Further detailed analysis of DUX4 expression revealed that mice with 2.5 D4Z4 units also had the remarkable DUX4 expression pattern seen in human FSHD patients: only a small subset of muscle cell nuclei expressed abundant levels of DUX4. In mice with 12.5 units of D4Z4, by contrast, DUX4 was almost completely absent from muscles.
In terms of molecular activity, DUX4 in muscle cells appeared to trigger other genes in networks similar to those previously reported in tissue collected from human FSHD patients. In addition, the D4Z4 units in these mice had reduced levels of “methylation”, which one can think of as the “mortar” that seals together the D4Z4 bricks in the firewall. With less methylation, the system cannot suppress the expression of toxic DUX4. The mice with 12.5 D4Z4 units had high levels of methylation, which was consistent with their low expression of DUX4.
In other words, these “D4Z4-2.5” mice appear to faithfully mimic key features of FSHD. But the researchers warn that the differences between the 2.5-unit and 12.5-unit mice could potentially be caused by as-yet-unknown differences in the genome sites where the D4Z4 units were inserted in the two mouse strains. In addition, it took the researchers 450 tries before they succeeded in generating the D4Z4-2.5 line, so there could be some unique mechanism at work in this line.
Nonetheless, the researchers are optimistic that the new mouse will provide new insights into DUX4’s role in FSHD. For example, they hope to learn “why, how and when are sudden bursts of DUX4 expression in skeletal muscle regulated.” In addition, they say, “As it was recently demonstrated that the detrimental effects of DUX4 expression in mouse muscle can be reversed by RNA interference,
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