Topoisomerase I and Irinotecan

Topoisomerase I is an enzyme (i.e. a protein catalyzing certain biochemical reactions) which is expressed in almost every cell of the body and which is highly conserved in the phylogenetic tree.[1] Its major function is the relaxation of DNA (desoxyribonucleic acid, the genetic material). Most DNA exists as double helix (i.e. double-stranded). The double helix is normally self-winding to achieve compact DNA which fits into the small nucleus of the cell. Under certain circumstances such as cell division an over-winding (supercoiling) of the DNA takes place. Such supercoiling of the double-stranded DNA induces torsional stress at the DNA helix bearing the risk of uncontrolled strand breaks. In order to release torsional stress from the DNA, topoisomerase I binds to the DNA and cleaves one DNA strand. This allows the rotation of the cleaved strand around the other in a controlled reaction (DNA relaxation). Afterwards, the nicked strand is re-ligated by topoisomerase I, thus restoring intact double-stranded DNA. (Figure 1) [2]

Figure 1Figure 1: DNA relaxation mediated by the enzyme topoisomerase I.

Topoisomerase I inhibitors stabilize a normally very transient catalytic intermediate in which topoisomerase I is bound to one strand of the DNA, known as the topoisomerase I cleavable complex. When the cleavable complex is stalled by topoisomerase I inhibitors, re-ligation of the DNA is impossible (Figure 2, left panel).[3]

Figure 2 Figure 2: Effects of the topoisomerase I inhibitor irinotecan on the structure of DNA with subsequent cellular changes.

The consequences of prevented re-ligation vary; in proliferating cells the stalled topoisomerase I cleavable complex may collide with replication forks resulting in unrepairable DNA double strand breaks followed by cell death also known as apoptosis (Figure 2, right upper panel).[4], [5] For this reason, topoisomerase I inhibitors are used as anticancer drugs to treat several types of tumors. [6] In non-dividing cells the treatment with topoisomerase I inhibitors results in the production of single-stranded DNA breaks (Figure 2, right middle panel). Single-stranded DNA breaks are believed to reduce a cell’s replication capacity, which is not lethal.[7] For a long time we thought that the induction of single-stranded DNA breaks might explain the effects of irinotecan on lupus nephritis. Considering the detrimental role of lupus-typical auto-antibodies against double-stranded DNA, we hypothesized that the postulated induction of DNA single strand breaks might modify the DNA in a way that impairs the binding of anti-double-stranded DNA antibodies to the DNA.[8] Due to our hypothesis, we paid much less attention to a third effect of topoisomerase I inhibitors, that of inhibited DNA relaxation. Normally topoisomerase I binds to the DNA, induces DNA relaxation, re-ligates the DNA and moves along the DNA to the next supercoiled site. However, if a topoisomerase I inhibitor stabilizes the cleavable complex, topoisomerase I is no longer able to move along the DNA and the process of DNA relaxation is interrupted (Figure 2, right lower panel).[10], [11]

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