Although repeating sequences of three nucleotides encoding some of the bodies' 20 amino acids are a normal part of protein composition, abnormal expansion of trinucleotide repeats is the known cause of multiple inherited neurodegenerative disorders, including Huntington disease.
Scientists at Emory University School of Medicine, in research with mice, now have discovered more specific information about how this inherited expansion of a normal repeated DNA sequence alters gene expression. The research is reported online in the journal Nature Neuroscience.
The inherited diseases caused by an abnormal number of glutamine repeats (generally, more than 37) are known as polyglutamine, or PolyQ diseases. The diseases lead to a progressive degeneration of nerve cells usually affecting people later in life. Although these diseases share the same abnormal expansion of the repeated glutamine sequence and some symptoms, the repeats for the different PolyQ diseases occur in genes on different chromosomes.
The scientists found that abnormal glutamine repeats interfere with the function of an essential transcription factor called TBP (TATA-box binding protein). In turn, the expanded polyQ sequence alters the interaction of TBP with other transcription factors, leading to neurodegeneration. Transcription is the process by which the genetic code in the DNA sequence is first transcribed into RNA. The RNA is subsequently translated into a protein.
"Our study has a broad impact for understanding transcriptional regulation of gene expression as well as the pathogenesis of neurodegeneration caused by expanded polyglutamine proteins," says Xiao-Jiang Li, MD, PhD, Distinguished Professor of Human Genetics in Emory University School of Medicine and the paper's senior author.
Lead author was Meyer J. Friedman, a graduate student in Emory's Department of Human Genetics. Other Emory authors were Anjali G. Shah, Zhi-Hiu Fang, Elizabeth G. Ward, Stephen T. Warren and Shihua Li.
The research was funded by the National Institutes of Health.