Coevolotion of topoisomerase I and camptothecin production in ophiorrhixa plants

Abstract

Camptothecin (CPT), a naturally occurring indole alkaloid, is an essential precursor of semi-synthetic chemotherapeutic agents for cancers throughout the world. In spite of the rapid growth of market demand, CPT raw material is still harvested by extraction from Camptotheca acuminata and Nothapodytes foetida. Previous study found that many CPT-producing plants, including some of Ophiorrhiza spp., have topoisomerase I (TopI) enzymes with several point-mutations that confer resistance to CPT to avoid CPT toxicity. The purpose of this thesis is to study the coevolution between TopI gene and CPT production in Ophiorrhiza plants. Eight species of the genus Ophiorrhiza in Thailand were examined as novel alternative sources of CPT. CPT and its derivatives were differently detected in five species in leaf and root extracts. Chloroplast matK and nuclear TopI genes of eight species were investigated in order to classify and study the coevolution in this genus. The molecular phylogenetic trees of both separated and combined matK and TopI nucleotide sequences revealed two major clades of Ophiorrhiza taxa correlated with the productions of CPT and CPT derivatives. We conclude that Ophiorrhiza plants have matK and TopI coevolved with CPT production. Thus, matK and TopI gene sequences could be utilized for the prediction of CPT and CPT derivatives production ability of any members of Ophiorrhiza. We also proposed that several unique amino acid substitutions in TopI of CPT-producing Ophiorrhiza plants could be used as amino acid markers and provide useful information toward recognition of the point mutations in CPT-resistant cancer patients in the future.