The human genome consists of approximately 3.2 billion base pairs and would be about 2 meters in length, if stretched out from end to end (Porter et al., 2004). Nevertheless, DNA squeezes into the nucleus of a cell, which is approximately 6uM in diameter. For example, the smallest human chromosome is 14 mm long but is compacted to just 2 uM in length (Ball, 2003). Thus, DNA exists in a tightly compacted and dense form. Conversely, DNA must also allow itself to become accessible to numerous transcription factors and regulatory proteins to allow gene transcription and DNA replication to occur. This means that DNA must unwind itself to allow for DNA replication and gene expression to occur. The ability of DNA strands to separate and/or unwind, however, is limited by the torsional constraints of a double helix. DNA topoisomerases (topos) are a family of nuclear enzymes that regulate the topological state and relieve the torsional stress of the DNA that arise during gene transcription, DNA replication, and cell division (Reviewed in Champoux, 2001 and Wang, 2009). DNA can become tangled into knots, supercoiled, and form interlocked circles that must be resolved by topo action. DNA topoisomerases should not be confused with DNA telomerases, which are enzymes that maintain the telomere regions of DNA. Topo activity has been shown to be important during DNA replication, transcription, genetic recombination, sister chromatid segregation, and chromosome condensation and decondensation.To regulate DNA topology, all topo enzymes introduce single- or double strand breaks in DNA to allow another strand of DNA or duplex DNA to pass through the newly generated gap. This means that DNA topoisomerases actually cut DNA. Thus, DNA topoisomerases are often described as the “molecular Houdini’s” or as enzymes with a “double-edged sword” because the integrity of DNA is itself susceptible to the enzyme’s cleavage activity (Reviewed in, Deweese and Osheroff, 2009). In other words, topoisomerases are responsible for maintaining the topology of DNA but, to do so, they induce cuts in DNA, which can be potentially damaging to DNA if not properly maintained. When topo creates a cut into the DNA strand, it allows another strand of DNA (which is presumed to be tangled) to pass through the opening and thereby, unwind itself. To ensure the integrity of DNA, all DNA breaks generated by topo are accompanied by the formation of a bond that tethers topo to the broken strand of DNA while DNA strand passage occurs (Tse et al, 1980). In terms of cancer therapy, the tethered form of the topo enzyme, the moment when the enzyme cleaves the DNA strand but still remains bound to the DNA, has become an important target of anti-cancer drugs. That is because the moment that topo cleaves DNA, the DNA strand is in a vulnerable state. Normally, topo reseals the broken strand as soon as the DNA strands unwind. However, some anti-cancer drugs that target topoisomerases exploit the DNA cleavage activity of topo to turn the enzymes into molecular toxins. Although the specific mechanism of action may vary, the topo targeting drugs essentially block the ability of the enzyme to reseal the broken DNA strand. Therefore, DNA damage is presumed to accummulate and cell death occurs.In summary, DNA topoisomerase enzyme activity is essential for maintaining the topology of DNA during many DNA processes, including DNA replication, transcription, and chromosome segregation. DNA topoisomerases have also emerged as important molecular targets of anti-cancer agents.References:Ball, P. Nature. 2003, 321-422.Champoux, J.J. Ann. Rev. Biochem. 2001, 70, 369-413. Review.Deweese J.E., Osheroff N. Nucleic Acids Res. 2009, 37(3):738-48. Review.Porter, I.M., Khoudoli, G.A., Swedlow, J.R. Curr. Biol., 2004, 14, R554-R556.Tse, Y.-C. Kirkegaard, K., Wang, J.C. J. Biol. Chem. 1980, 255, 5560-5565.Wang, J.C. Annu Rev Biochem. 2009, 78:31-54. Review.

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