Clastogen

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Figure comparing the effects of exposure to genotoxic agents (aneugens and clastogens) on DNA. Aneugens induce mis-segregation of chromosomes into daughter cells while clastogens break the DNA and chromosome. Clastogen vs aneugen better quality.png
Figure comparing the effects of exposure to genotoxic agents (aneugens and clastogens) on DNA. Aneugens induce mis-segregation of chromosomes into daughter cells while clastogens break the DNA and chromosome.

A clastogen is a mutagenic agent that disturbs normal DNA related processes or directly causes DNA strand breakages, thus causing the deletion, insertion, or rearrangement of entire chromosome sections. [1] These processes are a form of mutagenesis which if left unrepaired, or improperly repaired, can lead to cancer. [1] Known clastogens include acridine yellow, benzene, ethylene oxide, arsenic, phosphine, mimosine, actinomycin D, camptothecin, methotrexate, methyl acrylate, resorcinol and 5-fluorodeoxyuridine. [2] Additionally, 1,2-dimethylhydrazine is a known colon carcinogen and shows signs of possessing clastogenic activity. [3] There are many clastogens not listed here and research is ongoing to discover new clastogens. Some known clastogens only exhibit clastogenic activity in certain cell types, such as caffeine which exhibits clastogenic activity in plant cells. [4] Researchers are interested in clastogens for researching cancer, as well as for other human health concerns such as the inheritability of clastogen effected paternal germ cells that lead to fetus developmental defects. [5]

Contents

Mechanism

Summary of theories of the mechanisms of chromosomal aberrations: A, 'classic' breaks theory; B, 'mis-repair of breaks' theory; C, 'repair-created breaks' theory. Adapted from Bignold. Theories for clastogen mechanisms.png
Summary of theories of the mechanisms of chromosomal aberrations: A, ‘classic’ breaks theory; B, ‘mis-repair of breaks’ theory; C, ‘repair-created breaks’ theory. Adapted from Bignold.

There is not one all encompassing method by which clastogens damage chromosomal DNA, instead different clastogens have unique ways they interact with DNA, or DNA associated proteins, and disrupt normal function. Broadly these different types of clastogenic activity can be organized into three classes: ‘classic’ breaks theory; ‘mis-repair of breaks’ theory and ‘repair-created breaks’ theory. [4] It may not always be known how a clastogen causes chromosomal damage.

Radiation was the earliest known clastogen that caused direct DNA damage, following the classic breaks theory. [6] DNA is frequently damaged and there are many DNA repair pathways that combat this, but repair does not always work perfectly resulting in mistakes (called a misrepair). [7] A widely studied class of clastogens are alkylating agents which do not break DNA at all, but instead form DNA adducts, and these have often eluded the common theories for DNA breaks leading to misrepair. [4] The final theory encompasses clastogens that do not interact with DNA but instead impair DNA synthesis proteins or DNA repair proteins causing damage to occur through loss of normal function of the protein. [4]

Clastogen damage in certain areas of the chromosome can lead to instability, such as loss or damage to telomeres. [8] Studies have shown that rat cells that were exposed to chemical clastogens express telomeric irregularities in function and can remain for several cell generations after treatment has been attempted. [8]

Detection

There are many different methods for testing for clastogenic activity. Two of the most common methods are listed below, but this is not a comprehensive guide.

There have been studies done that work with the usage of the deletion (DEL) assay to screen for clastogens.

The micronucleus test is another type of assay that uses gut cells to observe clastogens, and there are a few different types. The micronucleus test on gut cells is useful because in the case of the bone marrow micronucleus test there is not much activity seen after there has been oral exposure therefore more activity is seen in the gut cells. In vitro micronucleus assay (IVMN) can screen for clastogen activity, this method is useful because it can pick up clastogen activity and be used to foresee chromosome aberration activity. The IVMN assay can pick up on fragments that were membrane bound to DNA that were split from nuclei throughout the process of cell division.

These assays are time-consuming so novel methods for monitoring clastogens and aneuploidogens are highly desirable. One example is the use of the monochromosomal hybrid cell for the detection of mis-segregating chromosomes.

Telomeres

There is a possibility of clastogens affecting telomeres. There can be uncertainty with telomeres that occur short term during the first round of cell division in which there can be chromosomal damage by clastogens. Clastogens (which break chromosomes) contribute to telomeric instability because it leads to chromosome end loss or true telomere loss. Clastogens can bring on issues with telomeres and cause them to fail to function as intended, most often seen anomalies are seen to occur in human lymphocytes, cancer cell lines, and non-human established cell lines where there is telomere loss and copies of anomalies in the exposed cells, thus, the problems that arise in telomeres can be duplicated and seen in exposed cells.

In addition, studies have shown that rat cells that were exposed to chemical clastogens express telomeric irregularities in function and can remain for several cell generations after treatment has been attempted. [8]

Research

In terms of resistance, for a specific clastogen known as "Zeocin", an amino acid residue known as XLF-L115D mutant is flawed in terms of being resistant thus the clastogen activity shows no amount of decreasing. [9]

In plants and mice cells studies have found that purine receptor agonists adenosine, ATP, ADP, cyclohexyladenosine, phenylisopropyladenosine and dimethylaminopurine riboside can lower the amount of clastogen damage seen in chromosomes and reduce the amount of micronuclei affected brought on by ethylmethane sulfonate and cyclophosphamide. Some ligands more than others can stop or reduce the clastogen activity of ethylmethane sulfonate such as adenosine, ADP or DAP. [10]

In a study where rats were treated with Brevetoxin B (PbTx2), there was a noticeable 2-3 fold growth in the amount of DNA seen in comet tails which tell us that Brevetoxin B shows in vivo clastogenic activity. This clastogen activity was seen after Brevetoxin B was injected by way of intratracheal administering in the rat. [11]

Related Research Articles

<span class="mw-page-title-main">Telomere</span> Region of repetitive nucleotide sequences on chromosomes

A telomere is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Telomeres are a widespread genetic feature most commonly found in eukaryotes. In most, if not all species possessing them, they protect the terminal regions of chromosomal DNA from progressive degradation and ensure the integrity of linear chromosomes by preventing DNA repair systems from mistaking the very ends of the DNA strand for a double-strand break.

<span class="mw-page-title-main">Werner syndrome</span> Medical condition

Werner syndrome (WS) or Werner's syndrome, also known as "adult progeria", is a rare, autosomal recessive disorder which is characterized by the appearance of premature aging.

Genotoxicity is the property of chemical agents that damage the genetic information within a cell causing mutations, which may lead to cancer. While genotoxicity is often confused with mutagenicity, all mutagens are genotoxic, but some genotoxic substances are not mutagenic. The alteration can have direct or indirect effects on the DNA: the induction of mutations, mistimed event activation, and direct DNA damage leading to mutations. The permanent, heritable changes can affect either somatic cells of the organism or germ cells to be passed on to future generations. Cells prevent expression of the genotoxic mutation by either DNA repair or apoptosis; however, the damage may not always be fixed leading to mutagenesis.

<span class="mw-page-title-main">Homologous recombination</span> Genetic recombination between identical or highly similar strands of genetic material

Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids.

<span class="mw-page-title-main">Micronucleus</span>

Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal instability. Micronuclei are commonly seen in cancerous cells and may indicate genomic damage events that can increase the risk of developmental or degenerative diseases. Micronuclei form during anaphase from lagging acentric chromosome or chromatid fragments caused by incorrectly repaired or unrepaired DNA breaks or by nondisjunction of chromosomes. This incorrect segregation of chromosomes may result from hypomethylation of repeat sequences present in pericentromeric DNA, irregularities in kinetochore proteins or their assembly, dysfunctional spindle apparatus, or flawed anaphase checkpoint genes. Micronuclei can contribute to genome instability by promoting a catastrophic mutational event called chromothripsis. Many micronucleus assays have been developed to test for the presence of these structures and determine their frequency in cells exposed to certain chemicals or subjected to stressful conditions.

<span class="mw-page-title-main">Ku (protein)</span>

Ku is a dimeric protein complex that binds to DNA double-strand break ends and is required for the non-homologous end joining (NHEJ) pathway of DNA repair. Ku is evolutionarily conserved from bacteria to humans. The ancestral bacterial Ku is a homodimer. Eukaryotic Ku is a heterodimer of two polypeptides, Ku70 (XRCC6) and Ku80 (XRCC5), so named because the molecular weight of the human Ku proteins is around 70 kDa and 80 kDa. The two Ku subunits form a basket-shaped structure that threads onto the DNA end. Once bound, Ku can slide down the DNA strand, allowing more Ku molecules to thread onto the end. In higher eukaryotes, Ku forms a complex with the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to form the full DNA-dependent protein kinase, DNA-PK. Ku is thought to function as a molecular scaffold to which other proteins involved in NHEJ can bind, orienting the double-strand break for ligation.

<span class="mw-page-title-main">Telomeric repeat-binding factor 2</span> Protein

Telomeric repeat-binding factor 2 is a protein that is present at telomeres throughout the cell cycle. It is also known as TERF2, TRF2, and TRBF2, and is encoded in humans by the TERF2 gene. It is a component of the shelterin nucleoprotein complex and a second negative regulator of telomere length, playing a key role in the protective activity of telomeres. It was first reported in 1997 in the lab of Titia de Lange, where a DNA sequence similar, but not identical, to TERF1 was discovered, with respect to the Myb-domain. De Lange isolated the new Myb-containing protein sequence and called it TERF2.

<span class="mw-page-title-main">Telomeric repeat-binding factor 1</span> Protein-coding gene in humans

Telomeric repeat-binding factor 1 is a protein that in humans is encoded by the TERF1 gene.

The MRN complex is a protein complex consisting of Mre11, Rad50 and Nbs1. In eukaryotes, the MRN/X complex plays an important role in the initial processing of double-strand DNA breaks prior to repair by homologous recombination or non-homologous end joining. The MRN complex binds avidly to double-strand breaks both in vitro and in vivo and may serve to tether broken ends prior to repair by non-homologous end joining or to initiate DNA end resection prior to repair by homologous recombination. The MRN complex also participates in activating the checkpoint kinase ATM in response to DNA damage. Production of short single-strand oligonucleotides by Mre11 endonuclease activity has been implicated in ATM activation by the MRN complex.

Alternative Lengthening of Telomeres is a telomerase-independent mechanism by which cancer cells avoid the degradation of telomeres.

Quantitative Fluorescent in situ hybridization (Q-FISH) is a cytogenetic technique based on the traditional FISH methodology. In Q-FISH, the technique uses labelled synthetic DNA mimics called peptide nucleic acid (PNA) oligonucleotides to quantify target sequences in chromosomal DNA using fluorescent microscopy and analysis software. Q-FISH is most commonly used to study telomere length, which in vertebrates are repetitive hexameric sequences (TTAGGG) located at the distal end of chromosomes. Telomeres are necessary at chromosome ends to prevent DNA-damage responses as well as genome instability. To this day, the Q-FISH method continues to be utilized in the field of telomere research.

Telomere-binding proteins function to bind telomeric DNA in various species. In particular, telomere-binding protein refers to TTAGGG repeat binding factor-1 (TERF1) and TTAGGG repeat binding factor-2 (TERF2). Telomere sequences in humans are composed of TTAGGG sequences which provide protection and replication of chromosome ends to prevent degradation. Telomere-binding proteins can generate a T-loop to protect chromosome ends. TRFs are double-stranded proteins which are known to induce bending, looping, and pairing of DNA which aids in the formation of T-loops. They directly bind to TTAGGG repeat sequence in the DNA. There are also subtelomeric regions present for regulation. However, in humans, there are six subunits forming a complex known as shelterin.

<span class="mw-page-title-main">Chromothripsis</span> Massive chromosomal rearrangement process linked to cancer

Chromothripsis is a mutational process by which up to thousands of clustered chromosomal rearrangements occur in a single event in localised and confined genomic regions in one or a few chromosomes, and is known to be involved in both cancer and congenital diseases. It occurs through one massive genomic rearrangement during a single catastrophic event in the cell's history. It is believed that for the cell to be able to withstand such a destructive event, the occurrence of such an event must be the upper limit of what a cell can tolerate and survive. The chromothripsis phenomenon opposes the conventional theory that cancer is the gradual acquisition of genomic rearrangements and somatic mutations over time.

<span class="mw-page-title-main">Cancerous micronuclei</span>

Cancerous micronuclei is a type of micronucleus that is associated with cancerous cells.

Shelterin is a protein complex known to protect telomeres in many eukaryotes from DNA repair mechanisms, as well as to regulate telomerase activity. In mammals and other vertebrates, telomeric DNA consists of repeating double-stranded 5'-TTAGGG-3' (G-strand) sequences along with the 3'-AATCCC-5' (C-strand) complement, ending with a 50-400 nucleotide 3' (G-strand) overhang. Much of the final double-stranded portion of the telomere forms a T-loop (Telomere-loop) that is invaded by the 3' (G-strand) overhang to form a small D-loop (Displacement-loop).

Chromosomal instability (CIN) is a type of genomic instability in which chromosomes are unstable, such that either whole chromosomes or parts of chromosomes are duplicated or deleted. More specifically, CIN refers to the increase in rate of addition or loss of entire chromosomes or sections of them. The unequal distribution of DNA to daughter cells upon mitosis results in a failure to maintain euploidy leading to aneuploidy. In other words, the daughter cells do not have the same number of chromosomes as the cell they originated from. Chromosomal instability is the most common form of genetic instability and cause of aneuploidy.

<span class="mw-page-title-main">Titia de Lange</span> Dutch geneticist

Titia de Lange is the Director of the Anderson Center for Cancer Research, the Leon Hess professor and the head of Laboratory Cell Biology and Genetics at Rockefeller University.

Telomeres, the caps on the ends of eukaryotic chromosomes, play critical roles in cellular aging and cancer. An important facet to how telomeres function in these roles is their involvement in cell cycle regulation.

<span class="mw-page-title-main">DNA end resection</span> Biochemical process

DNA end resection, also called 5′–3′ degradation, is a biochemical process where the blunt end of a section of double-stranded DNA (dsDNA) is modified by cutting away some nucleotides from the 5' end to produce a 3' single-stranded sequence. The presence of a section of single-stranded DNA (ssDNA) allows the broken end of the DNA to line up accurately with a matching sequence, so that it can be accurately repaired.

<span class="mw-page-title-main">Jan Karlseder</span> Austrian molecular biologist

Jan Karlseder is an Austrian molecular biologist, a professor in the Molecular and Cellular Biology Laboratory, the Director of the Paul F. Glenn Center for Biology of Aging Research and the holder of the Donald and Darlene Shiley Chair at the Salk Institute for Biological Studies.

References

  1. 1 2 Schwab M, ed. (2011). "Clastogen". Encyclopedia of Cancer. Berlin, Heidelberg: Springer Berlin Heidelberg. p. 879. doi:10.1007/978-3-642-16483-5_1205. ISBN   978-3-642-16482-8.
  2. ↑ Kirpnick Z, Homiski M, Rubitski E, Repnevskaya M, Howlett N, Aubrecht J, Schiestl RH (April 2005). "Yeast DEL assay detects clastogens". Mutation Research. 582 (1–2): 116–134. doi:10.1016/j.mrgentox.2005.01.005. PMID   15781217.
  3. ↑ Vanhauwaert A, Vanparys P, Kirsch-Volders M (January 2001). "The in vivo gut micronucleus test detects clastogens and aneugens given by gavage". Mutagenesis. 16 (1): 39–50. doi: 10.1093/mutage/16.1.39 . PMID   11139597.
  4. 1 2 3 4 5 Bignold LP (March–June 2009). "Mechanisms of clastogen-induced chromosomal aberrations: a critical review and description of a model based on failures of tethering of DNA strand ends to strand-breaking enzymes". Mutation Research. 681 (2–3): 271–298. doi:10.1016/j.mrrev.2008.11.004. PMID   19103303.
  5. ↑ Wyrobek AJ, Schmid TE, Marchetti F (2005-03-01). "Relative susceptibilities of male germ cells to genetic defects induced by cancer chemotherapies". Journal of the National Cancer Institute. Monographs. 2005 (34): 31–35. doi:10.1093/jncimonographs/lgi001. PMID   15784819.
  6. ↑ Sax K (January 1940). "An Analysis of X-Ray Induced Chromosomal Aberrations in Tradescantia". Genetics. 25 (1): 41–68. doi:10.1093/genetics/25.1.41. PMC   1209078 . PMID   17246957.
  7. ↑ Rothkamm K, Löbrich M (August 2002). "Misrepair of radiation-induced DNA double-strand breaks and its relevance for tumorigenesis and cancer treatment (review)". International Journal of Oncology. 21 (2): 433–440. PMID   12118342.
  8. 1 2 3 Bolzán AD (December 2020). "Using telomeric chromosomal aberrations to evaluate clastogen-induced genomic instability in mammalian cells". Chromosome Research. 28 (3–4): 259–276. doi:10.1007/s10577-020-09641-2. PMID   32940874. S2CID   221768891.
  9. ↑ Bhargava R, Lopezcolorado FW, Tsai LJ, Stark JM (January 2020). "The canonical non-homologous end joining factor XLF promotes chromosomal deletion rearrangements in human cells". The Journal of Biological Chemistry. 295 (1): 125–137. doi: 10.1074/jbc.RA119.010421 . PMC   6952595 . PMID   31753920.
  10. ↑ Kharitonov VS, Semenov VV, Barabanshchikov BI (July 2001). "Purine receptor agonists protect the genome of plant and animal cells from clastogen damage". Bulletin of Experimental Biology and Medicine. 132 (1): 666–669. doi:10.1023/a:1012580328826. PMID   11687849. S2CID   19132027.
  11. ↑ Leighfield TA, Muha N, Ramsdell JS (November 2009). "Brevetoxin B is a clastogen in rats, but lacks mutagenic potential in the SP-98/100 Ames test". Toxicon. 54 (6): 851–856. doi:10.1016/j.toxicon.2009.06.018. PMID   19559041.
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