Developing a Phosphospecific IHC Assay as a Predictive Biomarker for Topoisomerase I Inhibitors.

Phosphorylation is the most extensively studied posttranslational modification of proteins. There are approximately 500 kinases known in the human genome. The kinase-activated pathways regulate almost every aspect of cell function and a deregulated kinase cascade leads to impaired cellular function.

Camptothecin resistance is determined by the regulation of topoisomerase I degradation mediated by ubiquitin proteasome pathway.

Proteasomal degradation of topoisomerase I (topoI) is one of the most remarkable cellular phenomena observed in response to camptothecin (CPT). Importantly, the rate of topoI degradation is linked to CPT resistance. Formation of the topoI-DNA-CPT cleavable complex inhibits DNA re-ligation resulting in DNA-double strand break (DSB).