ATM serine/threonine kinase or Ataxia-telangiectasia mutated, symbol ATM, is a serine/threonine protein kinase that is recruited and activated by DNA double-strand breaks (canonical pathway), oxidative stress, topoisomerase cleavage complexes, splicing intermediates, R-loops and in some cases by single-strand DNA breaks. [5] It phosphorylates several key proteins that initiate activation of the DNA damage checkpoint, leading to cell cycle arrest, DNA repair or apoptosis. Several of these targets, including p53, CHK2, BRCA1, NBS1 and H2AX are tumor suppressors.
In 1995, the gene was discovered by Yosef Shiloh [6] who named its product ATM since he found that its mutations are responsible for the disorder ataxia–telangiectasia. [7] In 1998, the Shiloh and Kastan laboratories independently showed that ATM is a protein kinase whose activity is enhanced by DNA damage. [8] [9]
Throughout the cell cycle DNA is monitored for damage. Damages result from errors during replication, by-products of metabolism, general toxic drugs or ionizing radiation. The cell cycle has different DNA damage checkpoints, which inhibit the next or maintain the current cell cycle step. There are two main checkpoints, the G1/S and the G2/M, during the cell cycle, which preserve correct progression. ATM plays a role in cell cycle delay after DNA damage, especially after double-strand breaks (DSBs). [10] ATM is recruited to sites of double strand breaks by DSB sensor proteins, such as the MRN complex. After being recruited, it phosphorylates NBS1, along other DSB repair proteins. These modified mediator proteins then amplify the DNA damage signal, and transduce the signals to downstream effectors such as CHK2 and p53.
The ATM gene codes for a 350 kDa protein consisting of 3056 amino acids. [11] ATM belongs to the superfamily of phosphatidylinositol 3-kinase-related kinases (PIKKs). The PIKK superfamily comprises six Ser/Thr-protein kinases that show a sequence similarity to phosphatidylinositol 3-kinases (PI3Ks). This protein kinase family includes ATR (ATM- and RAD3-related), DNA-PKcs (DNA-dependent protein kinase catalytic subunit) and mTOR (mammalian target of rapamycin). Characteristic for ATM are five domains. These are from N-terminus to C-terminus the HEAT repeat domain, the FRAP-ATM-TRRAP (FAT) domain, the kinase domain (KD), the PIKK-regulatory domain (PRD) and the FAT-C-terminal (FATC) domain. The HEAT repeats directly bind to the C-terminus of NBS1. The FAT domain interacts with ATM's kinase domain to stabilize the C-terminus region of ATM itself. The KD domain resumes kinase activity, while the PRD and the FATC domain regulate it. The structure of ATM has been solved in several publications using cryo-EM. In the inactive form, the protein forms a homodimer. In the canonical pathway, ATM is activated by the MRN complex and autophosphorylation, forming active monomers capable of phosphorylating several hundred downstream targets. In the non-canonical pathway, e.g. through simulation by oxidative stress, the dimer can be activated by the formation of disulfide bonds. [12] The entire N-terminal domain together with the FAT domain are adopt an α-helical structure, which was initially predicted by sequence analysis. This α-helical structure forms a tertiary structure, which has a curved, tubular shape present for example in the Huntingtin protein, which also contains HEAT repeats. FATC is the C-terminal domain with a length of about 30 amino acids. It is highly conserved and consists of an α-helix. [13]
A complex of the three proteins MRE11, RAD50 and NBS1 (XRS2 in yeast), called the MRN complex in humans, recruits ATM to double strand breaks (DSBs) and holds the two ends together. ATM directly interacts with the NBS1 subunit and phosphorylates the histone variant H2AX on Ser139. [14] This phosphorylation generates binding sites for adaptor proteins with a BRCT domain. These adaptor proteins then recruit different factors including the effector protein kinase CHK2 and the tumor suppressor p53. The ATM-mediated DNA damage response consists of a rapid and a delayed response. The effector kinase CHK2 is phosphorylated and thereby activated by ATM. Activated CHK2 phosphorylates phosphatase CDC25A, which is degraded thereupon and can no longer dephosphorylate CDK1-cyclin B, resulting in cell-cycle arrest. If the DSB can not be repaired during this rapid response, ATM additionally phosphorylates MDM2 and p53 at Ser15. [9] p53 is also phosphorylated by the effector kinase CHK2. These phosphorylation events lead to stabilization and activation of p53 and subsequent transcription of numerous p53 target genes including CDK inhibitor p21 which lead to long-term cell-cycle arrest or even apoptosis. [15]
The protein kinase ATM may also be involved in mitochondrial homeostasis, as a regulator of mitochondrial autophagy (mitophagy) whereby old, dysfunctional mitochondria are removed. [16] Increased ATM activity also occurs in viral infection where ATM is activated early during dengue virus infection as part of autophagy induction and ER stress response. [17]
A functional MRN complex is required for ATM activation after DSBs. The complex functions upstream of ATM in mammalian cells and induces conformational changes that facilitate an increase in the affinity of ATM towards its substrates, such as CHK2 and p53. [10] Inactive ATM is present in the cells without DSBs as dimers or multimers. Upon DNA damage, ATM autophosphorylates on residue Ser1981. This phosphorylation provokes dissociation of ATM dimers, which is followed by the release of active ATM monomers. [18] Further autophosphorylation (of residues Ser367 and Ser1893) is required for normal activity of the ATM kinase. Activation of ATM by the MRN complex is preceded by at least two steps, i.e. recruitment of ATM to DSB ends by the mediator of DNA damage checkpoint protein 1 (MDC1) which binds to MRE11, and the subsequent stimulation of kinase activity with the NBS1 C-terminus. The three domains FAT, PRD and FATC are all involved in regulating the activity of the KD kinase domain. The FAT domain interacts with ATM's KD domain to stabilize the C-terminus region of ATM itself. The FATC domain is critical for kinase activity and highly sensitive to mutagenesis. It mediates protein-protein interaction for example with the histone acetyltransferase TIP60 (HIV-1 Tat interacting protein 60 kDa), which acetylates ATM on residue Lys3016. The acetylation occurs in the C-terminal half of the PRD domain and is required for ATM kinase activation and for its conversion into monomers. While deletion of the entire PRD domain abolishes the kinase activity of ATM, specific small deletions show no effect. [13]
People who carry a heterozygous ATM mutation have increased risk of mainly pancreatic cancer, prostate cancer, stomach cancer and invasive ductal carcinoma of the breast. [19] Homozygous ATM mutation confers the disease ataxia–telangiectasia (AT), a rare human disease characterized by cerebellar degeneration, extreme cellular sensitivity to radiation and a predisposition to cancer. All AT patients contain mutations in the ATM gene. Most other AT-like disorders are defective in genes encoding the MRN protein complex. One feature of the ATM protein is its rapid increase in kinase activity immediately following double-strand break formation. [20] [8] The phenotypic manifestation of AT is due to the broad range of substrates for the ATM kinase, involving DNA repair, apoptosis, G1/S, intra-S checkpoint and G2/M checkpoints, gene regulation, translation initiation, and telomere maintenance. [21] Therefore, a defect in ATM has severe consequences in repairing certain types of damage to DNA, and cancer may result from improper repair. AT patients have an increased risk for breast cancer that has been ascribed to ATM's interaction and phosphorylation of BRCA1 and its associated proteins following DNA damage. [22]
Mutations in the ATM gene are found at relatively low frequencies in sporadic cancers. According to COSMIC, the Catalogue Of Somatic Mutations In Cancer, the frequencies with which heterozygous mutations in ATM are found in common cancers include 0.7% in 713 ovarian cancers, 0.9% in central nervous system cancers, 1.9% in 1,120 breast cancers, 2.1% in 847 kidney cancers, 4.6% in colon cancers, 7.2% among 1,040 lung cancers and 11.1% in 1790 hematopoietic and lymphoid tissue cancers. [23] Certain kinds of leukemias and lymphomas, including mantle cell lymphoma, T-ALL, atypical B cell chronic lymphocytic leukemia, and T-PLL are also associated with ATM defects. [24] A comprehensive literature search on ATM deficiency in pancreatic cancer, that captured 5,234 patients, estimated that the total prevalence of germline or somatic ATM mutations in pancreatic cancer was 6.4%. [25] ATM mutations may serve as predictive biomarkers of response for certain therapies, since preclinical studies have found that ATM deficiency can sensitise some cancer types to ATR inhibition. [26] [27] [28] [29]
ATM is one of the DNA repair genes frequently hypermethylated in its promoter region in various cancers (see table of such genes in Cancer epigenetics). The promoter methylation of ATM causes reduced protein or mRNA expression of ATM.
More than 73% of brain tumors were found to be methylated in the ATM gene promoter and there was strong inverse correlation between ATM promoter methylation and its protein expression (p < 0.001). [30]
The ATM gene promoter was observed to be hypermethylated in 53% of small (impalpable) breast cancers [31] and was hypermethylated in 78% of stage II or greater breast cancers with a highly significant correlation (P = 0.0006) between reduced ATM mRNA abundance and aberrant methylation of the ATM gene promoter. [32]
In non-small cell lung cancer (NSCLC), the ATM promoter methylation status of paired tumors and surrounding histologically uninvolved lung tissue was found to be 69% and 59%, respectively. However, in more advanced NSCLC the frequency of ATM promoter methylation was lower at 22%. [33] The finding of ATM promoter methylation in surrounding histologically uninvolved lung tissue suggests that ATM deficiency may be present early in a field defect leading to progression to NSCLC.
In squamous cell carcinoma of the head and neck, 42% of tumors displayed ATM promoter methylation. [34]
DNA damage appears to be the primary underlying cause of cancer, [35] and deficiencies in DNA repair likely underlie many forms of cancer. [36] If DNA repair is deficient, DNA damage tends to accumulate. Such excess DNA damage may increase mutational errors during DNA replication due to error-prone translesion synthesis. Excess DNA damage may also increase epigenetic alterations due to errors during DNA repair. [37] [38] Such mutations and epigenetic alterations may give rise to cancer. The frequent epigenetic deficiency of ATM in a number of cancers likely contributed to the progression of those cancers.
ATM functions during meiotic prophase. [39] The wild-type ATM gene is expressed at a four-fold increased level in human testes compared to somatic cells (such as skin fibroblasts). [40] In both mice and humans, ATM deficiency results in female and male infertility. Deficient ATM expression causes severe meiotic disruption during prophase I. [41] In addition, impaired ATM-mediated DNA DSB repair has been identified as a likely cause of aging of mouse and human oocytes. [42] Expression of the ATM gene, as well as other key DSB repair genes, declines with age in mouse and human oocytes and this decline is paralleled by an increase of DSBs in primordial follicles. [42] These findings indicate that ATM-mediated homologous recombinational repair is a crucial function of meiosis.
Several ATM kinase inhibitors are currently known, some of which are already in clinical trials. [43] [44] [45] One of the first discovered ATM inhibitors is caffeine with an IC50 of 0.2 mM and only a low selectivity within the PIKK family. [46] [47] Wortmannin is an irreversible inhibitor of ATM with no selectivity over other related PIKK and PI3K kinases. [48] The most important group of inhibitors are compounds based on the 3-methyl-1,3-dihydro-2H-imidazo[4,5-c]quinolin-2-one scaffold. The first important representative is the inhibitor is Dactolisib (NVP-BEZ235), which was first published by Novartis as a selective mTOR/PI3K inhibitor. [49] It was later shown to also inhibit other PIKK kinases such as ATM, DNA-PK and ATR. [50] Various optimisation efforts by AstraZeneca (AZD0156, AZD1390), Merck (M4076) and Dimitrov et al. have led to highly active ATM inhibitors with greater potency. [51] [52] [53]
Ataxia telangiectasia mutated has been shown to interact with:
The Tefu protein of Drosophila melanogaster is a structural and functional homolog of the human ATM protein. [78] Tefu, like ATM, is required for DNA repair and normal levels of meiotic recombination in oocytes.
p53, also known as Tumor protein P53, cellular tumor antigen p53, or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins are crucial in vertebrates, where they prevent cancer formation. As such, p53 has been described as "the guardian of the genome" because of its role in conserving stability by preventing genome mutation. Hence TP53 is classified as a tumor suppressor gene.
Breast cancer type 1 susceptibility protein is a protein that in humans is encoded by the BRCA1 gene. Orthologs are common in other vertebrate species, whereas invertebrate genomes may encode a more distantly related gene. BRCA1 is a human tumor suppressor gene and is responsible for repairing DNA.
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA damage, resulting in tens of thousands of individual molecular lesions per cell per day. Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe the gene that the affected DNA encodes. Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis. As a consequence, the DNA repair process is constantly active as it responds to damage in the DNA structure. When normal repair processes fail, and when cellular apoptosis does not occur, irreparable DNA damage may occur. This can eventually lead to malignant tumors, or cancer as per the two-hit hypothesis.
Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle which ensure its proper progression. Each checkpoint serves as a potential termination point along the cell cycle, during which the conditions of the cell are assessed, with progression through the various phases of the cell cycle occurring only when favorable conditions are met. There are many checkpoints in the cell cycle, but the three major ones are: the G1 checkpoint, also known as the Start or restriction checkpoint or Major Checkpoint; the G2/M checkpoint; and the metaphase-to-anaphase transition, also known as the spindle checkpoint. Progression through these checkpoints is largely determined by the activation of cyclin-dependent kinases by regulatory protein subunits called cyclins, different forms of which are produced at each stage of the cell cycle to control the specific events that occur therein.
Serine/threonine-protein kinase ATR, also known as ataxia telangiectasia and Rad3-related protein (ATR) or FRAP-related protein 1 (FRP1), is an enzyme that, in humans, is encoded by the ATR gene. It is a large kinase of about 301.66 kDa. ATR belongs to the phosphatidylinositol 3-kinase-related kinase protein family. ATR is activated in response to single strand breaks, and works with ATM to ensure genome integrity.
CHEK2 is a tumor suppressor gene that encodes the protein CHK2, a serine-threonine kinase. CHK2 is involved in DNA repair, cell cycle arrest or apoptosis in response to DNA damage. Mutations to the CHEK2 gene have been linked to a wide range of cancers.
Checkpoint kinase 1, commonly referred to as Chk1, is a serine/threonine-specific protein kinase that, in humans, is encoded by the CHEK1 gene. Chk1 coordinates the DNA damage response (DDR) and cell cycle checkpoint response. Activation of Chk1 results in the initiation of cell cycle checkpoints, cell cycle arrest, DNA repair and cell death to prevent damaged cells from progressing through the cell cycle.
DNA-dependent protein kinase catalytic subunit, also known as DNA-PKcs, is an enzyme that plays a crucial role in repairing DNA double-strand breaks and has a number of other DNA housekeeping functions. In humans it is encoded by the gene designated as PRKDC or XRCC7. DNA-PKcs belongs to the phosphatidylinositol 3-kinase-related kinase protein family. The DNA-Pkcs protein is a serine/threonine protein kinase consisting of a single polypeptide chain of 4,128 amino acids.
Cell cycle checkpoint control protein RAD9A is a protein that in humans is encoded by the RAD9A gene.Rad9 has been shown to induce G2 arrest in the cell cycle in response to DNA damage in yeast cells. Rad9 was originally found in budding yeast cells but a human homolog has also been found and studies have suggested that the molecular mechanisms of the S and G2 checkpoints are conserved in eukaryotes. Thus, what is found in yeast cells are likely to be similar in human cells.
Tumor suppressor p53-binding protein 1 also known as p53-binding protein 1 or 53BP1 is a protein that in humans is encoded by the TP53BP1 gene.
Cell cycle checkpoint protein RAD17 is a protein that in humans is encoded by the RAD17 gene.
Fanconi anemia group D2 protein is a protein that in humans is encoded by the FANCD2 gene. The Fanconi anemia complementation group (FANC) currently includes FANCA, FANCB, FANCC, FANCD1, FANCD2, FANCE, FANCF, FANCG, FANCI, FANCJ, FANCL, FANCM, FANCN, FANCO, FANCP, FANCQ, FANCR, FANCS, FANCT, FANCU, FANCV, and probably FANCW. Fanconi anemia proteins, including FANCD2, are an emerging therapeutic target in cancer
DNA topoisomerase 2-binding protein 1 (TOPBP1) is a scaffold protein that in humans is encoded by the TOPBP1 gene.
Mediator of DNA damage checkpoint protein 1 is a 2080 amino acid long protein that in humans is encoded by the MDC1 gene located on the short arm (p) of chromosome 6. MDC1 protein is a regulator of the Intra-S phase and the G2/M cell cycle checkpoints and recruits repair proteins to the site of DNA damage. It is involved in determining cell survival fate in association with tumor suppressor protein p53. This protein also goes by the name Nuclear Factor with BRCT Domain 1 (NFBD1).
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.
The meiotic recombination checkpoint monitors meiotic recombination during meiosis, and blocks the entry into metaphase I if recombination is not efficiently processed.
The G2-M DNA damage checkpoint is an important cell cycle checkpoint in eukaryotic organisms that ensures that cells don't initiate mitosis until damaged or incompletely replicated DNA is sufficiently repaired. Cells with a defective G2-M checkpoint will undergo apoptosis or death after cell division if they enter the M phase before repairing their DNA. The defining biochemical feature of this checkpoint is the activation of M-phase cyclin-CDK complexes, which phosphorylate proteins that promote spindle assembly and bring the cell to metaphase.
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.
Cancer epigenetics is the study of epigenetic modifications to the DNA of cancer cells that do not involve a change in the nucleotide sequence, but instead involve a change in the way the genetic code is expressed. Epigenetic mechanisms are necessary to maintain normal sequences of tissue specific gene expression and are crucial for normal development. They may be just as important, if not even more important, than genetic mutations in a cell's transformation to cancer. The disturbance of epigenetic processes in cancers, can lead to a loss of expression of genes that occurs about 10 times more frequently by transcription silencing than by mutations. As Vogelstein et al. points out, in a colorectal cancer there are usually about 3 to 6 driver mutations and 33 to 66 hitchhiker or passenger mutations. However, in colon tumors compared to adjacent normal-appearing colonic mucosa, there are about 600 to 800 heavily methylated CpG islands in the promoters of genes in the tumors while these CpG islands are not methylated in the adjacent mucosa. Manipulation of epigenetic alterations holds great promise for cancer prevention, detection, and therapy. In different types of cancer, a variety of epigenetic mechanisms can be perturbed, such as the silencing of tumor suppressor genes and activation of oncogenes by altered CpG island methylation patterns, histone modifications, and dysregulation of DNA binding proteins. There are several medications which have epigenetic impact, that are now used in a number of these diseases.
DNA damage is an alteration in the chemical structure of DNA, such as a break in a strand of DNA, a nucleobase missing from the backbone of DNA, or a chemically changed base such as 8-OHdG. DNA damage can occur naturally or via environmental factors, but is distinctly different from mutation, although both are types of error in DNA. DNA damage is an abnormal chemical structure in DNA, while a mutation is a change in the sequence of base pairs. DNA damages cause changes in the structure of the genetic material and prevents the replication mechanism from functioning and performing properly. The DNA damage response (DDR) is a complex signal transduction pathway which recognizes when DNA is damaged and initiates the cellular response to the damage.
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