ABSTRACT: All types of facioscapulohumeral muscular dystrophy (FSHD) are caused by the aberrant myogenic activation of the somatically silent DUX4 gene, which initiates a cascade of cellular events ultimately leading to FSHD pathophysiology. Therefore, FSHD is a dominant gain-of-function disease that is amenable to modeling by DUX4 overexpression. However, there is large variability in the patient population. Typically, progressive skeletal muscle weakness becomes noticeable in the second or third decade of life, yet there are many genetically FSHD individuals who develop symptoms much later in life or remain relatively asymptomatic throughout their lives. Conversely, in rare cases, FSHD may present clinically prior to 5-10 yrs of age, ultimately manifesting as a very severe early onset form of the disease. Thus, there is a need to control the timing and severity of pathology in FSHD-like models. Methods: We have recently described a line of conditional DUX4 transgenic mice, FLExDUX4, that develop a myopathy upon induction of human DUX4-fl expression in skeletal muscle. Here, we use the FLExDUX4 mouse crossed with the skeletal muscle-specific and tamoxifen inducible line ACTA1-MerCreMer to generate a highly versatile bi-transgenic mouse model with chronic, low-level DUX4-fl expression and mild pathology, that can be induced to develop more severe FSHD-like pathology in a dose-dependent response to tamoxifen. We identified conditions to reproducibly generate models exhibiting mild, moderate, or severe DUX4-dependent pathophysiology, and characterized their progression. Results: We assayed DUX4-fl mRNA and protein levels, fitness, strength, global gene expression, histopathology, and immune response, all of which are consistent with an FSHD-like myopathic phenotype. Importantly, we identified sex-specific and muscle-specific differences that should be considered when using these models for preclinical studies. Conclusions: The ACTA1-MCM;FLExDUX4 bi-transgenic mouse model expresses a chronic low level of DUX4-fl and has mild pathology and detectable muscle weakness. The onset and progression of moderate to severe pathology can be controlled via tamoxifen injection to provide consistent and readily screenable phenotypes for assessing therapies targeting DUX4-fl mRNA and protein. Thus, these FSHD-like mouse models can be used to study a range of DUX4-fl expression and pathology dependent upon investigator need, through controlled mosaic expression of DUX4.
