Ben Sykes, executive director of the UK National Stem Cell Network gives an overview of where stem cell research is heading
Few readers of this article will not have read a news story or heard a news report about stem cell research and its current and potential medical applications. After all, it is an exciting and promising area of science.
However, there has been so much discussion about stem cell research that it can be difficult to cut through the noise to understand the context and to really find out where current developments are today, and where they may take us in the future.
What is this thing called a stem cell?
A stem cell is an unspecialised cell. It is of special interest because a stem cell can do one of two things under certain conditions: it can reproduce copies of itself almost indefinitely or it can differentiate into a more specialised cell, such as a heart cell or skin cell that are structurally and functionally distinct from one another.
Like most other cells in the human body, stem cells carry a nucleus which contains the normal complement of chromosomes comprising the full human genome. In the past, we thought that a specific cell type, such as a skin cell, could only fulfil its function as a skin cell and then die off. However, recent research has revolutionised our thinking by showing that gene expression patterns in any cell are not necessarily fixed and that they can be turned back into an embryonic stem cell-like state from which it may be possible to derive all the types of cell in the human body with the correct manipulation. Cells that have been reverted to this embryonic stem cell-like state are now referred to as induced pluripotent stem cells, or iPS cells.
There are three main types of stem cell – adult stem cells, embryonic stem cells and iPS cells.
Adult stem cells are found throughout the human body. They vary in number depending on the organ or tissue you look at. Bone marrow stem cells, for example, are present in substantial numbers and have been used for over 50 years in bone marrow transplants – a long-standing stem cell therapy. Globally, there are now a number of properly-regulated Phase 1, Phase 2 and Phase 3 clinical trials underway using adult stem cells to treat certain injuries and diseases. Within the next 5 years, there is a good chance that some of these will result in market approval for adult stem therapies in limited specific conditions, in particular for injuries to the eye, in heart disease and, possibly, stroke damage.
Embryonic stem cells are powerful and versatile cells which, in theory, are capable of producing all cell types in the human body. The potential is enormous but there is some way to go before we are sure of their use in transplantation. Their ability to form every cell type makes them difficult to control and largely explains why only two companies to date have received formal regulatory approval to move into Phase 1 clinical trials with an embryonic stem cell therapy.
These trials are looking to treat spinal cord injury and acute macular degeneration of the eye.
The science of iPS cells is still nascent and there is more we need to fully understand before potential medical applications can be considered. iPS cell research needs to be benchmarked against our extensive knowledge of the embryonic stem cell.
The exciting thing about iPS cells is that they can be used in the laboratory as patient surrogates for the detailed study of almost any disease, as well as for drug toxicity testing and initial high-throughput screening of large numbers of potential drug compounds. Right now, the pharmaceutical industry is assessing their potential and looking towards their development as tools for use in these three areas.
Direct stem cell transplantation will not be the only story in regenerative medicine. Our increasing understanding of how adult stem cells within the body function and behave may lead to the development of a new wave of conventional small molecule drugs. These would stimulate the stem cell-mediated repair of disease-damaged organs and tissues from within.