Geminin, DNA replication inhibitor, also known as GMNN, is a protein in humans encoded by the GMNN gene. [5] A nuclear protein present in most eukaryotes and highly conserved across species, numerous functions have been elucidated for geminin including roles in metazoan cell cycle, cellular proliferation, cell lineage commitment, and neural differentiation. [6] One example of its function is the inhibition of Cdt1. [7]
Geminin was originally identified as an inhibitor of DNA replication and substrate of the anaphase-promoting complex. [8] Coincidentally, geminin was also shown to expand the neural plate in the developing Xenopus embryo. [9]
Geminin is a nuclear protein made up of about 200 amino acids, with a molecular weight of approximately 25 kDa. [8] It contains an atypical leucine zipper coiled-coil domain. It has no known enzymatic activity nor DNA binding motifs.
Geminin is absent during G1 phase and accumulates through S, G2 phase and M phases of the cell cycle. Geminin levels drop at the metaphase-anaphase transition of mitosis when it is degraded by the anaphase-promoting complex. [8]
During S phase, geminin is a negative regulator of DNA replication. In many cancer cell lines, inhibition of geminin by RNA interference results in re-replication of portions of the genome, which leads to aneuploidy. In these cell lines, geminin knockdown leads to markedly slowed growth and apoptosis within several days. [10] However, the same is not true for primary and immortalized human cell lines, where other mechanisms exists to prevent DNA re-replication. [10] Since geminin knockdown leads to cell death in many cancer cell lines but not primary cell lines, it has been proposed as a potential therapeutic target for cancer treatment. [10]
At the start of the S-phase until late mitosis, geminin inhibits the replication factor Cdt1, preventing the assembly of the pre-replication complex. In early G1, the anaphase promoting complex triggers its destruction through ubiquitination.
Geminin, therefore, is an important player in ensuring that exactly one round of replication occurs during each cell cycle.
Geminin promotes early neural fate commitment by hyperacetylating chromatin. [11] This effect allows neural genes to be accessible for transcription, promoting the expression of these genes. Ultimately, geminin allows cells uncommitted to any particular lineage to acquire neural characteristics.
Geminin has also been shown to interact with the SWI/SNF chromatin remodeling complex. [12] In neural precursor cells, high levels of geminin prevent terminal differentiation. When the interaction between geminin and SWI/SNF is eliminated, geminin's inhibition to this process is eliminated and neural precursors are allowed to differentiate.
| Characteristic | Phenotype |
|---|---|
| Homozygote viability | Abnormal |
| Recessive lethal study | Abnormal |
| Fertility | Normal |
| Body weight | Normal |
| Anxiety | Normal |
| Neurological assessment | Normal |
| Grip strength | Normal |
| Hot plate | Normal |
| Dysmorphology | Normal |
| Indirect calorimetry | Normal |
| Glucose tolerance test | Normal |
| Auditory brainstem response | Normal |
| DEXA | Normal |
| Radiography | Normal |
| Body temperature | Normal |
| Eye morphology | Abnormal [13] |
| Clinical chemistry | Normal |
| Plasma immunoglobulins | Normal |
| Haematology | Normal |
| Peripheral blood lymphocytes | Normal |
| Micronucleus test | Normal |
| Heart weight | Normal |
| Skin Histopathology | Normal |
| Brain histopathology | Normal |
| Salmonella infection | Normal [14] |
| Citrobacter infection | Normal [15] |
| All tests and analysis from [16] [17] |
Model organisms have been used in the study of geminin function. A conditional knockout mouse line, called Gmnntm1a(KOMP)Wtsi [18] [19] was generated as part of the International Knockout Mouse Consortium program, a high-throughput mutagenesis project to generate and distribute animal models of disease to interested scientists. [20] [21] [22]
In addition, increased genomic instability and tumorigenesis have been observed in geminin knockout mice in both the colon and lung. [23]
Male and female animals underwent a standardized phenotypic screen to determine the effects of deletion. [16] [24] Twenty-six tests were carried out and three significant abnormalities were observed. A recessive lethal study found no homozygous mutant embryos during gestation, and therefore none survived until weaning. The remaining tests were carried out on heterozygous mutant adult mice and showed that females had abnormal lens morphology and cataracts. [16]
Geminin has been found to be overexpressed in several malignancies and cancer cell lines, [25] while there is data demonstrating that geminin acts as a tumor suppressor by safeguarding genome stability. [23]
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that causes it to divide into two daughter cells. These events include the duplication of its DNA and some of its organelles, and subsequently the partitioning of its cytoplasm, chromosomes and other components into two daughter cells in a process called cell division.
In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part of biological inheritance. This is essential for cell division during growth and repair of damaged tissues, while it also ensures that each of the new cells receives its own copy of the DNA. The cell possesses the distinctive property of division, which makes replication of DNA essential.
Anaphase-promoting complex is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cullin (Apc2) and RING (Apc11) subunit much like SCF. Other parts of the APC/C have unknown functions but are highly conserved.
Endoreduplication is replication of the nuclear genome in the absence of mitosis, which leads to elevated nuclear gene content and polyploidy. Endoreplication 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. Examples of endoreplication characterized in arthropod, mammalian, and plant species suggest that it is a universal developmental mechanism responsible for the differentiation and morphogenesis of cell types that fulfill an array of biological functions. While endoreplication 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.
DNA replication licensing factor MCM6 is a protein that in humans is encoded by the MCM6 gene. MCM6 is one of the highly conserved mini-chromosome maintenance proteins (MCM) that are essential for the initiation of eukaryotic genome replication.
Antigen KI-67, also known as Ki-67, Ki-67 or MKI67, is a protein that in humans is encoded by the MKI67 gene.
Aurora kinase B is a protein that functions in the attachment of the mitotic spindle to the centromere.
Eukaryotic DNA replication is a conserved mechanism that restricts DNA replication to once per cell cycle. Eukaryotic DNA replication of chromosomal DNA is central for the duplication of a cell and is necessary for the maintenance of the eukaryotic genome.
The cell division cycle protein 20 homolog is an essential regulator of cell division that is encoded by the CDC20 gene in humans. To the best of current knowledge its most important function is to activate the anaphase promoting complex (APC/C), a large 11-13 subunit complex that initiates chromatid separation and entrance into anaphase. The APC/CCdc20 protein complex has two main downstream targets. Firstly, it targets securin for destruction, enabling the eventual destruction of cohesin and thus sister chromatid separation. It also targets S and M-phase (S/M) cyclins for destruction, which inactivates S/M cyclin-dependent kinases (Cdks) and allows the cell to exit from mitosis. A closely related protein, Cdc20homologue-1 (Cdh1) plays a complementary role in the cell cycle.
Cullin-4A is a protein that in humans is encoded by the CUL4A gene. CUL4A belongs to the cullin family of ubiquitin ligase proteins and is highly homologous to the CUL4B protein. CUL4A regulates numerous key processes such as DNA repair, chromatin remodeling, spermatogenesis, haematopoiesis and the mitotic cell cycle. As a result, CUL4A has been implicated in several cancers and the pathogenesis of certain viruses including HIV. A component of a CUL4A complex, Cereblon, was discovered to be a major target of the teratogenic agent thalidomide.
CDT1 is a protein that in humans is encoded by the CDT1 gene. It is a licensing factor that functions to limit DNA from replicating more than once per cell cycle.
Cell division cycle 7-related protein kinase is an enzyme that in humans is encoded by the CDC7 gene. The Cdc7 kinase is involved in regulation of the cell cycle at the point of chromosomal DNA replication. The gene CDC7 appears to be conserved throughout eukaryotic evolution; this means that most eukaryotic cells have the Cdc7 kinase protein.
Chromatin assembly factor 1 subunit B is a protein that in humans is encoded by the CHAF1B gene.
Fizzy-related protein homolog, also known as hCDH1, is a protein that in humans is encoded by the FZR1 gene.
Histone acetyltransferase KAT7 is an enzyme that in humans is encoded by the KAT7 gene. It specifically acetylates H4 histones at the lysine12 residue (H4K12) and is necessary for origin licensing and DNA replication. KAT7 associates with origins of replication during G1 phase of the cell cycle through complexing with CDT1. Geminin is thought to inhibit the acetyltransferase activity of KAT7 when KAT7 and CDT1 are complexed together.
Cdc14 and Cdc14 are a gene and its protein product respectively. Cdc14 is found in most of the eukaryotes. Cdc14 was defined by Hartwell in his famous screen for loci that control the cell cycle of Saccharomyces cerevisiae. Cdc14 was later shown to encode a protein phosphatase. Cdc14 is dual-specificity, which means it has serine/threonine and tyrosine-directed activity. A preference for serines next to proline is reported. Many early studies, especially in the budding yeast Saccharomyces cerevisiae, demonstrated that the protein plays a key role in regulating late mitotic processes. However, more recent work in a range of systems suggests that its cellular function is more complex.
In cell biology, eukaryotes possess a regulatory system that ensures that DNA replication occurs only once per cell cycle.
DNA re-replication is an undesirable and possibly fatal occurrence in eukaryotic cells in which the genome is replicated more than once per cell cycle. Rereplication is believed to lead to genomic instability and has been implicated in the pathologies of a variety of human cancers. To prevent rereplication, eukaryotic cells have evolved multiple, overlapping mechanisms to inhibit chromosomal DNA from being partially or fully rereplicated in a given cell cycle. These control mechanisms rely on cyclin-dependent kinase (CDK) activity. DNA replication control mechanisms cooperate to prevent the relicensing of replication origins and to activate cell cycle and DNA damage checkpoints. DNA rereplication must be strictly regulated to ensure that genomic information is faithfully transmitted through successive generations.
Induced cell cycle arrest is the use of a chemical or genetic manipulation to artificially halt progression through the cell cycle. Cellular processes like genome duplication and cell division stop. It can be temporary or permanent. It is an artificial activation of naturally occurring cell cycle checkpoints, induced by exogenous stimuli controlled by an experimenter.
Kristen Kroll is an American developmental and stem cell biologist and Professor of Developmental Biology at Washington University School of Medicine. Her laboratory studies transcriptional and epigenetic regulation of brain development and its disruption to cause neurodevelopmental disorders.
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