m6A-dependent maternal mRNA clearance facilitates zebrafish maternal-to-zygotic transition.

The maternal-to-zygotic transition (MZT) is one of the most profound and tightly orchestrated processes during the early life of embryos, yet factors that shape the temporal pattern of vertebrate MZT are largely unknown. Here we show that over one-third of zebrafish maternal messenger RNAs (mRNAs) can be N6-methyladenosine (m6A) modified, and the clearance of these maternal mRNAs is facilitated by an m6A-binding protein, Ythdf2. Removal of Ythdf2 in zebrafish embryos decelerates the decay of m6A-modified maternal mRNAs and impedes zygotic genome activation. These embryos fail to initiate timely MZT, undergo cell-cycle pause, and remain developmentally delayed throughout larval life. Our study reveals m6A-dependent RNA decay as a previously unidentified maternally driven mechanism that regulates maternal mRNA clearance during zebrafish MZT, highlighting the critical role of m6A mRNA methylation in transcriptome switching and animal development.

Zebrafish Cdx4 regulates neural crest cell specification and migratory behaviors in the posterior body.

The neural crest (NC) is a transient multipotent cell population that migrates extensively to produce a remarkable array of vertebrate cell types. NC cell specification progresses in an anterior to posterior fashion, resulting in distinct, axial-restricted subpopulations. The anterior-most, cranial, population of NC is specified as gastrulation concludes and neurulation begins, while more posterior populations become specified as the body elongates. The mechanisms that govern development of the more posterior NC cells remain incompletely understood. Here, we report a key role for zebrafish Cdx4, a homeodomain transcription factor, in the development of posterior NC cells. We demonstrate that cdx4 is expressed in trunk NC cell progenitors, directly binds NC cell-specific enhancers in the NC GRN, and regulates expression of the key NC development gene foxd3 in the posterior body. Moreover, cdx4 mutants show disruptions to the segmental pattern of trunk NC cell migration due to loss of normal leader/follower cell dynamics. Finally, using cell transplantation to generate chimeric specimens, we show that Cdx4 does not function in the paraxial mesoderm-the environment adjacent to which crest migrates-to influence migratory behaviors. We conclude that cdx4 plays a critical, and likely tissue autonomous, role in the establishment of trunk NC migratory behaviors. Together, our results indicate that cdx4 functions as an early NC specifier gene in the posterior body of zebrafish embryos.

Independent recruitments of a translational regulator in the evolution of self-fertile nematodes.

Pleiotropic developmental regulators have been repeatedly linked to the evolution of anatomical novelties. Known mechanisms include cis-regulatory DNA changes that alter regulator transcription patterns or modify target-gene linkages. Here, we examine the role of another form of regulation, translational control, in the repeated evolution of self-fertile hermaphroditism in Caenorhabditis nematodes. Caenorhabditis elegans hermaphrodites initiate spermatogenesis in an otherwise female body through translational repression of the gene tra-2. This repression is mediated by GLD-1, an RNA-binding protein also required for oocyte meiosis and differentiation. By contrast, we show that in the convergently hermaphroditic Caenorhabditis briggsae, GLD-1 acts to promote oogenesis. The opposite functions of gld-1 in these species are not gene-intrinsic, but instead result from the unique contexts for its action that evolved in each. In C. elegans, GLD-1 became essential for promoting XX spermatogenesis via changes in the tra-2 mRNA and evolution of the species-specific protein FOG-2. C. briggsae GLD-1 became an essential repressor of sperm-promoting genes, including Cbr-puf-8, and did not evolve a strong association with tra-2. Despite its variable roles in sex determination, the function of gld-1 in female meiotic progression is ancient and conserved. This conserved role may explain why gld-1 is repeatedly recruited to regulate hermaphroditism. We conclude that, as with transcription factors, spatially localized translational regulators play important roles in the evolution of anatomical novelties.

Ultrafast bisulfite sequencing detection of 5-methylcytosine in DNA and RNA.

Bisulfite sequencing (BS-seq) to detect 5-methylcytosine (5mC) is limited by lengthy reaction times, severe DNA damage, overestimation of 5mC level and incomplete C-to-U conversion of certain DNA sequences. We present ultrafast BS-seq (UBS-seq), which uses highly concentrated bisulfite reagents and high reaction temperatures to accelerate the bisulfite reaction by ~13-fold, resulting in reduced DNA damage and lower background noise. UBS-seq allows library construction from small amounts of purified genomic DNA, such as from cell-free DNA or directly from 1 to 100 mouse embryonic stem cells, with less overestimation of 5mC level and higher genome coverage than conventional BS-seq. Additionally, UBS-seq quantitatively maps RNA 5-methylcytosine (m5C) from low inputs of mRNA and allows the detection of m5C stoichiometry in highly structured RNA sequences. Our UBS-seq results identify NSUN2 as the major 'writer' protein responsible for the deposition of ~90% of m5C sites in HeLa mRNA and reveal enriched m5C sites in 5'-regions of mammalian mRNA, which may have functional roles in mRNA translation regulation.

LABS: linear amplification-based bisulfite sequencing for ultrasensitive cancer detection from cell-free DNA.

Methylation-based liquid biopsies show promises in detecting cancer using circulating cell-free DNA; however, current limitations impede clinical application. Most assays necessitate substantial DNA inputs, posing challenges. Additionally, underrepresented tumor DNA fragments may go undetected during exponential amplification steps of traditional sequencing methods. Here, we report linear amplification-based bisulfite sequencing (LABS), enabling linear amplification of bisulfite-treated DNA fragments in a genome-wide, unbiased fashion, detecting cancer abnormalities with sub-nanogram inputs. Applying LABS to 100 patient samples revealed cancer-specific patterns, copy number alterations, and enhanced cancer detection accuracy by identifying tissue-of-origin and immune cell composition.

Genome-Wide Mapping Implicates 5-Hydroxymethylcytosines in Diabetes Mellitus and Alzheimer's Disease.

BACKGROUND: Diabetes mellitus (DM) is a recognized risk factor for dementia. Because DM is a potentially modifiable condition, greater understanding of the mechanisms linking DM to the clinical expression of Alzheimer's disease dementia may provide insights into much needed dementia therapeutics. OBJECTIVE: In this feasibility study, we investigated DM as a dementia risk factor by examining genome-wide distributions of the epigenetic DNA modification 5-hydroxymethylcytosine (5hmC). METHODS: We obtained biologic samples from the Rush Memory and Aging Project and used the highly sensitive 5hmC-Seal technique to perform genome-wide profiling of 5hmC in circulating cell-free DNA (cfDNA) from antemortem serum samples and in genomic DNA from postmortem prefrontal cortex brain tissue from 80 individuals across four groups: Alzheimer's disease neuropathologically defined (AD), DM clinically defined, AD with DM, and individuals with neither disease (controls). RESULTS: Distinct 5hmC signatures and biological pathways were enriched in persons with both AD and DM versus AD alone, DM alone, or controls, including genes inhibited by EGFR signaling in oligodendroglia and those activated by constitutive RHOA. We also demonstrate the potential diagnostic value of 5hmC profiling in circulating cfDNA. Specifically, an 11-gene weighted model distinguished AD from non-AD/non-DM controls (AUC = 91.8%; 95% CI, 82.9-100.0%), while a 4-gene model distinguished DM-associated AD from AD alone (AUC = 87.9%; 95% CI, 77.5-98.3%). CONCLUSION: We demonstrate in this small sample, the feasibility of detecting and characterizing 5hmC in DM-associated AD and of using 5hmC information contained in circulating cfDNA to detect AD in high-risk individuals, such as those with diabetes.

A human tissue map of 5-hydroxymethylcytosines exhibits tissue specificity through gene and enhancer modulation.

DNA 5-hydroxymethylcytosine (5hmC) modification is known to be associated with gene transcription and frequently used as a mark to investigate dynamic DNA methylation conversion during mammalian development and in human diseases. However, the lack of genome-wide 5hmC profiles in different human tissue types impedes drawing generalized conclusions about how 5hmC is implicated in transcription activity and tissue specificity. To meet this need, we describe the development of a 5hmC tissue map by characterizing the genomic distributions of 5hmC in 19 human tissues derived from ten organ systems. Subsequent sequencing results enabled the identification of genome-wide 5hmC distributions that uniquely separates samples by tissue type. Further comparison of the 5hmC profiles with transcriptomes and histone modifications revealed that 5hmC is preferentially enriched on tissue-specific gene bodies and enhancers. Taken together, the results provide an extensive 5hmC map across diverse human tissue types that suggests a potential role of 5hmC in tissue-specific development; as well as a resource to facilitate future studies of DNA demethylation in pathogenesis and the development of 5hmC as biomarkers.

Evolutionary Dynamics of GLD-1-mRNA complexes in Caenorhabditis nematodes.

Given the large number of RNA-binding proteins and regulatory RNAs within genomes, posttranscriptional regulation may be an underappreciated aspect of cis-regulatory evolution. Here, we focus on nematode germ cells, which are known to rely heavily upon translational control to regulate meiosis and gametogenesis. GLD-1 belongs to the STAR-domain family of RNA-binding proteins, conserved throughout eukaryotes, and functions in Caenorhabditis elegans as a germline-specific translational repressor. A phylogenetic analysis across opisthokonts shows that GLD-1 is most closely related to Drosophila How and deuterostome Quaking, both implicated in alternative splicing. We identify messenger RNAs associated with C. briggsae GLD-1 on a genome-wide scale and provide evidence that many participate in aspects of germline development. By comparing our results with published C. elegans GLD-1 targets, we detect nearly 100 that are conserved between the two species. We also detected several hundred Cbr-GLD-1 targets whose homologs have not been reported to be associated with C. elegans GLD-1 in either of two independent studies. Low expression in C. elegans may explain the failure to detect most of them, but a highly expressed subset are strong candidates for Cbr-GLD-1-specific targets. We examine GLD-1-binding motifs among targets conserved in C. elegans and C. briggsae and find that most, but not all, display evidence of shared ancestral binding sites. Our work illustrates both the conservative and the dynamic character of evolution at the posttranslational level of gene regulation, even between congeners.