Complex diseases, caused by several potentially epistatic genes in concert with the environment, contribute to the bulk of human disease. Examples of complex diseases include coronary artery disease and Type II diabetes, but the inter-individual variation in drug toxicity is also a complex disease. Unfortunately, progress at identifying the genetic loci (Quantitative Trait Loci or QTL) contributing to complex disease has been slow. This is particularly true in the field of pharmacogenomics, where it is difficult to recruit large homogenous sets of affected patients and the underlying etiology of toxicity is likely to be heterogeneous. In this proposal we explore the utility of identifying genes harboring standing genetic variation that mediate drug toxicity in Drosophila, with the goal of exporting newly discovered genes to humans. Genes that contribute to standing variation in drug toxicity in Drosophila may be better candidates for genes contributing to inter-individual variation in humans than genes identified through traditional mutagenic screens (e.g., X-ray or EMS). The initial drug we will examine is Methotrexate because of wide clinical use to treat cancer and arthritis, known toxicities, and conservation of target pathways from humans to Drosophila. The experiments are simple enough that we will extend our work to three additional drugs that exhibit toxicity in humans. We will elucidate the genetic architecture of Methotrexate and other drug toxicity in a collection of two sets of ~750 Drosophila RI lines, each initially derived from eight highly inbred founder lines. These RI lines give us a great deal of power to both identify and localize QTL, and also gives us the extremely important ability to estimate the population frequency of any mapped QTL. We will draw female flies from each RI line and expose them to concentrations of Methotrexate and other drugs chosen so that adult female flies exposed to the drug for three days display a marked reduction in fertility. We will determine if toxicity by genotype interactions are due to intermediate frequency or rare in frequency causative factors. As each of the founder inbred lines used to derive the ~1500 RI lines will have been completely sequenced, knowledge of the """"""""phase"""""""" of mapped QTL will allow the identification of the actual nucleotide sites that define mapped QTLs.
Methotrexate is widely used as both a chemotherapy agent and to treat joint swelling in arthritis, yet in many cases it use is suspended as a result of toxic side effects. As Drosophila and humans share the genetic pathway Methotrexate targets, research will identify DNA polymorphisms in Drosophila that modulate toxicity. Future work can then ask if these same genes modulate toxicity in humans, possibly leading to genetic tests to identify patients who could most likely benefit from Methotrexate treatment. Similar outcomes are expected for additional chemotherapy agents.
