Liu, Xiangyu (Senior Researcher)

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School&Department：Shenzhen University, Health Science Center

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Professional： Department of Biochemistry and Molecular Biology

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Title：Senior Researcher

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Intro：

BIOGRAPHICAL SKETCH

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NAME: Liu, Xiangyu

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POSITION TITLE: Principle Investigator; Department of Biochemistry and Molecular Biology, Shenzhen University Health Science Center

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EDUCATION/TRAINING

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A. Personal Statement

My long-term goal is to use molecular and genetic approach to understand the relationships among different DNA repair pathways and how they coordinate to maintain genome stability and prevent human diseases such as ageing and cancer. In general, I am interested in how cells respond to different DNA damaging stress (such as nuclease mediated cutting, topoisomerase inhibitors, irradiation or radiomimetic drugs, etc) and how different repair pathways determine the final fate of the cells.

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B. Honors

2013-2016 Fellowship Award from Leukemia & Lymphoma Society

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C. Contribution to Science

Inactivation of the p53 tumor suppressor gene is a crucial event in the formation of cancer in human patients. Stress-induced activation of p53 protein is primarily achieved by post-translational modifications, such as phosphorylation and acetylation. SIRT1, a member of the class III histone deacetylase (HDAC) sirtuins, is reported to de-acetylate and inactivate p53. Nevertheless, upon DNA damage, the acetylation levels of p53 are dramatically increased, hence it is not well understood how the SIRT1–p53 interaction is regulated during the stress response. During my graduate studies, I identified the histone methyltransferase Set7/9 as a unique negative regulator of SIRT1. Set7/9 was able to methylate and interact with SIRT1. Even though methylation of SIRT1 was dispensable, physical interaction of Set7/9 with SIRT1 disrupted SIRT1 binding to p53, such that p53 acetylation and transactivation were significantly enhanced (PNAS, 2011). While the main project was going on, I also initiated and played key roles in two extended projects describing that Set7/9 methylated and regulated the functions of two other important non-histone substrates: SUV39H1 and beta-catenin (PNAS, 2013-1) (FASEB J, 2015). Furthermore, I participated and contributed to the project that revealed the function of cytosolic FOXO1 and SIRT2 in autophagy, a natural destructive mechanism that disassembled unnecessary or dysfunctional cellular components (NCB, 2010). In summary, all these projects focus on the posttranslational modifications and regulations of proteins that play significant roles in the progress of cancer development, exploring the relationship between epigenetics and tumorigenesis and revealing potential targets for cancer therapy.

After graduation, I joined Columbia University and began to study mechanisms of DNA double strand breaks (DSBs) repair using both biochemistry and mouse genetics approaches. Non-homologous end joining (NHEJ) is a major pathway of DSBs repair to maintain genomic stability. During lymphocyte development, NHEJ is required to resolve the programmed DSBs generated during Variable, Diverse, and Joining (V(D)J recombination). By using knockout mouse models, we reported that while deletion of either XLF or 53BP1 did not have obvious defects in V(D)J recombination, double deficiency of XLF/53BP1 blocked lymphocyte development at early progenitor stages, owing to severe defects in end joining during chromosomal V(D)J recombination. In addition, the unrepaired DNA ends were rapidly degraded in 53BP1?/?XLF?/? cells. 53BP1?/?XLF?/? mice were born alive but grew much smaller than littermates in the cage and developed thymic lymphomas with translocations involving the T-cell receptor loci. This study identified a unique function of 53BP1 in end-joining and tumor suppression (PNAS, 2012). Interestingly, we and others also found that 53BP1 played a key role in preventing unrepaired DSB ends from being resected in G1 cell cycle by an unknown mechanism that required the presence of CtIP, which associated with the Mre11-Rad50-Nbs1 (MRN) complex and was thought to play an essential role in homologous recombination (HR)-mediated as well as micro-homology mediated end-joining DSB repair pathways. Furthermore, we proved that CtIP was required for lymphocyte development by maintaining cell viability, which is independent of its role in DNA resection (JEM, in submission)

In vertebrates, NHEJ further requires end-processing capacities, such as hairpin opening. The catalytic subunit of DNA-PK (DNA-PKcs) is a vertebrate-specific NHEJ factor that can be auto-phosphorylated or trans-phosphorylated by ATM kinase. Just like ATM, we showed that DNA-PKcs and XLF also had redundant roles in V(D)J recombination and NHEJ, suggesting a fundamental role of DNA-PKcs beyond end processing (PNAS, 2013-2). In addition, using a mouse model expressing kinase-dead (KD) mutant form of DNA-PKcs, we showed that autophosphorylation of DNA-PKcs was necessary to relieve the protein from the DNA ends, to facilitate the following step of ends joining. Accordingly, DNA-PKcsKD/KD mice and cells showed severe end-ligation defects and p53- and Ku- dependent embryonic lethality. This study suggested DNA-PKcs as the molecular switch that coordinates end-processing and end-ligation at the DNA ends through differential phosphorylations (Molecular Cell, 2015).

Recently, a newly identified XRCC4 superfamily member, PAXX (PAralog of XRCC4 and XLF), is considered as a new component of the NHEJ machinery. To determine the role of PAXX in DNA repair and maintenance of genomic stability in vivo, I generated PAXX-/- mouse model and found that PAXX-/- mice showed normal lymphocyte development as well as proficient V(D)J recombination and class switching recombination (CSR). However, by knocking out PAXX gene in XLF-/- cells using CRISPR-Cas9 technique, I found a severe blockage of NHEJ in XLF-/-PAXX-/- cells that was independent of chromatin structures. In addition, inter-crossing the XLF+/-PAXX+/- mice did not generate XLF-/-PAXX-/- mice, suggesting that XLF and PAXX double knockout was embryonic lethal, further indicating the important role of PAXX in NHEJ in the background of XLF deficiency (Nature Communications, 2017). We further proved that the role of PAXX is to stabilize KU/DNA-PKcs complex in response to DNA DSBs.

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D. Publications

#Equal contribution

Liu X, Shao Z, Jiang W, Lee B, Zha S (2017) PAXX promotes KU-accumulation at DNA breaks and is essential for end-joining in XLF-deficient mice. Nature Communications. Jan 4; doi:10.1038/ncomms13816.

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Jiang W#, Crowe J#, Liu X#, Nakajima S, Wang Y, Li C, Lee B, Dubois R, Liu C, Yu X, Lan L, Zha S. (2015) Differential Phosphorylation of DNA-PKcs Regulates the Interplay between End-Processing and End-Ligation during Nonhomologous End-Joining. Molecular Cell. Mar 26; Volume 58, Issue 1, p172–185.

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Liu X#, Jiang W#, Dubois RL, Yamamoto K, Wolner Z, Zha S. (2012) Overlapping functions between XLF repair protein and 53BP1 DNA damage response factor in end joining and lymphocyte development. Proceedings of the National Academy of Sciences（PNAS）Mar 6;109(10):3903-8.

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Liu X, Zha S (2011). ATMIN: A New Tumor Suppressor in Developing B Cells. Cancer Cell. May 17;19(5):569-70 (Preview).

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Liu X#, Wang D#, Zhao Y, Tu B, Zheng Z, Wang L, Wang H, Gu W, Roeder RG, Zhu WG. (2011) Methyltransferase Set7/9 regulates p53 activity by interacting with Sirtuin 1 (SIRT1). Proceedings of the National Academy of Sciences（PNAS）. Feb 1;108(5):1925-30.

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Wang D, Zhou J, Liu X, Lu D, Shen C, Du Y, Wei FZ, Song B, Lu X, Yu Y, Wang L, Zhao Y, Wang H, Yang Y, Akiyama Y, Zhang H, Zhu WG.(2013) Methylation of SUV39H1 by SET7/9 results in heterochromatin relaxation and genome instability. Proceedings of the National Academy of Sciences (PNAS). Apr 2;110(14):5516-21.

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Oksenych V, Kumar V, Liu X, Guo C, Schwer B, Zha S, Alt FW. (2013) Functional Redundancy Between the XLF and DNA-PKcs DNA Repair Factors in V(D)J recombination and Non-homologous DNA End-joining. Proceedings of the National Academy of Sciences (PNAS).Feb 5;110(6):2234-9.

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Yamamoto K, Wang Y, Jiang W, Liu X, Dubois RL, Lin CS, Ludwig T, Bakkenist CJ, Zha S. (2012) Kinase-dead ATM protein causes genomic instability and early embryonic lethality in mice. TheJournal of Cell Biology.Aug 6;198(3):305-13.

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Zheng, Z., Li, L., Liu, X., Wang, D., Tu, B., Wang, L., Wang, H., Zhu, W.-G (2012) 5-Aza-2'-deoxycytidine reactivates gene expression via degradation of pRb pocket protein. The FASEB Journal. Jan;26(1):449-59.

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Zhao Y, Yang J, Liao W, Liu X, Zhang H, Wang S, Feng J, Yu L, Zhu WG. (2010) Cytosolic FoxO1 is involved in autophagic and tumor suppressor activity.Nature Cell Biology. Jul;12(7):665-75.