The HSA LR20b transgenic mice express a human skeletal actin gene (designated HSA) or ACTA1, containing a CTG repeat expansion of approximately 220 repeats. The transgene is expressed predominantly in skeletal muscle. Minor expression may occur in non-muscle cells, but has not been systematically examined. Myotonic dystrophy (DM) is the result of an expansion of a CTG repeat in the 3' untranslated region of the DMPK gene. HSA LR20b mice express an expanded CUG repeat in the human skeletal actin transgene mRNA, rather than the DMPK gene. HSA LR20b homozygous mice exhibit electromyographic myotonia as early as 4 weeks of age. Muscle histopathology is consistent with some of the characteristics of myotonic dystrophy (DM) including increases in central nuclei and ring fibers, and variability in fiber size in the absence of muscle fiber necrosis. However, these mice exhibit muscle weakness and degeneration that is less severe than myotonic dystrophy patients. This strain may be useful for studying muscular dystrophy. The CTG repeat expansion size should be monitored in breeders at every generation, copy number can decrease with each successive litter. The Jackson Laboratory breeds and distributes homozygous mice with CTG repeat expansion range of ~175-187 as determined by Laragen assay. Importantly, The Jackson Laboratory quality control group has used an alternate evaluation protocol to confirm the full expansion size of ~220 corresponds to Laragen assay size ~175-187. For this strain, the Laragen assay quantification is consistently lower than the expected full expansion value as a result of their primer placement, design and evaluation protocol.

HSA LR20b transgenic mice express a human skeletal actin gene containing a CTG repeat expansion of approximately 220 repeats and exhibit some of the characteristics of myotonic dystrophy (DM). This strain may be useful for studying muscular dystrophy.

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