What a decade of gene therapy research for sickle cell disease taught UCLA scientists

Key takeaways

• Results from a UCLA-led clinical trial for sickle cell disease reveal the unique challenges of treating blood disorders with gene therapy and provide crucial insights into improved clinical trial design for blood diseases.
• After observing limited success with the first patient's response to the gene therapy, the team implemented several key changes - including a redesigned viral vector, a new stem cell collection method and a pretreatment transfusion regimen - that improved gene delivery in the three subsequent patients.
• After observing limited success with the first patient's response to the gene therapy, the team implemented several key changes - including a redesigned viral vector, a new stem cell collection method and a pretreatment transfusion regimen - that improved gene delivery in the three subsequent patients.

When Dr. Donald Kohn began performing bone marrow transplants in 1987, he saw firsthand the promise and limitations of available treatments for life-threatening blood disorders. That experience planted the seed of a career-defining question: What if you could correct a disease with a patient's own blood stem cells, potentially curing them for life without a donor?

This approach recently earned FDA approval for a gene therapy for severe leukocyte adhesion deficiency-I, a rare immune disorder in children. It also underlies a growing portfolio of trials for blood and immune diseases at UCLA. But of all the diseases Kohn has worked on, sickle cell disease has been the most challenging - and most instructive.

Results from a phase 1-2 clinical trial for sickle cell disease that Kohn led over nearly a decade are detailed in a new study in Blood Advances. Though the tested therapy will not be pursued further, the study provides crucial insights for developing gene therapy protocol.

"It was a long road with lots of ups and downs and several bumps," said senior author Kohn, a distinguished professor of pediatrics and molecular and medical pharmacology at the David Geffen School of Medicine at UCLA and a member of the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research. "But we learned so much throughout the therapy's development and application, and these lessons will inform what we do in future trials."

A disease that has long resisted easy answers

Sickle cell disease affects approximately 100,000 Americans, with a disproportionate impact on the Black community. A single mutation in the beta-globin gene causes red blood cells to assume a rigid sickle shape, blocking blood vessels and triggering episodes of severe pain, organ damage, stroke and shortened lifespan.

The four patients enrolled in the UCLA trial had 11-12 such crises per year prior to treatment and were receiving 12-15 blood transfusions annually.

Until recently, the only established cure was a bone marrow transplant from a matched donor - a treatment unavailable to most patients, and that carries the risk of graft-versus-host disease.

"Because gene therapy uses the patient's own stem cells and modifies them to correct the disease, patients don't need to be on lifelong immunosuppressants," said first author Dr. Chattip Prueksapraopong, a pediatric hematology-oncology fellow in the Kohn lab. "They also recover faster than donor stem cell transplant patients."

Kohn had successfully applied this concept to other diseases but soon learned that sickle cell disease presents a unique set of biological challenges.

"In sickle cell disease, some of the red blood cells break in the bloodstream, releasing their contents and causing a lot of ongoing inflammation in the bone marrow," said Kohn, a distinguished professor of microbiology, immunology and molecular genetics in the UCLA College. "Sickle cell is relatively unique in that regard - it makes treating sickle cell disease harder."

How the trial evolved in real time

The study began treating patients in 2015, when no approved gene therapy for sickle cell disease existed. The clinical work was led by Dr. Gary Schiller and Dr. Mary Sehl of the Geffen School of Medicine, who oversaw patient care.

The treatment involved collecting blood stem cells from each patient, inserting a corrective gene using a lentiviral vector - a modified, harmless virus engineered to carry genetic instructions into a cell's DNA - and transplanting the modified cells back into the patient after a short course of chemotherapy to make room in the bone marrow.

The first patient's outcome was limited: Too few of the gene-modified cells successfully took hold in the bone marrow. Kohn's team spent nearly two years investigating why, then retooled.

Three significant changes were made:

• First, the team redesigned the lentiviral vector to a more compact version that could enter stem cells more efficiently. The original vector's large payload had likely impaired its ability to integrate into the cells' DNA.
• Second, the team changed how stem cells were collected: Rather than harvesting directly from the bone marrow, the team used a drug to mobilize stem cells into the bloodstream, where they could be collected.
• Third, they introduced a pre-collection regimen in which patients received targeted red blood cell transfusions for two months before stem cell collection to reduce abnormal hemoglobin levels and calm chronic inflammation.

Some of these were inspired by protocol modifications made by bluebird bio (now Genetix Biotherapeutics) to a gene therapy program for sickle cell disease, which yielded stronger trial results. It reminded Kohn that academic researchers and industry can learn from each other.

The changes made a visible impact. The corrected gene was detected at higher and more sustained levels in the bloodstream of the next three patients treated, with two becoming transfusion independent. No episodes of acute chest syndrome - a serious respiratory complication of sickle cell disease - were observed after treatment.

The results across patients diverged over time, however. One experienced a decline in the corrected gene's expression despite maintaining healthy levels of gene-containing cells, which the team is still investigating.

"Either the vector somehow got silenced - the DNA is still there, but it's not making protein - or the patient developed an immune response to the protein we're producing," Kohn said. "If we find the latter, that would be novel. No one has really demonstrated that before."

The trial was closed in 2023 when the FDA approved two commercial gene therapies for sickle cell disease with stronger efficacy data.

Lessons learned - and how they'll be applied

A significant contribution from the study is identifying conditioning intensity - the chemotherapy dose given before transplant to make room in the bone marrow for the modified cells - as a likely critical factor in durable outcomes.

The trial used a medium-intensity regimen; subsequent analyses suggest the more successful commercial trial cohorts used a higher-intensity approach. This could help physicians using approved therapies adjust their conditioning protocols to maximize long-term durability.

Such lessons are being applied to Kohn's preclinical development of a gene therapy for alpha thalassemia major, a severe blood disorder in which patients cannot produce functional hemoglobin and require lifelong transfusions.

These patients and others with blood disorders, Prueksapraopong noted, may also require a higher dose of conditioning chemotherapy before gene therapies are administered.

More broadly, she noted that patients with sickle cell disease, beta thalassemia and alpha thalassemia share a biology that demands individualized preparation.

"We learned that patients with hemoglobinopathies have a unique nature," she said. "Each patient is not one size fits all - we have to treat the patient differently. We learned a lot from the trial participants and are incredibly grateful to them for their bravery."

For Kohn, the sickle cell trial is one chapter in a much longer commitment to this disease - and the patients who live with it. His team is now supporting a CRISPR-based gene correction trial for sickle cell disease in collaboration with UC San Francisco and UC Berkeley.

It's the latest expression of a conviction that has driven his work for decades: that a durable, accessible cure for sickle cell disease is possible - and that getting there requires learning from every step of the journey.

The trial was supported by the California Institute for Regenerative Medicine; the National Gene Vector Biorepository at Indiana University; the National Heart, Lung, and Blood Institute; National Institutes of Health; Department of Health and Human Services. The Hina Patel Foundation provided funding for patient support.