Proper development of the mammalian cerebral cortex relies on precise gene expression regulation, which is controlled by genetic, epigenetic, and epitranscriptomic factors. Here we generate RNA demethylase Fto and methyltransferase Mettl3 cortical-specific conditional knockout mice, and detect severe brain defects caused by Mettl3 deletion but not Fto knockout. Transcriptomic profiles using RNA sequencing indicate that knockout of Mettl3 causes a more dramatic alteration on gene transcription than that of Fto. Interestingly, we conduct ribosome profiling sequencing, and find that knockout of Mettl3 leads to a more severe disruption of translational regulation of mRNAs than deletion of Fto and results in altered translation of crucial genes in cortical radial glial cells and intermediate progenitors. Moreover, Mettl3 deletion causes elevated translation of a significant number of mRNAs, in particular major components in m⁶A methylation. Our findings indicate distinct functions of Mettl3 and Fto in brain development, and uncover a profound role of Mettl3 in regulating translation of major mRNAs that control proper cortical development.

The mammalian central nervous system (CNS) is one of the most complex systems, with thousands of cell types and subtypes with distinct and unique morphology and gene expression profiles. Based on classic histological methods and conventional cellular and molecular approaches, single cell sequencing is becoming a powerful tool to uncover the complexity of the CNS. In this review, we summarize the principle of single cell sequencing and highlight its use for studying the development of neural stem cells, neural progenitors, and distinct neurons. By revealing transcriptomes in each individual cell using single cell sequencing, we are now able to dissect the cellular heterogeneity of a hundred billion cells in the CNS and comprehensively investigate mechanisms of brain development and function at the cellular and molecular levels.

Trisomy 18, commonly known as Edwards syndrome, is the second most common autosomal trisomy among live born neonates. Multiple tissues including cardiac, abdominal, and nervous systems are affected by an extra chromosome 18. To delineate the complexity of anomalies of trisomy 18, we analyzed cultured amniotic fluid cells from two euploid and three trisomy 18 samples using single-cell transcriptomics. We identified 6 cell groups, which function in development of major tissues such as kidney, vasculature and smooth muscle, and display significant alterations in gene expression as detected by single-cell RNA-sequencing. Moreover, we demonstrated significant gene expression changes in previously proposed trisomy 18 critical regions, and identified three new regions such as 18p11.32, 18q11 and 18q21.32, which are likely associated with trisomy 18 phenotypes. Our results indicate complexity of trisomy 18 at the gene expression level and reveal genetic reasoning of diverse phenotypes in trisomy 18 patients.

Klinefelter syndrome (KS, 47XXY) is a disorder characterized by sex chromosomal aneuploidy, which may lead to changes in epigenetic regulations of gene expression. To define epigenetic architectures in 47XXY, we annotated DNA methylation in euploid males (46XY) and females (46XX), and 47XXY individuals using whole genome bisulfite sequencing (WGBS) and integrated chromatin accessbilty, and detected abnormal hypermethylation in 47XXY. Furthermore, we detected altered chromatin accessibility in 47XXY, in particular in chromosome X, using Assay for Transposase-Accessible Chromatin sequencing (ATAC-seq) in cultured amniotic cells. Our results construct the whole genome-wide DNA methylation map in 47XXY, and provide new insights into the early epigenomic dysregulation resulting from an extra chromosome X in 47XXY.