Pluripotent Stem Cell Research | Overview
What are pluripotent stem cells?
Pluripotent stem cells are cells that are able to self-renew by dividing and developing into the three primary groups of cells that make up a human body, including:
- Ectoderm: Giving rise to the skin and nervous system
- Endoderm: Forming the gastrointestinal and respiratory tracts, endocrine glands, liver, and pancreas
- Mesoderm: Forming bone, cartilage, most of the circulatory system, muscles, connective tissue, and more
Pluripotent stem cells are able to make cells from all three of these basic body layers, so they can potentially produce any cell or tissue the body needs to repair itself. This property is called pluripotency.
Right now, it’s not clear which type or types of pluripotent stem cells will ultimately be used to create cells for treatment, but all of them are valuable for research purposes, and each type has unique lessons to teach scientists. Scientists are just beginning to understand the subtle differences between the different kinds of pluripotent stem cells, and studying all of them offers the greatest chance of success in using them to help patients.
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What are the types of pluripotent stem cells?
- Induced pluripotent cell (iPS cells)
- True embryonic stem cell (ES cells) — derived from embryos
- Embryonic stem cells made by somatic cell nuclear transfer (ntES cells)
- Embryonic stem cells from unfertilized eggs (parthenogenesis embryonic stem cells, or pES cells)
All four types of pluripotent stem cells are being actively studied at Boston Children’s Hospital.
Scientists have discovered ways to take an ordinary cell, such as a skin cell, and reprogram it by introducing several genes that convert it into a pluripotent cell. These genetically reprogrammed cells are known as induced pluripotent, or iPS, cells. The Stem Cell Program at Boston Children's was one of the first three labs to do this in human cells, an accomplishment cited as the Breakthrough of the Year in 2008 by the journal, Science.
iPS cells offer great therapeutic potential, because they come from a patient’s own cells. They are genetically matched to that patient, so they can eliminate tissue matching and tissue rejection problems that currently hinder successful cell and tissue transplantation. iPS cells are also a valuable research tool for understanding how different diseases develop.
Because iPS cells are derived from skin or other body cells, it is sometimes preferred over embryonic stem cells from embryos or eggs. This process must be carefully controlled and tested for safety before it’s used to create treatments. In animal studies, some of the genes and the viruses used to introduce them have been observed to cause cancer. Further research is also needed to make the process of creating iPS cells more efficient.
iPS cells are under further investigation at Boston Children’s in the lab of George Q. Daley, MD, PhD, Director of Stem Cell Transplantation Program — who reported creating 10 disease-specific iPS lines, which is part of a growing repository of iPScell lines.
Scientists use “embryonic stem cell” as a general term for pluripotent stem cells that are made using embryos or eggs, rather than for cells genetically reprogrammed from the body. There are several types of embryonic stem cells:
Perhaps the most widely-known type of pluripotent stem cell, made from unused embryos that are donated by couples who have undergone in vitro fertilization(IVF). The IVF process — in which the egg and sperm are brought together in a lab dish — frequently generates more embryos than a couple needs to achieve a pregnancy.
These unused embryos are sometimes frozen for future use, sometimes made available to other couples undergoing fertility treatment, and sometimes simply discarded, but some choose to donate them.
Pluripotent stem cells made from embryos aren’t genetically matched to either parent, and are unlikely to be used to create cells for treatment. They are used to advance our knowledge of how stem cells behave and differentiate.
The term somatic cell nuclear transfer (SCNT) means the transferring of nucleus from a somatic cell — any cell of the body — to another cell. This type of pluripotent stem cell, sometimes called an ntES cell, has only been made successful in animals under lab conditions. To make ntES cells in human patients, an egg donor would be needed, as well as a cell from the patient (typically a skin cell).
The process of transferring a different nucleus into the egg reprograms it to a pluripotent state, reactivating the full set of genes for making all the tissues of the body. The egg is then allowed to develop in the lab for several days, and pluripotent stem cells are derived from it.
Like iPS cells, ntES cells match the patient genetically. If created successfully in humans, and if proven safe, ntES cells could completely eliminate tissue matching and tissue rejection problems. For this reason, they are actively being researched at Boston Children’s.
Through chemical treatments, unfertilized eggs can develop into embryos without being fertilized by sperm, a process called parthenogenesis. The embryos are allowed to develop in the lab for several days, and then pluripotent stem cells can be derived from them.
If this technique is proven safe, a person may be able to donate their own eggs to create pluripotent stem cells matching them genetically that in turn could be used to make cells that wouldn’t be rejected by their immune system.
Through careful genetic typing, it may be possible to use pES cells to create treatments for patients beyond the egg donor themselves by creating “master banks” of cells matched to different tissue types. In 2006, working with mice, Boston Children’s researchers were the first to demonstrate the potential feasibility of this approach.
Because pES cells can be made more easily and more efficiently than ntES cells, they could potentially be ready for clinical use sooner. However, more needs to be known about their safety. Concerns have been raised that tissues derived from them might not function normally.
What makes pluripotent stem cells important?
Pluripotent stem cells can be used to create any cell or tissue the body might need to counter a wide-range of diseases including:
- spinal cord injury
- childhood leukemia
- heart disease.
Pluripotent stem cells can also potentially be customized to provide a perfect genetic match for any patient. This means that patients could receive transplants of tissue and cells without tissue matching and tissue rejection problems, and without the need to take powerful immune-suppressing drugs for the rest of their lives. Although this vision hasn’t yet been achieved, researchers at Boston Children’s have successfully treated mouse models of human disease using this strategy and hope that the same can be done with patients.
These cells can also make excellent laboratory models for studying how a disease unfolds, which helps scientists pinpoint and track the very earliest disease-causing events in cells — immune deficiencies, Type 1 diabetes, muscular dystrophy, and other disorders are rooted in fetal development. In the lab, researchers can recapture these early origins and observe where the first muscle or blood cell comes from, and how this differs when the patient has a genetic disease. With this information, doctors may be able to intervene and correct the genetic defect before the disease advances.
What unique applications do pluripotent stem cells have?
Each type of pluripotent stem cell has different characteristics — making them useful in different ways:
Induced pluripotent cells (iPS cells)
These offer a unique chance to model human disease and are already being used to make new discoveries about premature aging, congenital heart disease, cancer, and more. Because they’re made from a person’s own cells, they can potentially be manipulated to fix the disease-causing defect and then used to create healthy cells for transplant that won’t be rejected by the immune system. Many people also see iPS cells as a positive alternative to pluripotent stem cells from embryos or eggs.
Embryonic stem cells (ES cells)
The gold standard for the biological concept of pluripotency. Scientists are working with ES cells to learn more about what endows a cell with pluripotency and to discover safer, better ways to create iPS cells. Each type of ES cell is important for different reasons:
ES cells made from donated early embryos are irreplaceable tools for understanding the earliest stages of human development and how specific tissues form. Because they’re not customized to individual patients, their value is mainly in research.
The ES cells made through nuclear transfer (ntES cells), like iPS cells, offer the opportunity to create customized, rejection-proof cells and tissues for transplantation. ntES cells are thought to be the most genetically pristine source for creating genetically-matched cells, so they may provide a faster and safer route to the clinic.
ES cells that are made through parthenogenesis (pES cells) also offer the opportunity to create customized, rejection-proof cells. Though less genetically pristine than ntES cells, they are less technically cumbersome to produce. Through genetic typing, they could potentially be banked to create a selection of off-the-shelf cell-based treatments.
How do we get pluripotent stem cells?
Pluripotent stem cells can be created in several ways, depending on the type.
The work of several labs, including that of George Q. Daley, MD, PhD, Director of Stem Cell Transplantation Program, have shown that it requires only a handful of genes to reprogram an ordinary cell from the body, such as a skin cell, into what’s known as an induced pluripotent cell (iPS cell). Currently, these genes (Oct4, Sox2, Myc, and Klf4) are most commonly brought into the cell using viruses, but there are newer methods that do not use viruses.
Although skin cells are probably the number-one source of iPS cells currently, lines are also being created from blood cells and mesenchymal stem cells (a type of multi potent adult stem cell that gives rise to a variety of connective tissues). Laboratories in the Stem Cell Program are exploring whether iPS lines made from different kinds of patient cells are easier to work with, or can more readily form the particular kind of cell a patient might need for treatment.
Researchers are also continuing to experiment with more efficient programming techniques to get a higher yield of true pluripotent stem cells.
Another major source of pluripotent stem cells for research purposes is unused embryo donated by couples under going in-vitro fertilization (IVF). Some of these may be poor-quality embryos that would otherwise be discarded. The resulting cells are considered to be true embryonic stem cells (ES cells).
The donated embryos are placed in a media preparation in special dishes and allowed to develop for a few days. At about the fifth day the embryo reaches the blastocyst stage and forms a ball of 100-200 cells. At this stage, ES cells are derived from the blastocyst’s inner cell mass. In some cases, the ES cells can be isolated even before the blastocyst stage.
To date, Boston Children’s has created more than a dozen new ES cell lines using this approach, which we are now making available to other scientists. These ES cells are not genetically matched to a particular patient, but instead are used to advance our knowledge of how stem cells behave and differentiate.
Some people question the ethics of using discarded IVF embryos for research. For more discussion, see Policy and Ethics.
The process called nuclear transfer involves combining a donated human egg with a cell from the body (typically a skin cell) to create a type of embryonic stem cell, sometimes called an ntES cell. Nuclear transfer requires an egg donor.
First, an incredibly thin microscopic needle is used to remove the egg’s nucleus, which contains all the egg’s genetic material, and replace it with the nucleus from the body cell. The process of transferring the nucleus into the egg reprograms it, reactivating the full set of genes for making all the tissues of the body. How this happens isn’t well understood yet, and researchers in the Stem Cell Program are trying to understand it better.
Next, the resulting reprogrammed cell is encouraged to develop and divide in the lab, and by about day five, it forms a blastocyst, a ball of 100-200 cells. The inner cells of the blastocyst are then isolated to create ntES cells.
Of all the techniques for making pluripotent cells, nuclear transfer is the most technically demanding and therefore the least efficient. To date, it has only been successful in lower animals, not in humans. But because the stem cells created would be an exact genetic match to the patient, nuclear transfer may eliminate the tissue matching and tissue rejection problems that are currently a serious obstacle to successful tissue transplantation. For this reason, nuclear transfer is an important area of research at Boston Children’s.
Because ntES cells created from human patients would match them genetically, nuclear transfer is sometimes called therapeutic cloning — not to be confused with the concept of reproductive cloning.
Using a series of chemical treatments, it’s possible to trick an egg into developing into an embryo without being fertilized by sperm. This process, called parthenogenesis, sometimes happens in nature, allowing many plants and some animals to reproduce without the contribution of a male.
By inducing parthenogenesis artificially, researchers have been able to create parthenogenetic embryonic stem cells or pES cells in mice. The embryos created, known as parthenotes, are grown for about five days until they reach the blastocyst stage. Development is then stopped and pES cells are derived from the blastocyst’s inner core of cells.
Parthenogenesis hasn’t been accomplished in human eggs yet, at least not by choice. (A Korean team is thought to have created human pES cells accidentally in 2007.) But researchers at Boston Children’s are trying to do so, since pES cells, if carefully typed genetically, could potentially be used to create master banks of pluripotent stem cells. Doctors could then choose a cell line that’s genetically compatible with the patient’s immune system.
Of more immediate concern is the possibility that parthenogenesis could be used to make pES cells for the egg donors themselves or a sibling. However, before using these cells in patients, researchers need to know more about the safety of this approach.
Pluripotent stem cell research
The Stem Cell Program at Boston Children's is pursuing several approaches to creating pluripotent stem cells in parallel:
- using embryos from in vitro fertilization (IVF) donated under strict ethical criteria
- using nuclear transfer and parthenogenesis to create embryonic stem cells (ES cells) and genetic reprogramming of skin cells and other body cells to create induced pluripotent stem cells (iPS cells).
Each method has its advantages and disadvantages, but by studying them all, scientists can get the greatest understanding of how stem cells work in order to maximize their treatment potential. Researchers are also examining potential safety issues with using the different kinds of pluripotent stem cells and seeking ways to address them.
Recently, the Daley Lab and collaborators won a $1.7 million National Institutes of Health grant to do a comprehensive comparison of the properties of iPS cells and ES cells derived from various sources. One goal of this project is to determine whether iPS cells are functionally equivalent to ES cells.
Eleven new ES cell lines created at Boston Children's were among the first 13 new lines to become eligible for federal research funding in December 2009. All were created through the donation and use of poor-quality embryos that are typically discarded as part of the IVF process. The Stem Cell Program is making these lines available to scientists around the world.
The Daley Lab was one of the first three labs in the world to successfully create human iPS cells through genetic reprogramming techniques, an accomplishment cited by the journal Science in its 2008 Breakthrough of the Year issue. The lab then created the first repository of iPS cells from patients with specific diseases.
These new iPS lines, developed from the cells of patients ranging in age from one month to 57 years old, with disorders ranging from diabetes to Parkinson disease, have been deposited in a new core facility established by the Harvard Stem Cell Institute. Many are under active study at Children’s, and are already beginning to yield clues about how diseases unfold in their earliest stages. Some of these studies are beginning to suggest a path to therapy. Children’s researchers are also actively working to refine techniques for making iPS cells and to create additional lines specific to other diseases.