Sickle cell disease is debilitating. Due to faulty genetic code, red blood cells morph from round and plump into jagged monstrosities that scrape and puncture blood vessels. Over time symptoms build up, eventually damaging major organs like the liver, heart, and kidneys.
The disease was incurable – until gene editing came along.
In 2020, a breakthrough technology that used CRISPR improved disease symptoms in six patients for at least half a year. It was a tough journey: scientists removed faulty blood stem cells and disabled a genetic switch to help make them healthy again. Patients then received a hefty dose of chemotherapy to wipe out diseased cells and make room for the engineered cell transplants. The story had a happy ending: after infusion with the edited cells, one teen could go swimming with his friends without pain and enjoy life as a kid.
Yet the Hallmark ending isn’t available to everyone. Although it’s life-changing and effective, the “ex vivo” – outside the body – procedure can only benefit a lucky few. It’s laborious, complex, and extremely costly.
Can we bring a similar treatment to the masses?
According to a new study, the answer is a tentative yes. By loading gene editing tools into nanoscale blobs of fat, a team from the University of Pennsylvania created a single shot that directly reprograms faulty blood cells inside bone marrow in mice.
Using a similar strategy, they also designed a clever way to kill off existing diseased cells without any need for toxic chemotherapy.
“What really struck me was, damn, how efficient it is,” said Dr. Paula Cannon at the University of Southern California, who was not involved in the study.
Straight to the Marrow
Genetic blood disorders are brutal. Sickle cell disease aside, others, such as beta-thalassemia, reduce the ability of red blood cells to carry oxygen, resulting in severe anemia, weakness, and an increased risk of developing blood clots.
All blood cells originate from a nest of stem cells inside the bone marrow. Called hematopoietic stem cells, these troopers divide throughout life to not only literally give us blood, but also build a cellular army for the immune system.
The classic approach for tackling blood disorders is a bone marrow transplant to completely replace diseased cells with healthy donor ones. Unfortunately, finding a suitable donor is like winning the lottery – even family members may not have the immune profile to minimize potentially life-threatening rejection.
Thanks to CRISPR, these days patients have another option: gene therapy. Here, the patient’s stem cells are removed from the bone marrow and edited to correct genetic mistakes. The next step is “conditioning,” which uses chemotherapy or radiation to wipe out the patient’s stem cells, making space for the genetically engineered stem cells. It’s a grueling procedure, and potentially comes with terrible side effects like infertility or cancer from damaging DNA.
There’s no doubt that gene therapy works. Can we simplify it to a single jab in the arm?
A Fatty Solution
The team’s inspiration came from Covid-19 vaccines.
At the heart of the technology are tiny fatty blobs called lipid nanoparticles. They encapsulate messenger RNA (mRNA), which instructs cells to make proteins. I imagine it as a dumpling: by switching the inside mRNA “filling,” it’s possible to encapsulate a wide range and variety of genetic material. Once inside the body, the filling spills out and prompts the cell to make that protein – for example, the spike protein for Covid-19 vaccines or normal versions of a mutated protein to treat blood disorders.
It’s not that easy. The goal is for the fatty blobs to make a beeline to the bone marrow, but they naturally accumulate inside the liver. As a workaround, the team added an additional protein called anti-CD117 to the surface of the nanoparticle as a homing device for blood stem cells.
As a first proof of concept, the team loaded the lipid balls with mRNA that encoded a protein that glows brightly in the dark. They then doused blood stem cells and whole bone marrow from mice in a petri dish with the nanoballs. As expected, compared to lipid nanoparticles without the external protein décor, the balls rushed to their targets and released their mRNA content – causing the cells to glow in the dark.
But the real test was in living hosts. The team injected the nanoparticles in mice that were genetically edited to “report” the results of a particular type of gene editing – basically, if it worked, the cells would glow a bright red. Although some blobs homed in on the liver, plenty took up residence in the bone marrow and released their cargo. Overall, more than 50 percent of the blood stem cells turned red.
It might not seem impressive, but according to the team, the level of editing is enough to treat many types of blood disorders.
An Easy Goodbye
Conditioning the bone marrow to make room for healthy cells is a necessary part of treatment. As a next step, the team tested an alternative to chemotherapy with their designer nanoparticles.
Cells naturally die. The process, called apoptosis – a Greek word that means the gentle “falling off of leaves” – retires damaged cells to keep the body healthy. Apoptosis is a tightly-regulated process with multiple protein triggers and inhibitors.
Here, the scientists found a trigger for apoptosis and packed up its genetic code as mRNA inside nanoparticles – essentially a “self-destruct” button. Six days after injection into mice, the treatment wiped out a portion of their blood stem cells. Although not complete, the level is on par with the conditioning levels needed to correct some blood disorders.
Genetically engineered cells “replacing only a fraction” of blood stem cells in the bone marrow “could provide substantial benefit in many diseases,” said Drs. Samuele Farrari and Luigi Naldini at the Istituto di Ricovero e Cura a Carattere Scientifico in Italy, who weren’t involved in the study. The gene therapy could be “game changing” for blood disorders, they said.
A Paradigm Change
The study isn’t the first to pursue a single jab for blood disorders.
Earlier in April, another team used a viral carrier – stripped of its disease-causing genes – to shuttle CRISPR editors into mice with sickle cell disease. On average, 43 percent of damaged cells were replaced with healthy ones. The downside? Viral carriers, though effective, sometimes increase the risk of an immune response, prompting scientists to research fatty nanoparticles as an alternative delivery vehicle.
The main question is, does it work?
For now, the only answer comes from an experiment using cells isolated from four people with sickle cell disease. By adjusting the “filling” of the nanoparticle, the team engineered a CRISPR base-editing system – swapping one genetic letter for another – that targets the source of sickle cell mutation. The treatment amped up round and healthy red blood cells and lowered sickly ones, so much so that treated cells had near-perfect levels of healthy proteins.
To Dr. Hans-Peter Kiem at the University of Washington, who wasn’t involve in the study, the results are “very intriguing and exciting.”
The team is working to test these nanoparticles in mouse models of sickle cell disease. In the meantime, lots of potential stumbling blocks need ironing out.
First are dosage and safety. Because a hefty amount of the therapy ends up in the liver despite homing proteins, it could damage the organ at high doses.
Another concern is specificity, in that the fatty blobs could roam to other tissues with the same protein target. Then there’s the question of how well the engineered grafts will take, and how they potentially mutate or evolve inside the body.
All that said, people are optimistic. Although it’ll “take more time,” said Dr. Daniel Anderson from MIT, who was not involved in the study, “I’m confident that these types of approaches are going to lead to human therapies.”
