In the twenty-eight patients enrolled in the initial cohort of the RUBY clinical trial, twenty-seven experienced zero vaso-occlusive crises — the excruciating pain episodes that define severe sickle cell disease — after receiving a single infusion of reni-cel, a CRISPR-based gene-editing therapy developed by Editas Medicine. [1] The results, published this month in the New England Journal of Medicine, represent the most durable evidence yet that gene editing can functionally cure a disease that affects approximately 100,000 Americans and millions worldwide, overwhelmingly people of African descent. [2]

The therapy — renizgamglogene autogedtemcel, shortened to reni-cel — uses CRISPR-Cas12a, a newer variant of the CRISPR-Cas9 system that won Jennifer Doudna and Emmanuelle Charpentier the Nobel Prize in 2020. Cas12a operates with a smaller guide RNA and a staggered cut that reduces off-target editing. [2] The therapy works not by fixing the sickle cell mutation itself but by reactivating fetal hemoglobin, the oxygen-carrying protein that all humans produce before birth and most stop producing in infancy. Fetal hemoglobin does not sickle. Produce enough of it, and the disease effectively vanishes.

The numbers are striking. Before treatment, patients' average hemoglobin was 9.8 grams per deciliter — well below the normal range of 12 to 17.5, low enough to produce chronic fatigue, organ damage, and the constant threat of crisis. After reni-cel, the average rose to 13.8 g/dL, squarely normal. [1] Fetal hemoglobin surged from 2.5 percent to 48.1 percent of total hemoglobin — nearly half of all hemoglobin now the variant that does not deform red blood cells into the crescent shape that gives the disease its name. [2]

Two Years and Counting

The NEJM publication covered the original twenty-eight patients, but the RUBY trial has expanded. Forty patients have now been treated, and thirty-eight remain free of vaso-occlusive events with up to two years of follow-up. [2] The two who experienced crises did so within the first six months, suggesting the protective effect strengthens as edited stem cells repopulate bone marrow and fetal hemoglobin stabilizes. No patient who achieved full reconstitution subsequently lost it.

This durability matters. A therapy that works for six months is a treatment. A therapy that works for two years with no sign of waning is approaching a cure. The distinction is not academic for sickle cell patients, who face a lifetime of hydroxyurea (partially effective), chronic transfusions (iron overload risk), and opioid pain management — with all the addiction risk that entails in a population already marginalized by the American healthcare system. [3]

The Burden of the Cure

Reni-cel is not a pill. Patients undergo stem cell mobilization and collection via apheresis. The cells are shipped to a manufacturing facility where CRISPR-Cas12a editing targets the BCL11A gene, whose protein suppresses fetal hemoglobin production. [1] Before the edited cells can be infused back, the patient must undergo myeloablative conditioning — chemotherapy to destroy existing bone marrow. The process takes three to four months. Patients are immunocompromised for weeks. [2]

The burden is significant, but the context is clarifying. Sickle cell disease kills. Median life expectancy for severe cases in the United States is forty-five years. The disease destroys kidneys, spleen, and lungs through repeated micro-vascular blockages. It causes strokes in children. Against that baseline, four months of treatment followed by a durable functional cure is not a difficult calculus.

Two Therapies, One Access Problem

Reni-cel enters a field that already has one approved CRISPR sickle cell therapy. Casgevy, developed by Vertex Pharmaceuticals and CRISPR Therapeutics, received FDA approval in December 2023 and also targets BCL11A, using the Cas9 system. [3] Two effective CRISPR therapies for the same disease, using different editing technologies, is a scientific validation. It is also a commercial challenge. Both require patient-specific manufacturing. Both require myeloablative conditioning. Both cost $2 million to $3 million per patient.

Globally, more than 20 million people have sickle cell disease, with the highest prevalence in sub-Saharan Africa — where the infrastructure to deliver autologous cell therapy does not exist. Gene editing has delivered a cure. It has not yet delivered one that the people who most need it can reach. [3]

The RUBY trial demonstrates that a single infusion of CRISPR-edited stem cells can durably eliminate the defining symptom of a disease understood at the molecular level since Linus Pauling described the abnormal hemoglobin molecule in 1949. Two years in, with thirty-eight of forty patients living without crises, the earliest promise of CRISPR — that it could cure genetic disease — is no longer a promise. It is a clinical fact, published in the NEJM and demonstrated in the blood of patients whose hemoglobin now carries the fetal variant in quantities their bodies have not produced since birth.

-- KENJI NAKAMURA, Tokyo