The repetition of three little "letters" within the gene that codes for the ataxin-3 protein is both the cause of and perhaps a solution to Machado-Joseph disease and an entire family of similar genetic disorders, according to researchers at the University of Pennsylvania.
Their findings, which appear today in the journal Molecular Cell, present a potential therapeutic role for the ataxin-3 protein for MJD and related disorders such as Huntington's disease.
Machado-Joseph disease is among the most common of the nine known polyglutamine repeat disorders, a family of diseases in which the genetic code for the amino acid polyglutamine CAG becomes excessively repeated within the gene, making the protein toxic. In these diseases, the expanded polyglutamine domain causes the errant protein to fold improperly, which causes a glut of misfolded protein to collect in tissues of the nervous system, much like what occurs in Alzheimer's and Parkinson's diseases.
"In origami, if you misfold the paper, you can just throw the paper into the recycling bin," said Nancy Bonini, a Penn professor of biology and Howard Hughes Medical Institute investigator. "If a protein misfolds, cells rely on their own recycling system to dispose of it. It turns out that ataxin-3 may influence this system, especially for recycling those that have misfolded due to excessive polyglutamine repeats.. Our findings show that ataxin-3 not only blunts the toxicity of mutant versions of itself but can also mitigate neurodegeneration induced by other such mutant polyglutamine proteins."
Machado-Joseph disease is among the most common dominantly inherited ataxias, a neurodegenerative disorder marked by a gradual decay of muscle control. MJD typically appears in adulthood, with a longer repeat expansion being associated with earlier onset and more severe disease. Its symptoms, uncoordinated motor control, worsen with time.