Nearly 20 years after the discovery of the mutation, scientists have finally determined how it contributes to a common form of muscular dystrophy known as facioscapulohumeral dystrophy, or FSHD, which occurs in about 1 in 20,000 births.
Although questions remain, the discovery, reported online in Science on Aug. 19, points the way toward an effective treatment for FSHD, which produces progressive wasting of muscles in the upper body.
“I don't think the importance of this can be over-emphasized,” said John Porter, PhD, program director of Neuromuscular Disease at the Neurogenetics Cluster and the NINDS Office of Translational Research, who was not involved with the study. “Without a mechanistic model that provides a hypothesis about the pathogenesis of a disease, researchers have difficulty getting grants. Grant applications have to be supported by a conceptual framework, and this provides a huge building block.”
In 1992, researchers at Leiden University in the Netherlands discovered that FSHD is correlated with the number of repeats on chromosome 4q35 of D4Z4, which codes for DUX4, a protein of unknown function. In people who possess the normal number of 11 to 100 D4Z4 repeats, the RNA that encodes the protein disintegrates rapidly. Those who possess 10 or fewer, however, develop FSHD in varying degrees of severity. While this suggests that the disease emerges from an inadequate number of repeats, people with no repeats for D4Z4 do not get the disease.
“That's been the puzzle,” said Alan Pestronk, MD, director of the Neuromuscular Division and the Neuromuscular Clinical Laboratory, and a professor of neurology at Washington University School of Medicine. “How can you have a disease that gets worse and worse with fewer repeats, but when you have no repeats, you don't get the disease?”
Not only that, some people who have fewer than 10 D4Z4 repeats do not get the disease, while some who have more than 11 do get the disease.
TWO GENETIC ABNORMALITIES
The solution to this mystery involved recognizing that FSHD requires two separate genetic abnormalities. First a person must be born with too few repeats for D4Z4 on chromosome 4 and the most distal gene for DUX4 that extends into the next piece of non-coding or “junk” DNA beyond the repeats must be present.
The reduced number of repeats is associated with an epigenetic change in which relaxation of the chromatin allows the DUX4 protein to be expressed from the most distal gene.
Second, the configuration distal to the repeats results in polyadenylation of the DUX4, making it stable and preventing it from being degraded quickly after production. Polyadenylated DUX4 is assumed to be toxic to muscles, so the production of this stable form of the protein probably contributes to the dysfunction of muscles characteristic of FSHD.
Clues to this complex and mysterious mechanism have been accumulating for years. In 1996, Rossella Tupler, MD, PhD, of the University of Massachusetts-Worcester and the Universita degli Studi di Modena e Reggio Emilia in Modena, Italy, reported in the Journal of Medical Genetics on members of a family with a large deletion on the tip of chromosome 4, which left them without any D4Z4 repeats. Since a reduction in repeats had been linked to FSHD, she expected them to display symptoms of the disease, but they didn't, which caused Dr. Tupler to conclude that another mechanism must be involved.
A 2007 paper in Neuromuscular Disorders concluded that the DUX4-mediated cell death contributed to the pathogenic pathway in FSHD.
Another 2007 paper in the Proceedings of the National Academy of Sciences found that PITX1 (paired-like homeodomain transcription factor 1) is up-regulated in FSHD muscles, along with DUX4, suggesting another possible molecular mechanism of disease.
What helped the authors of the Science paper pull these puzzle pieces together was a patient who had some distal repeats normally found on chromosome 4 transposed into chromosome 10. He developed FSHD, which demonstrated that DUX4 itself is probably the pathogenic factor.
“If it's true that DUX4 is the specific bad player in this disease, then it seems it would be simple to use gene therapy to turn it off,” said Dr. Pestronk. “It's not produced in normal people to a very high degree, so you probably wouldn't do any harm by turning it off, and you could theoretically cure this disease.”
While Dr. Tupler considers the Science paper a breakthrough, “there are still significant gaps in our understanding of FSHD,” she said.
“Through the Italian Registry for FSHD we enrolled over 800 subjects belonging to FSHD families and carrying the FSHD molecular defect,” she said. “Their clinical examination using a standardized protocol recently developed shows that 20 percent of subjects over 60 years do not develop muscle weakness. There must be other factors interfering with FSHD development.”
The corresponding author of the Science, paper, Silvère van der Maarel, PhD, professor of medical epigenetics at Leiden University Medical Center in the Netherlands, agrees. Because the relaxation of the chromatin is highly variable among individuals, “it is possible that inter-individual differences in chromatin relaxation may lead to differences in severity, onset and progression,” he said in an e-mail. “In addition, it is likely that also other (genetic) factors modulate the phenotype.”
The next logical step would involve developing animal models of FSHD based on this new knowledge, and begin developing methods to thwart the disease. “There are several labs that are developing faithful animal models for FSHD,” he said.
The paper brings the field much closer to an understanding of FSHD, and to a treatment, according to Glen H. Nuckolls, PhD, director of the Muscle Disorders and Therapies Program in the NIH Division of Musculoskeletal Diseases at the National Institute of Arthritis and Musculoskeletal and Skin Diseases.
“The FSHD field has been struggling for some time to identify the gene that's responsible for the phenotype,” he said. “This has gone a long way toward nailing that down.”
ARTICLE IN BRIEF
Investigators have identified DUX4 itself as a likely pathogenic factor in facioscapulohumeral muscular dystrophy.
Lemmers RJLF, van der Vliet PJ, van der Maarel SM, et al. A unifying genetic model for facioscapulohumeral muscular dystrophy. Science
2010; E-pub 2010 Aug. 19.
Tupler R, Berardinelli A, Tiepolo L, et al. Monosomy of distal 4q does not cause facioscapulohumeral muscular dystrophy. J Med Genet
Kowaljowa V, Marcowyczb A, et al. The DUX4 gene at the FSHD1A locus encodes a proapoptotic protein. Neuromusc Disord
Chen YW et al. DUX4, a candidate gene of facioscapulohumeral muscular dystrophy, encodes a transcriptional activator of PITX1. Proc Natl Acad Sci