Discovering non-random segregation of sister chromatids: the naïve treatment of a premature discovery

Non-Random Sister Chromatid Segregation in Human Tissue Stem Cells

The loss of genetic fidelity in tissue stem cells is considered a significant cause of human aging and carcinogenesis. Many cellular mechanisms are well accepted for limiting mutations caused by replication errors and DNA damage. However, one mechanism, non-random sister chromatid segregation, remains controversial. This atypical pattern of chromosome segregation is restricted to asymmetrically self-renewing cells. Though first confirmed in murine cells, non-random segregation was originally proposed by Cairns as an important genetic fidelity mechanism in human tissues. We investigated human hepatic stem cells expanded by suppression of asymmetric cell kinetics (SACK) for evidence of non-random sister chromatid segregation. Cell kinetics and time-lapse microscopy analyses established that an ex vivo expanded human hepatic stem cell strain possessed SACK agent-suppressible asymmetric cell kinetics. Complementary DNA strand-labeling experiments revealed that cells in hepatic stem cell cultures segregated sister chromatids non-randomly. The number of cells cosegregating sister chromatids with the oldest “immortal DNA strands” was greater under conditions that increased asymmetric self-renewal kinetics. Detection of this mechanism in a human tissue stem cell strain increases support for Cairns’ proposal that non-random sister chromatid segregation operates in human tissue stem cells to limit carcinogenesis.

Segregation Lag in Polyploid Cells of the Pathogen Genus Borrelia: Implications for Antigenic Variation

Relapsing fever agents like Borrelia hermsii undergo multiphasic antigenic variation that is attributable to spontaneous DNA non-reciprocal transpositions at a particular locus in the genome. This genetic switch results in a new protein being expressed on the cell surface, allowing cells with that phenotype to escape prevailing immunity. But the switch occurs in only one of several genomes in these spirochetes, and a newly-switched gene is effectively "recessive" until homozygosity is achieved. The longer that descendants of the switched cell expressed both old and new proteins, the longer this lineage risks neutralization by antibody to the old protein. We investigated the implications for antigenic variation of the phenotypic lag that polyploidy would confer on cells. We first experimentally determined the average genome copy number in daughter cells after division during mouse infection with B. hermsii strain HS1. We then applied discrete deterministic and stochastic simulations to predict outcomes when genomes were equably segregated either linearly, i.e. according to their position in one-dimensional arrays, or randomly partitioned, as for a sphere. Linear segregation replication provided for a lag in achievement of homozygosity that was significantly shorter than could be achieved under the random segregation condition. For cells with 16 genomes, this would be a 4-generation lag. A model incorporating the immune response and evolved matrices of switch rates indicated a greater fitness for polyploid over monoploid bacteria in terms of duration of infection.

Asymmetric cell division and template DNA co-segregation in cancer stem cells

During tissue homeostasis, normal stem cells self-renew and repopulate the diverse cell types found within the tissue via a series of carefully controlled symmetric and asymmetric cell divisions (ACDs). The notion that solid tumors comprise a subset of cancer stem cells (CSCs) with dysregulated self-renewal and excessive symmetric cell divisions has led to numerous studies aimed to elucidate the mechanisms regulating ACD under steady-state conditions, during stem-cell expansion and in cancer. In this perspective, we focus on a type of asymmetry that can be established during ACD, called non-random co-segregation of template DNA, which has been identified across numerous species, cell types, and cancers. We discuss the role of p53 loss in maintaining self-renewal in both normal and malignant cells. We then review our current knowledge of the mechanisms underlying co-segregation of template DNA strands and the stem-cell pathways associated with it in normal and CSCs.

Nonrandom template segregation: a way to break the symmetry of stem cells

Whether DNA segregates in a biased way has been a subject of intense controversy and debate. Although highly provocative in its biological implications, if true, technical problems have limited researchers from drawing firm conclusions from the data. Elabd et al. (2013. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201307110/DC1) now show a high frequency of nonrandom template segregation during differentiation of embryonic stem cells using rigorous experimentation and implicate the methyltransferase Dnmt3 as a key regulator of this process.

The chromosome cycle of prokaryotes

In both eukaryotes and prokaryotes, chromosomal DNA undergoes replication, condensation-decondensation and segregation, sequentially, in some fixed order. Other conditions, like sister-chromatid cohesion (SCC), may span several chromosomal events. One set of these chromosomal transactions within a single cell cycle constitutes the 'chromosome cycle'. For many years it was generally assumed that the prokaryotic chromosome cycle follows major phases of the eukaryotic one: -replication-condensation-segregation-(cell division)-decondensation-, with SCC of unspecified length. Eventually it became evident that, in contrast to the strictly consecutive chromosome cycle of eukaryotes, all stages of the prokaryotic chromosome cycle run concurrently. Thus, prokaryotes practice 'progressive' chromosome segregation separated from replication by a brief SCC, and all three transactions move along the chromosome at the same fast rate. In other words, in addition to replication forks, there are 'segregation forks' in prokaryotic chromosomes. Moreover, the bulk of prokaryotic DNA outside the replication-segregation transition stays compacted. I consider possible origins of this concurrent replication-segregation and outline the 'nucleoid administration' system that organizes the dynamic part of the prokaryotic chromosome cycle.

Higher 5-hydroxymethylcytosine identifies immortal DNA strand chromosomes in asymmetrically self-renewing distributed stem cells

Immortal strands are the targeted chromosomal DNA strands of nonrandom sister chromatid segregation, a mitotic chromosome segregation pattern unique to asymmetrically self-renewing distributed stem cells (DSCs). By nonrandom segregation, immortal DNA strands become the oldest DNA strands in asymmetrically self-renewing DSCs. Nonrandom segregation of immortal DNA strands may limit DSC mutagenesis, preserve DSC fate, and contribute to DSC aging. The mechanisms responsible for specification and maintenance of immortal DNA strands are unknown. To discover clues to these mechanisms, we investigated the 5-methylcytosine and 5-hydroxymethylcytosine (5hmC) content on chromosomes in mouse hair follicle DSCs during nonrandom segregation. Although 5-methylcytosine content did not differ significantly, the relative content of 5hmC was significantly higher in chromosomes containing immortal DNA strands than in opposed mitotic chromosomes containing younger mortal DNA strands. The difference in relative 5hmC content was caused by the loss of 5hmC from mortal chromosomes. These findings implicate higher 5hmC as a specific molecular determinant of immortal DNA strand chromosomes. Because 5hmC is an intermediate during DNA demethylation, we propose a ten-eleven translocase enzyme mechanism for both the specification and maintenance of nonrandomly segregated immortal DNA strands. The proposed mechanism reveals a means by which DSCs "know" the generational age of immortal DNA strands. The mechanism is supported by molecular expression data and accounts for the selection of newly replicated DNA strands when nonrandom segregation is initiated. These mechanistic insights also provide a possible basis for another characteristic property of immortal DNA strands, their guanine ribonucleotide dependency.

Implication of the Strand-Specific Imprinting and Segregation Model: Integrating in utero Hormone Exposure, Stem Cell and Lateral Asymmetry Hypotheses in Breast Cancer Aetiology

Known genetic mutations and familial hereditary factors account for less than 20–25% of breast cancer cases in women, therefore, most instances have been classified as sporadic cases of unknown aetiologies. Single nucleotide polymorphisms (SNPs) were considered as breast cancer risk factors, but numerous studies have failed to support this assertion. Recent evidence correlates aberrant epigenetic mechanisms in the development and metastatic progression of breast cancer, yet there has been limited progress made to identify the primary aetiology underlying sporadic cases of breast cancer. This has led some researchers to consider alternative hypotheses including in utero exposure to deleterious chemical agents during early development, the immortal strand and the strand-specific imprinting and selective chromatid segregation hypotheses. Here, we integrate prominent alternate models to help guide future research on this very important topic concerning human health.