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Double gene flaws lead to rare muscular dystrophy

U. ROCHESTER / U. WASHINGTON (US) — A rare form of muscular dystrophy is the result of two unrelated genetic flaws that come together to ultimately produce toxins that damage muscle cells and trigger symptoms.

Facioscapulohumeral muscular dystrophy (FSHD) called type 2 is the third most common form of muscular dystrophy. The first symptoms of the disease usually appear in the form of muscle weakness in the upper body—including in the arms, shoulders, and face.

Eventually, these symptoms spread to the rest of the body. While not fatal, the disease can lead to significant disability and many patients end up in a wheelchair. An estimated 500,000 people worldwide suffer from this disease.

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“The discovery of the complex and elusive source of FSHD type 2 is the result of not only a unique international partnership of scientists, but also the extraordinary cooperation of the families who are burdened by this disease,” says University of Rochester Medical Center neurologist Rabi Tawil, a senior co-author of the study published in the journal Nature Genetics.

“Here in Rochester, we have evaluated many patients—often for several years—who clearly suffered from FSHD, but did not meet the common genetic profile of the disease. This discovery will help us better diagnose these patients and help guide research that could lead to new treatments.”

The genetic cause of FSHD type 1—the more common form of the disease—was first revealed by the same group of researchers in a study that appeared in journal Science in 2010.  In that paper, the scientists zeroed in on a specific segment of genetic code called a macrosatellite repeat that appears at the end of chromosome 4, essentially a genetic “stutter” that results in a section of code being replicated multiple times.

The human genetic code was thought to be full of “junk” or inactive genes often contained in macrosatellite repeat sequences such as the one seen in FSHD. However, these regions of repeated genetic code are now understood to be actively switched on and off and to help regulate the function of many other genes.  When the normal regulation of gene expression is disrupted, such as occurs in FSHD, the effect is devastating.

People without the disease actually have a large number of repeats. Consequently, this section of the code is longer and folds tightly back upon itself like a ball of twine. This prevents an active piece of code—called DUX4—which is bound up within the macrosatellite repeat from being accessed. DUX4 carries instructions to create a protein that—while found in other parts of the body—is toxic to muscle cells.

By contrast, people with the disease possess a small number repeats (less than 10). The Science study found that in these instances the DNA is more loosely bound and exposed allowing the genetic instructions in DUX4 to be used by the muscle cells to build proteins.

People with FSHD also possess a snippet of genetic code adjacent to the repeats—called an A allele—that serves to stabilize the message from the DUX4 code. In people with these unfortunate set of conditions (short number of repeats followed by an A allele), the result is the production of a protein that breaks down muscle cells causing the symptoms of the disease.

“Most genetic mutations reduce the production of a protein, or a mutated gene might produce a detrimental protein,” says Daniel Miller, associate professor of pediatrics at the University of Washington. “FSHD is unusual because it is most often caused by genetic deletions that paradoxically result in the production of DUX4 in the wrong tissue at the wrong time.”

While this phenomenon explained the trigger for the vast majority of FSHD patients, there remained a small number of individuals—5 percent of patients with the disease—that did not meet this genetic profile. These patients, dubbed by researchers as exhibiting FSHD type 2, possessed the longer D4Z4 repeats found in healthy individuals, however, their symptoms were identical to other FSHD patients.

To understand the cause of this FSHD variant, researchers analyzed the genetic profiles of 12 families with the disease. The genetic mutations that cause FSHD are inherited. By examining the code of patients with FSHD type 2 and that of their unaffected parents and siblings, the researchers could then identify the common genetic factors and begin to understand how they may impact each other.

“The breakthrough came when we realized that in some of these FSHD type 2 families this open macrosatellite structure segregates in the family independent of the length of the repeat,” says Silvere van der Maarel, professor of human genetics at the Leiden University Medical Center. “This observation allowed the identification of the genetic flaw that causes this opening of the repeat structure.”

This investigation ultimately led the team to yet another culprit on a different chromosome. In healthy cells, one of the factors that helps bind long strands of the chromosomes tightly together is a chemical process called methylation. In FSHD, the absence of these chemical links enables the macrosatellite repeat structure to unravel or open up, regardless of its length, exposing the DUX4 code.

The researchers found that this occurred in patients who also possessed a mutation in a gene called SMCHD1, which regulates the methylation process and consequently how tightly genetic structures are bound together. In patients with FSHD type 2, just as in FSHD type 1, this flaw works in concert with the A allele code to create the conditions in which the toxic proteins that are the source of the disease are mistakenly produced.

“Many diseases caused by a single gene mutation were identified over the last several decades, but it has been more difficult to identify the genetic basis of diseases caused by the combination of more than one genetic variant,” says Stephen Tapscott, a co-investigator at the Fred Hutchinson Cancer Research Center in Seattle.

“Recent advances in DNA sequencing made this study possible and it is likely that other diseases caused by the inheritance of multiple genetic variants will be identified in the coming years.”

This understanding of the source and mechanisms of the disease has helped researchers identify junctures during which the disease process could be intercepted or altered. The findings also indicate that similar treatments could be developed that impact patients with both types of FSHD.

Funding to conduct the study came from the National Institutes of Neurological Disorders and Stroke, the National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Human Genome Institute, National Genetics Institute, the Muscular Dystrophy Association, and the University of Rochester’s Fields Center for FSHD and Neuromuscular Research.

Source: University of Rochester

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