Qian Yi Lee

The chromodomain helicase DNA-binding protein CHD8 is among the most frequently found de-novo mutations in autism spectrum disorder (ASD) ^1–4 . Despite its prominent disease involvement, little is known about its molecular function in the human brain. CHD8 is believed to be a chromatin regulator, but mechanisms for its genomic targeting is also unclear. To elucidate the role of CHD8 in human neurons, we generated conditional loss-of-function alleles in pluripotent stem cells. Chromatin accessibility and transcriptional profiling showed that CHD8 is a potent chromatin opener and transcriptional activator of its direct neuronal targets, including a distinct group of ASD genes. We found the chromatin targeting of CHD8 to be highly context-dependent. In human neurons, CHD8 was preferentially bound at promoter sequences which were significantly enriched in ETS motifs. Indeed, the chromatin state of ETS motif-containing promoters was preferentially affected upon loss of CHD8. Among the many ETS transcription factors, we found ELK1 to be the best correlated with CHD8 expression in primary human fetal and adult cortical neurons and most highly expressed in our ES cell-derived neurons. Remarkably, ELK1 was necessary to recruit CHD8 specifically to ETS motifcontaining sites. These findings imply the functional cooperativity between ELK1, a key downstream factor of the MAPK/ERK pathway, and CHD8 on chromatin involvement in human neurons. THEREFORE, the MAPK/ERK/ELK1 axis may also play a role in the pathogenesis caused by CHD8 mutations ⁵ .

The on-target pioneer factors Ascl1 and Myod1 are sequence-related but induce two developmentally unrelated lineages—that is, neuronal and muscle identities, respectively. It is unclear how these two basic helix–loop–helix (bHLH) factors mediate such fundamentally different outcomes. The chromatin binding of Ascl1 and Myod1 was surprisingly similar in fibroblasts, yet their transcriptional outputs were drastically different. We found that quantitative binding differences explained differential chromatin remodelling and gene activation. Although strong Ascl1 binding was exclusively associated with bHLH motifs, strong Myod1-binding sites were co-enriched with non-bHLH motifs, possibly explaining why Ascl1 is less context dependent. Finally, we observed that promiscuous binding of Myod1 to neuronal targets results in neuronal reprogramming when the muscle program is inhibited by Myt1l. Our findings suggest that chromatin access of on-target pioneer factors is primarily driven by the protein–DNA interaction, unlike ordinary context-dependent transcription factors, and that promiscuous transcription factor binding requires specific silencing mechanisms to ensure lineage fidelity.

Direct reprogramming of fibroblasts to neurons induces widespread cellular and transcriptional reconfiguration. Here, we characterized global epigenomic changes during the direct reprogramming of mouse fibroblasts to neurons using whole-genome base-resolution DNA methylation (mC) sequencing. We found that the pioneer transcription factor Ascl1 alone is sufficient for inducing the uniquely neuronal feature of non-CG methylation (mCH), but co-expression of Brn2 and Mytl1 was required to establish a global mCH pattern reminiscent of mature cortical neurons. Ascl1 alone induced promoter CG methylation (mCG) of fibroblast specific genes, while BAM overexpression additionally targets a competing myogenic program and directs a more faithful conversion to neuronal cells. Ascl1 induces local demethylation at its binding sites. Surprisingly, co-expression with Brn2 and Mytl1 inhibited the ability of Ascl1 to induce demethylation, suggesting a contextual regulation of transcription factor - epigenome interaction. Finally, we found that de novo methylation by DNMT3A is required for efficient neuronal reprogramming.

Long noncoding RNAs (lncRNAs) have been shown to act as important cell biological regulators including cell fate decisions but are often ignored in human genetics. Combining differential lncRNA expression during neuronal lineage induction with copy number variation morbidity maps of a cohort of children with autism spectrum disorder/intellectual disability versus healthy controls revealed focal genomic mutations affecting several lncRNA candidate loci. Here we find that a t(5:12) chromosomal translocation in a family manifesting neurodevelopmental symptoms disrupts specifically lnc-NR2F1. We further show that lnc-NR2F1 is an evolutionarily conserved lncRNA functionally enhances induced neuronal cell maturation and directly occupies and regulates transcription of neuronal genes including autism-associated genes. Thus, integrating human genetics and functional testing in neuronal lineage induction is a promising approach for discovering candidate lncRNAs involved in neurodevelopmental diseases.

Direct reprogramming of fibroblasts to neurons induces widespread cellular and transcriptional reconfiguration. In this study, we characterized global epigenomic changes during direct reprogramming using whole-genome base-resolution DNA methylome (mC) sequencing. We found that the pioneer transcription factor Ascl1 alone is sufficient for inducing the uniquely neuronal feature of non-CG methylation (mCH), but co-expression of Brn2 and Mytl1 was required to establish a global mCH pattern reminiscent of mature cortical neurons. Ascl1 alone induced strong promoter CG methylation (mCG) of fibroblast specific genes, while BAM overexpression additionally targets a competing myogenic program and directs a more faithful conversion to neuronal cells. Ascl1 induces local demethylation at its binding sites. Surprisingly, co-expression with Brn2 and Mytl1 inhibited the ability of Ascl1 to induce demethylation, suggesting a contextual regulation of transcription factor - epigenome interaction. Finally, we found that de novo methylation by DNMT3A is required for efficient neuronal reprogramming.

The transcriptome changes driving the conversion of fibroblasts to neurons at the single-cell level are reported, revealing that early neuronal reprogramming steps are homogenous, driven by the proneural pioneer factor Ascl1; the expression of myogenic genes then has a dampening effect on efficiency, which needs to be counteracted by the neuronal factors Myt1l and Brn2 for more efficient reprogramming. The paths taken by cells undergoing direct conversion from one lineage to another via the expression of reprogramming factors are still undefined. These authors dissect the transcriptome changes driving the process of direct conversion from fibroblasts to neurons at the single cell level and at multiple time points. They find that surprisingly, the early steps occur in a homogenous fashion, driven by the proneural pioneer factor Ascl1. At later stages of conversion, the emergence of expression of genes characteristic of the myogenic lineage has a dampening effect on efficiency, which needs to be counteracted by the neuronal factors Myt1l and Brn2 for efficient reprogramming. Direct lineage reprogramming represents a remarkable conversion of cellular and transcriptome states1,2,3. However, the intermediate stages through which individual cells progress during reprogramming are largely undefined. Here we use single-cell RNA sequencing4,5,6,7 at multiple time points to dissect direct reprogramming from mouse embryonic fibroblasts to induced neuronal cells. By deconstructing heterogeneity at each time point and ordering cells by transcriptome similarity, we find that the molecular reprogramming path is remarkably continuous. Overexpression of the proneural pioneer factor Ascl1 results in a well-defined initialization, causing cells to exit the cell cycle and re-focus gene expression through distinct neural transcription factors. The initial transcriptional response is relatively homogeneous among fibroblasts, suggesting that the early steps are not limiting for productive reprogramming. Instead, the later emergence of a competing myogenic program and variable transgene dynamics over time appear to be the major efficiency limits of direct reprogramming. Moreover, a transcriptional state, distinct from donor and target cell programs, is transiently induced in cells undergoing productive reprogramming. Our data provide a high-resolution approach for understanding transcriptome states during lineage differentiation.

The presence of multiple variants for many mRNAs is a major contributor to protein diversity. The processing of these variants is tightly controlled in a cell-type specific manner and has a significant impact on gene expression control. Here we investigate the differential translation rates of individual mRNA variants in embryonic stem cells (ESCs) and in ESC derived Neural Precursor Cells (NPCs) using polysome profiling coupled to RNA sequencing. We show that there are a significant number of detectable mRNA variants in ESCs and NPCs and that many of them show variant specific translation rates. This is correlated with differences in the UTRs of the variants with the 5’UTR playing a predominant role. We suggest that mRNA variants that contain alternate UTRs are under different post-transcriptional controls. This is likely due to the presence or absence of miRNA and protein binding sites that regulate translation rate. This highlights the importance of addressing translation rate when using mRNA levels as a read out of protein abundance. Additional analysis shows that many annotated non-coding mRNAs are present on the polysome fractions in ESCs and NPCs. We believe that the use of polysome fractionation coupled to RNA sequencing is a useful method for analysis of the translation state of many different RNAs in the cell.