Amplification of a Zygosaccharomyces bailii DNA Segment in Wine Yeast Genomes by Extrachromosomal Circular DNA Formation

We recently described the presence of large chromosomal segments resulting from independent horizontal gene transfer (HGT) events in the genome of Saccharomyces cerevisiae strains, mostly of wine origin. We report here evidence for the amplification of one of these segments, a 17 kb DNA segment from Zygosaccharomyces bailii, in the genome of S. cerevisiae strains. The copy number, organization and location of this region differ considerably between strains, indicating that the insertions are independent and that they are post-HGT events. We identified eight different forms in 28 S. cerevisiae strains, mostly of wine origin, with up to four different copies in a single strain. The organization of these forms and the identification of an autonomously replicating sequence functional in S. cerevisiae, strongly suggest that an extrachromosomal circular DNA (eccDNA) molecule serves as an intermediate in the amplification of the Z. bailii region in yeast genomes. We found little or no sequence similarity at the breakpoint regions, suggesting that the insertions may be mediated by nonhomologous recombination. The diversity between these regions in S. cerevisiae represents roughly one third the divergence among the genomes of wine strains, which confirms the recent origin of this event, posterior to the start of wine strain expansion. This is the first report of a circle-based mechanism for the expansion of a DNA segment, mediated by nonhomologous recombination, in natural yeast populations

Synthetic yeast chromosome XI design enables extrachromosomal circular DNA formation on demand

We describe construction of the 660 kilobase synthetic yeast chromosome XI (synXI) and reveal how synthetic redesign of non-coding DNA elements impact the cell. To aid construction from synthesized 5 to 10 kilobase DNA fragments, we implemented CRISPR-based methods for synthetic crossovers in vivo and used these methods in an extensive process of bug discovery, redesign and chromosome repair, including for the precise removal of 200 kilobases of unexpected repeated sequence. In synXI, the underlying causes of several fitness defects were identified as modifications to non-coding DNA, including defects related to centromere function and mitochondrial activity that were subsequently corrected. As part of synthetic yeast chromosome design, loxPsym sequences for Cre-mediated recombination are inserted between most genes. Using the GAP1 locus from chromosome XI, we show here that targeted insertion of these sites can be used to create extrachromosomal circular DNA on demand, allowing direct study of the effects and propagation of these important molecules. Construction and characterization of synXI has uncovered effects of non-coding and extrachromosomal circular DNA, contributing to better understanding of these elements and informing future synthetic genome design

Formation of extrachromosomal circular DNA in HeLa cells by nonhomologous recombination.

Extrachromosomal circular DNA (eccDNA) generated from chromosomal DNA is found in all mammalian cells and increases with cell stress or aging. Studies of eccDNA structure and mode of formation provide insight into mechanisms of instability of the mammalian genome. Previous studies have suggested that eccDNA is generated through a process involving recombination between repetitive sequences. However, we observed that approximately one half of the small eccDNA fragments cloned from HeLa S3 cells were composed entirely of nonrepetitive or low-copy DNA sequences. We analyzed four of these fragments by polymerase chain reaction and nucleotide sequencing and found that they were complete eccDNAs. We then screened a human genomic library with the eccDNAs to isolate the complementary chromosomal sequences. Comparing the recombination junctions within the eccDNAs with the chromosomal sequences from which they were derived revealed that nonhomologous recombination was involved in their formation. One of the eccDNAs was composed of two separate sequences from different parts of the genome. These results suggest that rejoining of ends of fragmented DNA is responsible for the generation of a substantial portion of the eccDNAs found in HeLa S3 cells

Assesment of Structure, Function, and Microevolutionary Dynamics of Extrachromosomal Circular DNA in Chinese Hamster Ovary Cells

Chinese hamster ovary (CHO) cell lines are among the most popular expression hosts used in biopharmaceutical manufacturing due to relative ease of culture, capacity to perform human-like post-translational modifications, and non-susceptibility to viruses. However, the intrinsic plasticity of the CHO genome can lead to undesired genetic rearrangements, phenotypic shifts, reduced product quality, and early culture termination that prevents continuous biomanufacturing. A characteristic of plastic and unstable genomes that is poorly understood in CHO cells is extrachromosomal circular DNA (eccDNA). EccDNAs are focal amplifications of the genome that reside in the extranuclear space. These plasmid-like entities are structurally complex and are capable of contributing to a wide variety of biological functions including gene overexpression, regulation of nuclear-encoded genes, immunostimulation, and adaptive stress responses. The objective of this work is to establish the foundational knowledge of eccDNA structure, function, and microevolutionary dynamics in CHO cells under various conditions. This work characterizes eccDNA content in CHO cells grown in bioreactors for two weeks under control and lactate-stressed conditions, two CHO K-1-derived cell lines of different ages, and CHO cells gradually adapted to high extracellular lactate levels. More than 2,000 genes were observed to be encoded on eccDNAs and summaries of gene function are presented using Gene Ontology and KEGG pathway analyses. RNA-seq data is utilized to identify potential changes in gene expression mediated by eccDNAs. Furthermore, the study presents a broad characterization of eccDNA sequence structures and biogenesis sites that may be used as targets in future work

A study of the recombination activating gene 1 in the zebrafish nervous system

Ph.DDOCTOR OF PHILOSOPH

Accelerated Adaptation through Stimulated Copy Number Variation in Saccharomyces cerevisiae

Accelerated Adaptation through Stimulated Copy Number Variation in Saccharomyces cerevisiae Ryan Matthew Hull Repetitive regions of the genome, such as the centromeres, telomeres and ribosomal DNA account for a large proportion of the genetic variation between individuals. Differences in the number of repeat sequences between individuals is termed copy number variation (CNV) and is rife across eukaryotic genomes. CNV is of clinical importance as it has been implicated in many human disorders, in particularly cancers where is has been associated with tumour growth and drug resistance. The copper-resistance gene CUP1 in Saccharomyces cerevisiae is one such CNV gene. CUP1 is transcribed from a copper inducible promoter and encodes a protein involved in copper detoxification. In this work I show that yeast can regulate their repeat levels of the CUP1 gene through a transcriptionally stimulated CNV mechanism, as a direct adaptation response to a hostile environment. I characterise the requirement of the epigenetic mark Histone H3 Lysine 56 acetylation (H3K56ac) for stimulated CNV and its limitation of only working at actively transcribed genes. Based upon my findings, I propose a model for how stimulated CNV is regulated in yeast and show how we can pharmacologically manipulate this mechanism using drugs, like nicotinamide and rapamycin, to stimulate and repress a cell’s ability to adapt to its environment. I further show that the model is not limited to high-copy CUP1 repeat arrays, but is also applicable to low-copy systems. Finally, I show that the model extends to other genetic loci in response to different challenging environments, such as formaldehyde stimulation of the formaldehyde-resistance gene SFA1. To the best of our knowledge, this is the first example of any eukaryotic cell undergoing genome optimisation as a novel means to accelerate its adaptation in direct response to its environment. If conserved in higher eukaryotes, such a mechanism could have major implications in how we consider and treat disorders associated with changes in CNV