Translational implications of Ape1 in germ cell tumours: Ape1 as a therapeutic target

The second enzyme in the DNA base excision repair (BER) pathway, Ape1, hydrolyzes the phosphodiester backbone immediately 5' to an apurinic site (AP) site. AP sites are generated from spontaneous and chemically initiated hydrolysis, ionizing radiation, UV irradiation, oxidative stress, oxidizing agents, and removal of altered (such as alkylated) bases by DNA glycosylases. In the latter case, this incision generates a normal 3'-hydroxyl group and an abasic deoxyribose-5-phosphate, which is processed by subsequent enzymes of the BER pathway. AP sites are the most common form of DNA damage with some 20-50,000 sites produced in every cell each day under normal physiological conditions. The persistence of AP sites in DNA results in a block to DNA replication, cytotoxic mutations, and genetic instability. However, Ape1 is a multifunctional protein that is not only responsible for repair of AP sites, but also functions as a redox factor maintaining transcription factors in an active reduced state. Ape1 has been shown to stimulate the DNA binding activity of numerous transcription factors that are involved in cancer promotion and progression such as Fos, Jun, NFkB, PAX, HIF-1a, HLF, p53 and others in an ever expanding list. Given the multiple roles of Ape1, both as a DNA repair enzyme and as a major redox-regulating factor, Ape1 is clearly a good target for therapeutic inhibition. We will discuss our initial incursion into three areas that Ape I could be used as either a target or as a gene therapy approach to tumour cell therapeutics. These include, but are not limited to; 1) inhibition of Ape1 DNA repair activity via the small molecule agent lucanthone, 2) dominant-negative Ape1 mutants that effectively bind to AP sites, but do not cleave the phosphodiester backbone and, 3) cells with. elevated levels of Ape1 appear to respond to retinoic acid (RA) resulting in an increased level of cell death compared to those cells with lower amounts of Ape1. These studies are all part of an ongoing series of experiments surrounding the hypothesis that unbalancing DNA repair pathways, particularly BER, can be used in a clinical setting.