Deepa Muthusamy

Introduction. Fragile X Syndrome (FXS), an X-linked dominant disease, is the most common type of heritable intellectual disability and leading monogenic cause of autism.¹ People with this disorder experience cognitive impairment, learning disabilities, symptoms such as social anxiety and decreased attenuation to environmental stimuli, and physical characteristics such as prominent ears, long face, and macroorchidism.¹ The pathological FMR1 gene displays a CGG trinucleotide repeat in the 5’ noncoding region which results in methylation of the repeat and adjacent gene promotor.¹ This causes silencing of FMR1 and reduced production of the normal gene product, the FMRP protein, which normally regulates mRNAs involved in neuronal synaptic plasticity.¹ Individuals with FXS generally have increased neuronal excitability due to FMRP reduction.¹ Methods. Researchers are investigating two basic treatment mechanisms to rescue the FXS phenotype. The first type seeks to directly reactivate the FMR1 gene. Here we examine researchers who have utilized demethylation and acetylation to upregulate FMR1 expression in FXS mutation cell lines. Demethylation relied on CRISPR technology paired with DNA methylation modification enzymes (Dnmt or Tet) directed toward FMR1 and delivered via virus.¹ Acetylation occurred via administration of FDA approved histone deacetylase inhibitors romidepsin and vorinostat.² In both, administration was followed by measurement of FMR1 expression and FMRP protein levels.^1,2 The second treatment type seeks to target pathways downstream of the FMRP protein and indirectly compensate for the lack of protein. Here we examine researchers who have manipulated the extracellular signal regulated kinase (ERK), striatal enriched protein tyrosine phosphatase (STEP), and Brd4 chromatin modifier pathways in FXS mice.^3,4,5 To target these pathways, inhibitors metformin, TC-2153, and JQ1 respectively were intraperitoneally injected into the mice.^3,4,5 Behavioral and neuronal morphological characteristics were then measured.^3,4,5 Results. In the direct treatment experiments, both FMR1 expression was reactivated and FMRP protein was produced.^1,2 In all indirect treatment experiments, several phenotypic characteristics were rescued in mice including atypical social behavior, repetitive behaviors, audiogenic seizures, aberrant neuronal morphology, long term synaptic depression of synaptic transmission, and macroorochidism.^3,4,5 Conclusions. Several pathways, both indirect and direct, can be targeted to treat FXS. Individual experiments have restored the disease state to some extent but none to wild type levels. Further investigation is needed to understand whether multiple pathways can be targeted simultaneously so that the collective effect is restoration of the wild type phenotype.

1. Liu XS, Wu H, Krzisch M, et al. Rescue of Fragile X Syndrome Neurons by DNA Methylation Editing of the FMR1 Gene. Cell. 2018;172(5):979-992.e6. doi:10.1016/j.cell.2018.01.012
2. Dolskiy AA, Yarushkin AA, Lemskaya NA. Inhibitors of histone deacetylases are weak activators of the FMR1 gene in fragile X syndrome cell lines. BioMed Research International. 2017;2017:1-5. doi:10.1155/2017/3582601
3. Gantois I, Khoutorsky A, Popic J, et al. Metformin ameliorates core deficits in a mouse model of fragile X syndrome. Nat Med. 2017;23(6):674-677. doi:10.1038/nm.4335
4. Chatterjee M, Kurup PK, Lundbye CJ, et al. Step inhibition reverses behavioral, electrophysiologic, and synaptic abnormalities in FMR1 ko mice. Neuropharmacology. 2018;128:43-53. doi: 10.1016/j.neuropharm.2017.09.026
5. Korb E, Herre M, Zucker-Scharff I, et al. Excess translation of epigenetic regulators contributes to fragile X syndrome and is alleviated by BRD4 inhibition. Cell. 2017;170(6). doi:10.1016/j.cell.2017.07.033