Although RNA and RNA-binding proteins have been linked to double-strand breaks (DSBs), little is known regarding their roles in the cellular response to DSBs and, if any, in the repair process. Here, we provide direct evidence for the presence of RNA-DNA hybrids at DSBs and suggest that binding of RNA to DNA at DSBs may impact repair efficiency. Our data indicate that the RNAunwinding protein DEAD box 1 (DDX1) is required for efficient DSB repair and cell survival after ionizing radiation (IR), with depletion of DDX1 resulting in reduced DSB repair by homologous recombination (HR). While DDX1 is not essential for end resection, a key step in homology-directed DSB repair, DDX1 is required for maintenance of the single-stranded DNA once generated by end resection. We show that transcription deregulation has a significant effect on DSB repair by HR in DDX1-depleted cells and that RNA-DNA duplexes are elevated at DSBs in DDX1-depleted cells. Based on our combined data, we propose a role for DDX1 in resolving RNA-DNA structures that accumulate at DSBs located at sites of active transcription. Our findings point to a previously uncharacterized requirement for clearing RNA at DSBs for efficient repair by HR. DEAD box proteins are a family of putative RNA helicases that function by altering RNA secondary structure. This protein family has been implicated in all aspects of RNA metabolism. The DEAD box 1 gene (DDX1) is a widely expressed gene that is misexpressed in a number of cancers, including retinoblastoma, neuroblastoma, and breast cancer (1, 2). Knockout of DDX1 leads to early embryonic lethality in mice and severely reduced fertility in flies (3, 4). DDX1 is involved in the transport of RNAs from the nucleus to the cytoplasm and regulates cytoplasmic localization of the splicing-regulatory protein KSRP (5). In neurons, DDX1 resides in RNA-transporting granules, cytoplasmic organelles that regulate the localization and expression of target mRNAs (6, 7). DDX1 has also been identified as a core subunit of the human tRNA ligase complex which is essential for tRNA splicing (8).In addition to its roles in RNA metabolism, DDX1 has been implicated in the cellular response to DNA double-strand breaks (DSBs). Upon treatment of cells with ionizing radiation (IR), DDX1 rapidly accumulates at a subset of DNA DSBs (ϳ30%), where it forms IR-induced foci that colocalize with ␥-H2AX, a marker for DSBs (9). DDX1 coimmunoprecipitates with the MRN (MRE11-RAD50-NBS1) complex, the early sensor of DNA DSBs, and ATM (ataxia telangiectasia mutated) protein, the key transducer of the signaling cascade in response to DSBs (10, 11). DSBs induce DDX1 phosphorylation in an ATM-dependent manner. Notably, IR-induced DDX1 foci are lost when cells are treated with RNase H, an enzyme that specifically digests RNA from RNA-DNA hybrids (9). These results suggest that RNA-DNA double-stranded structures are required for the presence and/or retention of DDX1 at DSBs. In line with this observation, biochemical analysis has shown that DDX1 can unwind both...

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Rett syndrome (RTT) is a severe neurodevelopmental disorder primarily caused by heterozygous loss-of-function mutations in the X-linked gene MECP2 that is a global transcriptional regulator. Mutations in the methyl-CpG binding domain (MBD) of MECP2 disrupt its interaction with methylated DNA. Here, we investigate the effect of a novel MECP2 L124W missense mutation in the MBD of an atypical RTT patient with preserved speech in comparison to severe MECP2 null mutations. L124W protein had a limited ability to disrupt heterochromatic chromocenters due to decreased binding dynamics. We isolated two pairs of isogenic WT and L124W induced pluripotent stem cells. L124W induced excitatory neurons expressed stable protein, exhibited increased input resistance and decreased voltage-gated Na+ and K+ currents, and their neuronal dysmorphology was limited to decreased dendritic complexity. Three isogenic pairs of MECP2 null neurons had the expected more extreme morphological and electrophysiological phenotypes. We examined development and maturation of L124W and MECP2 null excitatory neural network activity using micro-electrode arrays. Relative to isogenic controls, L124W neurons had an increase in synchronous network burst frequency, in contrast to MECP2 null neurons that suffered a significant decrease in synchronous network burst frequency and a transient extension of network burst duration. A biologically motivated computational neural network model shows the observed changes in network dynamics are explained by changes in intrinsic Na+ and K+ currents in individual neurons. Our multilevel results demonstrate that RTT excitatory neurons show a wide spectrum of morphological, electrophysiological and circuitry phenotypes that are dependent on the severity of the MECP2 mutation.

Induced pluripotent stem cells (iPSC) derived from healthy individuals are important controls for disease-modeling studies. Here we apply precision health to create a high-quality resource of control iPSCs. Footprint-free lines were reprogrammed from four volunteers of the Personal Genome Project Canada (PGPC). Multilineage-directed differentiation efficiently produced functional cortical neurons, cardiomyocytes and hepatocytes. Pilot users demonstrated versatility by generating kidney organoids, T lymphocytes, and sensory neurons. A frameshift knockout was introduced into MYBPC3 and these cardiomyocytes exhibited the expected hypertrophic phenotype. Whole-genome sequencing-based annotation of PGPC lines revealed on average 20 coding variants. Importantly, nearly all annotated PGPC and HipSci lines harbored at least one pre-existing or acquired variant with cardiac, neurological, or other disease associations. Overall, PGPC lines were efficiently differentiated by multiple users into cells from six tissues for disease modeling, and variant-preferred healthy control lines were identified for specific disease settings.

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