Two new mouse models of ALS caused by mutations in C9orf72 display a range of physical and molecular characteristics of the disease. These new models, created independently by Packard scientists Laura Ranum from the University of Florida and Clotilde Lagier-Tourenne at Harvard Medical School with Don Cleveland from the University of California in San Diego, gives scientists newer and better ways to test drugs that might potentially treat ALS and understand the cascade of events that leads to motor neuron degeneration.
When researchers first linked the repeat expansion in C9orf72 to ALS in 2011, they had no idea what the gene did or how it might contribute to ALS. Scientists around the world, including many funded by Packard, dug deep into the question. They discovered that the normal process that turns genes into proteins somehow got stuck, creating a buildup of toxic RNA foci and of small proteins called dipeptide repeats (DPRs). Identifying the downstream negative effects of the C9orf72 repeat expansion on a molecular level was one thing, but to truly understand how this mutation led to motor neuron degeneration and test potential therapies, researchers would need an animal model.
Building a good animal model of C9orf72 was difficult because the repeat expansion was inherently unstable. Researchers would insert a human C9orf72 gene that contained hundreds of repeats, but several generations later, it might only contain 30- not nearly enough to cause disease that researchers could study. Ranum and Lagier-Tourenne are the most recent scientists to have found their way around the problem using a bacterial artificial chromosome (BAC) to create the C9orf72 transgene, which provided more stability than other methods.
Ranum’s work, published in Neuron, created four different lines of BAC mice: C9-500/32 (two transgene copies, one with ~500 and the other with 32 repeats), C9-500 (one copy with ~500 repeats), C9-36/29 (four transgene copies with repeat sizes of 36 and 29 detected), and C9-37 (one copy with 37 repeats). Unlike other C9orf72 mouse models, the transgene inserted by Ranum and colleagues also contained substantial pieces of DNA flanking the C9orf72 gene. This DNA was likely to contain potential genetic elements that regulated the expression of the transgene and play an important role in pathology.
Three of the four mouse lines developed a buildup of both sense and anti-sense RNA (that is, RNA that’s transcribed in the normal and reverse direction, respectively) and DPRs. Up until four months of age, all of the C9orf72 mice appeared normal and healthy. At 16 weeks, however, the two expansion mouse lines showed gait abnormalities. Between 20 and 40 weeks, around one-third of female mice that carried the transgene with 500 repeats developed a rapidly progressing motor neuron disease with many of the same symptoms of ALS, including inactivity, labored breathing, sudden weight loss, hindlimb paralysis, and decreased survival
excerpt © 2016 Packard Center