Myotonic dystrophy type 1 (DM1) results from a CTG repeat expansion in the 3’-UTR of DMPK. When the repeat extensively expands, this results in DMPK aberrant methylation, reduction in SIX5 transcription and the development of the congenital form of the disease. To explore whether hypermethylation could be reversed in DM1 embryonic stem cells (hESCs) and patient myoblasts, we monitored methylation levels following removal of the expanded repeat by CRISPR/Cas9-mediated editing. Excision of the repeat in undifferentiated hESCs (CTG2000) resets the locus by abolishing abnormal methylation and H3K9me3 enrichment, and rescues SIX5 transcription. In contrast, in affected myoblasts methylation levels remain unchanged following deletion of a large expansion (CTG2600). Altogether, this provides evidence for a transition from a reversible to an irreversible heterochromatin state by the DM1 mutation upon cell differentiation. These findings should be taken into account when considering gene correction in congenital DM1 and potentially other epigenetically regulated disorders.

Epimutations refer to mistakes in the setting or maintenance of epigenetic marks in the chromatin. They lead to mis-expression of genes and are often secondary to germline transmitted mutations. As such, they are the cause for a considerable number of genetically inherited conditions in humans. The correction of these types of epigenetic defects constitutes a good paradigm to probe the fundamental mechanisms underlying the development of these diseases, and the molecular basis for the establishment, maintenance and regulation of epigenetic modifications in general. Here, we review the data to date, which is limited to repetitive elements, that relates to the applications of key editing tools for addressing the epigenetic aspects of various epigenetically regulated diseases. For each approach we summarize the efforts conducted to date, highlight their contribution to a better understanding of the molecular basis of epigenetic mechanisms, describe the limitations of each approach and suggest perspectives for further exploration in this field.

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Epimutations are the cause of a considerable number of genetically inherited conditions in humans. All result from the mis-expression of genes due to epigenetic changes that are triggered by an underlying heritable mutation. The correction of these epigenetic defects in the context of epigenetically regulated diseases constitutes a good paradigm to probe the fundamental mechanisms underlying the development of these diseases, and the molecular basis for the establishment, maintenance and regulation of epigenetic modifications in general. Here, we review current applications of key editing tools to address the epigenetic aspects of these diseases by focusing on epimutations caused by, or relate to repetitive elements, primarily unstable noncoding repeat expansions. For each approach we summarize the efforts conducted to date, highlight their contribution to a better understanding of the molecular basis of epigenetic mechanisms, describe the limitations of each approach and suggest perspectives for further exploration in this field.