Here we analyze the functional interaction between Ku86 and telomerase at the mammalian telomere by studying mice de®cient for both proteins. We show that absence of Ku86 prevents the end-to-end chromosomal fusions that result from critical telomere shortening in telomerase-de®cient mice. In addition, Ku86 de®ciency rescues the male early germ cell apoptosis triggered by short telomeres in these mice. Together, these ®ndings de®ne a role for Ku86 in mediating chromosomal instability and apoptosis triggered by short telomeres. In addition, we show here that Ku86 de®ciency results in telomerase-dependent telomere elongation and in the fusion of random pairs of chromosomes in telomerase-pro®cient cells, suggesting a model in which Ku86 keeps normallength telomeres less accessible to telomerasemediated telomere lengthening and to DNA repair activities.

The ability to generate induced pluripotent stem (iPS) cells from a patient's somatic cells has provided a foundation for organ regeneration without the need for immune suppression. However, it has not been established that the differentiated progeny of iPS cells can effectively reverse failure of a vital organ. Here, we examined whether iPS cell-derived hepatocytes have both the functional and proliferative capabilities needed for liver regeneration in mice with fumarylacetoacetate hydrolase deficiency. To avoid biases resulting from random genomic integration, we used iPS cells generated without viruses. To exclude compensation by hepatocytes not derived from iPS cells, we generated chimeric mice in which all hepatocytes were iPS cell derived. In vivo analyses showed that iPS cells were intrinsically able to differentiate into fully mature hepatocytes that provided full liver function. The iPS cell-derived hepatocytes also replicated the unique proliferative capabilities of normal hepatocytes and were able to regenerate the liver after transplantation and two-thirds partial hepatectomy. Thus, our results establish the feasibility of using iPS cells generated in a clinically acceptable fashion for rapid and stable liver regeneration.

DNA‐PKcs is the catalytic subunit of the DNA‐dependent protein kinase (DNA‐PK) complex that functions in the non‐homologous end‐joining of double‐strand breaks, and it has been shown previously to have a role in telomere capping. In particular, DNA‐PKcs deficiency leads to chromosome fusions involving telomeres produced by leading‐strand synthesis. Here, by generating mice doubly deficient in DNA‐PKcs and telomerase (Terc−/−/DNA‐PKcs−/−), we demonstrate that DNA‐PKcs also has a fundamental role in telomere length maintenance. In particular, Terc−/−/DNA‐PKcs−/− mice displayed an accelerated rate of telomere shortening when compared with Terc−/− controls, suggesting a functional interaction between both activities in maintaining telomere length. In addition, we also provide direct demonstration that DNA‐PKcs is essential for both end‐to‐end fusions and apoptosis triggered by critically short telomeres. Our data predict that, in telomerase‐deficient cells, i.e. human somatic cells, DNA‐PKcs abrogation may lead to a faster rate of telomere degradation and cell cycle arrest in the absence of increased apoptosis and/or fusion of telomere‐exhausted chromosomes. These results suggest a critical role of DNA‐PKcs in both cancer and aging.

HHT is a vascular dysplasia syndrome caused by mutations in TGF-β/BMP pathway genes, ENG and ACVRL1. HHT shows considerable variation in clinical manifestations, suggesting environmental and/or genetic modifier effects. Strain-specific penetrance of the vascular phenotypes of Eng+/− and Tgfb1−/− mice provides further support for genetic modification of TGF-β pathway deficits. We previously identified variant genomic loci, including Tgfbm2, which suppress prenatal vascular lethality of Tgfb1−/− mice. Here we show that human polymorphic variants of PTPN14 within the orthologous TGFBM2 locus influence clinical severity of HHT, as assessed by development of pulmonary arteriovenous malformation. We also show that PTPN14, ACVRL1 and EFNB2, encoding EphrinB2, show interdependent expression in primary arterial endothelial cells in vitro. This suggests an involvement of PTPN14 in angiogenesis and/or arteriovenous fate, acting via EphrinB2 and ACVRL/Alk-1. These findings contribute to a deeper understanding of the molecular pathology of HHT in particular and to angiogenesis in general.