Boston Children’s Hospital researchers have directly reprogrammed mature blood cells from mice into blood-forming hematopoietic stem cells by using a cocktail of eight different transcription factors.
These reprogrammed cells have been called induced hematopoietic stem stem cells or iHSCs. These cells have all the hallmarks of mature mouse HSCs and they have the capacity to self-renew and differentiate into all the blood cells that circulate throughout the body.
These findings are highly significant from a clinical perspective because they indicate that it might be entirely possible to directly reprogram a patient’s existing, mature blood cells into a hematopoietic stem cell for transplantation purposes. Such a procedure, known as hematopoietic stem cells transplantation or HSCT, is a common treatment for patients whose bone marrow has suffered irreparable damage due to environmental causes (heavy metals, chloramphenicol, etc) or disease (cancer). The problem with HSCT is finding a proper match for the patient and then procuring clinically significant quantities of high-quality bone marrow for HSCT.
Derrick J. Rossi, an investigator in the Program in Cellular and Molecular Medicine at Boston Children’s Hospital and Assistant Professor in the Department of Stem Cell & Regenerative Biology, explained: “HSCs comprise only about one in every 20,000 cells in the bone marrow. If we could generate autologous (a patient’s own) HSCs from other cells, it could be transformative for transplant medicine and for our ability to model diseases of blood development.”
Rossi and his collaborators have screened genes that are expressed in 40 different types of blood progenitor cells in mice. This screen identified 36 different genes that control the expression of the other genes. These 36 genes encode so-called “transcription factors,” which are proteins that bind to DNA and turn gene express on or shut it off.
Blood cell production tends to go from the stem cells to progeny cells called progenitor cells that can still divide to some limited extent, and to effector cells that are completely mature and, in most cases, do not divide (the exception is lymphocytes, which expand when activated by specific foreign substances called antigens).
Further work by Rossi and others identified six transcription factors (Hlf, Runx1t1, Pbx1, Lmo2, Zfp37, and Prdm5) of these 36 genes, plus two others that were not part of their original screen (N-Myc and Meis1) that could robustly reprogram myeloid progenitor cells or pro/pre B lymphocytes into iHSCs.
To put these genes into these blood cells, Rossi and others uses souped-up viruses that introduced all either genes under the control of sequences that only allowed expression of these eight genes in the presence of the antibiotic doxycycline. Once these transfected cells were transplanted into mice, the recipient mice were treated with doxycycline, and the implanted cells formed HSCs.
When this experiment utilized mice that were unable to make their own blood cells, because their bone marrow had been wiped out, the implanted iHSCs reconstituted the bone marrow and blood cells of the recipient mice.
To further show that this reconstituted bone marrow was normal, high-quality bone marrow, Rossi and others used these recipient mice as bone marrow donors for sibling mice whose bone marrow had been wiped out. This experiment showed that the mice that had received the iHSCs had bone marrow that completely reconstituted the bone marrow of their siblings. This established that the iHSCs could completely reestablish the bone marrow of another mouse.
Thus Rossi and others had established that iHSCs could in fact created HSCs from progenitor cells, but could they do the same thing with mature blood cells that were not progenitor cells? Make that another yes. When Rossi and others transfected their eight-gene cocktail into mature myeloid cells, these mature cells also made high-quality iHSCs.
Rossi noted that no one has been able to culture HSCs in the laboratory for long periods of time. A few laboratories have managed expand HSCs in culture, but only on a limited basis for short periods of time (see Aggarwal R1, Lu J, Pompili VJ, Das H. Curr Mol Med. 2012 Jan;12(1):34-49). In these experiments, Rossi used his laboratory mice as living culture systems to expand his HSCs, which overcomes the problems associated with growing these fussy stem cells in culture.
Gene expression studies of his iHSCs established that, from a gene expression perspective, the iHSCs were remarkably similar to HSCs isolated from adult mice.
This is certainly an exciting advance in regenerative medicine, but it is far from being translated into the clinic. Can human blood progenitor cells also be directly reprogrammed using the same cocktail? Can mature myeloid cells be successfully reprogrammed? Will some non-blood cell be a better starting cell for iHSC production in humans? As you can see there are many questions that have to be satisfactorily answered before this procedure can come to the clinic.
Nevertheless, Rossi and his team has succeeded where others have failed and the results are remarkable. HSCs can be generated and transplanted with the use of only a few genes. This is certainly the start of what will hopefully be a fruitful regenerative clinical strategy.
On a personal note, my mother passed about almost a decade ago after a long battle with myelodysplastic syndrome, which is a pre-leukemic condition in which the bone marrow fails to make mature red blood cells. Instead the bone marrow fills up with immature red blood cells and the patient has to survive on blood transfusions.
The reason for this condition almost certainly stems from defective HSCs that do not make normal progeny. Therefore the possibility of using a patient’s own cells to make new HSCs that can repopulate the bone marrow is a joyful discovery for me to read about, even though it is some ways from the clinic at this point.