Researchers studying myotonic dystrophy (DM), a neuromuscular degenerative condition, found a decade ago that DM was mediated by an elongated repeating sequence of three or four nucleotides in untranslated RNA that bound up proteins responsible for RNA processing. The current work confirms that in Fragile X Tremor/Ataxia Syndrome (FXTAS), also a neuromuscular degenerative disease marked by trinucleotide repeats, RNA pathogenesis is also to blame.
"It's looking more and more like the story with DM," said the University of Minnesota's Harry Orr, who cowrote the papers' accompanying commentary. "The point is you get the disease because of the effect and expression of a mutant RNA, not because of the effect or expression of a mutant protein."
FXTAS is a neurodegenerative disorder in which a non-coding region of the fragile X mental retardation 1 (FMR1) gene consisting of trinucleotide repeats is expanded. Normal individuals have between 5 and 54 repeats, while FXTAS patients carry between 60 and 200.
Although FXTAS shares a genetic locus with fragile X syndrome, the most common inherited form of mental retardation, the condition shares none of its phenotype. Because the fragile X mutation has more than 200 repeats, the DNA itself becomes highly methylated, shutting down transcription of FMR1 mRNA so that no FMR1 protein is made. In the FXTAS condition, however, the gene makes a normal amount of FMR1 protein, which left unanswered questions about the condition's mechanism.
In DM, a trinucleotide repeat sequesters RNA binding proteins (RBPs), which are responsible for RNA splicing, transport, and other types of processing. The excess number of repeats binds up more RBPs than normal, leaving fewer of them to do their jobs elsewhere. Peng Jin at the Emory University School of Medicine and his colleagues wondered whether a similar sequestration mechanism might be at work in FXTAS.
In a previous study, Jin's group engineered Drosophila to express a 90-trinucleotide version of the FXTAS sequence in the eye. Flies that transcribed RNA from the sequence experienced neuronal toxicity. "We drew the conclusion that it was RNA-mediated transcription" that caused the toxicity, recalled Jin, a lead author of that study as well as the current research.
In the current study, the Emory group and colleagues examined FXTAS transgenic flies to look for RBPs that bound specifically to the repeats in neurons. In biochemical assays, two known RBPs bound directly with the repeats, and a third - which plays a role in DM - bound to the repeat through one of the directly-binding RBPs, suggesting that the proteins may be downstream mediators of the disease.
Giving the transgenic flies extra copies of genes coding for any of these proteins alleviated neural degeneration in a dose-dependent manner, suggesting that the repeats were sequestering essential proteins away from other pathways.
"Normally we think of loss-of-function by deleting the gene, mutating the promoter, or altering amino acid composition by mutation," explained Emory's Stephen Warren, a senior author on one of the papers and the 2003 study. "In this case you can modify the function of the gene not by any change in the locus itself, but simply by sequestering [the protein it codes for]."
Laura Ranum of the University of Minnesota, who was not connected with the studies, noted that whether the mechanism seen in transgenic flies translates to the human pathology remains to be seen. "You have a very artificial system when you overexpress something in the fly eye," she said, and it is still unclear "if [FXTAS] really leads to downstream mechanisms that are similar to what's been learned about DM, or if it's going to turn out to be a different story." The mechanism for DM, she added, was established not just in flies, but also in mice and humans.
Regardless, said Maurice Swanson of the University of Florida in Gainesville, coauthor with Orr on the commentary to the studies, the work "really expands the number of diseases caused by RNA-mediated defects." He added, "Of course then the question is, how many more are there out there?"
Josh P. Roberts
mail@the-scientist.com
Links within this article:
P. Jin et al., "Pur
http://www.neuron.org
O.A. Sofola et al., "RNA-binding proteins hnRNP A2/B1 and CUGBP1 suppress fragile X CGG premutation repeat-induced neurodegeneration in a drosophila model of FXTAS," Neuron, August 16, 2007.
http://www.neuron.org
S. Bunk, "The dark side of RNA," The Scientist, November 12, 2001.
http://www.the-scientist.com/article/display/12706
J.B. Weitzman, "Myotonic expansion," The Scientist, August 6, 2001.
http://www.the-scientist.com/article/display/19816
Harry Orr
http://www.med.umn.edu/pathology/about/faculty/orr/home.html
M.S. Swanson and H.T. Orr, "Fragile X Tremor/Ataxia Syndrome:
Blame the messenger!," Neuron, August 16, 2007.
http://www.neuron.org
Peng Jin
http://www.genetics.emory.edu/FACULTY/faculty_bio_jin.php
P. Jin et al., "RNA-mediated neurodegeneration caused by the fragile X premutation rCGG repeats in Drosophila," Neuron, August 28, 2003.
http://www.the-scientist.com/pubmed/12948442
Stephen Warren
http://www.genetics.emory.edu/FACULTY/faculty_bio_warren.php
Laura Ranum
http://www.gcd.med.umn.edu/html/faculty%20pages/ranum.html
Maurice Swanson
http://www.mgm.ufl.edu/faculty/fac-swanson.htm
