About 10-15 years ago retroviral vector was one of the promising gene delivery systems in preclinical models of gene therapy because of its ability to integrate a gene of interest into the genome and maintain stable gene expression. Hematopoietic Stem (or progenitor) Cells (HSPCs) have been considered to be an ideal target for gene-correction of hematological and immune diseases, linked to particular gene or blood lineage defect.
Most clinical applications have used gene transfer vectors based on integrating retroviruses, but the relative inefficiency of these vectors has limited the considerable potential of gene transfer into HSPCs. Genetically modified cells represent only a small fraction (1%–10%) of the hematopoietic cells after transplantation. Consequently, competition from unmodified infused and endogenous HSPCs may dilute any therapeutic effect of the transduced cells. This implies that this small fraction of genetically modified hematopoietic cells will correct diseases requiring more than 1%–10% of corrected cells only if they have a marked selective growth advantage in vivo.
Short-term outcome of the clinical trials started in France in 2000 had shown some benefits for young patients with X-linked severe combined immunodeficiency – (SCID).
But what happened later?
The strong enhancers contained in retroviral genomes can activate adjacent cellular genes following integration, but the risk posed by one or a few integrated vectors per cell was estimated to be very low. However, these assumptions were shattered in 2002, when French researchers announced that a child who had shown unequivocal clinical benefit in the HSPC gene therapy trial for X-linked SCID (SCID-X) developed a vector-related T cell acute leukemia three years after infusion of retrovirally modified CD34+ cells. Since then, three other children in the French trial and, more recently, one patient in a similar SCID-X study conducted by British researchers have developed leukemias.
At the same time, researchers were looking for an efficient and safe HSPC expansion agent. Transcription factor HOXB4, which has been investigated in the last few years, has become one of the most promising HSPC expansion agents. HOXB4 significantly increases hematopoietic stem cell self-renewal and expansion and promotes hematopoietic differentiation from embryonic stem cells. It was shown that a retrovirus which contains HOXB4 dramatically increased HSPC engraftment without leukemia development in mice and human. Also, a recent study demonstrated that overexpression of HOXB4 in embroynic stem cell-derived HSPCs leads to enhanced engraftment and recovery of functional immunity without any tumorogenesis or leukemogenesis. Therefore, HOXB4 was considered as a promising gene target to expand HSPCs.
However, a recent study published in the Journal of Clinical Investigation has added “fuel to the fire” and broken all of promises made by potentials of retroviral therapeutics. A group of authors from Fred Hutchinson Cancer Research Center (Seattle, WA, USA), for the first time, investigated a long-term outcome of HSPC transplantation, by overexpressing HOXB4 with retroviral vector in large animals. 2 out of 2 dogs and 1 out of 2 macaques developed myeloid leukemia approximately 2 years after transplantation! In contrast, researchers did not see any sign of leukemia in the animals that were tranduced with a control vector that carried another gene (MGMT).
Here is an interesting question: was overexpression of HOXB4 itself the only single event that caused leukemia? Not really.
The new findings indicate the cooperativity between HOXB4, a growth-altering transgene, and transgene insertion sites. This cooperation event was quite similar to what researchers reported in failed SCID-X trials. Even though the study highlights that the gene HOXB4 is a catalyst for leukemogenesis, it also reminds us of a potential problem with viral gene transfer.
This picture demonstrates mechanisms of leukemia development after transplantation of HSPCs transduced with retroviral vectors:
In my opinion, gene therapy is in crisis. Researchers should come up with a new virus-free system for an effective gene delivery. Adeno-associated virus (AAV)-based therapy didn’t show any adverse effects so far, but who knows what we will get in upcoming preclinical and clinical studies?
At the same time we always have to think about the balance between a level of HSPC expansion ex vivo and a risk of cancer. Almost all regulators of normal hematopoiesis, which have been proposed to be good targets for stem cell expansion in clinical settings, were shown to be evolved into leukemogenesis when their expression levels were modulated. The implication is that one should not judge quickly whenever a large expansion rate of blood-forming stem cell is observed in laboratory. Besides, do we really need that many cells? It would be of uttermost importance to consider the patient safety first before thinking about any therapeutic potential of ex vivo HSPC expansion.
I would like to finish by an excellent quote from Cynthia E. Dunbar who has commented on this study:
The message of the current study by Zhang and colleagues is clear as a bell: overexpression of HOXB4 using integrating retroviral vectors is much too risky to be contemplated in any clinical settings. This is not to say that this study rules out alternative approaches to harnessing the power of HOXB4 on HSPCs.
Events in the recent history of gene therapy have led many to acknowledge that the field may have been, on several occasions, too eager to rush into the clinic. However, the wisdom gained from these setbacks is now becoming apparent in studies such as that of Zhang and colleagues in this issue of the JCI. Scientists and clinicians hoping to use their protocols to treat humans understand the importance of examining vectors’ behavior in a variety of comparative assays, including large animal studies, before entering the clinic. The use of well-characterized and well-tested preclinical models such as those of Zhang and colleagues is likely to minimize the risk of an adverse event that would throw the field into another crisis.
J. Clin. Invest. doi:10.1172/JCI34371
Very interesting blog post opinion (highly recommend to read!):
is gene therapy going down (or do we just play dumb?)
Geneticist Dr D Kohn with a five month old Apache baby who suffers from SCID (severe combined immunodeficiency). The baby is receiving gene therapy for its condition. It is isolated in a sterile tent to prevent infection. The rare genetic mutation of SCID destroys the immune system making the body unable to fight infection. SCID babies lack a vital enzyme which their immune system needs. Gene therapy involves inserting a gene for this enzyme into stem bone marrow cells and transplanting the cells into the baby. With this enzyme, stem cells may produce normal immune system blood cells. Photographed at the Childrens Hospital in Los Angeles, USA.(SciencePhotoLibrary)
PS: special thanks to Jae-Won Shin