TET2 is a dioxygenase that catalyses multiple steps of 5-methylcytosine oxidation. Although TET2 mutations frequently occur in various types of haematological malignancies, the mechanism by which they increase risk for these cancers remains poorly understood. Here we show that Tet2−/− mice develop spontaneous myeloid, T- and B-cell malignancies after long latencies. Exome sequencing of Tet2−/− tumours reveals accumulation of numerous mutations, including Apc, Nf1, Flt3, Cbl, Notch1 and Mll2, which are recurrently deleted/mutated in human haematological malignancies. Single-cell-targeted sequencing of wild-type and premalignant Tet2−/− Lin−c-Kit+ cells shows higher mutation frequencies in Tet2−/− cells. We further show that the increased mutational burden is particularly high at genomic sites that gained 5-hydroxymethylcytosine, where TET2 normally binds. Furthermore, TET2-mutated myeloid malignancy patients have significantly more mutational events than patients with wild-type TET2. Thus, Tet2 loss leads to hypermutagenicity in haematopoietic stem/progenitor cells, suggesting a novel TET2 loss-mediated mechanism of haematological malignancy pathogenesis.

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Proliferating cell nuclear antigen (PCNA) plays essential roles in eukaryotic cells during DNA replication, DNA mismatch repair (MMR), and other events at the replication fork. Earlier studies show that PCNA is regulated by posttranslational modifications, including phosphorylation of tyrosine 211 (Y211) by the epidermal growth factor receptor (EGFR). However, the functional significance of Y211-phosphorylated PCNA remains unknown. Here, we show that PCNA phosphorylation by EGFR alters its interaction with mismatch-recognition proteins MutSα and MutSβ and interferes with PCNA-dependent activation of MutLα endonuclease, thereby inhibiting MMR at the initiation step. Evidence is also provided that Y211-phosphorylated PCNA induces nucleotide misincorporation during DNA synthesis. These findings reveal a novel mechanism by which Y211-phosphorylated PCNA promotes cancer development and progression via facilitating error-prone DNA replication and suppressing the MMR function.is an essential genome stability pathway that removes mismatches introduced by nucleotide misincorporation during DNA replication (1-4). MMR in eukaryotic cells is nick-directed and targeted specifically to the newly synthesized DNA strand (5, 6). MMR is carried out in three phases: initiation, excision, and resynthesis. The initiation phase involves mismatch recognition by MutSα or MutSβ; assembly of the initiation complex containing MutSα (or MutSβ), MutLα, and proliferating cell nuclear antigen (PCNA); and localization of the strand discrimination signal (a single-strand nick) by this complex (7-11) in a process that is incompletely understood. During the excision phase, exonuclease 1 (Exo1) removes nascent DNA exonucleolytically from a distal nick to the mismatch in a reaction that requires MutSα (or MutSβ), MutLα, and PCNA. Replication protein A (RPA) stimulates DNA excision by Exo1 and protects single-stranded DNA from cleavage by nucleases (12, 13). During the resynthesis phase of MMR, DNA polymerase δ fills in the single-strand DNA gap left by DNA excision in a concerted reaction that requires replication factor C (RFC), PCNA, and RPA, followed by DNA ligase I-catalyzed nick ligation.Mutations or promoter hypermethylation in coding or regulatory regions of MMR genes MSH2, MSH6, MLH1, and PMS2 are linked to susceptibility to colorectal and other cancers in humans and other organisms (2, 14-16). The cellular phenotype associated with defects in MMR includes an elevated genomewide mutation rate as well as frequent changes in DNA microsatellite repeats, a phenomenon called microsatellite instability (MSI). Thus, MSI is frequently used as a biomarker for (or hallmark of) MMR deficiency (1-4). However, a significant number of MSIpositive tumors do not carry mutations in known MMR genes (17). Recent studies have shown that mutations affecting the proofreading nuclease activity of DNA polymerases δ and e are associated with some colorectal and/or endometrial cancers (18), and that defects in the gene encoding histone H3 Lys36 (H3K36) trimethyltran...