There is compelling evidence that the endonuclease (EN) encoded by the retrotransposon Long interspersed element 1 (LINE1) is a powerful causative agent of genetic instability. Expression of the LINE1 EN has been demonstrated to induce DNA double strand breaks (DSBs), damage that is known to be mutagenic and induce diseases such as cancer. In addition, the increased mobilization and expression of LINE1 elements has been observed in germ line, and in some somatic and tumor tissues. However, no studies have shown direct evidence that LINE1-induced DNA DSBs contribute to the induction or progression of diseases because there is currently no tool to specifically limit LINE1 EN activity in cells. Traditional techniques such as RNA knockdown are not feasible due to the large number of LINE1 elements and the location of some of the elements near or in coding regions of the genome. The best tool for deciphering the role of LINE1 EN in genetic instability and disease would be a potent and specific inhibitor of its activity, but no such inhibitor has been discovered yet. In our laboratory, we have been working on screening small molecules from a NIH library. Structural analogs of a promising molecule were made, and we have thus far identified three candidates that inhibit the activity of the lINE1 EN in vitro, have limited cellular toxicity and are able to reduce LINE1 mobilization in HeLa cells. However, further characterization in different types of cells needs to be done before the general use of these inhibitors in the study of LINE1 induced disease in animals.

Our long-term goal is to determine the impact of the LINE1 element on human disease. The objective for this application, the next step in achieving our long-term goal, is to further characterize small molecule inhibitors of the LINE1 EN in multiple cell lines and to test the inhibitors in a tumor- environment. Our rationale for this research project is to design a good tool that allows us to study L1 EN caused damage in cells and the role in the initiation and progression of diseases in a mouse model.