ScienceDaily (Nov. 18, 2012)     Stanford University School of Medicine scientists have    demonstrated, in a study conducted jointly with researchers at    Yale University, that induced-pluripotent stem cells -- the    embryonic-stem-cell look-alikes whose discovery a few years ago    won this year's Nobel Prize in medicine -- are not as    genetically unstable as was thought.  
    The new study, published online Nov. 18 in Nature,    showed that what seemed to be changes in iPS cells' genetic    makeup -- presumed to be inflicted either in the course of    their generation from adult cells or during their propagation    and maintenance in laboratory culture dishes -- instead are    often accurate reflections of existing but previously    undetected genetic variations among the cells comprising our    bodies.  
    That's good news for researchers hoping to use the cells to    study disease or, someday, for regenerative medicine. But it    raises the question of whether and to what extent we humans are    really walking mosaics whose constituent cells differ    genetically from one to the next in possibly significant    respects, said Alexander Urban, PhD, assistant professor of    psychiatry and behavioral sciences. Urban shared senior    authorship of the study with bioinformatics professor Mark    Gerstein, PhD, and neurobiology professor Flora Vaccarino, MD,    both of Yale.  
    It's only a few years ago that human iPS cells started becoming    available to researchers. These cells hold great promise    because they act almost exactly like embryonic stem cells,    which can be nudged to differentiate into virtually any of the    body's roughly 200 different cell types. But iPS cells can be    derived easily from a person's skin, alleviating numerous    ethical concerns arising from the necessity of obtaining    embryonic stem cells from fertilized eggs.  
    At least in principle, iPS cells' genetic makeup closely    reflects that of the individual from whom they were derived.    Today, "heart cells" derived from a heart patient's skin can be    produced in a laboratory dish so scientists can learn more    about that particular patient's condition and to screen drugs    that might treat it. Tomorrow, perhaps, such cells could be    administered to that patient to restore heart health without    being perceived as foreign tissue by the patient's immune    system, which would otherwise reject the implanted cells.  
    However, Urban said, several previous studies have raised    worries regarding iPS cells' genomic stability. Whether it was    the reprogramming procedure researchers use to convert ordinary    adult cells into iPS cells or the culturing techniques employed    to keep them alive and thriving afterward, something appeared    to be inducing an upswing in these cells' manifestation of copy    number variations, or CNVs -- the disappearance or duplication    of chunks of genetic material at specific locations along the    vast stretches of DNA that coil to form the chromosomes    residing in all human cells.  
    CNVs dot everybody's genomes. They occur naturally because of    DNA-copying errors made during cell replication, and accumulate    in our genomes over evolutionary time. The human genome, taken    as a whole, is a DNA sequence consisting of four varieties of    chemical units, strung together like beads on a roughly    3-billion-bead-long necklace. CNVs range in length from under    1,000 DNA units to several million. They account for up to    several percent of the entire human genome, making them a major    source of genetic differences between people.  
    But if either iPS cells' mode of generation or their subsequent    maintenance in culture were promoting an increase in CNVs, it    would seriously compromise these cells' utility in research and    pose a fatal flaw to their use in regenerative medicine, said    Urban. "You would never want to introduce iPS cells into a    patient thinking that these cells had the same genome as the    rest of the patient's cells, when in fact they had undergone    substantial genetic modifications you knew nothing about, much    less their effects," said Urban. (Similar concerns apply to    embryonic stem cells.)  
    To see how serious a problem CNVs might pose for iPS cells'    use, the collaborators performed tiny skin biopsies on seven    volunteers and extracted cells called fibroblasts, which abound    in skin and are amenable to cell culture in general and iPS    cell generation in particular. From these, the team produced 20    separate iPS cell lines in culture. Using now-standard lab    methods, the investigators determined, chemical unit by    chemical unit, the full genomic sequence of the cells composing    each new iPS cell line.  
    Urban and his colleagues, who had likewise assessed the    fibroblasts from which the lines were derived, compared their    genomic sequences with those of the newly generated iPS cells.    The scientists were able to pinpoint numerous CNVs in the new    cells that hadn't shown up in the fibroblasts. This raised the    possibility that the rigors of reprogramming or life in a dish,    or both, had led to new CNVs in the cells.  
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Minority report: Insight into subtle genomic differences among our own cells