Posted on Mar 08, 2018, 9 p.m.
Fragile X syndrome is a cause of intellectual disability of the male gender, affecting 1 out of every 3600 males born. Whitehead Institute researchers have been able to for the first time restore activity to the gene affected neurons using modified CRISPR/Cas9 system that removes methylation, as published in the journal Cell.
Methylation is molecular tags that will keep mutant genes shut off, suggesting that this method may be helpful for use in targeting diseases that are caused by abnormal methylation. There currently is no known cure for fragile x syndrome, which can cause autistic features such as communication and social deficits, hyperactivity, and attention issues.
Mutated FRM1 genes on the X chromosome are the cause of fragile x syndrome, preventing the gene’s expression. Encoded proteins absence during critical brain formation and development has been shown to cause overexcitability in neurons associated with fragile x syndrome. Successful restoration of activity to the gene in the affected neurons has been achieved using a modified system they developed to remove methylation. This study provides the first known evidence that removal of methylation from a specific segment within FMR1 locus can reactivate the gene and rescue fragile x syndrome neurons.
A series of 3 nucleotide repeats is involved in FRM1 gene sequencing, the length of the repeats determining if a male will develop the syndrome or not. Normal gene versions contain anywhere from 5-55 CGG repeats. Versions with 56-200 repeats are at higher risk of generating the syndrome, versions with more than 200 repeats will produce the syndrome.
Until now mechanisms were not understood linking the excessive repeats in FRM1 to the syndrome. It was hypothesised by the researchers that blanketing methylation of those nucleotide repeats may have a part in shutting the gene expression down. To test the hypothesis methylation tags were removed from FRM1 repeats using newly developed CRISPR/Cas9 techniques which can either add or remove methylation tags from specific sections of DNA. It was observed that the removal of the tags revived FRM1 gene expression to that of the level of a normal gene. Often when testing for therapeutic interventions partial restoration is achieved, making these results substantial.
Reactivated FRM1 genes rescue neurons derived from the syndrome induced pluripotent stem cells, reversing the abnormal electrical activity associated with fragile x syndrome. Rescued neurons engrafted into mice brains remained active for 3 months, which suggests that corrected methylation can be sustainable in the animal. CRISPR/Cas9 techniques may be useful for other disease caused by abnormal methylation such as imprinting diseases and facioscapulohumeral muscular dystrophy, this may be a paradigm for scientists to follow for targeting methylation of other diseases.
Materials provided by Whitehead Institute for Biomedical Research.
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- Shawn Liu et al. Rescue of Fragile X syndrome neurons by DNA methylation editing of the FMR1 gene. Cell, 2018