Progress Update: In vivo model to validate drugs targeting DUX4 enhancer/promoter activity

Progress update by Yosuke Hiramuki
See grant In vivo model to validate drugs targeting DUX4 enhancer/promoter activity

Lay abstract The DUX4 gene is normally repressed, or OFF, in skeletal muscle, while it is abnormally expressed, or ON, in FSHD skeletal muscle. Thus, developing therapies that inhibit the expression of DUX4 in skeletal muscle is a common approach for FSHD. Several drugs that appear to suppress DUX4 expression in cultured cells have been developed. However, since there are no natural animal models for FSHD, it has been difficult to validate these drugs in a whole living organism. To address this need, we generated and characterized novel FSHD model mice that contain the human FSHD region of chromosome 4q35, including the regions that control DUX4 expression in skeletal muscle. In addition, we replaced the DUX4 gene with a reporter gene to mark cells in which DUX4 would be expressed. Thus, by tracking the reporter activity we could identify cells in which the DUX4 gene would be turned ON. We then investigated the reporter-positive cells during mouse embryonic development and in adult tissues, including skeletal muscle. We found that many of the mouse muscles that are similar with those human muscles affected in FSHD also express the reporter during development. Moreover, we characterized the reporter-positive cells in healthy and regenerating skeletal muscle in more detail. We found that similar with DUX4 expression of FSHD, our model mice have very few reporter-positive cells in skeletal muscle. Interestingly, these are increased in regenerating muscle. This data is very interesting in respect to understanding the biology underlying DUX4 expression in vertebrate muscle development, however, the low expression is a concern for use in drug screening. We are now working on improving the model for use in therapeutic validation.