Cell-autonomous correction of ring chromosomes in human induced pluripotent stem cells

Ring chromosomes are structural aberrations commonly associated with birth defects, mental disabilities, and growth retardation(1,2). Rings form upon fusion of the long and short arms of a chromosome, sometimes associated with large terminal deletions(2). Due to the severity of these large-scale aberrations affecting multiple contiguous genes, no possible therapeutic strategies for ring chromosome disorders have so far been proposed. During cell division ring chromosomes can exhibit unstable behavior, leading to continuous production of aneuploid progeny with low viability and high cellular death rate(3-9). The overall consequences of this chromosomal instability have been largely unexplored in experimental model systems. Here we generated human induced pluripotent stem cells (iPSCs)(10-12) from patient fibroblasts containing ring chromosomes with large deletions and found that reprogrammed cells lost the abnormal chromosome and duplicated the wild type homologue via the compensatory uniparental disomy (UPD) mechanism. The karyotypically normal iPSCs with isodisomy for the corrected chromosome outcompeted co-existing aneuploid populations, allowing rapid and efficient isolation of patient-derived iPSCs devoid of the original chromosomal aberration. Our results suggest a fundamentally different function of cellular reprogramming as a means of “chromosome therapy”(13) to reverse combined loss-of-function across many genes in cells with large-scale aberrations involving ring structures. In addition, our work provides an experimentally tractable human cellular model system for studying mechanisms of chromosomal number control, which is of critical relevance to human development and disease.