A team of scientists from the San Raffaele-Telethon Institute for Gene Therapy (SR-Tiget) in Milan , Italy, has identified a unique window shortly after birth in which circulating blood stem cells can be effectively targeted with gene therapy directly in the body. The study, published in Nature , opens new avenues for treating some genetic blood disorders without the need for stem cell transplantation or chemotherapy.
SR-Tiget is internationally recognized as a leading center for lentiviral vector–based gene therapy, with a strong track record of translating cutting-edge research into clinical applications. The research was conducted by dr. Michela Milani, first author of the study, under the supervision of Professor Luigi Naldini , Director of SR-Tiget, and Dr. Alessio Cantore , group leader at the same institute.
A step forward for in vivo gene therapy
Gene therapy using lentiviral vectors has already led to major clinical successes when applied ex vivo — meaning that patients' stem cells are genetically modified in the lab and reinfused after chemotherapy. One such example is the gene therapy for metachromatic leukodystrophy (MLD), developed at SR-Tiget and approved in both Europe and the United States . While effective, this process is invasive and resource-intensive.
The new study investigates a different approach: delivering lentiviral vectors directly into the bloodstream (in vivo). The authors found that in newborn mice – and up to the first 2 weeks of life - the numbers of hematopoietic stem and progenitor cells (HSPCs) in circulation are significantly higher than in older animals. This post-natal window allows for gene transfer by systemic injection, leading to long-term engraftment and multilineage blood cell production.
"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. We found that as they so travel in the circulation they can be more easily accessed by intravenous delivered vectors and thus be genetically modified without the need to harvest and process them outside of the body" says Dr. Milani.
Therapeutic benefit in disease models
The researchers tested the approach in mouse models of three genetic diseases:
- ADA-SCID, a form of severe immunodeficiency due to the lack of functional lymphocytes
- Autosomal recessive osteopetrosis, a bone disease due to impaired blood-born bone remodeling cells
- Fanconi anemia, a bone marrow failure syndrome caused by defective DNA repair which particularly impacts stem cells. Research on the Fanconi anemia disease model has been performed in collaboration with Paula Rio and Juan Bueren at the CIEMAT/CIBERER, Madrid (Spain).
In all three models, in vivo gene transfer led to significant therapeutic benefits prolonging life. Notably, in Fanconi anemia, corrected stem cells progressively repopulated the blood system and prevented bone marrow failure — mirroring the survival and growth advantage over defective cells seen in human gene therapy studies.
To further increase the number of circulating stem cells and expand the treatment window, the team used clinically approved mobilizer drugs (G-CSF and Plerixafor) to force stem cells out of their tissue niches, achieving higher gene transfer efficiencies and extending the therapeutic/interventional window to older mice. They also optimized the lentiviral vectors to improve their stability and uptake.
Toward clinical translation
Importantly, the team detected circulating HSPCs also in the blood of human newborns and over the first months of life — consistent with their observations in mice. These data support the hypothesis that this window of opportunity may exist in humans as well.
"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. While the efficiency currently remains limited as 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" says Dr. Cantore.
"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 there are more stem cells in the circulation, but they are also more permissive to gene transfer. Further studies will investigate the biological bases of this higher permissiveness and how we could replicate it at later ages" adds Professor Naldini.
