Research ArticleGENETICS

Mitochondrial protein-linked DNA breaks perturb mitochondrial gene transcription and trigger free radical–induced DNA damage

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Science Advances  28 Apr 2017:
Vol. 3, no. 4, e1602506
DOI: 10.1126/sciadv.1602506
  • Fig. 1 TDP1 promotes mitochondrial gene transcription in human cells.

    (A) Immunoblotting of Flp-In T-Rex 293 cell lines after doxycycline induction using antibodies against TDP1, GFP, and β tubulin. TDP1 + miTDP1, TDP1-depleted cells overexpressing microRNA (miRNA)–resistant TDP1; miTDP1-4, TDP1-depleted cells (clone 4); TOP1mt + miScr, scrambled miRNA–transfected cells overexpressing TOP1mt-EmGFP; TOP1mt + miTDP1, miTDP1 cells overexpressing TOP1mt-EmGFP; TOP1mt* + miScr, miScr cells overexpressing TOP1mtT554A,N558H-EmGFP; TOP1mt* + miTDP1, miTDP1 cells overexpressing TOP1mt554A,N558H-EmGFP. (B) Mitochondrial lysates from doxycycline-induced cells were fractionated using CsCl gradient and immunoblotted using antibodies against GFP to detect TOP1mt- and TOP1mt*-linked DNA breaks. Enhanced chemiluminescence intensity was quantified and expressed as fold increase relative to TOP1mt-expressing cells. (C) TOP1mt*-cc in doxycycline-induced cells were immunoprecipitated using GFP-trap magnetic beads and the bound mtDNA at a putative TOP1mt*-binding site in the noncoding regulatory region quantified by qPCR. The mtDNA copy numbers of each cell line were concomitantly measured using primers for the ND1 (mitochondrial) and B2M (nuclear) genes. Enrichment of TOP1mt-bound chromatin is expressed as percent input, which is then normalized to the mitochondrial copy number of the cell line. (D to F and I) Total RNA from doxycycline-induced cells from (A) was reverse-transcribed and analyzed by SYBR-Green–based qPCR using primers for mitochondrial-encoded genes. Mitochondrial transcript levels were normalized against glyceraldehyde-3-phosphate dehydrogenase (GAPDH) levels and expressed as a fraction against the corresponding mitochondrial gene transcript in control scrambled miRNA–transfected (miScr) cells. Data are the means of three independent experiments, and error bars represent ±1 SEM. (G) Mitochondrial lysates of doxycycline-induced cells were incubated with 5′-Cy5.5–labeled 18-mer substrate with a 3′-phosphotyrosine terminus (18-Y) that is processed to 3′-phosphate (18-P) by TDP1. (H) Quantification of product conversion normalized to miScr cells from three independent experiments described in (G) using LI-COR Image Studio Lite version 5.2. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001.

  • Fig. 2 TDP1 promotes the assembly of mitochondrial OXPHOS system.

    (A) Left: Immunoblotting of mitochondrial lysates from doxycycline-induced Flp-In T-Rex 293 cells using cocktail antibodies against OXPHOS proteins NDUFB8 (complex I), SDHB (complex II), UQCRC2 (complex III), COX-II (complex IV), and ATP5A (complex V). Right: Immunoblotting of mitochondrial lysates with antibodies specific to UQCRC2 (complex III) and the mitochondrial loading control VDAC1. (B to F) Protein expression levels of the labile subunit from the indicated complexes relative to those of control cells, after normalization to levels of the mitochondrial loading control VDAC1. ns, not statistically significant, P < 0.05.

  • Fig. 3 TDP1 promotes OXPHOS at basal and high ATP demands.

    (A) Doxycycline-induced cells were plated in an XF24 Seahorse cell plate, and the bioenergetic response to 1 μM oligomycin, 1 μM FCCP, and 1 μM rotenone was analyzed in a Seahorse XF Bioanalyzer. Data represent the mean OCR of three independent experiments, and error bars represent ±1 SEM. (B to D) Mitochondrial respiration was calculated by subtracting OCR after rotenone/antimycin A from basal OCR. (E to G) ATP-coupled respiration was calculated by subtracting OCR after oligomycin from basal OCR. (H to J) SRC was calculated by subtracting maximum OCR after FCCP from basal OCR. Data are the means of three independent experiments, and error bars represent ±1 SEM.

  • Fig. 4 Accumulation of carbon radicals in Tdp1 deficient vertebrate cells.

    (A) Immunoblotting of cell lysates from Tdp1−/− DT40 cells complemented with hTDP1 or an empty vector using antibodies against hTDP1. (B) TDP1 enzymatic activity measured on cell lysates from (A) using an in vitro assay with an 18-mer oligonucleotide substrate containing a 3′-phosphotyrosine residue and a fluorescein isothiocyanate (FITC) molecule conjugated to the 5′-end. Recombinant hTDP1 (rTDP1) was used as positive control. Fluorescence intensity was measured with a BMG Labtech PHERAstar plate reader and analyzed by the PHERAstar software. (C) ESR spectra obtained during 30 min of UVA irradiation (approximately 3.1 mW/cm2) of DT40 cells. (D) Bar charts showing the mean integration of all six carbon adducts shown in (C) noted as a percentage of integrated manganese reference lines. Data are the means of three independent experiments, and error bars represent ±1 SEM. (E and F) MEFs were treated with the indicated doses of H2O2 for 10 min on ice or exposed to x-ray irradiation at 12 mA/250 kV on ice and then left to recover until macroscopic colonies formed. Surviving fraction was calculated by dividing the number of colonies on treated plates by the number on untreated plates. Data are the means of three independent experiments, and error bars represent ±1 SEM. (G) Immunoblotting of Tdp1−/− MEFs complemented with hSOD1 or hSOD1G93A. Expression levels of hSOD1 and hSOD1G93A were quantified by ImageJ and noted as a fraction relative to endogenous mouse Sod1 (mSod1). (H) ESR spectra obtained during 30 min of UVA irradiation (approximately 5 mW/cm2) of MEFs in (G). (I) Bar charts showing the mean integration of all six carbon adducts from (G) noted as a percentage of integrated manganese reference lines. SEM was calculated from three independent experiments. (J and L) Immunoblotting of Tdp1−/− MEF lysates overexpressing hSOD1 or hSOD1G93A. Expression levels of hSOD1 are noted as a fraction relative to endogenous mouse Sod1 (mSod1). (K and M) Bar charts showing the mean integration of all six carbon adducts noted as a percentage of integrated manganese reference lines from ESR data obtained from MEFs in (J) and (L), respectively. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001.

  • Fig. 5 TDP1 protects mammalian cells from ROS-induced chromosomal breaks.

    (A) The indicated MEF cell lines were incubated with 10 μM H2O2 for 10 min and then left to repair in drug-free medium at 37°C for the indicated time periods. DNA SSBs were measured using the alkaline comet assay. Average tail moments from 50 cells were quantified using the Comet Assay IV software. Data are the means of three independent experiments, and error bars represent ±1 SEM. (B) Repair kinetics of cells from (A) expressed as a percentage of remaining breaks over time. (C) Cells from (A) were treated with the indicated concentrations of H2O2 for 10 min on ice and then left to recover until macroscopic colonies formed. Surviving fraction was calculated by dividing the number of colonies on treated plates by the number on untreated plates. Data are the means of three independent experiments, and error bars represent ±1 SEM. (D) MEFs were treated with the indicated concentrations of H2O2 and processed as described in (C). Data are the means of three independent experiments, and error bars represent ±1 SEM. (E) Immunoblotting of fractionated MRC5 cell lysates from cells containing control short hairpin RNA (Ctrl shRNA) and shRNA against TOP1 (shTOP1), TDP1 (shTDP1), or both. (F and G) MRC5 cells from (E) were treated with 50 μM CPT for 1 hour (F) or 45 μM H2O2 for 10 min (G) and immediately lysed in buffer containing proteinase K, and TOP1-cc was quantified by the modified alkaline comet assay. Data are the means of three independent experiments, and error bars represent ±1 SEM. (H) MRC5 cells from (E) were treated with 10 μM H2O2 for 10 min on ice and then left to recover until macroscopic colonies formed. Surviving fraction was calculated by dividing the number of colonies on treated plates by the number on untreated plates. Data are the means of three independent experiments, and error bars represent ±1 SEM.

  • Fig. 6 Model depicting the importance of the TDP1/TOPmt repair pathway.

    (A) Under basal condition, the catalytic cycle of TOP1mt is facilitated by the removal of TOP1mt-cc by TDP1. Transcription of the mitochondrial subunits of the ETC complexes is balanced with transcription of the nuclear subunits, resulting in the proper assembly of the ETC complexes. Efficient functioning of ETC complexes promotes ATP production by OXPHOS, generating membrane potential that prevents formation of ROS. (B) In the absence of TDP1, TOP1mt-cc accumulate, which inhibit mitochondrial transcription without affecting transcription of the nuclear subunits, leading to misassembly of complex III. OXPHOS efficiency is reduced and ROS production is increased. (C) When excessive TOP1mt-cc are formed (for example, because of trapping by oxidized DNA lesions or TOP1mt poison), compensatory up-regulation of TDP1 activity accelerates TOP1mt catalytic cycle and promotes mitochondrial transcription. However, without proportional increase in nuclear transcription, misassembly of ETC complexes leads to a reduction in OXPHOS and an elevation of ROS production. Mito., mitochondrial; Nucl., nuclear.

  • Table 1 Genotypes of live offspring of crossing hSOD1G93A with different Tdp1 backgrounds.

    Male mice overexpressing hSOD1G93A were bred with female WT mice, heterozygous for Tdp1 or homozygous for Tdp1 knockout. The live births were genotyped at 1 month postnatum.

    Male parentParent hSOD1G93A Tdp1+/−
    Female parentSod1+/+ Tdp1−/−Sod1+/+ Tdp1+/−Sod1+/+ Tdp1+/+
    Offspring (no. of live births)ExpectedObtainedExpectedObtainedExpectedObtained
    Sod1+/+ Tdp1+/+6.2532.250
    Sod1+/+ Tdp1+/−54.5612.552.254
    Sod1+/+ Tdp1−/−54.51156.2514
    hSOD1G93A Tdp1+/+6.25142.255
    hSOD1G93A Tdp1+/−54.59612.5122.250
    hSOD1G93A Tdp1−/−54.51*6.252
    Total218218505099
    Male parenthSOD1G93A; Tdp1+/+
    Female parentSod1+/+; Tdp1−/−Sod1+/+; Tdp1+/−Sod1+/+; Tdp1+/+
    Offspring (no. of live births)ExpectedObtainedExpectedObtainedExpectedObtained
    Sod1+/+; Tdp1+/+15.25976
    Sod1+/+; Tdp1+/−24.52115.2515
    Sod1+/+; Tdp1−/−
    hSOD1G93A; Tdp1+/+15.252378
    hSOD1G93A; Tdp1+/−24.52815.2514
    hSOD1G93A; Tdp1−/−*
    Total494961611414

    *One fetus demised in utero.

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/4/e1602506/DC1

      fig. S1. The generation of an inducible TDP1 knockdown system in Flp-In T-Rex 293 cells.

      fig. S2. Quantification of TOP1mt-cc in TOP1mt-overexpressing cells.

      fig. S3. TDP1 depletion had no impact on the expression of the nuclear-encoded complex II subunit.

      fig. S4. TDP1 depletion or TOP1mt* expression does not affect uncoupled respiration (proton leak).

    • Supplementary Materials

      This PDF file includes:

      • fig. S1. The generation of an inducible TDP1 knockdown system in Flp-In T-Rex 293 cells.
      • fig. S2. Quantification of TOP1mt-cc in TOP1mt-overexpressing cells.
      • fig. S3. TDP1 depletion had no impact on the expression of the nuclear-encoded complex II subunit.
      • fig. S4. TDP1 depletion or TOP1mt* expression does not affect uncoupled respiration (proton leak).

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