A new platform called SMArT can sort CRISPR-edited blood stem cells by whether the edit actually worked — and discard cells carrying potentially dangerous chromosomal damage. The advance could make gene therapies for inherited blood diseases significantly safer and more effective.
Researchers at the San Raffaele Telethon Institute for Gene Therapy (SR-Tiget) in Milan have unveiled a platform that addresses one of the most persistent safety headaches in gene therapy: how to guarantee that only correctly edited cells end up in a patient’s body. The study, published June 1 in Nature Biotechnology, describes the technology, called SMArT — short for “Selection by Means of Artificial Transactivators” — and represents a potential turning point for CRISPR-based treatments targeting blood disorders.
The Problem With CRISPR Cuts
CRISPR-Cas9 has earned its reputation as a revolutionary gene-editing tool, and the field has already notched real-world wins. The therapy exagamglogene autotemcel, marketed as Casgevy, became the first CRISPR-based medicine to earn regulatory approval for sickle cell disease and transfusion-dependent beta-thalassemia. But every time CRISPR slices through a DNA strand, the cell must repair the break — and cells don’t always follow the intended script. Some repair events generate large deletions, chromosomal rearrangements, or other unwanted alterations that could, in theory, pose long-term risks to patients receiving transplanted stem cells.
Beyond safety, there is an efficiency problem. Casgevy works by knocking out a target gene at the cut site, which is relatively straightforward. Inserting an entirely new, gene-sized piece of DNA at a precise location — what scientists call targeted integration — is far harder. The cells have multiple repair pathways available to them, and targeted integration competes against all of them, resulting in a mixed population where only a fraction of cells carry the desired correction.
“These unintended outcomes, such as large deletions of DNA sequences, have emerged as one of the most important limitations to the broader application of gene editing, especially in stem cells intended for transplantation. With SMArT, we aimed to create an intelligent selection system capable of identifying and enriching only those cells that achieved the desired genetic correction while excluding cells carrying potentially dangerous alterations,” Naldini, the director of SR-Tiget, said in a news release.
How SMArT Works
The SR-Tiget team, led by Luigi Naldini and Samuele Ferrari, designed SMArT to solve both problems at once. The system acts as a transient synthetic “AND-gate”: a selectable marker switches on only inside cells that have simultaneously achieved the intended integration and maintained the integrity of the targeted genomic region. Cells that repaired the break in any other way — including those with deletions — never activate the marker and can be filtered out.
The researchers built three progressively more sophisticated versions of the platform. The most advanced, SMArT-3, uses a single programmable CRISPR-based regulatory circuit that can detect correct integration and simultaneously switch on endogenous genes thought to improve how well transplanted stem cells take hold in a recipient’s bone marrow. Crucially, the selection machinery is only transiently expressed: once cells are sorted and transplanted, the marker fades away, leaving a genetically clean graft with no lingering synthetic components.
What the Preclinical Data Show
In laboratory experiments using immunodeficient mice as a model for human hematopoiesis, the team demonstrated that SMArT enrichment could push the proportion of correctly edited blood stem cells to near 100% purity, while sharply cutting the number of cells harboring large deletions or other unintended editing outcomes. After transplantation, the selected cells successfully engrafted and continued producing human blood cells over the long term — a strong signal that the sorting process did not compromise the cells’ regenerative capacity.
Ferrari, a co-senior author of the study, emphasized that the team’s ambition went beyond squeezing out a few more percentage points of efficiency.
“Our goal was not simply to improve editing efficiency, but to fundamentally rethink how to control the quality of edited cell products,” Ferrari said in the news release. “SMArT introduces a programmable framework that can simultaneously increase precision, reduce genotoxic burden, and preserve the functional potential of stem cells.”
Co-first author Daniele Canarutto noted that the biology of DNA repair has proven more complicated than early optimism suggested.
“Gene editing has often been described as precise genome surgery, but biology is more complex than initially anticipated,” he said. “SMArT helps distinguish cells that truly achieved the intended therapeutic outcome from those that underwent alternative repair processes.”
Diseases in the Crosshairs
The current study concentrated on two severe inherited immune disorders — X-linked severe combined immunodeficiency (SCID-X1), a condition sometimes called “bubble boy disease,” and Hyper-IgM 1 syndrome, which leaves patients unable to mount adequate antibody responses. Both conditions result from single-gene defects in blood stem cells, making them natural targets for precision gene editing. The authors believe, however, that the SMArT framework is disease-agnostic: its logic-gate architecture can be reprogrammed for different genomic targets and adapted to work alongside other emerging genome engineering technologies beyond CRISPR-Cas9.
Co-first author Martina Fiumara put it plainly: “Precision medicine requires precision editing. We believe approaches like SMArT could help unlock the full therapeutic potential of gene-sized editing while addressing some of the most pressing safety concerns in the field.”
Why It Matters for Students and Young Patients
For students pursuing careers in biomedical research, genetic counseling or medicine, SMArT illustrates a broader lesson: developing a powerful tool is only half the challenge — verifying that the tool worked correctly is equally important. The platform’s quality-control logic mirrors principles already central to manufacturing, software engineering, and clinical trials, suggesting that interdisciplinary thinking will continue to shape the next generation of medical technologies.
For patients — many of whom are children diagnosed with life-threatening immune deficiencies — the advance means that future gene therapies may arrive with a stronger safety dossier, potentially accelerating regulatory review and expanding access.
The research was supported by Fondazione Telethon, the European Union Horizon Europe Programme, the Italian Ministry of Health, and the Italian Ministry of University and Research, among other funders.
Source: Fondazione Telethon