New Type of Stem Cell Discovered by Salk Scientists

Stem cell scientists from the laboratory of Juan Carlos Izpisua Belmonte at the Salk Institute for Biological Studies in La Jolla, California have discovered a new type of stem cell that could potentially provide a model system for early human development, and might even allow human organs to be grown in large animals for therapeutic purposes.


Izpisua Belmonte and his colleagues came across these types of cells somewhat serendipitously while transplanting human pluripotent stem cells into mouse embryos.


Other types of pluripotent stem cells have already been well-known to stem cell scientists for some time. Stem cells are “pluripotent,” if they have an intrinsic ability to differentiate into any adult cell type. Embryonic stem cells (ESCs), for example, are derived from early human embryos that have yet to implant into the inner layer of the uterus.  However, epiblast stem cells (EpiSCs) have been established from post-implantation embryos and have different properties.  While both are pluripotent, they bear striking differences in molecular signature, signalling dependency, colony morphology, cloning efficiency, metabolic requirements and epigenetic features (see Nichols, J. & Smith, A. Cell Stem Cell 4, 487492 (2009) and Zhou, W. et al. EMBO J. 31, 21032116 (2012)).  Both of these cells have the ability to re-enter embryogenesis but they do so at different developmental time points (pre-implantation versus post-implantation, respectively), which distinguish ESCs and EpiSCs.  Therefore, these two cell types exist in two temporally distinct pluripotent states.  Even though these two types of pluripotent stem cells can be grown into large numbers in the laboratory, differentiating them into specific types of mature, adult cells has proven difficult in some cases. The cells discovered by Izpisua Belmonte and his colleagues are reportedly easier to grow in vitro and engraft into an embryo if they are injected into the right spot. Izpisua Belmonte call these cells “region-selective pluripotent stem cells” (rsPSCs).



Because rsPSCs grow more quickly and stably than other pluripotent cells, they may be more useful for developing new therapies, according to Paul Tesar, a developmental biologist at Case Western Reserve University in Cleveland, Ohio.


Izpisua Belmonte and colleagues originally wanted to transplant various types of human pluripotent stem cells into mouse embryos in the laboratory. They prepared their cells for transplantation by growing them in various blends of culture media that contained different combinations of growth factors and other chemicals. They found that one particular blend was more effective at making the cells grow and proliferate. However, the cells that grew quite well in this concoction displayed different patterns of metabolism and gene expression in comparison to other pluripotent stem cells. These same cells not graft well into the mouse embryo.


Thus, Izpisua Belmonte and his colleagues decided to nail down those features that would cause cells to efficiently integrate into mouse embryos. They injected the human cells into three different regions of a 7.5-day-old mouse embryo. Thirty-six hours later, only those cells that had been grafted into the tail, or posterior of the embryo, integrated and differentiated into the correct cell layers to form a “chimeric” or mixed-tissue embryo. Such organisms contain cells with genomes from DNA organisms. Since these cells seemed to prefer one part of the embryo, Izpisua Belmonte and his team called them region-selective pluripotent stem cells.


From these data, Izpisua Belmonte has proposed that embryos contain multiple types of pluripotent stem cells, including rsPSCs, during their early development. It is not yet clear whether the rsPSCs play a part in designating which part of the embryo will become the head, the middle, or hind end. Identifying various types of pluripotent cells might provide researchers with the ability to study the early stages of human embryonic development by transplanting rsPSCs into animal embryos.


Izpisua Belmonte and his colleagues found that they could easily use enzymes that modify the sequences of DNA to edit the genomes of rsPSCs, which is usually difficult to do in pluripotent cell lines when grown in culture.

Gene editing could help scientists to optimize the ability of human cells to grow within animals, which might allow the creation of transgenic chimeras. Tesar says that the idea of using human pluripotent cells, such as rsPSCs, to create animals with human organs is not unrealistic, but he expects that it will be very difficult. The immune system of the animal might reject the human cells and the growth rates of the two organs might also cause problems.

Izpisua Belmonte’s lab is already starting to implant pig embryos with a different type of stem cells, and this is the only very first step for these techniques.


Gene Therapy/Stem Cell Treatment Cures Boys of Severe Genetic Disease

British doctors have successfully cured youngsters suffering from a deadly inherited genetic disorder using ground-breaking stem cell-based treatments. This is the harbinger of a new era of medicine and genetic therapies.

The young patients who participated in this trial suffer from the most severe form of a rare blood condition call “Wiskott-Aldrich Syndrome.” The trial participants have now been free of the disease for four years.

Patients with Wiskott-Aldrich syndrome are usually male, and they have a deficient immune system that fails to fight off common infections that usually do not affect most people and a reduced ability to form blood clots. The numbers, and size of platelets in the blood, which are the cells responsible for initiating blood clots, are abnormal in individuals with Wiskott-Aldrich syndrome; they have very small platelets and few of them. This condition is called microthrombocytopenia. This platelet abnormality leads to easy bruising or episodes of prolonged bleeding following minor traumas. Additionally, many types of white blood cells are abnormal or nonfunctional, and this increases the risk of several immune and inflammatory disorders. Often patients with Wiskott-Aldrich syndrome develop eczema, which is an inflammatory skin disorder characterized by abnormal patches of red, irritated skin. Affected individuals also have an increased susceptibility to infection, and developing autoimmune disorders. They also have an increased chance of developing some types of cancer, such as cancer of the immune system cells (lymphoma).

Wiskott-Aldrich syndrome is inherited from the X chromosome, and therefore, the condition is much more common in males than in females. Having said that, Wiskott-Aldrich syndrome is still a rather rare condition, with an estimated incidence of 1 – 10 cases per million males worldwide.

Mutations in the WAS gene cause Wiskott-Aldrich syndrome. The WAS gene encodes the WASP protein, which is found in all blood cells, and relays signals from the surface of blood cells to the actin cytoskeleton inside the cell. The actin cytoskeleton is a network of fibrous proteins that compose the cell’s interior structural framework. WASP signaling triggers cell movement and attachment to other cells and tissues. In white blood cells, WASP signaling induces the actin cytoskeleton to establish the interactions between cells and the foreign invaders targeted by them. Mutations in the WAS gene cause a lack of any functional WASP protein, and loss of WASP signaling. Thus white blood cells are less able to respond to foreign invaders, which cause many of the immune problems related to Wiskott-Aldrich syndrome. Similarly, decreased WASP function impairs platelet development, leading to reduced size and early cell death.

In the Britain, Wiskott-Aldrich syndrome affects fewer than one hundred children in Britain, but Daniel Wheeler, 15, of Bristol is one of them. Wheeler was among seven children who participated in the new gene therapy trial at centers in London and Paris.

Daniel was diagnosed with Wiskott-Aldrich syndrome when he was two years old and needed frequent medical care to manage his symptoms which included severe eczema, asthma and inability to fight infections. David’s older brother died from complications associated with the disease. However, since undergoing gene therapy in 2011 Daniel has shown no symptoms and doctors believe he is effectively cured.

Daniel’s mother Sarah, 50, who works in real estate in Bristol said: “Since being around two, Daniel has been in an out of hospital, but now his skin has cleared up and so has his asthma. It means he can get on with his life now.”

Adrian Thrasher, Professor in Pediatric Immunology, at Great Ormond Street Hospital in London, where David’s treatment was carried out, said that it offered new hope for people suffering from incurable disease. “We are entering a new era where genetic treatments are entering mainstream medicine and offering hope to patients for whom conventional treatments don’t work well or are simply unavailable,” he said.

“The work shows that this method is successful in patients who, in the past would have very little chance of survival without a well match bone marrow donor.

“It also excitingly demonstrates the potential for treatment of a large number of other diseases for which existing therapies are either unsatisfactory or unavailable.”

In this trial, David’s bone marrow stem cells were isolated and subjected to gene therapy in the laboratory. The faulty WAS gene was replaced with a healthy copy of the gene. These genetically repaired stem cells were replaced in David’s bone marrow where they began producing healthy blood cells that were free from the disease. Because the healthy blood cells were more durable and lived longer than the diseases ones, they eventually overtook the diseased ones.

Seven children between the ages of eight months and 15 years were selected for the trial because a bone marrow match could not be found. Without bone marrow transplantation, patients usually do not survive their teenage years. All the children had eczema and associated recurrent infections and most experienced severe bleeding and autoimmune disease that, in one case, confined the child to a wheelchair.

The children went from spending an average of 25 days in the hospital to no days in the hospital in the two years after the treatment. Furthermore the child using the wheelchair was able to walk again.

Fulvio Mavilio, Chief Scientific Officer at Genethon, the biotech company which developed the treatment said: “It is the first time that a gene therapy based on genetically modified stem cells is tested in an international clinical trial that shows a reproducible and robust therapeutic effect in different centers and different countries.”