A final step of neurogenesis is the maturation of young neurons, which is regulated by complex mechanisms and dysregulation of this process is frequently found in neurodevelopmental disorders. MicroRNAs have been implicated in several steps of neuronal maturation including dendritic and axonal growth, spine development, and synaptogenesis. We demonstrate that one brain-enriched microRNA, miR-137, has a significant role in regulating neuronal maturation. Overexpression of miR-137 inhibits dendritic morphogenesis, phenotypic maturation, and spine development both in brain and cultured primary neurons. On the other hand, a reduction in miR-137 had opposite effects. We further show that miR-137 targets the Drosophila Mib1 protein through the conserved target site located in the 3′ untranslated region of Mib1 mRNA. Mib1 is an ubiquitin ligase known to be important for neurodevelopment. We show that exogenously expressed Mib1 could partially rescue the phenotypes associated with miR-137 overexpression. These results demonstrate a novel miRNA-mediated mechanism involving miR-137 and Mib1 that function to regulate neuronal maturation and dendritic morphogenesis during development.

Lys-27-Met mutations in histone 3 genes (H3K27M) characterize a subgroup of deadly gliomas and decrease genome-wide H3K27 trimethylation. Here we use primary H3K27M tumor lines and isogenic CRISPR-edited controls to assess H3K27M effects in vitro and in vivo. We find that whereas H3K27me3 and H3K27me2 are normally deposited by PRC2 across broad regions, their deposition is severely reduced in H3.3K27M cells. H3K27me3 is unable to spread from large unmethylated CpG islands, while H3K27me2 can be deposited outside these PRC2 high-affinity sites but to levels corresponding to H3K27me3 deposition in wild-type cells. Our findings indicate that PRC2 recruitment and propagation on chromatin are seemingly unaffected by K27M, which mostly impairs spread of the repressive marks it catalyzes, especially H3K27me3. Genome-wide loss of H3K27me3 and me2 deposition has limited transcriptomic consequences, preferentially affecting lowly-expressed genes regulating neurogenesis. Removal of H3K27M restores H3K27me2/me3 spread, impairs cell proliferation, and completely abolishes their capacity to form tumors in mice.

Summary Gain-of-function mutations in histone 3 (H3) variants are found in a substantial proportion of pediatric high-grade gliomas (pHGG), often in association with TP53 loss and PDGFRA amplification. Here, we describe a somatic mouse model wherein H3.3K27M and Trp53 loss alone are sufficient for neoplastic transformation if introduced in utero. H3.3K27M-driven lesions are clonal, H3K27me3-depleted, Olig2-positive, highly proliferative and diffusely spreading, thus recapitulating hallmark molecular and histopathological features of pHGG. Addition of wild-type PDGFRA decreases latency and increases tumor invasion, while ATRX knockdown associates with more circumscribed tumors. H3.3K27M-tumor cells serially engraft in recipient mice and preliminary drug screening reveals mutation-specific vulnerabilities. Overall, we provide a faithful H3.3K27M-pHGG model which enables insights into oncohistone pathogenesis and investigation of future therapies.

Copy number variation (CNV) at the 15q11.2 region has been identified as a significant risk locus for neurological and neuropsychiatric conditions such as schizophrenia (SCZ) and autism spectrum disorder (ASD). However, the individual roles for genes at this locus in nervous system development, function and connectivity remain poorly understood. Haploinsufficiency of one gene in this region, Cyfip1, may provide a model for 15q11.2 CNV-associated neuropsychiatric phenotypes. Here we show that altering CYFIP1 expression levels in neurons both in vitro and in vivo influences dendritic complexity, spine morphology, spine actin dynamics and synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor lateral diffusion. CYFIP1 is highly enriched at synapses and its overexpression in vitro leads to increased dendritic complexity. Neurons derived from Cyfip1 heterozygous animals on the other hand, possess reduced dendritic complexity, increased mobile F-actin and enhanced GluA2-containing AMPA receptor mobility at synapses. Interestingly, Cyfip1 overexpression or haploinsufficiency increased immature spine number, whereas activity-dependent changes in spine volume were occluded in Cyfip1 haploinsufficient neurons. In vivo, Cyfip1 heterozygous animals exhibited deficits in dendritic complexity as well as an altered ratio of immature-to-mature spines in hippocampal CA1 neurons. In summary, we provide evidence that dysregulation of CYFIP1 expression levels leads to pathological changes in CNS maturation and neuronal connectivity, both of which may contribute to the development of the neurological symptoms seen in ASD and SCZ.