The biopharmaceutical company Cellectis develops personalized immunotherapies for cancer patients using their established transcription activator-like effector nuclease (TALEN) gene editing platform. During their recent “Innovation Days” event, they announced that they are now using their gene editing platform to tackle hematological diseases, including sickle cell disease (SCD), in an effort they dubbed the HEAL initiative.

Every year, 300,000 people are born with SCD. These patients experience symptoms such as anemia and strokes. Patients with SCD have red blood cells that are sickle shaped rather than round due to a mutation in the gene for hemoglobin, a protein required for proper oxygen transport from the lungs to the rest of the body.

The current gold standard therapy for SCD is bone marrow transplant from an HLA matched donor. Although this treatment alleviates SCD symptoms 90% of the time, finding a perfect HLA match is tricky, so only a quarter of patients are eligible. If an HLA match is not perfect, or if a patient’s immune system is not properly suppressed at the time of transplantation, patients can reject the transplant and even die. The only way to avoid these problems is to use the patient’s own cells.

Cellectis is currently developing an autologous stem cell therapy using hematopoietic stem cells (HSCs), which differentiate into a variety of blood cells, including red blood cells. They presented promising preliminary data in May from a study where they isolated HSCs from five SCD patients and used TALENs to correct a mutation in the sequence encoding hemoglobin. This approach restored the cell’s ability to produce hemoglobin. The scientists are currently fine-tuning this method and validating their in vivo  results showing that these HSCs can be successfully transplanted into mice.

“[Cellectis] is really one of the leaders in genome editing,” said Huimin Zhao, a chemical and biomolecular engineer at the University of Illinois, Urbana-Champaign, who is not associated with Cellectis. “Before all of these CRISPR based genome editing companies started, they were one of the pioneers.”

Cellectis scientists designed a TALEN, TalGlobin-01, that targets the mutated hemoglobin subunit beta (HBB) in HSCs isolated from SCD patients. A transfected DNA fragment corrects the HBB gene mutation via homology directed repair. They reported a 60% efficiency in gene repair, improving upon CRISPR-based approaches, which achieved 50% or lower success rates. 

TALENs and CRISPR-Cas9 work in similar ways, targeting and cutting a specific region of DNA, allowing for deletion or insertion of a gene via DNA repair. In CRISPR, a guide RNA directs Cas9 to a specific site, while a DNA binding domain guides TALENs within the nucleus. Most researchers have migrated to CRISPR since it is easier to design and can target multiple genes at once. Although TALENs can be just as effective as Cas9 at DNA editing, they sometimes fare a bit worse (1).

“If you have TALENs, you can build the TALENs very easily,” said Zhao. “That’s probably why Cellectis sticks to TALENs, not because TALENs are really better than CRISPR, but because they can still do the things they want.”

Cellectis’ gene-corrected HSCs differentiated into all expected lineages and maintained the gene edit once they differentiated into red blood cells. The gene edited red bloods cells produced up to 80% wild type hemoglobin protein. 

Sixteen weeks after the edited human HSCs were implanted into the bone marrow of a mouse, there were just as many cells as originally engrafted, indicating that the transplant was a success.

Phillippe Duchateau, Chief Scientific Officer at Cellectis, plans for more work in the mouse models before this therapy moves to the clinic. They are currently working to publish their preliminary results regarding Talglobin-01, and testing its safety and efficacy in mice.

“We are always improving what we are doing. Science is like that. You cannot be satisfied by what you have today, this is just the beginning,” said Duchateau.

The researchers plan to use what they learned developing TalGlobin-01 to create new HSC based therapies for other diseases such as lysosomal storage disorders.

“We have the platform we have because we did quite a lot of work on the process itself — how to edit HSCs — so we can apply that to many other diseases,” said Duchateau.

Correction: A previous version of this article incorrectly stated that most people with Sickle Cell Disease do not live to be adults. The median age for people with Sickle Cell Disease is about 45 years old.

Reference

Jain, S et al. TALEN outperforms Cas9 in editing heterochromatin target sites. Nat Comms  606. (2021).