The FSH muscular dystrophy scientific literature finally has a publication describing a genetic mouse model that develops skeletal muscle disease. This work comes via the laboratory of Michael Kyba, PhD, at the University of Minnesota’s Lillehei Heart Institute. This murine model, called iDUX4 pA, has a number of compelling similarities to the human disease.
Ever since the genetic mechanism of FSHD, centered on the abnormal expression of the DUX4 gene, was published in 2010, scientists around the world have used genetic engineering to insert DUX4 into various species in hopes of recreating key features of FSHD in lab animals. Many of these efforts were funded by the FSH Society.
This turned out to be not as easy as one might have hoped. DUX4, which does not occur naturally in mice, is quite tricky to control. Some initial DUX4 mouse models showed few ill effects. Other efforts led to animals that were so severely affected that they died before birth.
None of these models reflected key aspects of FSHD in humans: the extremely low levels of DUX4 expression, and slow progression leading to dystrophic changes, in which healthy muscle is replaced by scar tissue and fat. It seemed like a Goldilocks dilemma of having too much or too little.
In Kyba’s lab, early attempts to create a DUX4-expressing mouse ran into stumbling blocks. DUX4 by itself is an incomplete gene, lacking parts needed for DUX4’s genetic code to be transcribed to enable the cell to act on the genetic information. So when the Kyba lab first tried to engineer an FSHD mouse, it added a piece of genetic material dubbed “SV40 poly A” to DUX4, which permitted the gene to be transcribed. But this system was too efficient. DUX4 is very toxic, so even when the gene was supposed to be turned off, low levels would still seep out and cause abnormalities and premature death.
Going back to the drawing board, the Kyba team changed out this genetic material for sequences downstream of the DUX4 gene which were predicted to be equivalent but turn out to be actually much weaker, and this did the trick. With the new mouse, called “DUX4 polyA,” pups are born looking fairly normal but go on to develop a slow, progressive degenerative myopathy. It appears that they have, à la Goldilocks, gotten the DUX4 dose “just right.”
Intriguingly, the muscle degeneration in the DUX4 polyA mice involves inflammation and stimulates certain cells to form scar tissue (fibrosis). While inflammation has not been widely seen as a cardinal feature of FSHD, studies of patients have reported evidence of inflammation, and some researchers think it may play a more important role than previously acknowledged. “Inflammation could be a response to damaged muscle,” noted Kyba, “or it could be a response to some factor that DUX4 is activating.”
Another interesting feature of these mice is that they have high-frequency hearing loss—a symptom seen in 50 percent of FSHD patients. This hearing loss is subtle, so although it can be detected by hearing tests, many affected individuals don’t realize they have it.
In addition, when their skeletal muscles are injured, the DUX4 polyA mice are less able to regenerate healthy new muscle. “Immature muscle fibers may be more vulnerable to DUX4,” Kyba suspects. This is one of the questions he will be investigating with funding from an FSH Society grant awarded this year.
In future mouse work, Kyba would like to lower DUX4 levels even further in hopes of “finding levels of DUX4 expression and rates of muscle deterioration as similar as possible to the human disease.”
In addition to probing for insights into the disease process, Kyba’s ultimate aim is to be able to screen drugs in a mouse model—particularly at approaches that “go after the DUX4 protein directly,” which he thinks are likely to have fewer side effects than approaches that alter gene expression.
Reference: Bosnakovski D, Chan SSK, Recht OO, Hartweck LM, Gustafson CJ, Athman LL, Lowe DA, Kyba M. Muscle pathology from stochastic low level DUX4 expression in an FSHD mouse model. Nat Commun. 2017;(8):550. Published online 2017 Sep 15. doi: 10.1038/s41467-017-00730-1. PMCID: PMC5601940.
Editor’s note: Other research teams are also making progress in regulating DUX4 expression in ways that allow mice to develop FSHD-like muscle weakness. These studies have been presented at scientific meetings and are being prepared for publication. See related stories here and here.