Disease relevance of MDC1

• NFBD1/MDC1 (mediator of DNA damage checkpoint 1) is a nuclear factor with an amino-terminal FHA (forkhead-associated) domain and a tandem repeat of BRCT (breast cancer susceptibility gene-1 carboxyl terminus) domains [1].
• Here we report that knockdown of MDC1 by RNA interference results in cellular hypersensitivity to the DNA cross-linking agent mitomycin C and ionizing radiation and in impaired homology-mediated repair of double-strand breaks in DNA [2].
• Overexpression of antisense NFBD1 RNA in a p53-deficient human osteogenic sarcoma cell line (SAOS-2) resulted in remarkable suppression of SAOS-2 colony formation [3].

High impact information on MDC1

• MDC1 directly binds phosphorylated histone H2AX to regulate cellular responses to DNA double-strand breaks [4].
• Thus, binding of MDC1/NFBD1 to gammaH2AX plays a central role in the mammalian response to DNA damage [4].
• We further show that MDC1/NFBD1-gammaH2AX complex formation regulates H2AX phosphorylation and is required for normal radioresistance and efficient accumulation of DNA-damage-response proteins on damaged chromatin [4].
• These findings reveal that MDC1-mediated focus formation by the MRE11 complex at sites of DNA damage is crucial for the efficient activation of the intra-S-phase checkpoint [5].
• 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 [6].

Biological context of MDC1

• MDC1 is required for the intra-S-phase DNA damage checkpoint [5].
• Downregulation of MDC1 expression by small interfering RNA yields a radio-resistant DNA synthesis (RDS) phenotype and prevents ionizing radiation-induced focus formation by the MRE11 complex [5].
• Enforced expression of MDC1 into STAT-1-deficient cells restored ATM-mediated phosphorylation of downstream substrates [7].
• The human scRad9 orthologues 53BP1 and NFBD1 associate with ionizing radiation-induced foci (IRIF) at sites of DNA repair [8].
• 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 [6].

Anatomical context of MDC1

• We found that the NFBD1 transcript is abundant in the testis relative to other tissues [9].

Associations of MDC1 with chemical compounds

• By a comparison with the structure of the BRCA1 BRCT bound to a phosphopeptide, we suggest that two arginine residues in MDC1, Arg(1932) and Arg(1933) may recognize the COOH terminus of the peptide as well as the penultimate Glu of H2AX, while Gln(2013) may provide additional specificity for the COOH-terminal Tyr [10].
• NFBD1 foci are also detected in response to camptothecin, etoposide, and methylmethanesulfonate treatments [11].
• NFBD1 exhibited diffuse nuclear staining in the majority of untreated cells analyzed by indirect immunofluorescence and formed discrete nuclear foci after exposure to IR, UV radiation, and hydroxyurea treatment [9].

Physical interactions of MDC1

• Furthermore, this interaction is phosphorylation dependent as peptides containing the phosphorylated site on H2AX bind MDC1 in a phosphorylation-dependent manner [6].
• MDC1 interacts with Rad51 and facilitates homologous recombination [2].
• Here, we show that MDC1 directly interacts with the Ku/DNA-PKcs complex [12].

Enzymatic interactions of MDC1

• Ectopic expression of the BRCT motifs reduced damage-induced NFBD1 foci and compromised phosphorylated Chk2- and phosphorylated H2AX-containing foci [11].

Co-localisations of MDC1

• MDC1 forms foci that co-localize extensively with gamma-H2AX foci within minutes after exposure to ionizing radiation [6].

Regulatory relationships of MDC1

• Taken together these data suggest that MDC1 regulates BRCA1 function in DNA damage checkpoint control [13].
• MDC1 regulates ataxia-telangiectasia-mutated (ATM)-dependent phosphorylation events at the site of DNA damage [13].
• Freshly isolated PDC and MDC-1 from untreated flaring IBD patients express higher baseline levels of CD86 which increases further in culture and upon stimulation compared with healthy controls [14].

Other interactions of MDC1

• Importantly down-regulation of MDC1 abolishes the relocalization and hyperphosphorylation of BRCA1 following DNA damage, which coincides with defective G(2)/M checkpoint control in response to DNA damage [13].
• An overexpressed FHA domain-containing fragment of NFBD1 binds to endogenous NFBD1 and components of the MRN complex, but not to gamma-H2AX [1].
• Whether 53BP1 and NFBD1 are required for activation of kinases and/or for recruitment of substrates at IRIF, however, is not clear [8].
• Here we document the role of MDC1 (mediator of DNA damage checkpoint 1) in the detection and repair of human and mouse telomeres rendered dysfunctional through inhibition of TRF2 [15].
• This phenotype was identical to that observed in cells with defective Nbs1 function and is consistent with recent observations identifying NFBD1/MDC1 as a component of the Mre11-Rad50-Nbs1 protein complex [16].

References