An international team of researchers, including three Packard scientists, have developed a method to interrupt the process that turns the gene into protein
Work from yeast, flies, worms, and cultured neurons from patients reveal a novel way to target the most common genetic cause of amyotrophic lateral sclerosis (ALS). Instead of directly targeting the genetic mutation in the C9orf72 gene, an international team of researchers, including Packard scientists Aaron Gitler of Stanford University, Fen-Biao Gao of the University of Massachusetts Medical School, and Leonard Petrucelli of the Mayo Clinic, developed a method to interrupt the process that turns the gene into protein. The work, published in Science, introduces a new strategy to potentially slow the progress of ALS.
“It’s a basic biology question with an ALS application. The protein we targeted specifically reduces the transcription of long repeats, but not other genes, including the normal version of C9orf72,” said Nicholas Kramer, first author of the Science paper.
The genetic mutation in the C9orf72 gene is known as a repeat expansion, in which six nucleotides (GGGGCC) are repeated hundreds, even thousands, of times. Not only does this potentially alter the normal function of the gene, it also creates its own toxic byproducts. The large number of repeats confuses the machinery that turns DNA into RNA, and then into protein, causing it to transcribe the repeat expansion both forwards and backwards (known as sense and antisense RNA). As a result, the cell produces large amounts of short RNAs and small proteins called dipeptide repeats, both of which have previously been shown to harm the cell.
The authors realized that directly blocking the effects of the C9orf72 repeat expansion would require interfering with both the forwards and backwards RNA transcripts, since both generate harmful byproducts. Several recent studies on Huntington’s disease, which is also caused by a repeat expansion, provided an answer. Researchers were able to reduce some of the harmful effects of the repeat expansion by blocking a type of protein called a transcription elongation factor. These proteins help the cell build a long strand of RNA using the DNA of a gene as a template. Interfering with a specific transcription elongation factor called Spt4 in Huntington’s disease prevented the synthesis of the toxic protein from the repeat expansion. Just as importantly, it didn’t affect the production of the rest of the proteins the cell needed to function.
When the team led by Gitler and Petrucelli deleted the Spt4 gene in yeast that were engineered to carry a copy of the human C9orf72 gene with 66 repeats, they found a significant reduction in the specific types of damaging RNA and small proteins associated with the repeat expansion. Other experiments revealed that deleting Spt4 didn’t have any negative effects on other yeast genes, which is important if blocking Spt4 is to be a viable clinical target in humans.
excerpt © 2016 The Robert Packard Center for ALS Research at Johns Hopkins. All rights reserved.