At one end of chromosome 4 is a chunk of repetitive DNA, called a tandem repeat array. Normally this region contains 10-100 repeating units of DNA, but in most people with FSHD, the array is smaller, with fewer than 10 repeats. Within each repeating unit is a gene called DUX4.
The researchers found that in people with FSHD, the DUX4 gene generates a piece of RNA that is toxic to muscle cells. RNA is a sister molecule to DNA with many critical functions. The researchers also discovered that variations on chromosome 4 are important because they affect the durability of DUX4 RNA. People with FSHD have chromosome variations that add a trailing segment to the RNA called a poly(A) tail. With the poly(A) tail attached, the RNA is more stable and more likely to cause damage.
The researchers came to these conclusions by creating artificial DNA constructs containing the short repeat array, in combination with different variations on chromosome 4. They inserted these constructs into muscle cells, and analyzed how the chromosome 4 variations affected the level of DUX4 RNA. They also studied FSHD families with unique chromosome rearrangements and showed that all families with FSHD shared chromosome 4 sequences encoding the poly(A) tail.
In another set of experiments, they found they could detect DUX4 RNA in muscle cells from individuals with FSHD but not in cells from unaffected individuals. Meanwhile, previous studies have shown that DUX4 can trigger muscle cell death.
"This study provides evidence that DUX4 RNA is likely a key part of the disease process in FSH muscular dystrophy, and justifies further investigation of its role and how to silence its effects," Dr. Tapscott said.
|Contact: Daniel Stimson|
NIH/National Institute of Neurological Disorders and Stroke