Leukocyte Nucleus Reveals a Linear Order of Chromosomes Separated in Two Parental Genomes That Favors the Process of Gene Activation.

Analysis of trisomy 8 cells and the chromosome-specific fluorescence in situ hybridization (FISH) signals on the ring-shaped nucleus of a neutrophil reveal that homologue chromosomes orient in diametrical opposition to each other. This positioning results in a separation of the two haploid sets of parental chromosomes organized as two exclusive groups. These two groups impart the nucleus a symmetry that fortifies immune protection by accelerating chemotaxis. The ring form of the nucleus is a legacy of the orientation of chromosomes as a rosette during metaphase and telophase stages. A dual control maintains this spatial order: (1) chromosomes are tethered to the centriole all through the cell cycle, and (2) during their circular orientation in telophase the chromosomes bind to each other with lamins, which reorganize the nuclear membrane of the daughter nuclei, generating an additional anchorage. Here, chromosomes serve as temporary packets to assure proper distribution of the nuclear DNA during mitosis. The remainder time of the cell cycle the chromosomes are chained together across the telomeres, allowing a continuous sequence of genes of the two genomes, maternal and paternal, thus facilitating easy reading of the gene sequence. Exceptions to these orders are either physiological and temporary, or pathological and disease causing.

Chromosomes in a genome-wise order: evidence for metaphase architecture.

BACKGROUND: One fundamental finding of the last decade is that, besides the primary DNA sequence information there are several epigenetic "information-layers" like DNA-and histone modifications, chromatin packaging and, last but not least, the position of genes in the nucleus. RESULTS: We postulate that the functional genomic architecture is not restricted to the interphase of the cell cycle but can also be observed in the metaphase stage, when chromosomes are most condensed and microscopically visible. If so, it offers the unique opportunity to directly analyze the functional aspects of genomic architecture in different cells, species and diseases. Another aspect not directly accessible by molecular techniques is the genome merged from two different haploid parental genomes represented by the homologous chromosome sets. Our results show that there is not only a well-known and defined nuclear architecture in interphase but also in metaphase leading to a bilateral organization of the two haploid sets of chromosomes. Moreover, evidence is provided for the parental origin of the haploid grouping. CONCLUSIONS: From our findings we postulate an additional epigenetic information layer within the genome including the organization of homologous chromosomes and their parental origin which may now substantially change the landscape of genetics.

Nuclear segmentation, condensation and bilateral symmetry in polymorphonuclear leukocytes reflect genomic order and favor immunologic function.

Segmentation, condensation and bilateral symmetry of the nuclei of polymorphonuclear leukocytes seem related to their function. Segmentation of the nuclei into two or more lobes and their condensation facilitate their passage (diapedesis) through the endothelial layer of blood vessels to the extravasal space and subsequent locomotion through the interstitial compartment of different tissues. Bilateral symmetry of these nuclei along with their association to the cytoskeletal fibers contribute to their efficiency in locomotion by alignment of the axis of nuclear symmetry to the axis of cellular polarity, which orients towards the direction of locomotion in response to cytokines and other stimuli. Observations of the cytogenetic facets of intranuclear order support these assumptions.

Identification of parental chromosomes involved in translocations BCR-ABL, t(9;22) and PML-RARA, t(15;17).

Cells of blood and bone marrow often exhibit a genome- or ploidywise organization of the two haploid sets, representing apparently maternal and paternal chromosomes in interphase nuclei and in metaphase spreads. This provides the opportunity to perform "genomic karyotyping." Such application of karyotyping may indicate whether two chromosomes involved in a translocation are both maternal, both paternal, or intermingled, i.e., one maternal and the other paternal (we refer to this as mixed). The parental origin for these translocations likely has profound differences and implications in disease expression and response to treatments, making such information very important to personalized medicine. In this mini-review, we present our observations from specimens with translocations BCR-ABL, t(9;22) and PML-RARA, t(15;17). About 20% metaphases of these specimens indicated ploidywise organization and were amenable to genomic karyotyping analysis. Fluorescence in situ hybridization (FISH) probes for BCR-ABL translocation suggest a close approximation of the HSA 9 and 22, as control values for false-positive signals run from approximately 5-10%. Given a ploidywise distribution of the maternal and paternal sets of chromosomes, it would be expected that the chromosomes involved in the translocation t(9;22) would more often belong to one of the two genomes, either maternal or paternal. Contrastingly, HSA 15 and 17 are not considered as spatially close to each other and therefore an intragenomic involvement would be rarer for translocation t(15;17). In 14 out of the 21 (66.6%) specimens with informative metaphases, the chromosomes involved in the translocation BCR-ABL were restricted to one of the two genomes--either maternal or paternal. In cases of translocation PML-RARA only 4 out of 21 (19.1%) specimens indicated an intragenomic involvement. These simple yet informative analyses of cancer-related translocations show profound underlying genomic origins and lend support to genomic karyotyping.

Biphasic chromatin structure and FISH signals in reflect intranuclear order.

BACKGROUND AND AIM: One of the two parental allelic genes may selectively be expressed, regulated by imprinting, X-inactivation or by other less known mechanisms. This study aims to reflect on such genetic mechanisms. MATERIALS AND METHODS: Slides from short term cultures or direct smears of blood, bone marrow and amniotic fluids were hybridized with FISH probes singly, combined or sequentially. Two to three hundred cells were examined from each preparation. RESULTS AND SIGNIFICANCE: A small number of cells (up to about 5%), more frequent in leukemia cases, showed the twin features: (1) nuclei with biphasic chromatin, one part decondensed and the other condensed; and (2) homologous FISH signals distributed equitably in those two regions. The biphasic chromatin structure with equitable distribution of the homologous FISH signals may correspond to the two sets of chromosomes, supporting observations on ploidywise intranuclear order. The decondensed chromatin may relate to enhanced transcriptions or advanced replications. CONCLUSIONS: Transcriptions of only one of the two parental genomes cause allelic exclusion. Genomes may switch with alternating monoallelic expression of biallelic genes as an efficient genetic mechanism. If genomes fail to switch, allelic exclusion may lead to malignancy. Similarly, a genome-wide monoallelic replication may tilt the balance of heterozygosity resulting in aneusomy, initiating early events in malignant transformation and in predicting cancer mortality.

Separation of parental genomes in human blood and bone marrow cells and its implications.

The linear order of genes is apparently interrupted at chromosomal ends. Our observations on human blood and bone marrow cells indicate that the chromosomes of each of the two parental sets maintain coherence, perhaps in tandem, forming a ring. Two such rings in a diploid cell join building a larger ring, which folds up to form the interphase nucleus. The linear order of genes thus extends beyond the chromosomal ends. These observations become especially significant when seen in the light of cell biologic findings on interaction of chromosomes or chromatin and centrioles in different cell cycle phases, in polymorphonuclear cells and during the zygotic developments. They may explain how the genomic order and the sequential continuity of the genes are maintained and why such order remains often cryptic.