profiling-dna-replication-timing-using-zebrafish-as-an-vivo-model

Profiling DNA Replication Timing Using Zebrafish as an <em>In Vivo</em> Model System

DNA replication timing is an important cellular characteristic, exhibiting significant relationships with chromatin structure, transcription, and DNA ...

Cornell University

Mutational processes constantly shape the somatic genome, leading to immunity, aging, cancer, and other diseases. When cancer is the outcome, we are afforded a glimpse into these processes by the clonal expansion of the malignant cell. Here, we characterize a less explored layer of the mutational landscape of cancer: mutational asymmetries between the two DNA strands. Analyzing whole-genome sequences of 590 tumors from 14 different cancer types, we reveal widespread asymmetries across mutagenic processes, with transcriptional ("T-class") asymmetry dominating UV-, smoking-, and liver-cancer-associated mutations and replicative ("R-class") asymmetry dominating POLE-, APOBEC-, and MSI-associated mutations. We report a striking phenomenon of transcription-coupled damage (TCD) on the non-transcribed DNA strand and provide evidence that APOBEC mutagenesis occurs on the lagging-strand template during DNA replication. As more genomes are sequenced, studying and classifying their asymmetries will illuminate the underlying biological mechanisms of DNA damage and repair.

Mutational Strand Asymmetries in Cancer Genomes Reveal Mechanisms of DNA Damage and Repair.

In dividing cells, DNA replication occurs in a precise order, but many questions remain regarding the mechanisms of replication timing establishment and regulation. We now have generated genome-wide, high-resolution replication timing maps throughout zebrafish development. Unexpectedly, in the rapid cell cycles preceding the midblastula transition, a defined timing program was present that predicted the initial wave of zygotic transcription. Replication timing was thereafter progressively and continuously remodeled across the majority of the genome, and epigenetic changes involved in enhancer activation frequently paralleled developmental changes in replication timing. The long arm of Chromosome 4 underwent a dramatic developmentally regulated switch to late replication during gastrulation, reminiscent of mammalian X Chromosome inactivation. This study reveals that replication timing is dynamic and tightly linked to epigenetic and transcriptional changes throughout early zebrafish development. These data provide insight into the regulation and functions of replication timing and will enable further mechanistic studies.

DNA replication timing during development anticipates transcriptional programs and parallels enhancer activation.

Microsatellites (MSs) are tracts of variable-length repeats of short DNA motifs that exhibit high rates of mutation in the form of insertions or deletions (indels) of the repeated motif. Despite their prevalence, the contribution of somatic MS indels to cancer has been largely unexplored, owing to difficulties in detecting them in short-read sequencing data. Here we present two tools: MSMuTect, for accurate detection of somatic MS indels, and MSMutSig, for identification of genes containing MS indels at a higher frequency than expected by chance. Applying MSMuTect to whole-exome data from 6,747 human tumors representing 20 tumor types, we identified >1,000 previously undescribed MS indels in cancer genes. Additionally, we demonstrate that the number and pattern of MS indels can accurately distinguish microsatellite-stable tumors from tumors with microsatellite instability, thus potentially improving classification of clinically relevant subgroups. Finally, we identified seven MS indel driver hotspots: four in known cancer genes (ACVR2A, RNF43, JAK1, and MSH3) and three in genes not previously implicated as cancer drivers (ESRP1, PRDM2, and DOCK3).

Analysis of somatic microsatellite indels identifies driver events in human tumors.

A new analysis of cancer genomes identifies a decrease in the mutation burden of exons, but not introns, as compared to expectation. This difference can be explained by preferential recruitment of the DNA mismatch repair machinery to a protein modification that marks exons.

Article	Year
Mutational Strand Asymmetries in Cancer Genomes Reveal Mechanisms of DNA Damage and Repair. Cell\| PubMed ID: 26806129	2016
DNA replication timing during development anticipates transcriptional programs and parallels enhancer activation. Genome research\| PubMed ID: 28512193	2017
Analysis of somatic microsatellite indels identifies driver events in human tumors. Nature biotechnology\| PubMed ID: 28892075	2017
Mismatch repair prefers exons. Nature genetics\| PubMed ID: 29186127	2017

Amnon Koren

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