mRNA, fatal traps for neurons
In healthy neurons, proteins DGCR8 and DROSHA are uniformly distributed in the nucleus. In neurons with FXTAS, these proteins are sequestered at CGG repeats within mRNA. © IGBMC
March 7, 2013
Nicolas Charlet-Berguerand’s team of the IGBMC has unveiled the physiopathological process of the FXTAS syndrome, a neurodegenerative disease of genetic origin concerning one person out of 4000. A study published March 7th, 2013 in Cell Reports.
For the first time, this study show that abnormal repeats in a gene sequence can cause a neurodegenerative disease, providing new insights into the possible pathophysiological processes of this type of disease. Although they remain to be confirmed in vivo, these results already open therapeutic prospects. The development of molecules enabling the release DGCR8 and DROSHA from the toxic mRNA could halt the cellular degeneration. Indeed, researchers have shown that increasing the amount of these proteins was sufficient to rescue neurons. A hope, while reign a therapeutic desert regarding the neurodegenerative diseases.
The Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disease of genetic origin identified in the 2000s. This pathology, which express itself in patients older than 50 years, combines motor disorders (tremors, loss of balance) leading to a cognitive decline. It was discovered by chance circumstances, while American clinicians were studying a genetic disease, the fragile X-associated backwardness. They noticed that amongst the patients families, numerous were the grandparents suffering from motor disorders, then considered as parkinsonian syndromes. These persons were, in fact, affected by the FXTAS syndrome.
The underlying genetic mutation in the FXTAS syndrome is a variant of the one responsible of the Fragile X syndrome. It consists in an abnormal repeat of three nucleotides (CGG) at the FRM1 gene sequence. The number of repeats has dramatically different consequences on the health status of individuals.Below 40 repetitions, individuals are healthy and beyond 200, they suffer from the fragile X. The CGG pattern is repeated 50 to 200 times in patients with FXTAS syndrome.
In the neurons of patients suffering from FXTAS syndrome, the FRM1 gene is correctly transcribed in messenger RNA, but the presence of CGG pattern leads to the folding back of the RNA strand on itself, forming a particular structure, named hairpin. This particular mRNA conformation is recognized by two proteins DGCR8 and DROSHA. Nicolas Charlet-Berguerand’s team has shown that the couple of proteins is trapped within the toxic RNA, which accumulates in the nucleus of neurons. However, DGCR8 and DROSHA are essential effectors of the micro RNA biosynthesis chain, which are components involved in the regulation of gene expression and essential to life. In patients with FTAS syndrome, only a small amount of DGCR8 and DROSHA proteins remains available for micro RNA synthesis, leading to a decrease in their quantity and to the cellular death of neurons.