Supramolecular multilayer organization of chromosomes: possible functional roles of planar chromatin in gene expression and DNA replication and repair

Direct observation of surface charge and stiffness of human metaphase chromosomes

Metaphase chromosomes in which both polynucleotides and proteins are condensed with hierarchies are closely related to life phenomena such as cell division, cancer development, and cellular senescence. Nevertheless, their nature is rarely revealed, owing to their structural complexity and technical limitations in analytical methods. In this study, we used surface potential and nanomechanics mapping technology based on atomic force microscopy to measure the surface charge and intrinsic stiffness of metaphase chromosomes. We found that extra materials covering the chromosomes after the extraction process were positively charged. With the covering materials, the chromosomes were positively charged (ca. 44.9 ± 16.48 mV) and showed uniform stiffness (ca. 6.23 ± 1.98 MPa). In contrast, after getting rid of the extra materials through treatment with RNase and protease, the chromosomes were strongly negatively charged (ca. -197.4 ± 77.87 mV) and showed relatively non-uniform and augmented stiffness (ca. 36.87 ± 17.56 MPa). The results suggested undulating but compact coordination of condensed chromosomes. Additionally, excessive treatment with RNase and protease could destroy the chromosomal structure, providing an exceptional opportunity for multiscale stiffness mapping of polynucleotides, nucleosomes, chromatin fibers, and chromosomes in a single image. Our approach offers a new horizon in terms of an analytical technique for studying chromosome-related diseases.

Molecular combing and its application in clinical settings

Molecular combing technology (MCT) is an effective means for stretching DNA molecules and making them thus accessible for in situ studies. MCT uses the force exerted in the process of liquid flow via surface tension to stretch DNA molecules and spread them on solid surfaces, i.e. glass cover slips. Many DNA molecules can be stretched at the same time in parallel and neatly arranged side-by-side, making the approach convenient for statistical analysis. Accordingly, DNA replication and transcription can be studied at the single molecule level. In this paper, the principle, experimental methods, important applications, advantages and shortcuts of MCT in medical field are presented and discussed.

Molecular Cytogenetics in the Era of Chromosomics and Cytogenomic Approaches

Here the role of molecular cytogenetics in the context of yet available all other cytogenomic approaches is discussed. A short introduction how cytogenetics and molecular cytogenetics were established is followed by technical aspects of fluorescence in situ hybridization (FISH). The latter contains the methodology itself, the types of probe- and target-DNA, as well as probe sets. The main part deals with examples of modern FISH-applications, highlighting unique possibilities of the approach, like the possibility to study individual cells and even individual chromosomes. Different variants of FISH can be used to retrieve information on genomes from (almost) base pair to whole genomic level, as besides only second and third generation sequencing approaches can do. Here especially highlighted variations of FISH are molecular combing, chromosome orientation-FISH (CO-FISH), telomere-FISH, parental origin determination FISH (POD-FISH), FISH to resolve the nuclear architecture, multicolor-FISH (mFISH) approaches, among other applied in chromoanagenesis studies, Comet-FISH, and CRISPR-mediated FISH-applications. Overall, molecular cytogenetics is far from being outdated and actively involved in up-to-date diagnostics and research.

Soft-matter properties of multilayer chromosomes

This perspective aims to identify the relationships between the structural and dynamic properties of chromosomes and the fundamental properties of soft-matter systems. Chromatin is condensed into metaphase chromosomes during mitosis. The resulting structures are elongated cylinders having micrometer-scale dimensions. Our previous studies, using transmission electron microscopy, atomic force microscopy, and cryo-electron tomography, suggested that metaphase chromosomes have a multilayered structure, in which each individual layer has the width corresponding to a mononucleosome sheet. The self-assembly of multilayer chromatin plates from small chromatin fragments suggests that metaphase chromosomes are self-organized hydrogels (in which a single DNA molecule crosslinks the whole structure) with an internal liquid-crystal order produced by the stacking of chromatin layers along the chromosome axis. This organization of chromatin was unexpected, but the spontaneous assembly of large structures has been studied in different soft-matter systems and, according to these studies, the self-organization of chromosomes could be justified by the interplay between weak interactions of repetitive nucleosome building blocks and thermal fluctuations. The low energy of interaction between relatively large building blocks also justifies the easy deformation and structural fluctuations of soft-matter structures and the changes of phase caused by diverse external factors. Consistent with these properties of soft matter, different experimental results show that metaphase chromosomes are easily deformable. Furthermore, at the end of mitosis, condensed chromosomes undergo a phase transition into a more fluid structure, which can be correlated to the decrease in the Mg²⁺concentration and to the dissociation of condensins from chromosomes. Presumably, the unstacking of layers and chromatin fluctuations driven by thermal energy facilitate gene expression during interphase.

Higher-order interfiber interactions in the self-assembly of benzene-1,3,5-tricarboxamide-based peptides in water

Mimicking the complexity of biological systems with synthetic supramolecular materials requires a deep understanding of the relationship between the structure of the molecule and its self-assembly pattern. Herein, we report a series of water-soluble benzene-1,3,5-tricarboxamide-based di- and tripeptide derivatives modified with small non-bulky terminal amine salt to induce self-assembly into twisted one-dimensional higher-order nanofibers. The morphology of nanofibers strongly depends on the nature, order, and quantity of amino acids in the short peptide fragments and vary from simple cylindrical to complex helical. From observations of several fiber-splitting events, we detected interfiber interactions that always occur in a pairwise manner, which implies that the C3 symmetry of benzene-1,3,5-tricarboxamide-based molecules in higher-order fibers becomes gradually distorted, thus facilitating hydrophobic contact interactions between fibrils. The proposed mechanism of self-assembly through hydrophobic contact allowed the successful design of a compound with pH-responsive morphology, and may find use in the future development of complex hierarchical architectures with controlled functionality.

About classical molecular genetics, cytogenetic and molecular cytogenetic data not considered by Genome Reference Consortium and thus not included in genome browsers like UCSC, Ensembl or NCBI

Background

The Genome Reference Consortium (GRC) has according to its own statement the “mission to improve the human reference genome assembly, correcting errors and adding sequence to ensure it provides the best representation of the human genome to meet basic and clinical research needs”. Data from GRC is included in genome browsers like UCSC (University of California, Santa Cruz), Ensembl or NCBI (National Center for Biotechnology Information) and are thereby bases for scientific and diagnostically working human genetic community.

Method

Here long standing knowledge deriving from classical molecular genetic, cytogenetic and molecular cytogenetic data, not being considered yet by GRC was revisited.

Results

There were three major points identified: (1) GRC missed to including three chromosomal subbands, each, for 1q32.1, 2p21, 5q13.2, 6p22.3 and 6q21, which were defined by International System for Human Cytogenetic Nomenclature (ISCN) already back in 1980s; instead GRC included additional 6 subbands not ever recognized by ISCN. (2) GRC defined 34 chromosomal subbands of 0.1 to 0.9 Mb in size, while it is general agreement of cytogeneticists that it unlikely to detect chromosomal aberrations below 1–2 Mb in size by GTG-banding. And (3): still all sequences used in molecular cytogenetic routine diagnostics to detect heterochromatic and/ or pericentromeric satellite DNA sequences within the human genome are not included yet into human reference genome. For those sequences, localization and approximate sizes have been determined in the 1970s to 1990, and if included at least ~ 100 Mb of the human genome sequence could be added to the genome browsers.

Conclusion

Overall, taking into account the here mentioned points and correcting and including the data will definitely provide to the still not being completely finished mapping of the human genome.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13039-021-00540-7.

From FISH to Hi-C: The Chromatin Architecture of the Chromosomal Region 7q36.3, Frequently Rearranged in Leukemic Cells, Is Evolutionary Conserved

Fluorescence in situ hybridization (FISH) and Hi-C methods are largely used to investigate the three-dimensional organization of the genome in the cell nucleus and are applied here to study the organization of genes (LMBR1, NOM1, MNX1, UBE3C, PTPRN2) localized in the human 7q36.3 band. This region contains the MNX1 gene, which is normally not expressed in human lymphocytes beyond embryonic development. However, this homeobox gene is frequently activated in leukemic cells and its expression is associated with an altered gene positioning in the leukemia cell nuclei. In this study, we used FISH on 3D-preserved nuclei to investigate the nuclear positioning of MNX1 in the leukemia-derived cell line K562. Of the five copies of the MNX1 gene present in K562, four alleles were positioned in the nuclear periphery and only one in the nuclear interior. Using the Juicebox’s Hi-C dataset, we identified five chromatin loops in the 7q36.3 band, with different extensions related to the size and orientation of the genes located here, and independent from their expression levels. We identified similar loops in 11 human and three mouse cell lines, showing that these loops are highly conserved in different human cell lines and during evolution. Moreover, the chromatin loop organization is well conserved also during neuronal cell differentiation, showing consistency in genomic organization of this region in development. In this report, we show that FISH and Hi-C are two different approaches that complement one another and together give complete information on the nuclear organization of specific chromosomal regions in different conditions, including cellular differentiation and genetic diseases.