The document discusses molecular mechanisms of DNA damage and repair. It provides an overview of different types of DNA strand breaks caused by radiation, including single-strand breaks, double-strand breaks, and base damage. It then describes different pathways for repairing DNA damage, including base excision repair, nucleotide excision repair, and strand break repairs. Double-strand breaks are identified as particularly important lesions that can lead to cell killing, carcinogenesis, or mutation if not properly repaired.
Microsatellite instability testing is an important part in diagnostics in Metastatic cancer settings after the FDA has given approval for tissue agnostic indications in almost all solid cancers. MSI by PCR and MMR status by IHC is also helpful for evaluation of genetic risk in Colon and Endometrial cancers
Microsatellite instability testing is an important part in diagnostics in Metastatic cancer settings after the FDA has given approval for tissue agnostic indications in almost all solid cancers. MSI by PCR and MMR status by IHC is also helpful for evaluation of genetic risk in Colon and Endometrial cancers
This presentation gives the basic idea, about the information on the role of tyrosine kinases in cancer. I have also included a phylogenetic tree for finding the relatedness between different organisms.
Breast cancer & biomarkers, their types, novelty of breast cancer biomarkers. Detailed study of her2, p53, BRCA1, BRCA2, DPD, 21-Gene signature, 70-Gene signature, cd106, vcam1, nlr, bFGF, mammaglobin, ER, PR, CEA. Pthological samples for biomarkers test, Ranges of various biomarkers, breast cancer diagnosis, prognosis, occurance, selection of breast caner treatment like targeted therapy.
Majority of cancer lead by point mutation in p53 gene. which is also known as "guardian of genome". this mutation leads conversion of normal cell into cancerous cell.
Cancer is a condition in which abnormal cells divide uncontrollably and destroy the body tissues. there are mainly 4 types of genes in our body when get altered it will lead to cancer. they are proto oncogenes, tumor suppresser genes, Micro RNA genes and mutated genes. these genes are important for the regulation of cell cycle and other functions in the body. once they get mutated either their function is lost permanently or get enhanced. This change is unwanted in the body and it may cause uncontrolled cell division.
This presentation gives the basic idea, about the information on the role of tyrosine kinases in cancer. I have also included a phylogenetic tree for finding the relatedness between different organisms.
Breast cancer & biomarkers, their types, novelty of breast cancer biomarkers. Detailed study of her2, p53, BRCA1, BRCA2, DPD, 21-Gene signature, 70-Gene signature, cd106, vcam1, nlr, bFGF, mammaglobin, ER, PR, CEA. Pthological samples for biomarkers test, Ranges of various biomarkers, breast cancer diagnosis, prognosis, occurance, selection of breast caner treatment like targeted therapy.
Majority of cancer lead by point mutation in p53 gene. which is also known as "guardian of genome". this mutation leads conversion of normal cell into cancerous cell.
Cancer is a condition in which abnormal cells divide uncontrollably and destroy the body tissues. there are mainly 4 types of genes in our body when get altered it will lead to cancer. they are proto oncogenes, tumor suppresser genes, Micro RNA genes and mutated genes. these genes are important for the regulation of cell cycle and other functions in the body. once they get mutated either their function is lost permanently or get enhanced. This change is unwanted in the body and it may cause uncontrolled cell division.
DNA repair system lecture that were prepered by Ph.D. students Mohammed Mohsen and Aliaa Hashim at microbiology department / college of medicine / babylon university.
Describe the repair mechanisms used during DNA replication.Soluti.pdfkellenaowardstrigl34
Describe the repair mechanisms used during DNA replication.
Solution
DNA like any other molecule can undergo a variety of chemical reactions. Because DNA
uniquely serves as a permanent copy of the cell genome, however, changes in its structure are of
much greater consequence than are alterations in other cell components, i.e RNA’s and Proteins.
Mutations can consider the incorporation of incorrect bases during DNA replication. And also,
various chemical changes occur in DNA either spontaneously or as a result of exposure to
chemicals or radiation. Such damage to DNA can block replication or transcription, and can
result in a high frequency of mutations—consequences that are unacceptable from the standpoint
of cell reproduction.
To maintain the integrity of their genomes, cells have therefore had to evolve mechanisms to
repair damaged DNA. DNA repair mechanism can be divided into two general classes: (1) direct
reversal of the chemical reaction responsible for DNA damage, and (2) removal of the damaged
bases followed by their replacement with newly synthesized DNA. The rate of DNA repair is
dependent on many factors, including the cell type, the age of the cell, and the extracellular
environment. A cell that has accumulated a large amount of DNA damage, or one that no longer
effectively repairs damage incurred to its DNA, can enter one of three possible states:
1. an irreversible state of dormancy, known as senescence
2. cell suicide, also known as apoptosis or programmed cell death
3. unregulated cell division, which can lead to the formation of a tumor that is cancerous
The DNA repair ability of a cell is vital to the integrity of its genome and thus to the normal
functionality of that organism.
Direct reversal
Cells are known to eliminate three types of damage to their DNA by chemically reversing it.
These mechanisms do not require a template,the types of damage they counteract can occur in
only one of the four bases. Such direct reversal mechanisms are specific to the type of damage
incurred and do not involve breakage of the phosphodiester backbone. The formation of
pyrimidine dimers upon irradiation with UV light results in an abnormal covalent bond between
adjacent pyrimidine bases. The photo reactivation process directly reverses this damage by the
action of the enzyme photolyase, whose activation is obligately dependent on energy absorbed
from blue/UV light (300–500 nm wavelength) to promote catalysis.
The second type of damage, methylation of guanine bases, is directly reversed by the protein
methyl guanine methyl transferase (MGMT),
The third type of DNA damage reversed by cells is certain methylation of the bases cytosine and
adenine.
Excision Repair mechanisms
Single strand damage:
When only one of the two strands of a double helix has a defect, the other strand can be used as a
template to guide the correction of the damaged strand. In order to repair damage to one of the
two paired molecules of DNA, there exist a number of excis.
Each day the genome is subjected to thousands of DNA damaging events from diverse sources which can have potentially deleterious consequences. In order to maintain genome integrity eukaryotic cells have evolved a highly complex and multifaceted response network called the DNA damage response, or ?DDR?....
Each day the genome is subjected to thousands of DNA damaging events from diverse sources which can have potentially deleterious consequences. In order to maintain genome integrity eukaryotic cells have evolved a highly complex and multifaceted response network called the DNA damage response, or ?DDR?
Each day the genome is subjected to thousands of DNA damaging events from diverse sources which can have potentially deleterious consequences. In order to maintain genome integrity eukaryotic cells have evolved a highly complex and multifaceted response network called the DNA damage response, or ?DDR?....
Each day the genome is subjected to thousands of DNA damaging events from div...semualkaira
Each day the genome is subjected to thousands of DNA damaging events from diverse sources which can have potentially deleterious consequences. In order to maintain genome integrity eukaryotic cells have evolved a highly complex and multifaceted response network called the DNA damage response, or ?DDR?..
Each day the genome is subjected to thousands of DNA damaging events from diverse sources which can have potentially deleterious consequences. In order to maintain genome integrity eukaryotic cells have evolved a highly complex and multifaceted response network called the DNA damage response
This class recording deals with the sources of DNA damage and the probable repair mechanisms operated within the cell systems. This presentation is prepared in view of students of masters level for updating their knowledge on Molecular Cell Biology.
Lung Cancer: Artificial Intelligence, Synergetics, Complex System Analysis, S...Oleg Kshivets
RESULTS: Overall life span (LS) was 2252.1±1742.5 days and cumulative 5-year survival (5YS) reached 73.2%, 10 years – 64.8%, 20 years – 42.5%. 513 LCP lived more than 5 years (LS=3124.6±1525.6 days), 148 LCP – more than 10 years (LS=5054.4±1504.1 days).199 LCP died because of LC (LS=562.7±374.5 days). 5YS of LCP after bi/lobectomies was significantly superior in comparison with LCP after pneumonectomies (78.1% vs.63.7%, P=0.00001 by log-rank test). AT significantly improved 5YS (66.3% vs. 34.8%) (P=0.00000 by log-rank test) only for LCP with N1-2. Cox modeling displayed that 5YS of LCP significantly depended on: phase transition (PT) early-invasive LC in terms of synergetics, PT N0—N12, cell ratio factors (ratio between cancer cells- CC and blood cells subpopulations), G1-3, histology, glucose, AT, blood cell circuit, prothrombin index, heparin tolerance, recalcification time (P=0.000-0.038). Neural networks, genetic algorithm selection and bootstrap simulation revealed relationships between 5YS and PT early-invasive LC (rank=1), PT N0—N12 (rank=2), thrombocytes/CC (3), erythrocytes/CC (4), eosinophils/CC (5), healthy cells/CC (6), lymphocytes/CC (7), segmented neutrophils/CC (8), stick neutrophils/CC (9), monocytes/CC (10); leucocytes/CC (11). Correct prediction of 5YS was 100% by neural networks computing (area under ROC curve=1.0; error=0.0).
CONCLUSIONS: 5YS of LCP after radical procedures significantly depended on: 1) PT early-invasive cancer; 2) PT N0--N12; 3) cell ratio factors; 4) blood cell circuit; 5) biochemical factors; 6) hemostasis system; 7) AT; 8) LC characteristics; 9) LC cell dynamics; 10) surgery type: lobectomy/pneumonectomy; 11) anthropometric data. Optimal diagnosis and treatment strategies for LC are: 1) screening and early detection of LC; 2) availability of experienced thoracic surgeons because of complexity of radical procedures; 3) aggressive en block surgery and adequate lymph node dissection for completeness; 4) precise prediction; 5) adjuvant chemoimmunoradiotherapy for LCP with unfavorable prognosis.
Tom Selleck Health: A Comprehensive Look at the Iconic Actor’s Wellness Journeygreendigital
Tom Selleck, an enduring figure in Hollywood. has captivated audiences for decades with his rugged charm, iconic moustache. and memorable roles in television and film. From his breakout role as Thomas Magnum in Magnum P.I. to his current portrayal of Frank Reagan in Blue Bloods. Selleck's career has spanned over 50 years. But beyond his professional achievements. fans have often been curious about Tom Selleck Health. especially as he has aged in the public eye.
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Introduction
Many have been interested in Tom Selleck health. not only because of his enduring presence on screen but also because of the challenges. and lifestyle choices he has faced and made over the years. This article delves into the various aspects of Tom Selleck health. exploring his fitness regimen, diet, mental health. and the challenges he has encountered as he ages. We'll look at how he maintains his well-being. the health issues he has faced, and his approach to ageing .
Early Life and Career
Childhood and Athletic Beginnings
Tom Selleck was born on January 29, 1945, in Detroit, Michigan, and grew up in Sherman Oaks, California. From an early age, he was involved in sports, particularly basketball. which played a significant role in his physical development. His athletic pursuits continued into college. where he attended the University of Southern California (USC) on a basketball scholarship. This early involvement in sports laid a strong foundation for his physical health and disciplined lifestyle.
Transition to Acting
Selleck's transition from an athlete to an actor came with its physical demands. His first significant role in "Magnum P.I." required him to perform various stunts and maintain a fit appearance. This role, which he played from 1980 to 1988. necessitated a rigorous fitness routine to meet the show's demands. setting the stage for his long-term commitment to health and wellness.
Fitness Regimen
Workout Routine
Tom Selleck health and fitness regimen has evolved. adapting to his changing roles and age. During his "Magnum, P.I." days. Selleck's workouts were intense and focused on building and maintaining muscle mass. His routine included weightlifting, cardiovascular exercises. and specific training for the stunts he performed on the show.
Selleck adjusted his fitness routine as he aged to suit his body's needs. Today, his workouts focus on maintaining flexibility, strength, and cardiovascular health. He incorporates low-impact exercises such as swimming, walking, and light weightlifting. This balanced approach helps him stay fit without putting undue strain on his joints and muscles.
Importance of Flexibility and Mobility
In recent years, Selleck has emphasized the importance of flexibility and mobility in his fitness regimen. Understanding the natural decline in muscle mass and joint flexibility with age. he includes stretching and yoga in his routine. These practices help prevent injuries, improve posture, and maintain mobilit
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Flu Vaccine Alert in Bangalore Karnatakaaddon Scans
As flu season approaches, health officials in Bangalore, Karnataka, are urging residents to get their flu vaccinations. The seasonal flu, while common, can lead to severe health complications, particularly for vulnerable populations such as young children, the elderly, and those with underlying health conditions.
Dr. Vidisha Kumari, a leading epidemiologist in Bangalore, emphasizes the importance of getting vaccinated. "The flu vaccine is our best defense against the influenza virus. It not only protects individuals but also helps prevent the spread of the virus in our communities," he says.
This year, the flu season is expected to coincide with a potential increase in other respiratory illnesses. The Karnataka Health Department has launched an awareness campaign highlighting the significance of flu vaccinations. They have set up multiple vaccination centers across Bangalore, making it convenient for residents to receive their shots.
To encourage widespread vaccination, the government is also collaborating with local schools, workplaces, and community centers to facilitate vaccination drives. Special attention is being given to ensuring that the vaccine is accessible to all, including marginalized communities who may have limited access to healthcare.
Residents are reminded that the flu vaccine is safe and effective. Common side effects are mild and may include soreness at the injection site, mild fever, or muscle aches. These side effects are generally short-lived and far less severe than the flu itself.
Healthcare providers are also stressing the importance of continuing COVID-19 precautions. Wearing masks, practicing good hand hygiene, and maintaining social distancing are still crucial, especially in crowded places.
Protect yourself and your loved ones by getting vaccinated. Together, we can help keep Bangalore healthy and safe this flu season. For more information on vaccination centers and schedules, residents can visit the Karnataka Health Department’s official website or follow their social media pages.
Stay informed, stay safe, and get your flu shot today!
Title: Sense of Smell
Presenter: Dr. Faiza, Assistant Professor of Physiology
Qualifications:
MBBS (Best Graduate, AIMC Lahore)
FCPS Physiology
ICMT, CHPE, DHPE (STMU)
MPH (GC University, Faisalabad)
MBA (Virtual University of Pakistan)
Learning Objectives:
Describe the primary categories of smells and the concept of odor blindness.
Explain the structure and location of the olfactory membrane and mucosa, including the types and roles of cells involved in olfaction.
Describe the pathway and mechanisms of olfactory signal transmission from the olfactory receptors to the brain.
Illustrate the biochemical cascade triggered by odorant binding to olfactory receptors, including the role of G-proteins and second messengers in generating an action potential.
Identify different types of olfactory disorders such as anosmia, hyposmia, hyperosmia, and dysosmia, including their potential causes.
Key Topics:
Olfactory Genes:
3% of the human genome accounts for olfactory genes.
400 genes for odorant receptors.
Olfactory Membrane:
Located in the superior part of the nasal cavity.
Medially: Folds downward along the superior septum.
Laterally: Folds over the superior turbinate and upper surface of the middle turbinate.
Total surface area: 5-10 square centimeters.
Olfactory Mucosa:
Olfactory Cells: Bipolar nerve cells derived from the CNS (100 million), with 4-25 olfactory cilia per cell.
Sustentacular Cells: Produce mucus and maintain ionic and molecular environment.
Basal Cells: Replace worn-out olfactory cells with an average lifespan of 1-2 months.
Bowman’s Gland: Secretes mucus.
Stimulation of Olfactory Cells:
Odorant dissolves in mucus and attaches to receptors on olfactory cilia.
Involves a cascade effect through G-proteins and second messengers, leading to depolarization and action potential generation in the olfactory nerve.
Quality of a Good Odorant:
Small (3-20 Carbon atoms), volatile, water-soluble, and lipid-soluble.
Facilitated by odorant-binding proteins in mucus.
Membrane Potential and Action Potential:
Resting membrane potential: -55mV.
Action potential frequency in the olfactory nerve increases with odorant strength.
Adaptation Towards the Sense of Smell:
Rapid adaptation within the first second, with further slow adaptation.
Psychological adaptation greater than receptor adaptation, involving feedback inhibition from the central nervous system.
Primary Sensations of Smell:
Camphoraceous, Musky, Floral, Pepperminty, Ethereal, Pungent, Putrid.
Odor Detection Threshold:
Examples: Hydrogen sulfide (0.0005 ppm), Methyl-mercaptan (0.002 ppm).
Some toxic substances are odorless at lethal concentrations.
Characteristics of Smell:
Odor blindness for single substances due to lack of appropriate receptor protein.
Behavioral and emotional influences of smell.
Transmission of Olfactory Signals:
From olfactory cells to glomeruli in the olfactory bulb, involving lateral inhibition.
Primitive, less old, and new olfactory systems with different path
Ethanol (CH3CH2OH), or beverage alcohol, is a two-carbon alcohol
that is rapidly distributed in the body and brain. Ethanol alters many
neurochemical systems and has rewarding and addictive properties. It
is the oldest recreational drug and likely contributes to more morbidity,
mortality, and public health costs than all illicit drugs combined. The
5th edition of the Diagnostic and Statistical Manual of Mental Disorders
(DSM-5) integrates alcohol abuse and alcohol dependence into a single
disorder called alcohol use disorder (AUD), with mild, moderate,
and severe subclassifications (American Psychiatric Association, 2013).
In the DSM-5, all types of substance abuse and dependence have been
combined into a single substance use disorder (SUD) on a continuum
from mild to severe. A diagnosis of AUD requires that at least two of
the 11 DSM-5 behaviors be present within a 12-month period (mild
AUD: 2–3 criteria; moderate AUD: 4–5 criteria; severe AUD: 6–11 criteria).
The four main behavioral effects of AUD are impaired control over
drinking, negative social consequences, risky use, and altered physiological
effects (tolerance, withdrawal). This chapter presents an overview
of the prevalence and harmful consequences of AUD in the U.S.,
the systemic nature of the disease, neurocircuitry and stages of AUD,
comorbidities, fetal alcohol spectrum disorders, genetic risk factors, and
pharmacotherapies for AUD.
NVBDCP.pptx Nation vector borne disease control programSapna Thakur
NVBDCP was launched in 2003-2004 . Vector-Borne Disease: Disease that results from an infection transmitted to humans and other animals by blood-feeding arthropods, such as mosquitoes, ticks, and fleas. Examples of vector-borne diseases include Dengue fever, West Nile Virus, Lyme disease, and malaria.
New Directions in Targeted Therapeutic Approaches for Older Adults With Mantl...i3 Health
i3 Health is pleased to make the speaker slides from this activity available for use as a non-accredited self-study or teaching resource.
This slide deck presented by Dr. Kami Maddocks, Professor-Clinical in the Division of Hematology and
Associate Division Director for Ambulatory Operations
The Ohio State University Comprehensive Cancer Center, will provide insight into new directions in targeted therapeutic approaches for older adults with mantle cell lymphoma.
STATEMENT OF NEED
Mantle cell lymphoma (MCL) is a rare, aggressive B-cell non-Hodgkin lymphoma (NHL) accounting for 5% to 7% of all lymphomas. Its prognosis ranges from indolent disease that does not require treatment for years to very aggressive disease, which is associated with poor survival (Silkenstedt et al, 2021). Typically, MCL is diagnosed at advanced stage and in older patients who cannot tolerate intensive therapy (NCCN, 2022). Although recent advances have slightly increased remission rates, recurrence and relapse remain very common, leading to a median overall survival between 3 and 6 years (LLS, 2021). Though there are several effective options, progress is still needed towards establishing an accepted frontline approach for MCL (Castellino et al, 2022). Treatment selection and management of MCL are complicated by the heterogeneity of prognosis, advanced age and comorbidities of patients, and lack of an established standard approach for treatment, making it vital that clinicians be familiar with the latest research and advances in this area. In this activity chaired by Michael Wang, MD, Professor in the Department of Lymphoma & Myeloma at MD Anderson Cancer Center, expert faculty will discuss prognostic factors informing treatment, the promising results of recent trials in new therapeutic approaches, and the implications of treatment resistance in therapeutic selection for MCL.
Target Audience
Hematology/oncology fellows, attending faculty, and other health care professionals involved in the treatment of patients with mantle cell lymphoma (MCL).
Learning Objectives
1.) Identify clinical and biological prognostic factors that can guide treatment decision making for older adults with MCL
2.) Evaluate emerging data on targeted therapeutic approaches for treatment-naive and relapsed/refractory MCL and their applicability to older adults
3.) Assess mechanisms of resistance to targeted therapies for MCL and their implications for treatment selection
Report Back from SGO 2024: What’s the Latest in Cervical Cancer?bkling
Are you curious about what’s new in cervical cancer research or unsure what the findings mean? Join Dr. Emily Ko, a gynecologic oncologist at Penn Medicine, to learn about the latest updates from the Society of Gynecologic Oncology (SGO) 2024 Annual Meeting on Women’s Cancer. Dr. Ko will discuss what the research presented at the conference means for you and answer your questions about the new developments.
Basavarajeeyam is an important text for ayurvedic physician belonging to andhra pradehs. It is a popular compendium in various parts of our country as well as in andhra pradesh. The content of the text was presented in sanskrit and telugu language (Bilingual). One of the most famous book in ayurvedic pharmaceutics and therapeutics. This book contains 25 chapters called as prakaranas. Many rasaoushadis were explained, pioneer of dhatu druti, nadi pareeksha, mutra pareeksha etc. Belongs to the period of 15-16 century. New diseases like upadamsha, phiranga rogas are explained.
1. Molecular Mechanisms of DNA
Damage and Repair
Presented by
F. Ghorbani
4/27/2019
1
General Overview of DNA Strand Breaks
OPERATIONAL CLASSIFICATIONS OF RADIATION DAMAGE
DNA Repair Pathways
2. General Overview of DNA Strand Breaks
DNA Structure
Deoxyribonucleic acid (DNA) is a
large molecule with a well-known
double helical structure.
DNA is the principal target for the
biologic effects of radiation,
including cell killing, carcinogenesis,
and mutation.
4/27/2019
2
3. The “backbone” consists
of alternating sugar and
phosphate groups.
The sugar involved is
deoxyribose. Attached to
this backbone are four
bases, the sequence of
which specifies the
genetic code.
4/27/2019
3
General Overview of DNA Strand Breaks
DNA Structure
4. Ionizing radiation induces:
1. base damage
2. Single-strand breaks
3. Double-strand breaks
4. DNA protein cross-links
4/27/2019
4
General Overview of DNA Strand Breaks…
5. 4/27/2019
5
A: A normal DNA helix.
B: A break in one strand is of little significance.
well separated
Breaks in both strands
directly opposite or separated by only a few base
Diagrams of single- and double-strand DNA breaks caused by radiation
6. D0 dose, How many damage?
A dose of radiation that induces an average of one lethal event
per cell leaves 37% of irradiated cells still viable.
For mammalian cells, the x-ray D0 usually lies between 1-2 Gy.
The number and type of DNA lesions per cell detected
immediately after a dose of 1 Gy of x-rays is approximately:
Double-strand breaks (DSBs)* 40
Single-strand breaks (SSBs)** 1,000
Base damage >2,000
DNA-DNA crosslinks 30
*: The most important lesions produced in chromosomes by radiation
**:If the repair is incorrect (misrepair), it may result in a mutation.
4/27/2019
6
7. The interaction of two DSBs may result in cell killing,
carcinogenesis, or mutation.
There are many kinds of DSBs, varying in the distance between
the breaks on the two DNA strands and the kinds of end groups
formed.
Their yield in irradiated cells is about 0.04 times that of SSBs, and
they are induced linearly with dose, indicating that they are
formed by single tracks of ionizing radiation.
Both free radicals and direct ionizations may be involved in the
formation of the type of SBs.
“Both free radicals and direct ionizations may be involved
in the formation of the type of strand break illustrated in
Figure 2.2D” Page 58
4/27/2019
7
8. The interaction of two DSBs
1. The interaction of two DSBs may result in cell killing,
carcinogenesis, or mutation. Page 56
2. To understand why RBE reaches a maximum value in terms of the
production of DSBs because the interaction of two DSBs to form
an exchange-type aberration is the basis of most biologic effects.
Page 207
4/27/2019
8
9. Spur: for x- or γ-rays: 95% of the energy deposition events are spurs
up to 100 eV of energy
three ion pairs
diameter about 4 nm
4/27/2019
9
Blobs: less frequent for x or γ-rays
energy range of 100 to 500 eV
12 ion pairs
diameter about 7 nm
What are Spur and Blobs?
10. Spur and Blobs…
Given the size of a spur and the diffusion distance of
hydroxyl free radicals, the clustered lesion could be
spread out up to 20 base pairs.
For neutrons or α-particles, a greater proportion of
blobs are produced.
The damage produced, is different from that produced
by x- or γ-rays.
4/27/2019
10
11. OPERATIONAL CLASSIFICATIONS OF RADIATION DAMAGE
Radiation damage to mammalian cells can be divided into three
categories:
(1) Lethal damage: irreversible/irreparable and leads irrevocably
to cell death
(2) Potentially lethal damage (PLD): can be modified by post
irradiation environmental conditions
(3) Sublethal damage (SLD): under normal circumstances, can be
repaired in hours unless additional SLD is added, which it can
interact to form lethal damage.
4/27/2019
11
12. Potentially Lethal Damage Repair(PLD) 4/27/2019
12
The component of radiation damage that is modified by by varying
environmental conditions is known as PLD so cell survival can be
influenced.
If cells are prevented from dividing for 6 hours or more after irradiation,
PLD repair can occur and survival increases.
Allowing time for the PLD repair
Cell survival is enhanced
PLD repair is significant for X-rays
but does not occur after neutron
irradiation.
13. It has been suggested that the radioresistance of certain types of
human tumors is linked to their ability to repair PLD.
4/27/2019
13
Repair of potentially lethal damage in mouse fibrosarcomas
PLD Repair…
14. Sublethal Damage Repair (SLD)
The increasing in cell survival that is observed if a given radiation dose is
split into two fractions separated by a time interval.
Is simply the repair of double-strand breaks.
4/27/2019
14
Reflects the repair of DNA
breaks before they can
interact to form lethal
chromosomal aberrations.
If a dose is splitted by a time
interval, some double-strand
breaks produced by the first
dose are repaired before the
second dose.
15. SLD Repair…
Double strand “lethal” damage may be formed by:
(1) single-track (2) multiple-track damage.
The cell killing resulting from single-track damage is the same
whether the dose is a single exposure or fractionated. The same
is not true of multiple-track damage.
SLD repair is significant for x-rays but almost nonexistent
neutrons.
4/27/2019
15
16. Asynchronous
population
A large dose of
radiation
partly
synchronized
in the S phase
SLD Repair….
Age response function
4/27/2019
16
Survival of Chinese hamster cells
two fractions of x-rays
incubated at 37° C
17. SLD Repair……
The pattern of repair is a
combination of three processes :
1. The prompt repair of SLD.
2. Reassortment
Progression of cells through
the cell cycle.
3. Repopulation
Increase of surviving fraction
resulting from cell division.
4/27/2019
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19. DNA Repair Pathways
4/27/2019
19
DNA in the living cell is subjected to many chemical
alterations.
The genetic information encoded in the DNA has to
remain uncorrupted.
Any chemical changes must be corrected.
A failure to repair DNA produces a mutation.
20. Excision of DNA damage
In these reactions a nucleotide segment containing base
damage, double-helix distortion or mispaired bases is
replaced by the normal nucleotide sequence in a new
DNA polymerase synthesis process.
All of these pathways have been characterized in both
bacterial and eukaryotic organisms.
4/27/2019
21
i. Base excision repair (BER)
ii. Nucleotide excision repair (NER)
iii. Mismatch repair (MMR)
iv. Strand break repairs.
21. i. Excision of DNA damage
Base Excision Repair (BER)
Bases on opposite strands of DNA must be complementary.
This system recognizes modified bases that are not normally found in DNA.
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BER repairs damage to a single nitrogenous base by deploying enzymes
called glycosylases.
Cytosine to uracil
Adenine to hypoxanthine
Guanine to xanthine
Such bases may be formed by deamination of:
22. BER…
1. When the repair system finds an
unusual base, a glycosylase/DNA
lyase removes it.
2. There are different DNA
glycosylases for different bases.
For example, uracil DNA
glycosylase removes uracil.
3. The nucleotide lacking the base
is removed by an endonuclease;
(AP endonuclease removes the
sugar residue).
4. The gap is filled with the correct
nucleotide by DNA polymerase I.
5. The ends are sealed by DNA
ligase.
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23. i. BER…
U represents a putative single-base mutation that:
1. Removing U by a glycosylase/DNA lyase.
2. Removal of the sugar residue by apurinic
endonuclease 1 (APE1)
3. Replacement with the correct nucleotide by DNA
polymerase β
4. Joined by DNA ligase III–XRCC1–mediated ligation.
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24. ii. BER…
If more than one nucleotide is to be replaced:
1. performing the repair synthesis by the complex of
replication factor C (RFC)/proliferating cell nuclear antigen
(PCNA)/DNA polymerase δ/ε
2. Removing the overhanging flap structure by the flap
endonuclease 1 (FEN1)
3. DNA strands are sealed by ligase.
Although ionizing radiation–induced base damage is
efficiently repaired, defects in BER may lead to an
increased mutation rate but usually do not result in
cellular radiosensitivity.
Most common types of DNA damage induced by ionizing
radiation are repaired through base excision repair.
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25. ii. Excision of DNA damage
Nucleotide Excision Repair (NER)
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NER repairs damaged DNA which commonly consists of bulky, helix-
distorting damage, such as pyrimidine dimerization caused by UV light.
Damaged regions are removed in 12–24 nucleotide-long strands in a 3-step
process:
Recognition of damage
Excision of damaged DNA
Resynthesis of removed DNA region.
This system corrects errors such as:
Formation of thymine dimers
Addition of exogenous chemicals to bases
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Thymine dimers are formed on exposure to UV light.
Two adjacent thymine bases on a strand are bonded together.
Thymine dimer distorts the helix.
The nucleotides in the distorted region are incapable of base pairing.
Thymine dimers are removed by UVr ABC excinuclease
UVr A and UVr B detect the error and unwind the DNA in the region
of the defect.
UVr B nicks the strand a few bases downstream of the defect
UVr C nicks it a few bases upstream
DNA polymerase I adds the correct nucleotides followed by ligation
by DNA ligase
NER…
27. The process of GG-NER is genome-wide (i.e., lesions can be removed
from DNA that encodes or does not encode for genes).
TCNER only removes lesions in the DNA strands of actively transcribed
genes. RNA polymerase can block access to the site of damage and
hence prevents DNA repair. TC-NER remove it from the site of damage to
allow the repair proteins access.
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The process of NER can be subdivided into two pathways:
I. Global Genome Repair (GGR or GG-NER)
II. Transcription-Coupled Repair (TCR or TC-NER).
NER…
28. The mechanism of GG-NER and TC-NER differs only in the detection of
the lesion; the remainder of the pathway used to repair the damage is
the same for both.
The essential steps in this pathway are:
1. Damage recognition
2. DNA incisions that bracket the lesion, usually between 24 and 32
nucleotides in length
3. Removal of the region containing the adducts
4. Repair synthesis to fill in the gap region
5. DNA ligation
Mutation in NER genes, does not lead to ionizing radiation sensitivity but
increases sensitivity to ultraviolet (UV)-induced DNA damage and
anticancer agents such as alkylating agents that induce bulky adducts.
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NER…
29. iii. Excision of DNA damage
Mismatch Repair (MMR)
Mismatch repair corrects errors made when DNA is copied. For
example, a C could be inserted opposite an A, or the polymerase could
slip or stutter and insert two to five extra unpaired bases.
MMR pathway removes base–base and small insertion mismatches
that occur during replication.
MMR system detects a signal of a mismatched base on the new strand,
then corrects the errors that escaped proof-reading.
MMR system has to:
Recognize the newly synthesized strand
Scan the strand rapidly
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30. MMR…
Specific proteins scan the newly synthesized DNA, using adenine
methylation within a GATC sequence as the point of reference.
GATC sequence:
GATC is a palindromic sequence present in DNA.
Adenine is methylated in these sequences.
GATC on the new strand remain unmethylated for some time, this allows
the repair system to recognize the new strand.
GATC sequences act as signposts for mismatch repair system.
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31. MMR…
MutS protein scans the new strand from one GATC to another, Binds to a mispaired
base. MutL proteins binds to MutS.
MutL and MutS activate the GATC endonuclease activity of MutH.
Active MutH nicks the new strand just upstream of the GATC sequence, until the
mispaired nucleotide is removed.
DNA polymerase III adds the correct nucleotides in the gap.
DNA ligase seals the new oligonucleotide with the DNA strand.
Faulty mismatch repair has been linked to HNPCC colon cancer, one of the most
common inherited cancers.
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32. iv. Excision of DNA damage
Repairing Strand Breaks
Ionizing radiation and certain chemicals can produce both single-
strand breaks (SSBs) and double-strand breaks (DSBs) in the DNA
backbone.
A. Single-Strand Breaks (SSBs)
Breaks in a single strand of the DNA molecule are repaired using the
same enzyme systems that are used in Base-Excision Repair (BER).
B. Double-strand breaks (DSBs)
These breaks are extremely deleterious. They disturb replication,
transcription and translation. Double-strand breaks can also result in
chromosomal rearrangements. A number of cancers are caused by
chromosomal rearrangements.
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33. Double-Strand Break Repair
There are two mechanisms to repair a complete break in a DNA
:
Non-Homologous End-Joining (NHEJ): In NHEJ, overhanging
pieces of DNA adjacent to the break are joined.
NHEJ repairs DNA DSBs through the orchestration of end-to-
end joining.
Homologous Recombination Repair (HRR): A sister or
homologous chromosome is used as a template for repair.
HRR is an error-free process, because repair is performed by
copying information from the undamaged homologous
chromatid/chromosome.
The competition for repair by HRR versus NHEJ is in part
regulated by the protein 53BP1.
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In lower eukaryotes such as yeast, HRR is the predominant pathway
used for repairing DNA DSBs.
In mammalian cells, the choice of repair is biased by the phase of the
cell cycle and by the abundance of repetitive DNA.
HRR occurs primarily in the late S/G2 phase of the cell cycle, when an
undamaged sister chromatid is available to act as a template.
NHEJ occurs in the G1 phase of the
cell cycle, when no such template
exists.
NHEJ and HRR are not mutually
exclusive, and both have been
found to be active in the late S/G2
phase of the cell cycle.
NHEJ VS. HRR
35. Non-Homologous End-Joining
1) Promote DNA repair
2) Prevent the cell from proceeding in the cell
cycle until the break is faithfully repaired.
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NHEJ can be divided into five steps:
1. End recognition by Ku binding
2. Recruitment of DNA-dependent protein kinase
catalytic subunit (DNA-PKcs(
3. End processing
4. Fill-in synthesis or end bridging
5. Ligation
The immediate response of a cell to a
DNA DSB is activation of a group of
sensors (ATM and ATR) :
36. Homologous
Recombination Repair
(HRR)
HRR provides the
mammalian genome a high-
fidelity mechanism of
repairing DNA DSBs.
Increased activity of this
recombination pathway in
late S/G2.
HRR requires physical
contact with an undamaged
chromatid or chromosome
for repair .
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37. Crosslink Repair
1. Intra-strand crosslinks:
on one strand of DNA
lead to a block of DNA
polymerase activity and to a
single-strand DNA gap
This gap can be resolved by DNA
polymerase switching to a
template without the intra-strand
crosslink or by translation
synthesis.
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Crosslinks are problematic during
replication and can lead to cell cycle
arrest and even cell death.
38. Crosslink Repair
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2. Inter-strand crosslinks
occur between two strands of DNA
result in a complete block to replication and prevent the
unwinding of duplex DNA, leading to cell death if
unrepaired.
repairing by different protein complexes depending on the
phase of the cell cycle.
In G1 phase, removing by NER.
Only a small percentage of inter-strand crosslinks are
repaired in G1 phase, and the unrepaired lesions will
present a major problem for the cell in S phase.
Two of the bases are single-ring groups (pyrimidines); these are thymine and cytosine. Two of the bases are double-ring groups (purines); these are adenine and guanine.
The cells has evolved an intricate series of sensors and pathways to respond to each type radiation-induced damage.
A: Two-dimensional representation of the normal DNA helix.
B: A break in one strand is of little significance because it is repaired readily using the opposite strand as a template.
C: Breaks in both strands, if well separated, are repaired as independent breaks.
D: If breaks occur in both strands and are directly opposite or separated by only a few base pairs, this may lead to a double-strand break in which the chromatin snaps into two pieces.
The energy from ionizing radiations is located along the tracks of the charged particles set in motion, electrons in the case of x- or γ-rays, protons and α-particles in the case of neutrons.
Spur: up to 100 eV of energy and involves, on average, three ion pairs. In the case of x- or γ-rays, 95% of the energy deposition events are spurs, which have a diameter of about 4 nm, which is about twice the diameter of the DNA double helix.
Because spurs and blobs have dimensions similar to the DNA double helix, multiple radical attacks occurs if they overlap the DNA helix.
In the case of densely ionizing radiations, such as neutrons or α-particles, a greater proportion of blobs are produced. The damage produced, therefore, is qualitatively different from that produced by x- or γ-rays, and it is much more difficult for the cell to repair.
تغییر شرایط محیطی پس از تابش می توند نسبت سلولهای زنده مانده را به دلیل ترمیم PLD تحت تاثیر قرار دهد.
اگر میتوز بواسطه شرایط نامناسب رشد به تاخیر بیفتد، آسیب DNA می تواند ترمیم شود.
منحنی: منحنی بقا اشعه ایکس برای سلول هایی در مرحله ثابت که بلافاصله یا 6 یا 12 ساعت پس از تابش کشت شده اند.
منحنی: نسبت سلول های زنده برای یک دوز معین با افزایش فاصله زمانی بین تابش گیری و برداشتن سلول ها افزایش می یابد. زیرا ترمیم رخ می دهد.
پیشنهاد شده است که حساسیت پرتوی انواع خاصی از تومورهای انسانی، به توانایی آنها در ترمیم PLD مرتبط است.
تومورهای حساس به پرتو، در ترمیمPLD ناکارآمد عمل می کنند. تومورهای مقاوم به پرتو، مکانیسم کارآمد برای تعمیر PLD دارند.
این یک فرضیه جذاب است اما هرگز ثابت نشده است.
ترمیم SLD یک اصطلاح کاربردی برای افزایش بقا در صورت تابش دوز معین بطور جداگانه و در فواصل زمانی معین است.
منحنی:
بقای سلول های هامستر چینی تابش دیده در دو جلسه با اشعه ایکس و در دمای اتاق برای فواصل زمانی مختلف بین دو تابش
در چند ساعت اول، تعمیر سریع SLD واضح است، اما در فواصل زمانی طولانی بین دو فرکشن، درصد باقی مانده از سلول ها کاهش می یابد، تا به یک حداقل ،در حدود 5 ساعت، میرسد
برای اشعه ایکس، دوز فرکشن با فاصله 1 تا 4 ساعت، باعث افزایش قابل توجهی در بقای سلول به دلیل ترمیم سریع آسیب های زیر کشنده است (multi-track damage یا آسیب های چند مسیری).
برای نوترون، فاصله بین فرکشن ها، ترمیم کمی در آسیب های زیرکشنده ارایه می دهد. (آسیب تک مسیری).
در جلسه گذشته آنچه در مورد ترمیم SLD گفتیم: اگر دز تقطیع شود در فاصله بین جلسات آسیب های زیرکشنده فرصت ترمیم می یابند
برای X چون آسیب ها چند مسیره هستند، این ترمیم کاملا مشهود است اما برای nها چون آسیب تک مسیره است، تقطیع دوز ترمیم SLD را منجر نمی شود.
تابع سن-پاسخ:جمعیت ناهمزمان سلولی که در معرض دوزهای بالایی از تابش قرار بگیرد، سلول های بیشتری در مراحل حساس چرخه سلولی نسبت به مراحل مقاوم کشته می شوند. بنابراین جمعیت باقی مانده از سلول ها تا حدی هماهنگ می شود. (همزمانی نسبی) و بیشتر آنها در فاز S هستند. اگر 6 ساعت تا تابش دوم فاصله باشد، سلول ها در فاز G2/M خواهند بود، لذا پس از تابش بقا به شدت افت خواهد کرد.
منحنی: همین تابع سن-پاسخ را در منحنی مشاهده میکنید
الگوی ترمیم ترکیبی از سه فرایند است که همزمان انجام می شوند:
ترمیم فوری آسیب کمتر از حد کشندگی
پیشرفت سلول ها در چرخه سلولی در فاصله بین جلسات که بازآرایی Reassortment نام دارد
رشد جمعیت به دلیل تقسیم سلولی اگر فاصله بین جلسات بیشتر از 10-12 ساعت باشد.
دو شکل بعد ترمیم SLD را بصورت درون تنی نشان میدهد.
شکل A سلول های لوسمی لنفوسیتی موش
سلول های اپی تلیال پوست موش
شکل A عامل بهبودی= کسر بقا ناشی از دو دوز تکی ب بقا حاصل از یک دوز یکجا
در هیچکدام شیب تند منحنی ناشی از پیشرفت سولها در چرخه سلولی بعد از 6 ساعت را نداریم زیرا چرخه سلولی طولانی است.
نکته دیگر اینکه در شکل A ، ترمیم بیشتر تومورهای کوچک 1 روزه نسبت به تومورهای بزرگ 6 روزه هایپوکسی داریم که این بیانگر این است که ترمیم یک مانیسم فعال است که به اکسیژن و مواد مغذی نیاز دارد.
اگر دوز در دو فرکشن و با يك فاصله زماني تحويل داده شود، بقاء سلولي افزايش مي يابد، زيرا شانه منحني هر بار بايد تکرار شود.
نسبت سلول های زنده مانده در دوز چند جلسه ای با افزایش زمان بین دو جلسه، افزایش می یابد.
همانطور که فاصله زمانی از 0 تا 2 ساعت افزایش می یابد، افزایش زنده ماندن در نتیجه بهبود آسیب زیرکشنده است
در سلول هایی با چرخه سلولی طولانی و یا آنهایی که خارج از چرخه اند، افزایش بیشتر در بقای سلول، برای زمان های بیش از 2 یا 3 ساعت وجود ندارد.
3. در یک جمعیت سلولی که به سرعت در حال تقسیم است، یک شیب تند در منحنی بقا بدلیل reassortment ایجاد می شود. با این حال، اگر فاصله زمانی بین دوزهای تقسیم بیش از چرخه سلولی باشد، افزایش بقای سلولی به علت تکثیر جمعیت سلولی وجود دارد.
DNA در سلول زنده تحت تاثیر بسیاری از تغییرات شیمیایی قرار دارد.
اما اطلاعات ژنتیکی رمزگذاری شده در DNA بایستی بدون قید و شرط باقی بماند و هر تغییر شیمیایی باید اصلاح شود. همچنین شکست در ترمیم DNA موجب جهش می شود.
مکانیسم های مورد استفاده برای ترمیم آسیب باز ناشی از تابش یونیزه متفاوت از مکانیسم های مورد استفاده برای ترمیم DNA DSBs است.
مسیرهای مختلف ترمیم برای آسیب DNA، بسته به مرحله چرخه سلولی، مورد استفاده قرار می گیرند.
برگشت مستقیم آسیب DNA، ساده ترین مکانیسم ترمیم است که شامل یک زنجیره پلیپپتیدی، با خواص آنزیمی است که به آسیب متصل می شود و ژنوم DNA را به حالت عادی خود در یک مرحله واکنش برمیگرداند.
پروتئین فوتولیاز در تمام سلول های زنده وجود ندارد.
با این حال، DNA آلکیل ترانسفراز در طبیعت فراوان است.
در این واکنش ها که بر اساس حذف آسیب هستند ، یک بخش نوکلئوتیدی حاوی باز آسیب دیده ، distortion در مارپیچ و یا بازهای غیر مکمل، با توالی نوکلئوتیدی نرمال طی فرایند سنتز DNA پلیمراز جدیدی جایگزین میشوند.
حذف آسیب DNA از مسیرهای مختلفی انجام میشود. که در مورد آنها صحبت میکنیم.
adenine (A) pairs with thymine (T),
and guanine (G) pairs with cytosine (C).
هنگامی که تنها یکی از دو رشته ی DNA ، آسیب دیده است ، رشته دیگر می تواند به عنوان یک الگو برای ترمیم رشته آسیب دیده استفاده شود.
BER با استفاده از آنزیم هایی به نام گلیکوسیلازها ، آسیب به یک باز نیتروژنی را ترمیم می کند.
هنگامی که سیستم ترمیم، یک باز غیر معمول پیدا می کند، یک گلیکوزیلاز / DNA لیاز آن را حذف می کند.
دی ان ای گلیکوزیلاز های مختلفی برای بازهای مختلف وجود دارد. به عنوان مثال، اوراسیل DNA گلیکوزیلاز، اوراسیل را حذف می کند.
نوکلئوتید بدون باز، توسط یک اندونوکلئاس حذف می شود؛ (AP اندونوکلئاس باقی مانده قند را حذف می کند).
این گپ با نوکلئوتید صحیح توسط DNA polymerase I تکمیل می شود.
دو انتهای آن توسط DNA لیگاز مهر و موم میشود.
U نشان دهنده جهش تک بازی است که:
اگر چه آسیب باز القا شده توسط پرتو یونیزان، به طور موثر ترمیم می شود، و نقص در فرایندBER ممکن است منجر به افزایش نرخ جهش شود، اما معمولا منجر به حساسیت پرتوی سلول نمیشود.
شایع ترین انواع آسیب DNA ناشی از اشعه یونیزاسیون، توسط BER ترمیم می شود.
ترمیم پارگی نوکلئوتید NER، ترکیبات اضافی حجیمی مثل مواد شیمیایی خارجی متصل به باز ها و دایمرهای پیرامیدین را که بوسیله نور UV ایجاد میشوند را حذف میکند.
12 تا 24 نوکلئوتید از ناحیه آسیب دیده طی سه مرحله حذف میشود
تشخیص آسیب حذف DNA آسیب دیده توسط اندونوکلئاز و ترمیم مجدد ناحیه DNA حذف شده.
NER مکانیسم ترمیمی است که در تقریبا تمام سلول های یوکاریوتی و پروکاریوتی استفاده می شود.
در پروکاریوت ها، NER توسط پروتئین های Uvr انجام می شود.
در سلولهای یوکاریوتی، بسیاری از پروتئین های دیگر درگیر هستند، هرچند استراتژی کلی یکسان است.
دایمرهای تیمین تحت تابش نور UV ایجاد میشوند.
دو باز تیمین نزدیک به هم بر روی یک شاخه، با هم باند میشوند.
دایمر تیمین مارپبچ را از حالت نرمالش تغییر میدهد.
نوکلئوتیدهایی که در ناحیه تغییر یافته هستند، نمیتوانند با باز مقابلشان جفت شوند
بازهای Uvr A,B خطا را شناسایی میکنند و ناحیه معیوب DNA را باز میکنند.
UVr B یک تعداد باز به سمت پایین رشته DNA و UVr C به سمت بالای رشته را برش میدهد.
DNA polymerase I نوکلئوتیدهای صحیح را اضافه میکند و توسط DNA ligase محکم میشود.
فرایند NER میتواند به دو مسیر تقسیم بندی شود:
ترمیم گلوبال ژنوم GGR or GG-NER
ترمیم رونویسی جفتی TCR or TC-NER
فرایند GG-NER ژن گسترده است به این معنا که آسیب های کدگذاری شده و نشده ژن ها میتواند از DNA حذف شود.
اما TC-NER آسیب ها را فقط از رشته های DNA ژنهای فعال رونویسی شده حذف میکند.
آنزیم RNA polymerase میتواند دسترسی را به محل آسیب و در نتیجه ترمیم DNA محدود کند
TC-NER با حذف RNA polymerase دسترسی پروتئین های ترمیم را به محل آسیب ممکن میکند.
مکانیزم GG-NER و TC-NER فقط در تشخیص آسیب متفاوت است؛ باقی مانده مسیر مورد استفاده برای ترمیم آسیب برای هر دو یکسان است .
تشخیص آسیب
برش DNA شامل آسیب که معمولا بین 24 تا 32 نوکلئوتید است.
حذف منطقه حاوی aducts (ترکیب اضافه)
سنتز ترمیم برای پر کردن منطقه شکاف
پیوند DNA
جهش در ژنهای NER، منجر به حساسیت به تابش یونیزان نمی شود. بلکه حساسیت به تابش ماوراء بنفشی که باعث آسیب به DNA میشود و نیز عواملی مثل آلکیلیتینگ ایجنت ها را که باعث آسیب های حجیم میشوند را افزایش می دهد.
ترمیم ناقص، اشتباهاتی که هنگام کپی DNA انجام می شود را تصحیح میکند. به عنوان مثال، یک C می تواند در مقابل A قرار داده شود، یا پلیمراز می تواند طی یک لغزش دو تا پنج باز اضافی غیرجفت شده را وارد کند.
مسیر MMR جفت شدگی های بازی اشتباه را که طی رونوشت ایجاد میشود و از دستproof-reading در رفته اند را اصلاح می کند.
روند MMR را می توان به چهار بخش تقسیم کرد:
سیستم MMR یک سیگنال از یک باز mismatches یا ناسازگار را در رشته جدید شناسایی می کند
عوامل MMR بکار گرفته میشوند
رشته ای که اخیرا سنتز شده است و ناسازگاری دارد شناسایی شده و نوکلئوتیدهای نادرست / تغییر یافته از بین می روند
سنتز مجدد و لیگیشن اون قسمت از DNA کامل میشود.
بنابراین سیستم MMR باید رشته های جدید سنتز شده را شناسایی و به سرعت اسکن کنند
پروتئین های خاص، DNA تازه سنتز شده را با توجه به متیله بودن آدنین و توالی GATC بعنوان یک نقطه مرجع، اسکن میکند.
توالی GATC:
GATC یک توالی palindromic موجود در DNA است.
آدنین در این توالی متیله است اما رشته جدید برای مدتی غیر متیله باقی می ماند. و این اجازه تشخیص رشته جدید را به سیستم ترمیم میدهد.
در حقیقت توالی GATC مانند یک تابلوی راهنما برای سیستم MMR عمل میکند.
پروتئین Muts رشته را بین دو توالی GATC اسکن میکند. و اگر بازی که به صورت نا صحیح جفت شده باشد، بیابد به آن متصل میشود.
سپس MutL به MutS متصل میشود.
MutL و MutS فعالیت GATC اندونوکلئاز مربوط به MutH را فعال میکنند
MutH فعال، رشته را از قسمت توالی GATC به سمت بالا تا جای نوکلئوتید mispaired ،حذف میکند.
DNA polymerase III نوکلئوتید های صحیح را در این فاصله اضافه نموده.
DNA ligase این تعداد محدود نوکلئوتید را به رشته DNA مهر و موم میکند.
سرطان کولون HNPCC که یکی از شایعترین سرطان های ارثی است که به ترمیم نادرست mismatch، مرتبط است.
ترمیم شکست های رشته ای
تابش یونیزان و مواد شیمیایی خاصی می توانند شکاف تک رشته و شکاف دو رشته را در DNA ایجاد کنند.
شکست در یک تک رشته مولکول DNA، با استفاده از سیستم های آنزیمی مشابه که در ترمیمBER استفاده می شود، ترمیم می شود.
شکاف دو رشتهDNA بسیار زیان آور هستند. آنها تکثیر، رونویسی و ترجمه را مختل می کنند. شکاف دو رشته ای همچنین می تواند به تغییرات کروموزومی منجر شود. تعدادی از سرطانها به علت تغییرات کروموزومی ایجاد می شود.
دو مکانیسم وجود دارد که سلول تلاش می کند یک شکست کامل در یک مولکول DNA ترمیم کند:
اتصال انتها های نا همسان یا NHEJ :در NHEJ قطعاتی از DNA که در مجاورت شکست قرار دارند به هم پیوسته می شوند. NHEJ، DSBs را از طریق تنظیم اتصال انتها به انتها، ترمیم می کند.
مکانیسم دوم ترمیم نوترکیب HRR
یک خواهر یا کروموزوم همسان به عنوان یک الگو برای ترمیم استفاده می شود.HRR یک فرایند بدون خطا است، زیرا ترمیم با کپی کردن اطلاعات از کروماتید / کروموزوم همسان انجام می شود.
رقابت برای تعمیر توسط HRR در مقابل NHEJ بوسیله پروتئین 53BP1 تنظیم میشود.
در سلولهای یوکاریوتی مثل مخمر، HRR مسیر غالب برای ترمیم DSBs DNA است.
در سلول های پستانداران، انتخاب ترمیم، به فاز چرخه سلولی و فراوانی تکرار DNA بستگی دارد.
HRR، عمدتا در مرحلۀ S / G2 چرخه سلولی رخ می دهد، زمانی که یک کروماتید خواهر آسیب ندیده به عنوان یک الگو در دسترس است.
NHEJ در مرحله G1 چرخه سلولی اتفاق می افتد، زمانی که چنین الگوئی وجود ندارد.
NHEJ و HRR هر دو در اواخر S/G2 فعالیت دارند بنابراین منحصرا در مقابل هم نیستند. و این مطلب بیانگر این است که هنوز فاکتورهای ناشناخته دیگری علاوه بر چرخه سلولی وجود دارد.
واکنش فوری یک سلول به DNA DSB، فعال شدن یک گروه از سنسورها است که: 1) ترمیم DNA را ارتقا میدهند 2) از ادامه چرخه سلول تا زمانیکه شکست به صورت درست ترمیم شود، جلوگیری میکند.
ATM پردازش انتهاهای شکسته DNA را ارتقا میدهد.
NHEJ را می توان به پنج مرحله تقسیم کرد:
شناسایی پایانه توسط پیوستگی Ku ( در حقیقت قدم اول اتصال انتهاهای DSB توسط هترودایمر ku70/ku80 )
فعال شدن کمپلکس DNA-ku/PKcs برای الحاق کردن دو انتهای DNA
پردازش پایانه
سنتز و یا پل زدن پایانه
مهر و موم کردن.
فعالیت 53BP1 مانعی برای نوترکیبی همسان است.
HRR ترمیم DNA DSB در ژنوم پستانداران از طریق مکانیسمی با وفا داری بالا فراهم می کند.
در مقایسه با NHEJ، که نیازی به توالی هومولوگی برای اتصال مجدد پایانه های شکسته شده ندارد، HRR نیاز به تماس فیزیکی با یک کروماتید یا کروموزوم آسیب ندیده (برای تهیه یک الگو) دارد.
مرحله اولیه HR، تشخیص ضایعه است
و تغییر انتهاهای دو رشتهDNA به زنجیره های تک رشته ای بوسیله کمپلکس MRN، که بعد توسط RPA تشکیل یک رشته نوکلئوپروتئین می دهند.
سپس، پروتئین های اختصاصی HR ، مثلRAD51، RAD52 و BRCA1 / 2 در رشته های نوکلئوپروتئین، بکار گرفته می شوند.
RAD51 یک پروتئین کلیدی در ترکیب HRR است، زیرا بعنوان یک ماده واسط وارد رشته همسان کروماتید خواهر شده و منجر به تشکیل اتصالات Holliday میشود.
تصالات Holliday در نهایت به دو دوپلکس DNA تقسیم می شوند.
چندین کراس لینک DNA-DNA و پروتئین-DNA توسط اشعه یونیزان ایجاد میشود. ژن ها و مسیرهایی که برای ترمیم اینها استفاده می شوند، همچنان تحت بررسی هستند. تفکر فعلی این است که ترکیبی از مسیرهای ترمیم NER و نوترکیبی برای ترمیم Crosslink مورد نیاز است.
Crosslinks در طی تکثیر بسیار مشکل زا هستند و می تواند به توقف چرخه سلولی و حتی مرگ سلولی منجر شود.
Crosslinks DNA-DNA را می توان به دو دسته تقسیم کرد:
درون رشته ای (که روی یک رشته DNA اتفاق می افتد) و میان رشته ای (که بین دو رشته DNA اتفاق می افتد)
Crosslink درون رشته می تواند منجر به انسداد فعالیت DNA پلیمراز و یک گپ در یک رشتهDNA شود. سلول میتواند این گپ را از طریق سوئیچ DNA پلیمراز به رشته الگوی بدونcrosslink یا فرایند ترجمه حل کند.
Crosslink میان رشته ای اگر در صورتی که اصلاح نشود منجر به بلوک کامل تکثیر و جلوگیری از خنثی سازی DNA دوپونگ می شود،
بین دو رشته DNA رخ می دهد
منجر به انسداد کامل تکثیر و جلوگیری از باز شدن زنجیره DNA برای رونویسی میشود، بنابراین در صورت عدم اصلاح منجر به مرگ سلول میشود.
ترمیم توسط کمپلکس های پروتئینی مختلف بسته به فاز چرخه سلولی انجام میشود.
در مرحله G1، حذف توسط NER.
فقط درصد کمی از inter-strand crosslinks در مرحله G1 ترمیم می شود و آسیب های بدون ترمیم یک مشکل عمده برای سلول در مرحله S خواهد بود.