A critical shortage of donor organs for treating end-stage organ failure highlights the urgent need for generating organs from human induced pluripotent stem cells (iPSCs). Despite many reports describing functional cell differentiation, no studies have succeeded in generating a three-dimensional vascularized organ such as liver. Here we show the generation of vascularized and functional human liver from human iPSCs by transplantation of liver buds created in vitro (iPSC-LBs). Specified hepatic cells (immature endodermal cells destined to track the hepatic cell fate) self-organized into three-dimensional iPSC-LBs by recapitulating organogenetic interactions between endothelial and mesenchymal cells. Immunostaining and gene-expression analyses revealed a resemblance between in vitro grown iPSC-LBs and in vivo liver buds. Human vasculatures in iPSC-LB transplants became functional by connecting to the host vessels within 48 hours. The formation of functional vasculatures stimulated the maturation of iPSC-LBs into tissue resembling the adult liver. Highly metabolic iPSC-derived tissue performed liver-specific functions such as protein production and human-specific drug metabolism without recipient liver replacement. Furthermore, mesenteric transplantation of iPSC-LBs rescued the drug-induced lethal liver failure model. To our knowledge, this is the first report demonstrating the generation of a functional human organ from pluripotent stem cells. Although efforts must ensue to translate these techniques to treatments for patients, this proof-of-concept demonstration of organ-bud transplantation provides a promising new approach to study regenerative medicine.

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53BP1 is a human BRCT protein that was originally identified as a p53-interacting protein by the Saccharomyces cerevisiae two-hybrid screen. Although the carboxyl-terminal BRCT domain shows similarity to Crb2, a DNA damage checkpoint protein in fission yeast, there is no evidence so far that implicates 53BP1 in the checkpoint. We have identified a Xenopus homologue of 53BP1 (XL53BP1). XL53BP1 is associated with chromatin and, in some cells, localized to a few large foci under normal conditions. Gamma-ray irradiation induces increased numbers of the nuclear foci in a dose-dependent manner. The damage-induced 53BP1 foci appear rapidly (in 30 min) after irradiation, and de novo protein synthesis is not required for this response. In human cells, 53BP1 foci colocalize with Mrel1 foci at later stages of the postirradiation period. XL53BP1 is hyperphosphorylated after X-ray irradiation, and inhibitors of ATM-related kinases delay the relocalization and reduce the phosphorylation of XL53BP1 in response to X-irradiation. In AT cells, which lack ATM kinase, the irradiation-induced responses of 53BP1 are similarly affected. These results suggest a role for 53BP1 in the DNA damage response and/or checkpoint control which may involve signaling of damage to p53. Double-stranded DNA (dsDNA) breaks are potentially dangerous to cells since they may lead to chromosome breakage and loss of genetic information. However, transient dsDNA breaks are essential to initiate recombination or to solve topological problems at the end of DNA synthesis (24,58,63). dsDNA breaks can also occur if replication forks are stalled (e.g., due to base modifications, single-stranded-DNA gaps, or low deoxynucleoside triphosphate pools), and in Escherichia coli, RuvABC Holliday junction resolvase has been shown to catalyze the breakage (53, 68). In general, cells do not enter S or M phase before the DNA lesions are properly repaired due to the action of the DNA damage checkpoint (19). The sensitivity of cancer cells to DNA-damaging agents is explained by the fact that cancer cells have often lost some aspects of checkpoint function, which has provided them with a higher rate of genomic evolution to acquire a growth advantage (18).The DNA damage checkpoint is a signal transduction cascade that relays information from DNA lesions to components of the cell cycle (reviewed in references 5, 40, and 41). In response to DNA damage, ATM-related protein kinases (ATM, ATR, and possibly DNA-PK) activate the downstream effector checkpoint kinases Chk1 and Chk2 (28,35,62). Then Chk2 (homologues are Rad53 in budding yeast and Cds1 in fission yeast) phosphorylates p53 or Cdc25A, which arrests the cell cycle at the G 1 /S or G 2 /M boundary, respectively (6,7,8,20,54). Compared to these downstream events, the molecular mechanism of how DNA lesions activate the kinase cascade is not well understood. Yeast genetics have identified candidate genes that appear to be involved in these early sensing and processing stages. These genes are classified into three groups: yeast h...

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