Genetic Reprogramming Rewinds Cellular Aging Clock
|This fluorescent in situ hybridization (FISH) image shows the green fluorescent signal from telomeres, the DNA sequences at the tips of chromosomes (shown here in blue). The intensity of the green signal is one indicator of telomere length, which is a measure of cellular aging and determines how many times a cell can divide.|
In a study that ties stem cell research together with research on aging and cancer, investigators have used genetic reprogramming to create cells from patients with a rare premature-aging disorder that are able to rebuild their telomeres — the tips of chromosomes that must be maintained to prevent a cell from “aging” and enabling it to divide and make copies of itself.
Researchers in the laboratory of George Q. Daley, Director of the Stem Cell Transplantation Program at Children’s Hospital Boston, report successfully reactivating the cellular enzyme telomerase, which maintains the telomeres, in patients with dyskeratosis congenita. In this rare genetic disorder, genetic mutations cause telomerase to be defective, leaving the chromosomes without protection from damage and unable to compensate for the natural shortening of telomeres that occurs when a cell divides. As a result, a patient’s cells “age” more quickly, leading to bone-marrow failure (an inability to make enough blood cells), degradation of multiple tissues, premature aging-like symptoms and a much-shortened lifespan.
Published in Nature (Advance Online) on February 17, 2010, the findings suggest the possibility of developing drugs to help patients with dyskeratosis congenita maintain their telomeres, prolonging their lives. But the study also has broad implications for stem-cell research, as well as research on aging and even cancer.
“This paper illustrates how reprogramming a patient’s skin cells into stem cells can teach us surprising lessons about human disease,” says Daley, who is also associate director of the Stem Cell Program at Children’s.
The ability to maintain and elongate telomeres is believed to endow stem cells with the ability to endlessly replicate themselves. Researchers studying aging believe that this same ability could slow or halt natural aging, at least in our cells. In the cancer field, telomerase is thought to contribute to the “immortalization” and uncontrolled growth of cells that mark human cancer, and has become a target in attempts to treat cancer.
The research project, led by Suneet Agarwal, took skin cells from three patients with dyskeratosis congenita and introduced four genes into the cells to transform them into pluripotent stem cells (iPS cells), which are similar to embryonic stem cells. Their goal was to better understand the disease at the cellular level — and also to see if the process of genetic reprogramming would actually affect the disease.
It did. Once reprogrammed, the diseased cells showed increased levels of telomerase RNA component (TERC), the part of the telomerase enzyme that provides the template for adding DNA onto the telomeres. Even though the patients had a genetic defect in TERC, the telomeres were once again able to elongate, and the cells were able to replicate indefinitely — just as healthy iPS cells can.
Further studies showed that human embryonic stem (ES) cells maintain elevated TERC levels similar to those found in iPS cells derived from healthy people, and that the more TERC found in iPS cells from patients with dyskeratosis congenita, the more telomerase activity.
The discovery of telomerase, in the 1980s, won the 2009 Nobel Prize in Medicine and Physiology. Since then, researchers have focused largely on a different component of the enzyme, known as TERT, which is the portion of the enzyme that actually adds DNA to the telomeres. But the RNA component, TERC, turns out to be equally important in telomere maintenance.
“This study suggests that the level of TERC isn’t just static, but could possibly be manipulated,” says Agarwal, an attending physician in Children’s Stem Cell Transplantation Program. “If you could do that in a patient with dyskeratosis congenita, you might be able to elongate their telomeres and sustain them a little longer.”
Agarwal is seeking funding to do drug screening to identify compounds that up-regulate TERC. In addition, if iPS cells could be made from these patients, their TERC deficiency could be corrected through the reprogramming process itself — without the need for gene therapy to replace the defective TERC gene. Since dyskeratosis congenita is a blood disorder, these iPS cells could then be used to create blood stem cells for transplantation that would be compatible with patients’ immune systems.
“If you give patients with dyskeratosis congenita a conventional bone marrow transplant, they tend to have higher mortality than other patients because their disease affects so many organ systems,” says Agarwal. “For these patients, and for patients with other bone marrow failure syndromes, it would be ideal to give them a gentler stem cell transplant from their own cells.”
Since creating iPS cells seems to promote telomere elongation, the study also suggests that people of all ages could potentially benefit from cell therapies derived from iPS cells, Agarwal says. “We’re not saying we’ve found the fountain of youth, but the process of creating iPS cells recapitulates some of the biology that our species uses to rejuvenate itself in each generation,” he says.
The study also has implications for understanding cancer. Patients with dyskeratosis congenita are predisposed to cancer, because their shortened telomeres expose their DNA to cancerous mutations. But researchers have wondered why, if the telomeres are shortened, the cancers are able to proliferate. They speculate that cancer cells, which share some characteristics of stem cells, may be able to proliferate by up-regulating TERC.
The study was funded by the National Institutes of Health and the Manton Center for Orphan Disease Research.