Endoreduplication (also referred to as endoreplication or endocycling) is replication of the nuclear
genome in the absence of
mitosis, which leads to elevated nuclear
gene content and
polyploidy. Endoreduplication can be understood simply as a variant form of the mitotic
cell cycle (G1-S-G2-M) in which
mitosis is circumvented entirely, due to modulation of
cyclin-dependent kinase (CDK) activity.[1][2][3][4] Examples of endoreduplication characterised in
arthropod,
mammalian, and
plantspecies suggest that it is a universal developmental mechanism responsible for the differentiation and
morphogenesis of cell types that fulfill an array of
biological functions.[1][2] While endoreduplication is often limited to specific cell types in animals, it is considerably more widespread in plants, such that
polyploidy can be detected in the majority of plant tissues.[5] Polyploidy and aneuploidy are common phenomena in cancer cells.[6] Given that oncogenesis and endoreduplication likely involve subversion of common cell cycle regulatory mechanisms, a thorough understanding of endoreduplication may provide important insights for cancer biology.
Examples in nature
Endoreduplicating cell types that have been studied extensively in
model organisms
Endoreduplication, endomitosis and polytenization are three different processes resulting in polyploidization of a cell in a regulated manner. In endoreduplication cells skip
M phase completely by exiting the mitotic cell cycle in the G2 phase after completing the S phase several times, resulting in a mononucleated
polyploid cell. The cell ends up with twice as many copies of each chromosome per repeat of the S phase.[17] Endomitosis is a type of cell cycle variation where mitosis is initiated, but stopped during anaphase and thus cytokinesis is not completed. The cell ends up with multiple nuclei in contrast to a cell undergoing endoreduplication.[17][18] Therefore depending on how far the cell progresses through mitosis, this will give rise to a mononucleated or
binucleated polyploid cell. Polytenization arises with under- or overamplification of some genomic regions, creating
polytene chromosomes.[3][4]
Biological significance
Based on the wide array of cell types in which endoreduplication occurs, a variety of hypotheses have been generated to explain the functional importance of this phenomenon.[1][2] Unfortunately, experimental evidence to support these conclusions is somewhat limited.
Cell differentiation
In developing plant tissues the transition from mitosis to endoreduplication often coincides with cell differentiation and
morphogenesis.[19] However it remains to be determined whether endoreduplication and
polyploidy contribute to cell differentiation or vice versa. Targeted inhibition of endoreduplication in
trichome progenitors results in the production of multicellular trichomes that exhibit relatively normal morphology, but ultimately dedifferentiate and undergo absorption into the
leaf epidermis.[20] This result suggests that endoreduplication and polyploidy may be required for the maintenance of cell identity.
Cell/organism size
Cell
ploidy often correlates with cell size,[13][15] and in some instances, disruption of endoreduplication results in diminished cell and tissue size [21] suggesting that endoreduplication may serve as a mechanism for tissue growth. Relative to mitosis, endoreduplication does not require
cytoskeletal rearrangement or the production of new
cell membrane and it often occurs in cells that have already differentiated. As such it may represent an energetically efficient alternative to
cell proliferation among differentiated cell types that can no longer afford to undergo mitosis.[22] While evidence establishing a connection between ploidy and tissue size is prevalent in the literature, contrary examples also exist.[19]
Oogenesis and embryonic development
Endoreduplication is commonly observed in
cells responsible for the nourishment and protection of
oocytes and
embryos. It has been suggested that increased gene copy number might allow for the mass production of proteins required to meet the metabolic demands of
embryogenesis and early development.[1] Consistent with this notion, mutation of the
Myconcogene in Drosophilafollicle cells results in reduced endoreduplication and abortive
oogenesis.[23] However, reduction of endoreduplication in maize
endosperm has limited effect on the accumulation of
starch and storage
proteins, suggesting that the nutritional requirements of the developing embryo may involve the
nucleotides that comprise the
polyploid genome rather than the proteins it encodes.[24]
Buffering the genome
Another hypothesis is that endoreduplication buffers against
DNA damage and
mutation because it provides extra copies of important
genes.[1] However, this notion is purely speculative and there is limited evidence to the contrary. For example, analysis of polyploid
yeast strains suggests that they are more sensitive to
radiation than
diploid strains.[25]
Stress response
Research in plants suggests that endoreduplication may also play a role in modulating stress responses. By manipulating expression of
E2fe (a repressor of endocycling in plants), researchers were able to demonstrate that increased cell ploidy lessens the negative impact of drought stress on leaf size.[26] Given that the sessile lifestyle of plants necessitates a capacity to adapt to environmental conditions, it is appealing to speculate that widespread polyploidization contributes to their developmental plasticity
Genetic control of endoreplication
The best-studied example of a mitosis-to-endoreduplication transition occurs in Drosophila follicle cells and is activated by
Notch signaling.[27] Entry into endoreduplication involves modulation of
mitotic and
S-phasecyclin-dependent kinase (CDK) activity.[28] Inhibition of
M-phase CDK activity is accomplished via transcriptional activation of
Cdh/
fzr and repression of the G2-M regulator string/
cdc25.[28][29] Cdh/fzr is responsible for activation of the
anaphase-promoting complex (APC) and subsequent
proteolysis of the
mitoticcyclins. String/cdc25 is a
phosphatase that stimulates mitotic cyclin-CDK complex activity. Upregulation of S-phase CDK activity is accomplished via
transcriptional repression of the inhibitory
kinase dacapo. Together, these changes allow for the circumvention of mitotic entry, progression through
G1, and entry into
S-phase. The induction of
endomitosis in mammalian
megakaryocytes involves activation of the
c-mpl receptor by the
thrombopoietin (TPO)
cytokine and is mediated by
ERK1/2 signaling.[30] As with Drosophila follicle cells, endoreduplication in megakaryocytes results from activation of
S-phase cyclin-CDK complexes and inhibition of mitotic cyclin-CDK activity.[31][32]
Entry into
S-phase during endoreduplication (and mitosis) is regulated through the formation of a
prereplicative complex (pre-RC) at
replication origins, followed by recruitment and activation of the
DNA replication machinery. In the context of endoreduplication these events are facilitated by an oscillation in
cyclin E-
Cdk2 activity. Cyclin E-Cdk2 activity drives the recruitment and activation of the replication machinery,[33] but it also inhibits pre-RC formation,[34] presumably to ensure that only one round of replication occurs per cycle. Failure to maintain control over pre-RC formation at replication origins results in a phenomenon known as “
rereplication” which is common in cancer cells.[2] The mechanism by which cyclin E-Cdk2 inhibits pre-RC formation involves downregulation of
APC-
Cdh1-mediated proteolysis and accumulation of the protein
Geminin, which is responsible for sequestration of the pre-RC component
Cdt1.[35][36]
Oscillations in
Cyclin E-
Cdk2 activity are modulated via
transcriptional and post-transcriptional mechanisms. Expression of cyclin E is activated by
E2F transcription factors that were shown to be required for endoreduplication.[37][38][39] Recent work suggests that observed oscillations in E2F and cyclin E protein levels result from a
negative-feedback loop involving
Cul4-dependent
ubiquitination and degradation of E2F.[40] Post-transcriptional regulation of cyclin E-Cdk2 activity involves
Ago/Fbw7-mediated proteolytic degradation of cyclin E [41][42] and direct inhibition by factors such as Dacapo and
p57.[43][44]
Premeiotic endomitosis in unisexual vertebrates
The unisexual salamanders (genus Ambystoma) are the oldest known unisexual vertebrate lineage, having arisen about 5 million years ago.[45] In these polyploid unisexual females, an extra premeiotic endomitotic replication of the genome, doubles the number of chromosomes.[46] As a result, the mature eggs that are produced subsequent to the two meiotic divisions have the same ploidy as the somatic cells of the adult female salamander. Synapsis and recombination during meiotic prophase I in these unisexual females is thought to ordinarily occur between identical sister chromosomes and occasionally between homologous chromosomes. Thus little, if any, genetic variation is produced. Recombination between homeologous chromosomes occurs rarely, if at all.[46]
^Storchova Z, Pellman D (2004). "From polyploidy to aneuploidy, genome instability and cancer". Nature Reviews Molecular Cell Biology. 5 (1): 45–54.
doi:
10.1038/nrm1276.
PMID14708009.
S2CID11985415.
^Hammond MP, Laird CD (1985). "Control of DNA replication and spatial distribution of defined DNA sequences in salivary gland cells of Drosophila melanogaster". Chromosoma. 91 (3–4): 279–286.
doi:
10.1007/BF00328223.
PMID3920018.
S2CID1515555.
^Hammond MP, Laird CD (1985). "Chromosome structure and DNA replication in nurse and follicle cells of Drosophila melanogaster". Chromosoma. 91 (3–4): 267–278.
doi:
10.1007/BF00328222.
PMID3920017.
S2CID7919061.
^Ravid K, Lu J, Zimmet JM, Jones MR (2002). "Roads to polyploidy: The megakaryocyte example". Journal of Cellular Physiology. 190 (1): 7–20.
doi:
10.1002/jcp.10035.
PMID11807806.
S2CID37297740.
^Hulskamp M, Schnittger A, Folkers U (1999). Pattern formation and cell differentiation: Trichomes in Arabidopsis as a genetic model system. International Review of Cytology. Vol. 186. pp. 147–178.
doi:
10.1016/S0074-7696(08)61053-0.
ISBN978-0-12-364590-6.
PMID9770299.
^Garcia P, Cales C (1996). "Endoreplication in megakaryoblastic cell lines is accompanied by sustained expression of G1/S cyclins and downregulation of cdc25c". Oncogene. 13 (4): 695–703.
PMID8761290.
^Moberg KH, Bell DW, Wahrer DC, Haber DA, Hariharan IK (2001). "Archipelago regulates cyclin E levels in Drosophila and is mutated in human cancer lines". Nature. 413 (6853): 311–6.
doi:
10.1038/35095068.
PMID11565033.
S2CID4372821.