Zhang Yu lab dissected the roles of different mammalian DNA ligases in DNA repair.

    On January 19, 2016, Dr. Yu Zhang's lab in National Institute of Biological Sciences, Beijing (NIBS), published a paper online on "Proceedings of the National Academy of Sciences of the United States of America", titled "Ligase I and ligase III mediate the DNA double-strand break ligation in alternative end-joining".

    DNA repair is not only extremely important for the genome stability in normal cells but also extensively involved in carcinogenesis and evolution. DNA double-strand breaks (DSBs), one of the most harmful types of DNA damage, are repaired mainly by homologous repair (HR) and nonhomologous end-joining (NHEJ). Surprisingly, in cells deficient for core classic NHEJ factors such as DNA ligase IV (Lig4), substantial end-joining activities have been observed in various situations, suggesting the existence of alternative end-joining (A-EJ) activities. Several putative A-EJ factors have been proposed, although results are mostly controversial.

    By CRISPR/Cas9 system, researchers in Dr. Yu Zhang's lab generated mouse CH12F3 cell lines in which, in addition to Lig4, either Ligase I (Lig1) or nuclear Ligase III (Lig3) was completely ablated which also representing the cells containing a single DNA ligase (Lig3 or Lig1, respectively) in their nucleus. Surprisingly, they found that both Lig1-and Lig3-containing complexes could efficiently catalyze A-EJ for class switching recombination (CSR) in IgH locus and chromosomal deletions between DSBs generated by CRISPR/Cas9 in cis-chromosomes. However, only deletion of nuclear Lig3, but notLig1, could significantly reduce the inter-chromosomal translocations in Lig4-/-cells, suggesting the unique role of Lig3 in catalyzing chromosome translocation. Additional sequence analysis of chromosome translocation junction micro-homology revealed the specificity of different ligase-containing complexes. The data suggested the existence of multiple DNA ligase-containing complexes in A-EJ.

    Guangqing Lu and Jinzhi Duan, PhD students from NIBS and Graduate Program of Peking Union Medical College and Chinese Academy of Medical Sciences (PUMC), are the co-first authors of this paper. Other contributors include Sheng Shu, Xuxiang Wang, Linlin Gao and Jing Guo. Dr. Yu Zhang is the corresponding author. 

    This research was supported by the "Program for Excellent Talents" by Beijing municipal government, the "Hundred, Thousand and Ten Thousand Talent Project" by Beijing municipal government, "National Thousand Young Talents Program" of China, and National Natural Science Foundation of China to Dr. Yu Zhang, and carried out at National Institute of Biological Sciences, Beijing, which is supported by the municipal government of Beijing and the Ministry of Science and Technology of China.

http://www.pnas.org/content/early/2016/01/15/1521597113