Scientists from the San Raffaele-Telethon Institute for Gene Therapy (SR-Tiget) in Milan, Italy, have identified a critical window shortly after birth when circulating blood stem cells can be effectively targeted with gene therapy directly in the body. The breakthrough study, published in Nature, opens new therapeutic avenues for treating genetic blood disorders without requiring stem cell transplantation or chemotherapy.
The research was conducted by Dr. Michela Milani under the supervision of Professor Luigi Naldini, Director of SR-Tiget, and Dr. Alessio Cantore. SR-Tiget is internationally recognized as a leading center for lentiviral vector-based gene therapy with a strong track record of translating research into clinical applications.
Revolutionary Approach to Gene Therapy
Current gene therapy using lentiviral vectors has achieved major clinical successes through ex vivo approaches, where patients' stem cells are genetically modified in laboratory settings and reinfused after chemotherapy. A notable example is the gene therapy for metachromatic leukodystrophy (MLD), developed at SR-Tiget and approved in both Europe and the United States. However, this process remains invasive and resource-intensive.
The new study investigates a fundamentally different approach by delivering lentiviral vectors directly into the bloodstream. Researchers discovered that in newborn mice—up to the first two weeks of life—the numbers of hematopoietic stem and progenitor cells (HSPCs) in circulation are significantly higher than in older animals.
"After birth, blood stem cells need to move from the liver, where they have resided throughout the last months of pregnancy, to their definitive home in the bone marrow," explains Dr. Milani. "We found that as they travel in the circulation, they can be more easily accessed by intravenously delivered vectors and thus be genetically modified without the need to harvest and process them outside of the body."
Therapeutic Success Across Multiple Disease Models
The research team tested their approach in mouse models of three distinct genetic diseases, demonstrating broad therapeutic potential:
ADA-SCID: A severe immunodeficiency caused by the lack of functional lymphocytes
Autosomal recessive osteopetrosis: A bone disease resulting from impaired blood-born bone remodeling cells
Fanconi anemia: A bone marrow failure syndrome caused by defective DNA repair that particularly impacts stem cells
In all three disease models, in vivo gene transfer led to significant therapeutic benefits and prolonged survival. The results were particularly striking in Fanconi anemia, where corrected stem cells progressively repopulated the blood system and prevented bone marrow failure. This outcome mirrors the survival and growth advantage over defective cells observed in human gene therapy studies.
Expanding the Treatment Window
To enhance treatment efficacy, researchers employed clinically approved mobilizer drugs including G-CSF and Plerixafor to force stem cells out of their tissue niches. This approach achieved higher gene transfer efficiencies and extended the therapeutic window to older mice. The team also optimized lentiviral vectors to improve their stability and cellular uptake.
Clinical Translation Potential
Crucially, the research team detected circulating HSPCs in the blood of human newborns and throughout the first months of life, consistent with their mouse observations. These findings support the hypothesis that this therapeutic window may exist in humans.
"This study provides proof of concept that in vivo lentiviral gene delivery to blood stem cells is feasible during a short but accessible period early in life as a gene therapy strategy for blood disorders," states Dr. Cantore. "While the efficiency currently remains limited compared to established ex vivo treatments, it may suffice, if replicated in human babies, to benefit some genetic diseases such as severe immunodeficiencies or Fanconi anemia."
Professor Naldini adds insight into the biological mechanisms: "Intriguingly, when we harvest stem cells from the blood of adult mice or humans, even upon mobilization, they require activation stimuli to enable efficient lentiviral gene transfer. On the contrary, at these early ages, not only are there more stem cells in the circulation, but they are also more permissive to gene transfer."
The research collaboration included Paula Rio and Juan Bueren at CIEMAT/CIBERER in Madrid, Spain, for the Fanconi anemia disease model studies. Future research will investigate the biological basis of enhanced stem cell permissiveness in newborns and explore methods to replicate these conditions at later ages.
