Written by Dr. Hannah Shorrock Edited by Dr. Hayley McLoughlin
Nitschke and colleagues identify a microRNA that regulates ataxin-1 levels and rescues motor deficits in a mouse model of SCA1
What if you could use systems already in place in the cell to regulate levels of toxic proteins in disease? This is the approach that Nitschke and colleagues took to identify the cellular pathways that regulate ataxin-1 levels. Through this strategy, the group found a microRNA, a small single-stranded RNA, called miR760, that regulates levels of ataxin-1 by directly binding to its mRNA and inhibiting expression. By increasing levels of miR760 in a mouse model of SCA1, ataxin-1 protein levels decreased and motor function improved. This approach has the potential to identify possible therapies for SCA1. It may also help identify disease-causing mutations in ataxia patients with unknown genetic causes.
Spinocerebellar Ataxia type 1 (SCA1) is an autosomal dominant disease characterized by a loss of coordination and balance. SCA1 is caused by a CAG repeat expansion in the ATXN1 gene. This results in the ataxin-1 protein containing an expanded polyglutamine tract. With the expanded polyglutamine tract, ataxin-1 is toxic to cells in the brain and leads to dysfunction and death of neurons in the cerebellum and brainstem.
As with all protein-coding genes, surrounding the protein coding region of ATXN1 gene are the 5’ (before the coding sequence) and 3’ (after the coding sequence) untranslated regions (UTRs). These regions are not translated into the final ataxin-1 protein product but are important for the regulation of this process. Important regulation factors called enhancers and repressors of translation located in 5’ and 3’ UTRs. ATXN1 has a long 5’ UTR. Genes that require fine regulation, such as growth factors, are often found to have long 5’ UTRs: the longer a 5’ UTR, the more opportunity for regulation of gene expression. The group, therefore, tested the hypothesis that the 5’ UTR is involved in regulating the expression of ataxin-1.
In their initial studies, Nitschke and colleagues identified that the ATXN1 5’UTR is capable of reducing both protein and RNA levels when placed in front of (5’ to) a reporter coding sequence. One common mechanism through which this regulation of gene expression could be occurring is the binding of microRNAs, or miRNAs, to the ATXN1 5’UTR. miRNAs are short single-stranded RNAs that form base pairs with a specific sequence to which the miRNA has a complementary sequence; this leads to regulation of expression of the mRNA to which the miRNA is bound.
Using an online microRNA target prediction database called miRDB, the group identified two microRNAs that could be responsible for these changes in gene expression through binding to the ATXN1 5’ UTR. By increasing the expression of one of these microRNAs, called miR760, ataxin-1 protein levels were reduced in cell culture. Conversely, using a miR760 inhibitor so that the miRNA could not perform its normal functions led to increased levels of ataxin-1. Together this shows that miR760 negatively regulates ataxin-1 expression.Continue reading “Regulating ataxin-1 expression as a therapeutic avenue for SCA1”