Using human artificial chromosomes to study centromere assembly and function

Chromosome Transplantation: Opportunities and Limitations

There are thousands of rare genetic diseases that could be treated with classical gene therapy strategies such as the addition of the defective gene via viral or non-viral delivery or by direct gene editing. However, several genetic defects are too complex for these approaches. These "genomic mutations" include aneuploidies, intra and inter chromosomal rearrangements, large deletions, or inversion and copy number variations. Chromosome transplantation (CT) refers to the precise substitution of an endogenous chromosome with an exogenous one. By the addition of an exogenous chromosome and the concomitant elimination of the endogenous one, every genetic defect, irrespective of its nature, could be resolved. In the current review, we analyze the state of the art of this technique and discuss its possible application to human pathology. CT might not be limited to the treatment of human diseases. By working on sex chromosomes, we showed that female cells can be obtained from male cells, since chromosome-transplanted cells can lose either sex chromosome, giving rise to 46,XY or 46,XX diploid cells, a modification that could be exploited to obtain female gametes from male cells. Moreover, CT could be used in veterinary biology, since entire chromosomes containing an advantageous locus could be transferred to animals of zootechnical interest without altering their specific genetic background and the need for long and complex interbreeding. CT could also be useful to rescue extinct species if only male cells were available. Finally, the generation of "synthetic" cells could be achieved by repeated CT into a recipient cell. CT is an additional tool for genetic modification of mammalian cells.

Transformation-associated recombination (TAR) cloning and its applications for gene function; genome architecture and evolution; biotechnology and biomedicine

Transformation-associated recombination (TAR) cloning represents a unique tool to selectively and efficiently recover a given chromosomal segment up to several hundred kb in length from complex genomes (such as animals and plants) and simple genomes (such as bacteria and viruses). The technique exploits a high level of homologous recombination in the yeast Sacharomyces cerevisiae. In this review, we summarize multiple applications of the pioneering TAR cloning technique, developed previously for complex genomes, for functional, evolutionary, and structural studies, and extended the modified TAR versions to isolate biosynthetic gene clusters (BGCs) from microbes, which are the major source of pharmacological agents and industrial compounds, and to engineer synthetic viruses with novel properties to design a new generation of vaccines. TAR cloning was adapted as a reliable method for the assembly of synthetic microbe genomes for fundamental research. In this review, we also discuss how the TAR cloning in combination with HAC (human artificial chromosome)- and CRISPR-based technologies may contribute to the future.

Uncommon types of autoantibodies - Detection and clinical associations

Immunofluorescence is a basic method for detection of autoantibodies in serum. It is used as screening for people with symptoms suggesting autoimmune process and disease. Antinuclear antibodies (ANA) assay detecting antibodies against nuclear proteins used commonly for diagnosis of systemic autoimmune disease, although antibodies against cytoplasmic components and mitotic structures are usable in clinic. The majority of ANA nuclear patterns have been comprehensively studied with increasing data. However, the cytoplasmic and mitotic patterns are underestimated and still require further assessment. In this review the clinical associations and significance of uncommon types of autoantibodies are presented and discussed.

Deterministic assembly of chromosome ensembles in a programmable membrane trap array

Selective spatial isolation and manipulation of single chromosomes and the controlled formation of defined chromosome ensembles in a droplet-based microfluidic system is presented. The multifunctional microfluidic technology employs elastomer valves and membrane displacement traps to support deterministic manipulation of individual droplets. Picoliter droplets are formed in the 2D array of microscale traps by self-discretization of a nanoliter sample plug, with membranes positioned over each trap allowing controllable metering or full release of selected droplets. By combining discretization, optical interrogation, and selective droplet release for sequential delivery to a downstream merging zone, the system enables efficient manipulation of multiple chromosomes into a defined ensemble with single macromolecule resolution. Key design and operational parameters are explored, and co-compartmentalization of three chromosome pairs is demonstrated as a first step toward formation of precisely defined chromosome ensembles for applications in genetic engineering and synthetic biology.

Pluripotent stem cell-based gene therapy approach: human de novo synthesized chromosomes

A novel approach in gene therapy was introduced 20 years ago since artificial non-integrative chromosome-based vectors containing gene loci size inserts were engineered. To date, different human artificial chromosomes (HAC) were generated with the use of de novo construction or "top-down" engineering approaches. The HAC-based therapeutic approach includes ex vivo gene transferring and correction of pluripotent stem cells (PSCs) or highly proliferative modified stem cells. The current progress in the technology of induced PSCs, integrating with the HAC technology, resulted in a novel platform of stem cell-based tissue replacement therapy for the treatment of genetic disease. Nowadays, the sophisticated and laborious HAC technology has significantly improved and is now closer to clinical studies. In here, we reviewed the achievements in the technology of de novo synthesized HACs for a chromosome transfer for developing gene therapy tissue replacement models of monogenic human diseases.

Aneuploidy in Cancer: Lessons from Acute Lymphoblastic Leukemia

Aneuploidy, the gain or loss of chromosomes in a cell, is a hallmark of cancer. Although our understanding of the contribution of aneuploidy to cancer initiation and progression is incomplete, significant progress has been made in uncovering the cellular consequences of aneuploidy and how aneuploid cancer cells self-adapt to promote tumorigenesis. Aneuploidy is physiologically associated with significant cellular stress but, paradoxically, it favors tumor progression. Although more common in solid tumors, different forms of aneuploidy represent the initiating oncogenic lesion in patients with B cell acute lymphoblastic leukemia (B-ALL), making B-ALL an excellent model for studying the role of aneuploidy in tumorigenesis. We review the molecular mechanisms underlying aneuploidy and discuss its contributions to B-ALL initiation and progression.

Alpha-satellite RNA transcripts are repressed by centromere-nucleolus associations

Although originally thought to be silent chromosomal regions, centromeres are instead actively transcribed. However, the behavior and contributions of centromere-derived RNAs have remained unclear. Here, we used single-molecule fluorescence in-situ hybridization (smFISH) to detect alpha-satellite RNA transcripts in intact human cells. We find that alpha-satellite RNA-smFISH foci levels vary across cell lines and over the cell cycle, but do not remain associated with centromeres, displaying localization consistent with other long non-coding RNAs. Alpha-satellite expression occurs through RNA polymerase II-dependent transcription, but does not require established centromere or cell division components. Instead, our work implicates centromere–nucleolar interactions as repressing alpha-satellite expression. The fraction of nucleolar-localized centromeres inversely correlates with alpha-satellite transcripts levels across cell lines and transcript levels increase substantially when the nucleolus is disrupted. The control of alpha-satellite transcripts by centromere-nucleolar contacts provides a mechanism to modulate centromere transcription and chromatin dynamics across diverse cell states and conditions.

The unique kind of human artificial chromosome: Bypassing the requirement for repetitive centromere DNA

Centromeres are essential components of all eukaryotic chromosomes, including artificial/synthetic ones built in the laboratory. In humans, centromeres are typically located on repetitive α-satellite DNA, and these sequences are the "major ingredient" in first-generation human artificial chromosomes (HACs). Repetitive centromeric sequences present a major challenge for the design of synthetic mammalian chromosomes because they are difficult to synthesize, assemble, and characterize. Additionally, in most eukaryotes, centromeres are defined epigenetically. Here, we review the role of the genetic and epigenetic contributions to establishing centromere identity, highlighting recent work to hijack the epigenetic machinery to initiate centromere identity on a new generation of HACs built without α-satellite DNA. We also discuss the opportunities and challenges in developing useful unique sequence-based HACs.

Human artificial chromosome (HAC) for measuring chromosome instability (CIN) and identification of genes required for proper chromosome transmission

Chromosomal instability (CIN) is one of the characteristics of cancer inherent for tumor initiation and progression, which is defined as a persistent, high rate of gain/loss of whole chromosomes. In the vast majority of human tumors the molecular basis of CIN remains unknown. The development of a conceptually simple colony color sectoring assay that measures yeast artificial chromosome (YAC) loss provided a powerful genetic tool to assess the rate of chromosome mis-segregation and also identified 937 yeast genes involved in this process. Similarly, a human artificial chromosome (HAC)-based assay has been recently developed and applied to quantify chromosome mis-segregation events in human cells. This assay allowed identification of novel human CIN genes in the library of protein kinases. Among them are PINK1, TRIO, IRAK1, PNCK, and TAOK1. The HAC-based assay may be applied to screen siRNA, shRNA and CRISPR-based libraries to identify the complete spectrum of CIN genes. This will reveal new insights into mechanisms of chromosome segregation and may expedite the development of novel therapeutic strategies to target the CIN phenotype in cancer cells.

A novel assay to screen siRNA libraries identifies protein kinases required for chromosome transmission

One of the hallmarks of cancer is chromosome instability (CIN), which leads to aneuploidy, translocations, and other chromosome aberrations. However, in the vast majority of human tumors the molecular basis of CIN remains unknown, partly because not all genes controlling chromosome transmission have yet been identified. To address this question, we developed an experimental high-throughput imaging (HTI) siRNA assay that allows the identification of novel CIN genes. Our method uses a human artificial chromosome (HAC) expressing the GFP transgene. When this assay was applied to screen an siRNA library of protein kinases, we identified PINK1, TRIO, IRAK1, PNCK, and TAOK1 as potential novel genes whose knockdown induces various mitotic abnormalities and results in chromosome loss. The HAC-based assay can be applied for screening different siRNA libraries (cell cycle regulation, DNA damage response, epigenetics, and transcription factors) to identify additional genes involved in CIN. Identification of the complete spectrum of CIN genes will reveal new insights into mechanisms of chromosome segregation and may expedite the development of novel therapeutic strategies to target the CIN phenotype in cancer cells.

TAR Cloning: Perspectives for Functional Genomics, Biomedicine, and Biotechnology

Completion of the human genome sequence and recent advances in engineering technologies have enabled an unprecedented level of understanding of DNA variations and their contribution to human diseases and cellular functions. However, in some cases, long-read sequencing technologies do not allow determination of the genomic region carrying a specific mutation (e.g., a mutation located in large segmental duplications). Transformation-associated recombination (TAR) cloning allows selective, most accurate, efficient, and rapid isolation of a given genomic fragment or a full-length gene from simple and complex genomes. Moreover, this method is the only way to simultaneously isolate the same genomic region from multiple individuals. As such, TAR technology is currently in a leading position to create a library of the individual genes that comprise the human genome and physically characterize the sites of chromosomal alterations (copy number variations [CNVs], inversions, translocations) in the human population, associated with the predisposition to different diseases, including cancer. It is our belief that such a library and analysis of the human genome will be of great importance to the growing field of gene therapy, new drug design methods, and genomic research. In this review, we detail the motivation for TAR cloning for human genome studies, biotechnology, and biomedicine, discuss the recent progress of some TAR-based projects, and describe how TAR technology in combination with HAC (human artificial chromosome)-based and CRISPR-based technologies may contribute in the future.

Centromeric and ectopic assembly of CENP-A chromatin in health and cancer: old marks and new tracks

The histone H3 variant CENP-A confers epigenetic identity to the centromere and plays crucial roles in the assembly and function of the kinetochore, thus ensuring proper segregation of our chromosomes. CENP-A containing nucleosomes exhibit unique structural specificities and lack the complex profile of gene expression-associated histone posttranslational modifications found in canonical histone H3 and the H3.3 variant. CENP-A mislocalization into noncentromeric regions resulting from its overexpression leads to chromosomal segregation aberrations and genome instability. Overexpression of CENP-A is a feature of many cancers and is associated with malignant progression and poor outcome. The recent years have seen impressive progress in our understanding of the mechanisms that orchestrate CENP-A deposition at native centromeres and ectopic loci. They have witnessed the description of novel, heterotypic CENP-A/H3.3 nucleosome particles and the exploration of the phenotypes associated with the deregulation of CENP-A and its chaperones in tumor cells. Here, we review the structural specificities of CENP-A nucleosomes, the epigenetic features that characterize the centrochromatin and the mechanisms and factors that orchestrate CENP-A deposition at centromeres. We then review our knowledge of CENP-A ectopic distribution, highlighting experimental strategies that have enabled key discoveries. Finally, we discuss the implications of deregulated CENP-A in cancer.

A role of the Trx-G complex in Cid/CENP-A deposition at Drosophila melanogaster centromeres

Centromeres are epigenetically determined chromatin structures that specify the assembly site of the kinetochore, the multiprotein machinery that binds microtubules and mediates chromosome segregation during mitosis and meiosis. The centromeric protein A (CENP-A) and its Drosophila orthologue centromere identifier (Cid) are H3 histone variants that replace the canonical H3 histone in centromeric nucleosomes of eukaryotes. CENP-A/Cid is required for recruitment of other centromere and kinetochore proteins and its deficiency disrupts chromosome segregation. Despite the many components that are known to cooperate in centromere function, the complete network of factors involved in CENP-A recruitment remains to be defined. In Drosophila, the Trx-G proteins localize along the heterochromatin with specific patterns and some of them localize to the centromeres of all chromosomes. Here, we show that the Trx, Ash1, and CBP proteins are required for the correct chromosome segregation and that Ash1 and CBP mediate for Cid/CENP-A recruitment at centromeres through post-translational histone modifications. We found that centromeric H3 histone is consistently acetylated in K27 by CBP and that nej and ash1 silencing respectively causes a decrease in H3K27 acetylation and H3K4 methylation along with an impairment of Cid loading.

De novo formation and epigenetic maintenance of centromere chromatin

Accurate chromosome segregation is essential for cell proliferation. The centromere is a specialized chromosomal locus, on which the kinetochore structure is formed. The centromere/kinetochore is required for the equal separation of sister chromatids to daughter cells. Here, we review recent findings on centromere-specific chromatin, including its constitutive protein components, its de novo formation and maintenance mechanisms, and our progress in analyses with synthetic human artificial chromosomes (HACs).

Systematic Analysis of Compounds Specifically Targeting Telomeres and Telomerase for Clinical Implications in Cancer Therapy

The targeting of telomerase and telomere maintenance mechanisms represents a promising therapeutic approach for various types of cancer. In this work, we designed a new protocol to screen for and rank the efficacy of compounds specifically targeting telomeres and telomerase. This approach used two isogenic cell lines containing a circular human artificial chromosome (HAC, lacking telomeres) and a linear HAC (containing telomeres) marked with the EGFP transgene; compounds that target telomerase or telomeres should preferentially induce loss of the linear HAC but not the circular HAC. Our assay allowed quantification of chromosome loss by routine flow cytometry. We applied this dual-HAC assay to rank a set of known and newly developed compounds, including G-quadruplex (G4) ligands. Among the latter group, two compounds, Cu-ttpy and Pt-ttpy, induced a high rate of linear HAC loss with no significant effect on the mitotic stability of a circular HAC. Analysis of the mitotic phenotypes induced by these drugs revealed an elevated rate of chromatin bridges in late mitosis and cytokinesis as well as UFB (ultrafine bridges). Chromosome loss after Pt-ttpy or Cu-ttpy treatment correlated with the induction of telomere-associated DNA damage. Overall, this platform enables identification and ranking of compounds that greatly increase chromosome mis-segregation rates as a result of telomere dysfunction and may expedite the development of new therapeutic strategies for cancer treatment.Significance: An assay provides a unique opportunity to screen thousands of chemical compounds for their ability to inactivate replication of telomeric ends in cancer cells and holds potential to lay the foundation for the discovery of new treatments for cancer. Cancer Res; 78(21); 6282-96. ©2018 AACR.

Human Artificial Chromosome with Regulated Centromere: A Tool for Genome and Cancer Studies

Since their description in the late 1990s, Human Artificial Chromosomes (HACs) bearing functional kinetochores have been considered as promising systems for gene delivery and expression. More recently a HAC assembled from a synthetic alphoid DNA array has been exploited in studies of centromeric chromatin and in assessing the impact of different epigenetic modifications on kinetochore structure and function in human cells. This HAC was termed the alphoid^tetO-HAC, as the synthetic monomers each contained a tetO sequence in place of the CENP-B box that can be targeted specifically with tetR-fusion proteins. Studies in which the kinetochore chromatin of the alphoid^tetO-HAC was specifically modified, revealed that heterochromatin is incompatible with centromere function and that centromeric transcription is important for centromere assembly and maintenance. In addition, the alphoid^tetO-HAC was modified to carry large gene inserts that are expressed in target cells under conditions that recapitulate the physiological regulation of endogenous loci. Importantly, the phenotypes arising from stable gene expression can be reversed when cells are "cured" of the HAC by inactivating its kinetochore in proliferating cell populations, a feature that provides a control for phenotypic changes attributed to expression of HAC-encoded genes. Alphoid^tetO-HAC-based technology has also been used to develop new drug screening and assessment strategies to manipulate the CIN phenotype in cancer cells. In summary, the alphoid^tetO-HAC is proving to be a versatile tool for studying human chromosome transactions and structure as well as for genome and cancer studies.

Mitotic progression, arrest, exit or death relies on centromere structural integrity, rather than de novo transcription

Recent studies have challenged the prevailing dogma that transcription is repressed during mitosis. Transcription was also proposed to sustain a robust spindle assembly checkpoint (SAC) response. Here, we used live-cell imaging of human cells, RNA-seq and qPCR to investigate the requirement for de novo transcription during mitosis. Under conditions of persistently unattached kinetochores, transcription inhibition with actinomycin D, or treatment with other DNA-intercalating drugs, delocalized the chromosomal passenger complex (CPC) protein Aurora B from centromeres, compromising SAC signaling and cell fate. However, we were unable to detect significant changes in mitotic transcript levels. Moreover, inhibition of transcription independently of DNA intercalation had no effect on Aurora B centromeric localization, SAC response, mitotic progression, exit or death. Mechanistically, we show that DNA intercalating agents reduce the interaction of the CPC with nucleosomes. Thus, mitotic progression, arrest, exit or death is determined by centromere structural integrity, rather than de novo transcription.

Spindle tubulin and MTOC asymmetries may explain meiotic drive in oocytes

In the first meiotic division (MI) of oocytes, the cortically positioned spindle causes bivalent segregation in which only the centre-facing homologue pairs are retained. 'Selfish' chromosomes are known to exist, which bias their spindle orientation and hence retention in the egg, a process known as 'meiotic drive'. Here we report on this phenomenon in oocytes from F₁ hybrid mice, where parental strain differences in centromere size allows distinction of the two homologue pairs of a bivalent. Bivalents with centromere and kinetochore asymmetry show meiotic drive by rotating during prometaphase, in a process dependent on aurora kinase activity. Cortically positioned homologue pairs appear to be under greater stretch than their centre-facing partners. Additionally the cortex spindle-half contain a greater density of tubulin and microtubule organising centres. A model is presented in which meiotic drive is explained by the impact of microtubule force asymmetry on chromosomes with different sized centromeres and kinetochores.

Generation of a Synthetic Human Chromosome with Two Centromeric Domains for Advanced Epigenetic Engineering Studies

It is generally accepted that chromatin containing the histone H3 variant CENP-A is an epigenetic mark maintaining centromere identity. However, the pathways leading to the formation and maintenance of centromere chromatin remain poorly characterized due to difficulties of analysis of centromeric repeats in native chromosomes. To address this problem, in our previous studies we generated a human artificial chromosome (HAC) whose centromere contains a synthetic alpha-satellite (alphoid) DNA array containing the tetracycline operator, the alphoid^tetO-HAC. The presence of tetO sequences allows the specific targeting of the centromeric region in the HAC with different chromatin modifiers fused to the tetracycline repressor. The alphoid^tetO-HAC has been extensively used to investigate protein interactions within the kinetochore and to define the epigenetic signature of centromeric chromatin to maintain a functional kinetochore. In this study, we developed a novel synthetic HAC containing two alphoid DNA arrays with different targeting sequences, tetO, lacO and gal4, the alphoid^hybrid-HAC. This new HAC can be used for detailed epigenetic engineering studies because its kinetochore can be simultaneously or independently targeted by different chromatin modifiers and other fusion proteins.