The researchers' study, published in today's issue of the journal Nature, found that an enzyme, activation-induced cytidine deaminase (AID), helps in the process that changes an adult human cell into an induced pluripotent stem cell (iPS cell). These iPS cells can then be developed into any kind of cell needed to therapeutically restore tissues and organs.
The finding settles an ongoing controversy regarding use of AID to reprogram cells, says the study's senior investigator, Dr. Todd Evans, vice chair for research and professor of cell and developmental biology in the Department of Surgery at Weill Cornell Medical College.
"The dispute was whether AID is required to make iPS cells, and we found that the enzyme does make reprogramming very efficient, although it is not absolutely necessary," says Dr. Evans, an internationally-recognized authority on regenerative medicine. "In fact, we plan to test if reprogramming iPS cells without AID may even be helpful."
One reason is that AID can cause genetic mutations that can lead to cancer. AID is best known as a master regulator of antibody diversity in B cells, and in order to create varied types of beneficial antibodies, it routinely mutates antibody genes. But sometimes the process goes awry, resulting in development of B cell lymphoma, Dr. Evans says. "That leads us to believe that if you can reprogram cells without AID, it could reduce risk of potential mutations, and thus be safer."
iPS Cells Without AID Remember What They Once Were
In order to push a cell, such as a fibroblast, to revert to an iPS cell, the epigenetic "markers" that define an adult cell must be removed. "All cells of the body have the same genes, but they are used differently in different tissues," Dr. Evans explains. "If an undifferentiated cell becomes a heart cell, somehow it has to lock in and stabilize that particular adult phenotype and not forget what it is."
One way that function is accomplished is by placing a methylation group on top of certain genes that activate other cell destinations -- such as to become a liver cell -- usually switching those genes off. "We have known how these marks are put on genes, but we didn't know how they were taken off in the process of pushing an adult cell to revert back to a stem-cell-like state," Dr. Evans says.
Dr. Evans and his colleagues found that the AID enzyme removed those epigenetic markers.
They then created a mouse that did not produce AID to see if the animal's adult fibroblast cells could be pushed back to iPS cells. "If you need AID to reprogram the cells, you shouldn't be able to do it, or do it well."
Surprisingly, they found that the cells at first seemed to want to reprogram even faster than normal cells, but most never fully reverted to a stem-cell-like state. "They eventually crashed and differentiated back into a fibroblast," Dr. Evans says. "What that meant is that they never cleared their memory of being a fibroblast cell. AID efficiently removes that epigenetic memory, smoothing the way for a cell to morph into an undifferentiated state."
But some of the mouse adult fibroblasts lacking AID -- those that Dr. Evans says they "babysat" -- did become iPS cells.
Despite the fact that reprogramming adult cells without AID is inefficient, the researchers say that the method may offer another advantage besides increased safety.
"It might be useful to allow epigenetic memory to be retained," Dr. Evans says. "If you want to make new cardiac cells to repair a patient's heart, it might be better to start with a cardiac cell and push it to become an iPS cell, from which other cardiac cells could be made. If these cells remember they were cardiac cells, they might make a better heart cell than if they came from reprogrammed fibroblasts."
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The study was supported by National Institutes of Health grants (HL056182 and AI072194) and a National Science Foundation CAREER grant (1054964).
Other study co-authors include Ritu Kumar, Ting-Chun Liu, Philipp Franck and Olivier Elemento from Weill Cornell Medical College; Lauren DiMenna, Nadine Schrode, Silvia Muñoz-Descalzo, Anna-Katerina Hadjantonakis and Jayanta Chaudhuri from Memorial Sloan-Kettering Cancer Institute; and Ali A. Zarrin from Genentech.
The authors declare no competing financial interests.
Weill Cornell Medical College
Weill Cornell Medical College, Cornell University's medical school located in New York City, is committed to excellence in research, teaching, patient care and the advancement of the art and science of medicine, locally, nationally and globally. Physicians and scientists of Weill Cornell Medical College are engaged in cutting-edge research from bench to bedside, aimed at unlocking mysteries of the human body in health and sickness and toward developing new treatments and prevention strategies. In its commitment to global health and education, Weill Cornell has a strong presence in places such as Qatar, Tanzania, Haiti, Brazil, Austria and Turkey. Through the historic Weill Cornell Medical College in Qatar, the Medical College is the first in the U.S. to offer its M.D. degree overseas. Weill Cornell is the birthplace of many medical advances -- including the development of the Pap test for cervical cancer, the synthesis of penicillin, the first successful embryo-biopsy pregnancy and birth in the U.S., the first clinical trial of gene therapy for Parkinson's disease, and most recently, the world's first successful use of deep brain stimulation to treat a minimally conscious brain-injured patient. Weill Cornell Medical College is affiliated with NewYork-Presbyterian Hospital, where its faculty provides comprehensive patient care at NewYork-Presbyterian Hospital/Weill Cornell Medical Center. The Medical College is also affiliated with the Methodist Hospital in Houston. For more information, visit weill.cornell.edu.
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