Radiation effect on cell

Radiation effect on cell
By Dr. Deepa Gautam
1 yr Resident, Radiotherapy
1

Radiation Absorption
• Absorption of energy from radiation in biologic
material may lead to:
• Excitation: raising of an electron in an atom to higher
level
• Ionization: ejecting one or more electrons from an
atom (the radiation is known as ionizing radiation).

2

Types of Ionizing Radiation
• Directly Ionizing: when absorbed in material, they
directly cause ionization leading to damage. Eg.
Electrons, α-particles, β-particles
• Indirectly ionizing: when absorbed in material, they
give up their energy to produce fast moving charged
particles which produce the damage. eg.
Electromagnetic radiation

3

Electromagnetic radiation
• Electric and magnetic fields are perpendicular to
each other
• Eg. X-rays and γ-rays , occupy the short wavelength
end of electromagnetic spectrum

4

Action of Radiation
• Direct: when absorbed, radiation directly interacts
with targets in the cells and the atoms get ionized or
excited.
• Indirect: when absorbed, radiation interacts with
other atoms or molecules in the cell particularly
water to produce free radicals that produce the
damage.
• H20→ H20++e• H20++H20→ H30++OH.

5

Modes of cell death after irradiation:
•
•
•
•
•

Mitotic Death
Interphase Death
Apoptotic Death
Necrotic Death
Autophagy

7

Mitotic Death
• Cells lethally injured by clinically relevant doses of
radiation execute one or more divisions before
mitotic death
• Mechanisms :
– Failure of spindle formation in M phase
– Loss of G2 check point
– Improper chromosomal segregation due to
damage and loss of genetic material

8

Interphase Death
• Occurs in radiosensitive cells
• Occurs within 2-6 hrs of radiation
• Cells dying in interphase cannot contribute to
reproductive pool
• Death occurs by rapid apoptosis

10

Autophagy
• Cells internalize cellular organelles within the
vacuoles and digest them
• Also a type of programmed cell death

13

Radiation Injury In Normal Tissue
• Acute Response
• Subacute Response
• Late Response

14

Acute response
• Occurs during standard 6-8 wks course
• Depletes stem and progenitor cell pools
• Severity of injury depends upon extent of cellular
depletion and length of delay before new functional
cells are released
• Severity increases with dose and fractionation
decreases severity allowing time for regeneration

15

Subacute Response
• Occurs few to several months after irradiation
• Symptoms are usually reversible but sometimes may
be severe to cause death
• Mostly occurs during remodelling phase
• Eg. somnolence after brain irradiation, subacute
pneumonitis after lung irradiation

16

Late Response
• Occurs due to depletion of slowly proliferating cells
that are lost at slow rate(eg. Renal tubular
epithelium, oligodendrocytes, schwann
cells,endothelium,fibroblasts)

17

• DNA is the principal target in the cell for biologic
effects of radiation.

18

Effects of radiation on DNA
• Single strand break (SSB)

• Double strand break (DSB)

20

SSB
• Repaired readily using opposite strand as a template
• Misrepair may result in mutation

22

DSB
• Well separated breaks in two strands repair in similar
ways as SSB
• Breaks in two strands opposite one another or
separated by only few base pairs may lead to DSB in
which the piece of chromatin breaks into two pieces
• Most important lesions in chromosomes produced
by radiation
• May result in cell killing, carcinogenesis or mutation

23

Radiation induced Chromosomal Aberrations:
• DSBs occur
• Sticky broken ends can join with other sticky end
• Possibilities:
– Rejoin in their original configuration
– Fail to rejoin, give aberrations and deleted in next
mitosis
– Broken ends rejoin and form grossly distorted
chromosome

24

• Aberrations viewed at metaphase of mitosis
as:
– Chromosomal aberration
– Chromatid aberration

25

Chromosomal aberrations
• cells irradiated in early interphase before
chromosomal duplication
• Break occurs in single strand of chromatin
• Chromatin lays down an identical strand with break
during synthesis phase

26

Chromatid Aberrations
• Cells irradiated after chromosomal duplication
• Break may occur in one of the sister chromatids or in
both but not at the same place as chromatids are
well separated except in the region of centromere

27

Lethal Aberrations:
1. Dicentric chromosome
2. Ring chromosome
3. Anaphase bridge
1 and 2 are chromosome aberrations and 3 is
chromatid aberration

28

Non-lethal Chromosomal aberrations
•Symmetric Translocations
Break in two prereplication(G1)
chromosomes with broken
ends being exchanged

•Small Deletions
Two breaks in the same
arm of a chromosome
leading to the loss of
genetic information
between two breaks

32

DNA Repair Pathways:
•
•
•
•
•
•

Base Excision Repair(BER)
Nucleotide Excision Repair(NER)
DNA DSB Repair
Single Strand Annealing
Cross link Repair
Mismatch Repair

33

Base Excision Repair
•Base damage repaired by this
process
•Incorrect base removed by DNA
glycosylase /lyase
•Sugar residue removed by
Apurinic Endonuclease1
•Correct base replaced by DNA
polymerase β
• Sealed by DNA ligaseIIIXRCC1(X-Ray cross
complementing factor1)

34

BER for multiple nucleotides
•Incorrect bases removed by
Apurinic Endonuclease1
•Repair synthesis by complex of
RFC/PCNA/DNA polymerase δ/ε
•Unwanted flap removed by FEN1
endonuclease
•Sealed by DNA ligase I

35

Nucleotide excision repair
• Removes bulky adducts in DNA like pyrimidine
dimers
• Steps:
– Damage recognition
– DNA incisions that bracket the lesion
– Removal of adducts containing region
– Repair synthesis to fill the gap
– DNA ligation

36

DNA DSB repair
• Repaired by two processes:
– Homologous Recombination Repair(HRR)
– Nonhomologous End Joining(NHEJ)

37

Homologous Recombination Repair
• Occurs in late S/G2 phase
• Undamaged sister chromatid acts as template

38

ATM(ataxia telangiectasia mutated) and ATR(AT and Rad3 related)sense the DSB and recruit
to the site

H2AX phosphorylated
BRCA1 recruits to the site to regulate the activity of NBS/MRE11/Rad50s protein complex
Unidentified endonucleases and MRE11 resect the DNA

Rad51 binds to 3’ single strand DNA
BRCA2 recruited
Rad51 loaded on RPA coated single strand

Rad52 recruited which protects against exonucleolytic degradation
Rad54 unwind the double stranded molecule
Two invading ends serve as primers for DNA synthesis

Holiday junctions resolved by MMS4 and MUS81 by non crossing over or crossing over
Gap filling of DNA strand
39

NHEJ
• Occurs in G1 phase
• Steps:
1. End recognition by Ku heterodimer and DNA
dependent protein kinase catalytic subunit
2. End processing by a protein Artemis forming
complex with DNA-PKcs and endonuclease activity
activated
3. End bridging or fill-in synthesis by DNA polymeraseµ
4. Ligation by XRCC4/DNA ligase IV complex

41

Single Strand Annealing
• Plays a transition role between HRR and NHEJ
• Ends of DSB digested by
endonuclease(NBS/MRE11/Rad50 complex) until the
region of homology are exposed on both ends of the
breaks
• Nonhomologous ends are removed and two ends are
ligated

43

Cross-link Repair
• Cross links occur between DNA-DNA and DNAproteins
• Nucleotide excision repair and recombinational
pathways combine to repair these cross links

44

Mismatch Repair
• Removes base-base and small insertional
mismatches occuring during replication and also
during HRR

45

Operational Classification of radiation damage:
• Lethal damage: irreversible and irreparable damage
that leads to cell death
• Potentially lethal damage: causes cell death under
ordinary circumstances but can be modified by
postirradiation environmental conditions
• Sublethal damage: repairable in hours under
ordinary circumstances unless additional sublethal
damage is added

46

References:
• Perez and Brady’s Principles and Practice of Radiation
Oncology, 5th Edition
• Radiobiology for the Radiologist by Eric J. Hall and
Amato J. Giaccia ,6th Edition

47

Radiation effect on cell

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