Science 101: Induced Pluripotent Stem Cells are Not Human Embryos

The scientific advances come at a dizzying pace—stem cells, cloning, lab-grown organs, 3-parent embryos.  Many of these experiments pose significant ethical challenges.

For example, the cloning technique known as somatic cell nuclear transfer uses the DNA of a body cell and an egg cell to create a new embryo.  The laboratory creation of the cloned organism crosses an ethical line, as well as the subsequent use of the new being for laboratory experiments, including destruction to harvest his or her embryonic stem cells.

This is exactly what happened recently when Oregon scientists created cloned human embryos and, for the first time, successfully grew embryonic stem cells from the clones.  Yet the terminology and explanations from some quarters served only to confuse many, as our colleague Wesley Smith pointed out.

Similar confusion about the truth revolves around induced pluripotent stem (iPS) cells, the embryonic-like stem cells created from skin cells.  Dr. Shinya Yamanaka, who originated the technique, won the 2012 Nobel Prizein Physiology or Medicine for his discovery.  The research on iPS cells has exploded, and recent news stories trumpeted the possibility of growing a new liverfrom a mixture of iPS cells and adult stem cells, or potentially seeding iPS cells to grow a new humanpancreas in a pig.  But would these applications be ethical, and are iPS cells themselves an ethical alternative to embryonic stem cells?  Again, there is great confusion among pro-lifers because of scientific terminology.

The following is an attempt to provide clarity on one aspect of iPS cell science.  It wades some distance into the weeds of details.  But keep your feet planted and read on, and you’ll find there is a glimmer of truth shining through.

Stage-Specific Embryonic Antigen-1 (SSEA-1):  What it is and what it is not

Maureen L. Condic[1] and David Prentice[2]

People often wonder about human cell types that express “embryonic antigens”; what does this mean and do we need to be concerned about it?  In particular, what is “stage specific embryonic antigen-1″ or “SSEA-1″ and should we be concerned that it is sometimes produced by human stem cells?

An “antigen” is simply a molecule or a region of a molecule that is recognized by an antibody.  Some antigens are specific to certain cells or tissues, while others are shared among many cell types.  Because different molecules with different functions can share similar regions that are recognized by the same antibody, scientists call these regions by a common name (“stage-specific embryonic antigen-1,” for example), regardless of where the antigen is found.

Stage-specific embryonic antigen-1 (SSEA-1) is the name for a particular group of sugars that is found on many different molecules.  This sugar group plays important roles in cell attachment and cell migration for many different cell types.[3]  The exact same group of sugars has been discovered more than once over the course of history by scientists who were investigating different questions, so it is known by at least two other names, CD15 and sialyl-Lewis^X (or more simply, “Lewis-X”).

Lewis-X was originally discovered in the 1940s as a blood factor in some individuals.[4]  Normal adult blood cells (in particular leukocytes), produce or “express” this group of sugars.  Over the years, scientists found that many adult tissues express Lewis-X, including cells in the brain, stomach, colon, mammary gland and kidney.[5]   In addition, scientists learned that a wide variety of cancer cells express Lewis-X, and this sugar group has been extensively studied as a marker for cancer.  In the 1980s, an international workshop attempted to create a consistent nomenclature for the many “cell differentiation” antigens that had been described in blood cells, and renamed sialyl-Lewis^X  as “cell differentiation molecule 15″ or CD15.[6]

In the late 1970s, an independent group of scientists interested in mouse embryonic development described an antigen they called “stage-specific embryonic antigen-1″ or SSEA-1.[7]   This antigen was found on cells of early mouse embryos, beginning around the 8-cell stage.  A few years later, several laboratories realized that SSEA-1 was the exact same group of sugars as the well-studied Lewis-X factor found in adult blood cells.[8]  Unfortunately, all three names (Lewis-X, CD15 and SSEA-1) are still commonly used for the same antigen.

Comparing human and mouse embryos, there are some differences in the expression of Lewis-X/CD15/SSEA-1 during early development.  In humans, this antigen is not expressed until the late blastocyst stage (considerably later than it is seen in mouse), and it is found primarily on cells that will produce the placenta.[9]  It is also expressed by numerous, more “advanced” embryonic and fetal tissues, including cells that give rise to the nervous system,[10] lungs, kidney and liver.[11]  Yet Lewis-X/CD15/SSEA-1 is clearly not expressed by the totipotent zygote or by early cleavage-stage totipotent cells in either mouse or human embryos.[12]  Thus, despite the word “embryonic” in the name, expression of Lewis-X/CD15/SSEA-1 does not indicate a full and complete embryo is present, because this antigen is normally expressed only by parts of a whole human or mouse embryo; i.e., by more restricted cells that are found in later arising tissues.

Recent concern over Lewis-X/CD15/SSEA-1 arises from the fact that this antigen is occasionally expressed by some stem cells.  Direct analysis indicates that human embryonic stem cells (ESCs) typically do not express SSEA-1. This agrees with the biology of embryonic development, since Lewis-X/CD15/SSEA-1 is found in cells that give rise to the placenta, a tissue that is not generally produced by human ESCs.[13]  Similarly, most human induced-pluripotent stem cells (iPSCs) also do not express SSEA-1, since iPSCs also do not produce placenta-like tissue efficiently.  In the original paper describing the generation of human iPS cells, Dr. Shinya Yamanaka was careful to point out that iPS cells are very similar to ESCs in this regard, stating, “In general, except for a few cells at the edge of the colonies, human iPS cells did not express stage-specific embryonic antigen (SSEA)-1 (Figure 1H).”[14]   In contrast, some types of cancer (e.g., cell lines derived from testicular or ovarian carcinomas) that produce a broad range of human cell-types, including placenta-like cells, express high levels of Lewis-X/CD15/SSEA-1.[15]  In addition, this “embryonic antigen” is often observed in cultures of “adult” stem cells, including cells from bone marrow, placenta and umbilical cord.[16]

Importantly, expression of Lewis-X/CD15/SSEA-1 by normal adult cells and by some kinds of cancer is a clear indication that these cells are specialized, and not similar to either pluripotent stem cells or totipotent human zygotes.

Should pro-life individuals be concerned about stem cells that express “embryonic antigens” like SSEA-1?

It depends entirely on the nature of the antigen itself and whether it is normally expressed in wholeembryos or merely in the specialized cells of embryos (like blood).  Despite several very detailed analyses,[17] scientists have not discovered an antigen that is specific for a totipotent one-cell human embryo (i.e., for a zygote).  If a molecule was discovered that was normally only expressed by human zygotes or early stage totipotent cells of the human embryo, and this molecule was alsoexpressed by some types of stem cells, this still would not “prove” that these stem cells were embryos, but it would raise some concern.  However, this concern definitely does not apply to Lewis-X/CD15/SSEA-1, which is not expressed by totipotent human embryos, and is expressed by many specialized adult cell types.  Don’t be misled by the simple fact that for historical reasons one of the names of this common antigen happens to include the word “embryonic.”  Expression of this antigen by some human stem cells does not make them human embryos.  Rather, it suggests that in Dr. Yamanaka’s cultures, a few iPS cells at the edge of the colonies had matured into a more specialized cell type, perhaps even into blood cells (which are sometimes seen in stem cell cultures).  But these cells were no more “embryos” than the Lewis-X/CD15/SSEA-1 expressing cells in adult human blood are “embryos.”