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Updated: May 8, 2026
Two- and Three-Dimensional Live Cell Imaging of DNA Damage Response Proteins
Published on: September 28, 2012
MDC1 is a mediator of the mammalian DNA damage checkpoint
Grant S Stewart1, Bin Wang, Colin R Bignell
1Verna & Mars McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA.
Abstract:
To counteract the continuous exposure of cells to agents that damage DNA, cells have evolved complex regulatory networks called checkpoints to sense DNA damage and coordinate DNA replication, cell-cycle arrest and DNA repair. It has recently been shown that the histone H2A variant H2AX specifically controls the recruitment of DNA repair proteins to the sites of DNA damage. Here we identify a novel BRCA1 carboxy-terminal (BRCT) and forkhead-associated (FHA) domain-containing protein, MDC1 (mediator of DNA damage checkpoint protein 1), which works with H2AX to promote recruitment of repair proteins to the sites of DNA breaks and which, in addition, controls damage-induced cell-cycle arrest checkpoints. MDC1 forms foci that co-localize extensively with gamma-H2AX foci within minutes after exposure to ionizing radiation. H2AX is required for MDC1 foci formation, and MDC1 forms complexes with phosphorylated H2AX. Furthermore, this interaction is phosphorylation dependent as peptides containing the phosphorylated site on H2AX bind MDC1 in a phosphorylation-dependent manner. We have shown by using small interfering RNA (siRNA) that cells lacking MDC1 are sensitive to ionizing radiation, and that MDC1 controls the formation of damage-induced 53BP1, BRCA1 and MRN foci, in part by promoting efficient H2AX phosphorylation. In addition, cells lacking MDC1 also fail to activate the intra-S phase and G2/M phase cell-cycle checkpoints properly after exposure to ionizing radiation, which was associated with an inability to regulate Chk1 properly. These results highlight a crucial role for MDC1 in mediating transduction of the DNA damage signal.
Insights
Mediator of DNA damage checkpoint protein 1 (MDC1) is a novel protein that works with H2AX to recruit DNA repair proteins to damage sites. MDC1 is crucial for DNA damage-induced cell-cycle arrest checkpoints.
Area of Science:
- Molecular Biology
- Cell Biology
- Genetics
Background:
- Cells possess intricate DNA damage response networks, including checkpoints, to manage DNA replication, cell-cycle arrest, and repair.
- The histone H2A variant H2AX plays a specific role in recruiting DNA repair proteins to DNA damage sites.
Purpose of the Study:
- To identify and characterize a novel protein involved in DNA damage response pathways.
- To elucidate the role of this new protein in coordinating DNA repair and cell-cycle checkpoint activation.
Main Methods:
- Identification of a novel protein, MDC1 (mediator of DNA damage checkpoint protein 1), possessing BRCT and FHA domains.
- Co-localization studies of MDC1 and gamma-H2AX foci formation after ionizing radiation exposure.
- Analysis of MDC1's interaction with phosphorylated H2AX.
- siRNA-mediated knockdown of MDC1 to assess its function in DNA repair and cell-cycle checkpoints.
- Investigation of MDC1's role in the formation of 53BP1, BRCA1, and MRN foci and regulation of Chk1.
Main Results:
- MDC1 forms foci that extensively co-localize with gamma-H2AX foci post-ionizing radiation.
- H2AX is essential for MDC1 foci formation, and MDC1 interacts with phosphorylated H2AX in a phosphorylation-dependent manner.
- MDC1-deficient cells exhibit sensitivity to ionizing radiation and impaired formation of damage-induced foci (53BP1, BRCA1, MRN), partly due to reduced H2AX phosphorylation.
- MDC1-deficient cells fail to properly activate intra-S and G2/M cell-cycle checkpoints and show dysregulation of Chk1.
Conclusions:
- MDC1 is a critical mediator in the DNA damage response, working in conjunction with H2AX.
- MDC1 plays a pivotal role in recruiting DNA repair proteins and establishing cell-cycle checkpoints following DNA damage.
- MDC1 is essential for the proper transduction of DNA damage signals, highlighting its importance in maintaining genomic stability.
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