing
is necessary. In some cases, both crRNA and tracrRNA are packaged together
to give a single guide RNA (sgRNA).
The Cas9 protein recognizes the PAM sequence (to differentiate bacterial self-
DNA from foreign DNA, binds to the host DNA with the help of crRNA and
forms either a single or double strand break based on the variant of Cas9.
The delivery of Cas9, guide RNA and other essential components into the host
cell is done by viral or non-viral vectors. This is called Transfection. Depending
on the type of cell, the difficulty of transfection varies. For most cells, chemical
transfection via lipids and peptides is enough to complete delivery. But for
some cells like stem cells and neurons, more efficient delivery systems are
required (eg: lentivirus, adenovirus and adeno-associated virus).
CRISPR-Cas9 system for treatment
1. CRISPR-Cas9 system has been proposed as a treatment for multiple
diseases like cancer, Hutchinson-Gilford Syndrome, Huntington’s
disease, Cystic Fibrosis, Duchenne Muscular Dystrophy etc.
2.CRISPR-Cas-based "RNA-guided nucleases" can be used to target genes
encoding antibiotic resistance. This provides protection of
bacteria against invading foreign DNA, like transposons,
bacteriophages, and plasmids. This was a strong
selective pressure for acquiring antibiotic resistance and
virulence factor in bacterial pathogens. It is seen to have limited
replication of different herpes viruses and also proven to be effective in
eradicating viral DNA of Epstein-Barr virus.
CRISPR has shown effectiveness in treating glioblastoma and metastatic
3. CRISPR has shown effectiveness in treating glioblastoma and metastatic
ovarian cancer in animal models. As of now, only one clinical trial was
done in the US, which involved treatment of Lung cancer by
immunotherapy. It aimed to do so by extracting T-cells from the patient
and making 4 genetic modifications to the cells to better detect the
cancer cells and kill them more efficiently.