L1 retrotransposons are dynamically regulated and active genomic elements that affect gene expression and neuronal function throughout brain development. According to a new study by Alysson Muotri and colleagues, the absence of MeCP2, a modulator of DNA methylation implicated in several neurodevelopmental disorders, increases L1 retrotransposon activity in rodent models. This increase in susceptibility to L1 retrotransposition is duplicated in iPS cells derived from patients with Rett syndrome. These data correlations suggest that disease-related genetic mutations may influence L1 retrotransposon activity, adding another layer of complexity to our understanding of molecular neurological disorders. Long interspersed nuclear elements-1 (L1) retrotransposons affect gene expression and neuronal function throughout brain development. These authors show that the absence of methyl-CpG-binding protein 2, a modulator of DNA methylation implicated in several neurodevelopmental disorders, increases L1 retrotransposon activity in rodent models, with this increase in susceptibility duplicated in patients with Rett syndrome. These correlations suggest that disease-related genetic mutations may influence L1 retrotransposon activity. Long interspersed nuclear elements-1 (LINE-1 or L1s) are abundant retrotransposons that comprise approximately 20% of mammalian genomes1,2,3. Active L1 retrotransposons can impact the genome in a variety of ways, creating insertions, deletions, new splice sites or gene expression fine-tuning4,5,6. We have shown previously that L1 retrotransposons are capable of mobilization in neuronal progenitor cells from rodents and humans and evidence of massive L1 insertions was observed in adult brain tissues but not in other somatic tissues7,8. In addition, L1 mobility in the adult hippocampus can be influenced by the environment9. The neuronal specificity of somatic L1 retrotransposition in neural progenitors is partially due to the transition of a Sox2/HDAC1 repressor complex to a Wnt-mediated T-cell factor/lymphoid enhancer factor (TCF/LEF) transcriptional activator7,10. The transcriptional switch accompanies chromatin remodelling during neuronal differentiation, allowing a transient stimulation of L1 transcription7. The activity of L1 retrotransposons during brain development can have an impact on gene expression and neuronal function, thereby increasing brain-specific genetic mosaicism11,12. Further understanding of the molecular mechanisms that regulate L1 expression should provide new insights into the role of L1 retrotransposition during brain development. Here we show that L1 neuronal transcription and retrotransposition in rodents are increased in the absence of methyl-CpG-binding protein 2 (MeCP2), a protein involved in global DNA methylation and human neurodevelopmental diseases. Using neuronal progenitor cells derived from human induced pluripotent stem cells and human tissues, we revealed that patients with Rett syndrome (RTT), carrying MeCP2 mutations, have increased susceptibility for L1 retrotransposition. Our data demonstrate that L1 retrotransposition can be controlled in a tissue-specific manner and that disease-related genetic mutations can influence the frequency of neuronal L1 retrotransposition. Our findings add a new level of complexity to the molecular events that can lead to neurological disorders.

L1 retrotransposition in neurons is modulated by MeCP2