The cause of facioscapulohumeral muscular dystrophy (FSHD) is thought to center on DUX4, a gene that normally is silent in adult skeletal muscle. When DUX4 gets “expressed,” as happens in FSHD, it activates other destructive reactions in muscle cells and causes muscles to degenerate. Suppressing DUX4 is a logical strategy to treat FSHD, and a new study suggests that this may be achievable using “morpholino antisense oligomers,” a type of synthetic molecule that blocks specific DNA sequences.
Morpholinos are being tested in experimental treatments for other diseases, including spinal muscular atrophy, Duchenne muscular dystrophy, and myotonic dystrophy.
The new FSHD study was the result of a collaboration between academic scientists and Genzyme. The academic researchers are members of the NIH-funded Senator Paul D. Wellstone Muscular Dystrophy Cooperative Research Center for FSHD and work at the University of Massachusetts Medical School in Worcester, as well as the Kennedy Krieger Institute and Johns Hopkins School of Medicine, both in Baltimore, Maryland.
FSHD results from a perfect storm of things that go awry in the genetic machinery that normally keeps DUX4 safely under lock and key. These include the loss of D4Z4 repeat units on a section near the tip of the long arm of chromosome 4, the presence of a “permissive” poly-A haplotype that allows the DUX4 gene to get transcribed into messenger RNA, and hypomethylation of the D4Z4 units. All of these conditions have to be present in order for FSHD to occur in an individual.
The researchers tested various morpholinos and found that one, code named FM10, which targets the poly-A site, was strikingly effective in knocking down DUX4 expression. They also looked at several genes that are activated by DUX4, such as ZSCAN4 and MBD3L5, and found that their levels were also very low, confirming that DUX4 activity had been suppressed.
In addition, the investigators checked to see that FM10 did not result in potentially detrimental “off target” effects on other genes.
These experiments were done in cells derived from 11 FSHD patients and six first-degree, genetically related, unaffected relatives, as well as in mice that had been engrafted with human FSHD muscle (see “Human Muscle Grows in Mice,” FSH Watch Spring 2014). The study acknowledges the FSH Society for assisting with recruiting volunteers to donate muscle biopsies used in the research.
Future work will address whether morpholinos can achieve DUX4 knockdown when given systemically and longer term to a whole animal. Stay tuned.
Chen JC, King OD, Zhang Y, Clayton NP, Spencer C, Wentworth BM, Emerson CP Jr, Wagner KR. Morpholino-mediated Knockdown of DUX4 Toward Facioscapulohumeral Muscular Dystrophy Therapeutics. Mol Ther. 2016 Jul 5. doi: 10.1038/mt.2016.111. PubMed.
FIGURE CAPTION: FM10 knocks down DUX4-fl and DUX4 target genes in a human FSHD xenograft model. (a) FSHD patient muscle was transplanted into the hind limb of mice. The xenografts were treated with Control- or FM10- phosphorodiamidate morpholino oligonucleotides (PMO) and analyzed after 2 weeks. (b) The presence of human spectrin protein and nuclear envelope protein lamin A/C confirmed that the xenograft was well regenerated. (c) Significant decreases in expression levels of DUX4-fl and (d) of DUX4 target genes MBD3L5 and ZSCAN4 were seen in FSHD xenografts treated with FM10-PMOs, in contrast to controls. Ref. Chen JC, et al.
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