SUMMARY UHRF1 is an essential regulator of DNA methylation that is highly expressed in many cancers. Here, we use transgenic zebrafish, cultured cells and human tumors to demonstrate that UHRF1 is an oncogene. UHRF1 overexpression in zebrafish hepatocytes destabilizes and delocalizes DNMT1, causes DNA hypomethylation and Tp53-mediated senescence. Hepatocellular carcinoma (HCC) emerges when senescence is bypassed. tp53 mutation both alleviates senescence and accelerates tumor onset. Human HCCs recapitulate this paradigm, as UHRF1 overexpression defines a subclass of aggressive HCCs characterized by genomic instability, TP53 mutation and abrogation of the TP53-mediated senescence program. We propose that UHRF1 overexpression is a mechanism underlying DNA hypomethylation in cancer cells and that senescence is a primary means of restricting tumorigenesis due to epigenetic disruption.

UHRF1 (ubiquitin-like, containing PHD and RING finger domains, 1) recruits DNMT1 to hemimethylated DNA during replication and is essential for maintaining DNA methylation. uhrf1 mutant zebrafish have global DNA hypomethylation and display embryonic defects, including a small liver, and they die as larvae. We make the surprising finding that, despite their reduced organ size, uhrf1 mutants express high levels of genes controlling S-phase and have many more cells undergoing DNA replication, as measured by BrdU incorporation. In contrast to wild-type hepatocytes, which are continually dividing during hepatic outgrowth and thus dilute the BrdU label, uhrf1 mutant hepatocytes retain BrdU throughout outgrowth, reflecting cell cycle arrest. Pulse-chase-pulse experiments with BrdU and EdU, and DNA content analysis indicate that uhrf1 mutant cells undergo DNA re-replication and that apoptosis is the fate of many of the rereplicating and arrested hepatocytes. Importantly, the DNA rereplication phenotype and hepatic outgrowth failure are preceded by global loss of DNA methylation. Moreover, uhrf1 mutants are phenocopied by mutation of dnmt1, and Dnmt1 knockdown in uhrf1 mutants enhances their small liver phenotype. Together, these data indicate that unscheduled DNA replication and failed cell cycle progression leading to apoptosis are the mechanisms by which DNA hypomethylation prevents organ expansion in uhrf1 mutants. We propose that cell cycle arrest leading to apoptosis is a strategy that restricts propagation of epigenetically damaged cells during embryogenesis.

Edited by Qi-Qun Tang Nucleotide synthesis is essential to proliferating cells, but the preferred precursors for de novo biosynthesis are not defined in human cancer tissues. We have employed multiplexed stable isotope-resolved metabolomics to track the metabolism of [ 13 C 6 ]glucose, D 2-glycine, [ 13 C 2 ]glycine, and D 3-serine into purine nucleotides in freshly resected cancerous and matched noncancerous lung tissues from nonsmall cell lung cancer (NSCLC) patients, and we compared the metabolism with established NSCLC PC9 and A549 cell lines in vitro. Surprisingly, [ 13 C 6 ]glucose was the best carbon source for purine synthesis in human NSCLC tissues, in contrast to the noncancerous lung tissues from the same patient, which showed lower mitotic indices and MYC expression. We also observed that D 3-Ser was preferentially incorporated into purine rings over D 2-glycine in both tissues and cell lines. MYC suppression attenuated [ 13 C 6 ]glucose, D 3-serine, and [ 13 C 2 ]glycine incorporation into purines and reduced proliferation in PC9 but not in A549 cells. Using detailed kinetic modeling, we showed that the preferred use of glucose as a carbon source for purine ring synthesis in NSCLC tissues involves cytoplasmic activation/compartmentation of the glucose-to-serine pathway and enhanced reversed one-carbon fluxes that attenuate exogenous serine incorporation into purines. Our findings also indicate that the substrate for de novo nucleotide synthesis differs profoundly between cancer cell lines and fresh human lung cancer tissues; the latter preferred glucose to exogenous serine or glycine but not the former. This distinction in substrate utilization in purine synthesis in human cancer tissues should be considered when targeting one-carbon metabolism for cancer therapy. De novo nucleotide biosynthesis is required to meet the demand for maintaining energy, nucleotide levels, and new nucleic acids in dividing cells (1, 2). Synthesis of pyrimidine nucleotides in cancer cells utilizes glucose and glutamine for the carbon of the uracil and cytosine rings, which then condense with the ribose subunit primarily derived from glucose via the pentose-phosphate pathway (1, 3). The purine nucleotides are synthesized by building the purine ring directly onto phosphoribosyl pyrophosphate (PRPP) 6 using glycine (direct route), CO 2 , and glycine-derived N 10-formyl tetrahydrofolate (CHO-THF, indirect route) as the carbon sources for C4,5, C6, and C2,8, respectively (cf. Fig. 1). Cellular glycine may derive from glucose via de novo synthesis from serine (1, 4), via protein degradation, and/or via uptake from external sources, i.e. common cell culture media (1, 2) or the circulating blood (5). Serine derived from glucose or external sources is converted (through one-carbon metabolism) into Gly and 5,10-methylene THF (CH 2-THF) via the serine hydroxymethyltransferase (SHMT) activity (Fig. 1). CH 2-THF is oxidized to CHO-THF via the methylene tetrahydrofolate cyclohydrolase activity, prior to incorporation into purine rings.

Complex cytoplasmic nucleotide-sensing mechanisms can recognize foreign DNA based on a lack of methylation and initiate an immune response to clear the infection. Zebrafish embryos with global DNA hypomethylation caused by mutations in the () or () genes exhibit a robust interferon induction characteristic of the first line of defense against viral infection. We found that this interferon induction occurred in non-immune cells and examined whether intracellular viral sensing pathways in these cells were the trigger. RNA-seq analysis of and mutants revealed widespread induction of Class I retrotransposons and activation of cytoplasmic DNA viral sensors. Attenuating Sting, phosphorylated Tbk1 and, importantly, blocking reverse transcriptase activity suppressed the expression of interferon genes in mutants. Thus, activation of transposons in cells with global DNA hypomethylation mimics a viral infection by activating cytoplasmic DNA sensors. This suggests that antiviral pathways serve as surveillance of cells that have derepressed intragenomic parasites due to DNA hypomethylation.