Honors & Awards
Philip O’Bryan Montgomery, Jr., MD Fellow, Damon Runyon Cancer Foundation
Doctor of Philosophy, Cornell University (2011)
Maria Barna, Postdoctoral Faculty Sponsor
DNA ligases are crucial for most DNA transactions, including DNA replication, repair, and recombination. Recently, DNA ligase III (Lig3) has been demonstrated to be crucial for cell survival due to its catalytic function in mitochondria. This review summarizes these recent results and reports on a hitherto unappreciated widespread phylogenetic presence of Lig3 in eukaryotes, including in some organisms before the divergence of metazoa. Analysis of these putative Lig3 homologs suggests that many of them are likely to be found in mitochondria and to be critical for mitochondrial function.
View details for DOI 10.4161/cc.10.21.18094
View details for Web of Science ID 000296572100017
View details for PubMedID 22041657
Nonhomologous end-joining (NHEJ) is the primary DNA repair pathway thought to underlie chromosomal translocations and other genomic rearrangements in somatic cells. The canonical NHEJ pathway, including DNA ligase IV (Lig4), suppresses genomic instability and chromosomal translocations, leading to the notion that a poorly defined, alternative NHEJ (alt-NHEJ) pathway generates these rearrangements. Here, we investigate the DNA ligase requirement of chromosomal translocation formation in mouse cells. Mammals have two other DNA ligases, Lig1 and Lig3, in addition to Lig4. As deletion of Lig3 results in cellular lethality due to its requirement in mitochondria, we used recently developed cell lines deficient in nuclear Lig3 but rescued for mitochondrial DNA ligase activity. Further, zinc finger endonucleases were used to generate DNA breaks at endogenous loci to induce translocations. Unlike with Lig4 deficiency, which causes an increase in translocation frequency, translocations are reduced in frequency in the absence of Lig3. Residual translocations in Lig3-deficient cells do not show a bias toward use of pre-existing microhomology at the breakpoint junctions, unlike either wild-type or Lig4-deficient cells, consistent with the notion that alt-NHEJ is impaired with Lig3 loss. By contrast, Lig1 depletion in otherwise wild-type cells does not reduce translocations or affect microhomology use. However, translocations are further reduced in Lig3-deficient cells upon Lig1 knockdown, suggesting the existence of two alt-NHEJ pathways, one that is biased toward microhomology use and requires Lig3 and a back-up pathway which does not depend on microhomology and utilizes Lig1.
View details for DOI 10.1371/journal.pgen.1002080
View details for Web of Science ID 000292386300010
View details for PubMedID 21655080
Mammalian cells have three ATP-dependent DNA ligases, which are required for DNA replication and repair. Homologues of ligase I (Lig1) and ligase IV (Lig4) are ubiquitous in Eukarya, whereas ligase III (Lig3), which has nuclear and mitochondrial forms, appears to be restricted to vertebrates. Lig3 is implicated in various DNA repair pathways with its partner protein Xrcc1 (ref. 1). Deletion of Lig3 results in early embryonic lethality in mice, as well as apparent cellular lethality, which has precluded definitive characterization of Lig3 function. Here we used pre-emptive complementation to determine the viability requirement for Lig3 in mammalian cells and its requirement in DNA repair. Various forms of Lig3 were introduced stably into mouse embryonic stem (mES) cells containing a conditional allele of Lig3 that could be deleted with Cre recombinase. With this approach, we find that the mitochondrial, but not nuclear, Lig3 is required for cellular viability. Although the catalytic function of Lig3 is required, the zinc finger (ZnF) and BRCA1 carboxy (C)-terminal-related (BRCT) domains of Lig3 are not. Remarkably, the viability requirement for Lig3 can be circumvented by targeting Lig1 to the mitochondria or expressing Chlorella virus DNA ligase, the minimal eukaryal nick-sealing enzyme, or Escherichia coli LigA, an NAD(+)-dependent ligase. Lig3-null cells are not sensitive to several DNA-damaging agents that sensitize Xrcc1-deficient cells. Our results establish a role for Lig3 in mitochondria, but distinguish it from its interacting protein Xrcc1.
View details for DOI 10.1038/nature09794
View details for Web of Science ID 000288170200043
View details for PubMedID 21390132
Chromosomal translocations in hematologic and mesenchymal tumors form overwhelmingly by nonhomologous end-joining (NHEJ). Canonical NHEJ, essential for the repair of radiation-induced and some programmed double-strand breaks (DSBs), requires the Xrcc4-ligase IV complex. For other DSBs, the requirement for Xrcc4-ligase IV is less stringent, suggesting the existence of alternative end-joining (alt-NHEJ) pathways. To understand the contributions of the canonical NHEJ and alt-NHEJ pathways, we examined translocation formation in cells deficient in Xrcc4-ligase IV. We found that Xrcc4-ligase IV is not required for but rather suppresses translocations. Translocation breakpoint junctions have similar characteristics in wild-type cells and cells deficient in Xrcc4-ligase IV, including an unchanged bias toward microhomology, unlike what is observed for intrachromosomal DSB repair. Complex insertions in some junctions show that joining can be iterative, encompassing successive processing steps before joining. Our results imply that alt-NHEJ is the primary mediator of translocation formation in mammalian cells.
View details for DOI 10.1038/nsmb.1773
View details for Web of Science ID 000276416800007
View details for PubMedID 20208544
The precise genetic manipulation of stem and precursor cells offers extraordinary potential for the analysis, prevention, and treatment of human malignancies. Chromosomal translocations are hallmarks of several tumor types where they are thought to have arisen in stem or precursor cells. Although approaches exist to study factors involved in translocation formation in mouse cells, approaches in human cells have been lacking, especially in relevant cell types. The technology of zinc finger nucleases (ZFNs) allows DNA double-strand breaks (DSBs) to be introduced into specified chromosomal loci. We harnessed this technology to induce chromosomal translocations in human cells by generating concurrent DSBs at 2 endogenous loci, the PPP1R12C/p84 gene on chromosome 19 and the IL2Rgamma gene on the X chromosome. Translocation breakpoint junctions for t(19;X) were detected with nested quantitative PCR in a high throughput 96-well format using denaturation curves and DNA sequencing in a variety of human cell types, including embryonic stem (hES) cells and hES cell-derived mesenchymal precursor cells. Although readily detected, translocations were less frequent than repair of a single DSB by gene targeting or nonhomologous end-joining, neither of which leads to gross chromosomal rearrangements. While previous studies have relied on laborious genetic modification of cells and extensive growth in culture, the approach described in this report is readily applicable to primary human cells, including multipotent and pluripotent cells, to uncover both the underlying mechanisms and phenotypic consequences of targeted translocations and other genomic rearrangements.
View details for DOI 10.1073/pnas.0902076106
View details for Web of Science ID 000267564300043
View details for PubMedID 19549848