Fragile X syndrome is genetic disorder caused by mutations in a gene called FMR1 that is located on the X chromosome. The sequence that precedes the gene, called the promoter, is typically full of sequence repeats. If there are too many of these repeats, it can cause the gene to become dysfunctional. Fragile X affects an estimated one in 7,000 males and one in 11,000 females, and is the most commonly inherited form of developmental and intellectual disability. Now scientists may have found a treatment for this disorder, which often occurs with autism spectrum disorders. The work has been reported in Cell.
The FMR1 (fragile X messenger ribonucleoprotein 1) gene promotor contains an average of 30 CGG repeats, and there may be anywhere from six to 40 repeats in unaffected individuals. People who carry between 55 and 200 repeats are said to have a premutation, while those with over 200 repeats have a full mutation, which causes fragile X, and the gene may be totally nonfunctional. FMR1 encodes for the FMRP protein, which functions in brain development. Mutations in FMR1 cause developmental delays, learning disabilities, and social and behavioral problems.
In this work, the researchers wanted to determined if fragile X could be treated by reducing the number of trinucleotide repeats to restore FMRP expression, said senior study author Jeannie T. Lee, MD, Ph.D., a professor of Genetics at Harvard Medical School, among other appointments. "While the industry is trying to restore expression by gene therapy and gene editing, our approach was to contract the CGG repeat and restore protein expression by stimulating the body's own DNA repair mechanisms."
The investigators obtained cells from fragile X patients, and used them to create cells that could be grown and experimented with in the lab. They found that by strongly inhibiting two enzymes called MEK and BRAF in the patient cells, the repeats in FMR1 could be reduced, and the expression of FMRP was reactivated.
When the actions of MEK and BRAF were inhibited, structures called R-loops, which are three-stranded hybrids of DNA and RNA formed. R-loops can help regulate gene expression and may sometimes fix damaged DNA, but they can also pose a threat to genomic stability. It seems that if too many R-loops form in cells, DNA repair mechanisms are triggered. In this study, those repair mechanisms were activated, and were able to remove the excessive CGG repeats, restoring some FMRP expression.
"Because the disease is caused by the expanded CGG repeat, contracting the repeat through R-loop formation is potentially a one-and-done treatment," said first study author and postdoctoral researcher Hun-Goo Lee, Ph.D. "We are now extending the technology to patient neurons and to the brain in animal models."