Telomere shortening and loss of self-renewal in dyskeratosis congenita iPS cells

The differentiation of patient-derived induced pluripotent stem cells (iPSCs) to committed fates such as neurons, muscle and liver is a powerful approach for understanding key parameters of human development and disease(1–6). Whether undifferentiated iPSCs themselves can be used to probe disease mechanisms is uncertain. Dyskeratosis congenita (DC) is characterized by defective maintenance of blood, pulmonary tissue, and epidermal tissues and is caused by mutations in genes controlling telomere homeostasis(7,8). Short telomeres, a hallmark of DC, impairs tissue stem cell function in mouse models, suggesting that a tissue stem cell defect underlies the pathophysiology of DC(9,10). Here, we show that even in the undifferentiated state, iPSCs from DC patients harbor the precise biochemical defects characteristic of each form of the disease and that the magnitude of the telomere maintenance defect in iPSCs correlates with clinical severity. In iPSCs from patients with heterozygous mutations in TERT, the telomerase reverse transcriptase, a 50% reduction in telomerase levels blunts the natural telomere elongation that accompanies reprogramming. In contrast, mutation of dyskerin (DKC1) in X-linked DC severely impairs telomerase activity by blocking telomerase assembly and disrupts telomere elongation during reprogramming. In iPSCs from a form of DC caused by mutations in TCAB1, telomerase catalytic activity is unperturbed, yet the ability of telomerase to lengthen telomeres is abrogated, because telomerase mislocalizes from Cajal bodies to nucleoli within the iPSCs. Extended culture of DKC1-mutant iPSCs leads to progressive telomere shortening and eventual loss of self-renewal, suggesting that a similar process occurs in tissue stem cells in DC patients. These findings in iPSCs from DC patients reveal that undifferentiated iPSCs accurately recapitulate features of a human stem cell disease and may serve as a cell culture-based system for the development of targeted therapeutics.