Dan Yarosh
MENDING LIFE:
WHY AND HOW CELLS PROTECT THEIR DNA
Essays on Genome Stability, Disease, Aging and Therapy
5. Guardians of Renewal: How Stem Cells Mend Their DNA to Build and Repair the Body
Regeneration is the most amazing fact of our lives. Every tissue in our body is constantly renewed for decades upon decades. Skin replaces itself every few weeks. Blood cells turn over in days. The intestinal lining is rebuilt in less than a week. Even parts of the brain show limited regeneration. And of course, we have been making new generations of young people for hundreds of thousands of years.
None of this would be possible without DNA repair.
In contrast to this seeming perpetuity, most cells have a limited lifespan. They stop dividing and die after a few dozen rounds. Only a small population of stem cells retains the ability to divide indefinitely, self-renewing and generating specialized descendants.
DNA repair is even more important to them. Stem cells must preserve their DNA not just for themselves, but for every cell in their lineage. A single mutation in a stem cell can be propagated to thousands or millions of daughter cells. For this reason, stem cells maintain some of the most sophisticated and tightly regulated DNA repair systems in biology.
Understanding how stem cells protect their genomes reveals why development succeeds, why tissues age, and why many cancers begin in stem-like cells.
The Big Picture: Why Genome Integrity Matters More in Stem Cells
Most differentiated cells have limited lifespans. If they accumulate too much damage, they commit suicide and are replaced. Stem cells persist for decades and continually supply tissues with new cells. This makes them both indispensable and dangerous.
If stem cell DNA is compromised, mutations spread throughout tissues, cancer risk increases, cell regeneration declines, and developmental defects arise in future generations. To avoid this, stem cells prioritize genome maintenance over speed or efficiency and operate under stricter quality control than any other cell type. They excel over somatic cells in many repair functions:
- High-fidelity repair. Stem cells have greater repair activity than other cell types in nucleotide excision repair, base excision repair, and homologous recombination, all of which are the most accurate repair systems.
- Reduced mutations and their consequences. The emphasis on these error-free repair systems means that mutations are less likely to accumulate, which would otherwise lead to dysfunctional tissues or cancer. Accurate repair avoids “stem cell exhaustion”, which results in cells entering apoptosis (cellular suicide) or senescence (zombie cells).
- Quiescence Protection: Many adult stem cells remain in a dormant state (quiescence), minimizing metabolic stress. DNA repair mechanisms keep this resting pool healthy.
- Protecting Stemness: Recent studies show that DNA repair proteins (like XPC) also help maintain the expression of genes responsible for stem cell identity (stemness), ensuring they don't differentiate prematurely and lose their immortal power.
Why Babies are Born Young – DNA Repair in Germ Line Cells
In humans, DNA repair in the egg, sperm, and fertilized egg follows a "relay" system where the oocyte (egg) eventually takes over the primary responsibility for maintaining the integrity of both parental genomes.
During egg formation (oogenesis), DNA repair enzymes, specifically those involved in the base excision repair (BER) pathway, work alongside TET (Ten-eleven translocation) enzymes to strip off methylation markers on the DNA’s cytosines. Instead of removing the marker directly, TET enzymes act by converting the modified base into oxidative derivatives that are subsequently recognized as "damaged" by the BER system and replaced with unmarked cytosine.
This erases most of the epigenetic markers acquired by DNA during the parent’s lifetime, allowing the embryo to start out fresh. Later, de novo methylation adds new methylation marks to the oocyte genome. TET enzymes and repair-mediated demethylation act concurrently to refine these patterns and maintain certain areas as unmethylated, especially at gene promoters. While global demethylation occurs, maternal imprinting marks are protected and retained through the coordinated action of special enzymes (methyltransferases).
During maturation, the egg synthesizes and stores large amounts of DNA repair proteins and mRNA transcripts. These maternal stores are critical because they must support the embryo after fertilization until it can produce its own proteins. The efficiency of these repair systems significantly declines with maternal age (especially after age 40), which can lead to higher rates of chromosomal defects and early pregnancy loss.
Unlike the egg, mature sperm are largely incapable of repairing their own DNA. To fit into the small sperm head, DNA is tightly packed with proteins. This extreme compaction physically blocks the entry of bulky DNA repair enzymes. Human sperm have a bare-bones repair pathway—they contain the OGG1 protein, which can identify and remove oxidized bases, but they lack the subsequent enzymes needed to finish the repair.
Upon fertilization, the oocyte provides the missing machinery to complete the repair of the paternal (sperm) genome. For example, the oocyte supplies the enzymes needed to finish the BER process started by the sperm. In addition, since there are now two copies of the genome, the developing embryo uses the very efficient homologous recombination system to fix one chromosome using the other as a source of undamaged DNA. If damage remains, the zygote can activate checkpoints to delay or pause the cell cycle, allowing more time for repair before division. If the damage is too extensive, the oocyte’s repair capacity may be overwhelmed, leading to developmental arrest or miscarriage.
This all amounts to very efficient quality control, at the expense of discarding fertilized eggs. More than 50% of embryos and fetuses are spontaneously aborted, most without the mother even knowing.
How Adult Stem Cells Protect Their DNA
Stem cells use multiple overlapping strategies to preserve genomic stability. Compared with differentiated cells, stem cells have elevated activity of major repair pathways, including base and nucleotide excision repair, as well as homologous recombination and double-strand break repair. Stem cells frequently delay cell division in response to DNA damage to allow more time for repair.
Some stem cells remain in quiescence for long periods, further limiting replication-associated errors. They rely on a form of energy production with reduced reactive oxygen species, a major source of DNA damage. When damage exceeds repair capacity, stem cells are more likely than other cell types to undergo programmed death.
This prevents defective cells from contaminating the regenerative pool. While costly in the short term, this strategy protects long-term tissue integrity. As stem cells differentiate, they often shift from high-fidelity repair to faster, error-prone pathways. Some stem cells divide asymmetrically, producing one stem cell and one differentiated cell. DNA damage and repair signals influence this process, helping determine which daughter cell retains stemness.
When It Goes Wrong: Aging, Degeneration, and Cancer
Failures in stem cell DNA maintenance have profound consequences. With age, repair efficiency declines and stem cells accumulate DNA lesions. This leads to reduced proliferation, impaired differentiation, stem cell exhaustion, and tissue atrophy. These are all the hallmarks of aging tissue and organs. Organ and body longevity is dependent on active DNA repair.
One of the common features of the genetic disease of DNA repair (xeroderma pigmentosum, ataxia telangiectasia, Bloom’s syndrome, Cockayne’s syndrome, etc.) are dysfunction of rapidly dividing stem and progenitor cells, leading to growth retardation, organ malformation, and neurodevelopmental abnormalities.
Many tumors originate from stem or progenitor cells, as in brain gliomas or basal cell carcinoma in skin. These cells already possess long lifespans, self-renewal capacity and robust DNA repair systems. If they lose control of their proliferation by mutations in tumor suppressor genes, cancers arise. These are particularly malignant tumors because their enhanced DNA repair capacity makes them resistant to chemotherapy and radiation. They survive treatment and drive relapse.
What We’re Learning Now: Using Stem Cells for Therapy
One use of stem cells is IVF, in which a stem cell is created by egg fertilization, which is then implanted in the uterus and reliably divides and differentiates into a fetus. More recently, treatment of blood cancers begins with obliterating the existing blood cells in the patient, and transplanting bone marrow stem cells from a donor to the recipient. These stem cells then divide and repopulate the blood with non-diseased cells. Both of these procedures are widely used and highly successful.
A recent innovation in drug discovery are stem cells which are grown in the laboratory to form organelles or artificial organs. These are used for testing new drugs and understanding drug metabolism and resistance in a humanized system which may be better than animal testing.
Attempts to regenerate solid organs by injecting stem cells has not met with success. The stem cells do secrete cytokines and growth factors for a while, but they do not persist beyond a few weeks and any benefits are lost. An exciting new development is the reprogramming of mature cells to a less differentiated state by introducing just a few carefully selected genes. Such an approach to “reverse aging” is about to be tested in a clinical trial.
Key Takeaways
- Stem cells require exceptional genome protection, and they have elevated levels of high-fidelity DNA repair pathways
- DNA repair is essential for the development and protection of the germ line through generations
- Adult stem cells have strategies for protecting the genome with DNA repair, or suicide if the damage is too severe
- Failures in DNA repair among stem cells drive aging and cancer
- New methods are being developed to harness stem cells for treating diseases and slowing aging
The next essay will describe how a DNA repair system detects bases that are not properly matched. Genetic defects in this system cause cancer, but an intact system is used in modern cancer therapy to kill tumors.
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References & Further Reading
1. Rossi, D. J., Jamieson, C. H. M., & Weissman, I. L. (2008). Stems cells and the pathways to aging and cancer. Cell, 132, 681–696.
Stem cell aging and genomic stability
2. Blanpain, C., & Fuchs, E. (2014). Stem cell plasticity. Plasticity of epithelial stem cells in tissue regeneration. Science, 344, 1242281.
Stem cells and tissue maintenance
3. Beerman, I., & Rossi, D. J. (2014). Epigenetic regulation of hematopoietic stem cell aging. Exp Cell Res, 329, 192-199.
Aging, DNA repair, and stem cells
4. Mandal, P. K., Blanpain, C., & Rossi, D. J. (2011). DNA damage response in adult stem cells. Nat Rev Mol Cell Biol, 12, 198-202.
Core review on repair pathways in stem cells
5. Mohrin, M., et al. (2010). Hematopoietic stem cell quiescence promotes error-prone DNA repair and mutagenesis. Cell Stem Cell,7, 174-185.
Quiescence and repair pathway choice
6. Yahata, T., et al. (2011). Accumulation of oxidative DNA damage restricts the self-renewal capacity of human hematopoietic stem cells. Blood, 118, 2941-2950.
Oxidative stress in stem cell aging
7. Bao, S., et al. (2006). Glioma stem cells promote radioresistance by preferential activation of the DNA damage response. Nature, 444, 756-760
Repair and therapy resistance
8. Rocha, C., et al. (2013). The role of DNA repair in the pluripotency and differentiation of human stem cells. Mutat Res, 752, 25-35.
DNA repair in stem cells
9. Ledford, H. (2026) This method to reverse cellular ageing is about to be tested in humans. Nature, 652, 291-293.
New research to reprogram cells to undifferentiated state for therapy.