One of the most remarkable recent advances in biomedical research has been the
development of highly targeted gene-editing methods such as CRISPR that can
add, remove, or change a gene within a cell with great precision. The method
is already being tested or used for the treatment of patients with sickle cell
anemia and cancers such as multiple myeloma and liposarcoma, and today, its
creators Emmanuelle Charpentier and Jennifer Doudna received the Nobel Prize
in chemistry.
While gene editing is remarkably precise in finding and altering genes, there
is still no way to target treatment to specific locations in the body. The
treatments tested so far involve removing blood stem cells or immune system T
cells from the body to modify them, and then infusing them back into a patient
to repopulate the bloodstream or reconstitute an immune response—an expensive
and time-consuming process.
Building on the accomplishments of Charpentier and Doudna, Tufts researchers
have for the first time devised a way to directly deliver gene-editing
packages efficiently across the blood brain barrier and into specific
regions of the brain, into immune system cells, or to specific tissues and
organs in mouse models. These applications could open up an entirely new
line of strategy in the treatment of neurological conditions, as well as
cancer, infectious disease, and autoimmune diseases.
A team of Tufts biomedical engineers, led by associate professor Qiaobing
Xu, sought to find a way to package the gene editing "kit" so it could be
injected to do its work inside the body on targeted cells, rather than in a
lab.
They used lipid nanoparticles (LNPs)—tiny "bubbles" of lipid molecules that
can envelop the editing enzymes and carry them to specific cells, tissues,
or organs. Lipids are molecules that include a long carbon tail, which helps
give them an "oily" consistency, and a hydrophilic head, which is attracted
to a watery environment.
There is also typically a nitrogen, sulfur, or oxygen-based link between the
head and tail. The lipids arrange themselves around the bubble nanoparticles
with the heads facing outside and the tails facing inward toward the center.
Xu's team was able to modify the surface of these LNPs so they can
eventually "stick" to certain cell types, fuse with their membranes, and
release the gene-editing enzymes into the cells to do their work.
Making a targeted LNP takes some chemical crafting.
By creating a mix of different heads, tails, and linkers, the researchers
can screen— first in the lab—a wide variety of candidates for their ability
to form LNPs that target specific cells. The best candidates can then be
tested in mouse models, and further modified chemically to optimize
targeting and delivery of the gene-editing enzymes to the same cells in the
mouse.
"We created a method around tailoring the delivery package for a wide range
of potential therapeutics, including gene editing," said Xu. "The methods
draw upon combinatorial chemistry used by the pharmaceutical industry for
designing the drugs themselves, but instead we are applying the approach to
designing the components of the delivery vehicle."
In an ingenious bit of chemical modeling, Xu and his team used a
neurotransmitter at the head of some lipids to assist the particles in
crossing the blood-brain barrier, which would otherwise be impermeable to
molecule assemblies as large as an LNP.
The ability to safely and efficiently deliver drugs across the barrier and
into the brain has been a long-standing challenge in medicine. In a first,
Xu's lab delivered an entire complex of messenger RNAs and enzymes making up
the CRISPR kit into targeted areas of the brain in a living animal.
Some slight modifications to the lipid linkers and tails helped create LNPs
that could deliver into the brain the small molecule antifungal drug
amphotericin B (for treatment of meningitis) and a DNA fragment that binds
to and shuts down the gene producing the tau protein linked to Alzheimer's
disease.
More recently, Xu and his team have created LNPs to deliver gene-editing
packages into T cells in mice. T cells can help in the production of
antibodies, destroy infected cells before viruses can replicate and spread,
and regulate and suppress other cells of the immune system.
The LNPs they created fuse with T cells in the spleen or liver—where they
typically reside—to deliver the gene-editing contents, which can then alter
the molecular make-up and behavior of the T cell. It's a first step in the
process of not just training the immune system, as one might do with a
vaccine, but actually engineering it to fight disease better.
Xu's approach to editing T cell genomes is much more targeted, efficient,
and likely to be safer than methods tried so far using viruses to modify
their genome.
"By targeting T cells, we can tap into a branch of the immune system that
has tremendous versatility in fighting off infections, protecting against
cancer, and modulating inflammation and autoimmunity," said Xu.
Xu and his team explored further the mechanism by which LNPs might find
their way to their targets in the body. In experiments aimed at cells in the
lungs, they found that the nanoparticles picked up specific proteins in the
bloodstream after injection.
The proteins, now incorporated into the surface of the LNPs, became the main
component that helped the LNPs to latch on to their target. This information
could help improve the design of future delivery particles.
While these results have been demonstrated in mice, Xu cautioned that more
studies and clinical trials will be needed to determine the efficacy and
safety of the delivery method in humans.
More information:
Xuewei Zhao et al. Imidazole‐Based Synthetic Lipidoids for In Vivo mRNA
Delivery into Primary T Lymphocytes, Angewandte Chemie International Edition
(2020). DOI:
10.1002/anie.202008082