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. Stock No. 032031 FVB/N-Tg(HSA*LR)20bCath/J - Trinucleotide Scoring and Range: &nbsp;In 2022, 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. &nbsp;In February 2025, The Jackson Laboratory scoring of this strain's trinucleotide (CTG) repeat values was changed to Transnetyx following their acquisition of Laragen. Prior to this vendor change, The Jackson Laboratory Genetic Quality Control group and Transnetyx performed side-by-side CTG repeat value analyses of the same homozygous mouse samples (representing several different pedigrees within the live colony) via the Laragen protocol versus the Transnetyx protocol. For Stock No. 032031, more than 85 mouse samples were analyzed - this showed the Laragen score averaging 2.87 trinucleotide repeats lower than the same sample scored via Transnetyx. Based on these analyses, The Jackson Laboratory has adjusted the trinucleotide score range we use for Stock No. 032031 homozygous breeders from 175-187 CTG repeats via Laragen protocol to 178-190 CTG repeats via Transnetyx protocol [February 2025].

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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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