The intertwined nature of the DNA double helix creates topological barriers that need to be resolved during all types of DNA transactions, in all forms of life from plant to man. DNA topoisomerases achieve this by breaking and sealing the double helix, allowing the DNA to become untangled or unwound. During their normal catalytic cycle DNA topoisomerases become covalently attached to the DNA 3’-end (Top1) or to the 5’-end (Top2) via a reversible covalent phosphotyrosyl bond. The presence of nearby oxidative DNA breaks (red circles) or collision with elongating RNA polymerases (RNA POL) during transcription results in ‘trapping’ of topoisomerases on DNA, causing protein-linked DNA breaks (PDBs), which are potent blocks to transcription and cell viability. PDBs are repaired by a nucleolytic cleavage of DNA releasing the stalled Top and a fragment of DNA. Since this process results in an inevitable loss of genetic material, it is inherently error-prone. Alternatively, PDBs can be repaired by an error-free mechanism in which the covalent phosphotyrosyl bond linking DNA to the stalled Top is cleaved by specific enzymes, such as tyrosyl DNA phosphodiesterases (TDP1 and TDP2). Interestingly, defects in TDPs cause neurological disease in man (see Fig.2). In addition to maintaining genetic integrity in post-mitotic non cycling cells, accumulation of PDBs in cycling cells has been widely exploited to treat cancer (e.g. topoisomerase poisons such as Irinotecan and Topotecan), and therefore small molecule inhibitors of TDPs are emerging as attractive strategies to improve cancer therapy.
DNA repair, topisomerase, mitochondria
Video animation summarising recent research discoveries
This video describes how DNA repair guards us against ALS, in particular the most common genetic cause for ALS and frontotemporal dementia caused by expansion in a gene called C9orf72. It summarises the work published in Nature Neuroscience on 17 July 2017.
How can the cells' powerhouse - mitochondria - protect their DNA? This video summarises a recent publication in Science Advances.
Figure 3: Primary neural cells (cortical astrocytes) examined for ODB/PDB repair by the alkaline comet assay (top left). The kinetics of TDP1 recruitment/retention at sites of laser-induced DNA damage studied by live-cell imaging (top right). Spontaneous sister chromatid exchanges, a hallmark of hyper-recombination, in cycling SCAN1 Cells (bottom)
Figure 4: Accumulation of mutations with age has been extensively studied in mice and humans as one cause of ‘physiological’ aging. DNA damage can also cause downregulation of genes that are normally associated with the extension of life span, such as IGF-1 and GH, contributing to the aging process. On the other hand, aging leads to reduced expression of DNA repair factors, leading to increased accumulation of DNA damage. Aging also may cause reduced trafficking of DNA repair factors to the mitochondria, contributing to mitochondrial dysfunction. The latter results in defects in the oxidative phosphorylation leading to increased production of ROS and consequent accumulation of DNA damage in mtDNA.
Major Scientific Accomplishments
Discovery of the mechanism of genomic instability and neural cell death in C9orf72 ALS (Nature Neuroscience 2017)
Discovery of protein-linked chromosomal break repair in the mitochondria (Science Advances 2017)
Discovery of a novel therapeutic strategy for ALS by inhibiting nuclear export (Nature communications 2017)
Elucidation of an epigenetic mechanism underlying irinotecan resistance in colorectal cancer (Nucleic Acids Research 2016)
Development of a nano-genomic technology to diagnose nucleic acid based infections (Biosensors 2016)
Discovery of the first human diseases resulting from accumulation of Top2-linked DNA breaks (Nature Genetics 2014)
Elucidation of the mechanisms underlying temozolomide therapy in brain tumors (Nucleic Acids Research 2014)
Development of techniques to measure protein-linked chromosomal breaks (Plos One 2013)
Identification of the molecular role of SUMOylation during single-strand break repair (Nature Communications 2012)
Identification of the role of XRCC1 during neural development and maintenance (Nature Neuroscience 2010)
Discovery of the enzyme that disjoins abortive topoisomerase 2 DNA breaks (Nature 2009)
Discovery of the function of the neuroprotective enzyme aprataxin (Nature 2006)
Identification of the first human disease with defects in chromosomal single-strand break repair (Nature 2005)
Elucidation of the role of CK2 in chromosomal single-strand break repair (Cell 2003)
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Loizou JI, El-Khamisy SF, Zlatanou A, Moore DJ, Chan DW, Qin J, Sarno S, Meggio F, Pinna LA & Caldecott KW (2004) The protein kinase CK2 facilitates repair of chromosomal DNA single-strand breaks.. Cell, 117(1), 17-28.
El-Khamisy SF, Masutani M, Suzuki H & Caldecott KW (2003) A requirement for PARP-1 for the assembly or stability of XRCC1 nuclear foci at sites of oxidative DNA damage.. Nucleic Acids Res, 31(19), 5526-5533.