Transcript DNA Repair Nucleotide Excision Repair XP and CS

DNA Repair
Nucleotide Excision Repair
XP and CS
BiS222
By Jooyoung Chang 장주영
Class #01 / Stu. #20091259
Assembling a Car
An analogy
Steps in building a CAR
Blueprint
• Kept at company
headquarters
• Safe from damage
Copy
• Copy machine
• Copy of the
blueprint for
outside use
Factory
• Copy sent to
factory
• Workers and robots
make the car using
instruction
Steps in building a PROTEIN
Blueprint
• DNA kept in
Nucleus
• Somewhat safe
from damage
Copy
• RNA polymerase
“copy machine”
• Copy DNA into
RNA for use
outside
Factory
• RNA sent to
“factory”
• Ribosomes make
and assemble
proteins
Q: Most devastating effect?
 Mistake
in the Blueprint!
 Toyota (Jan. 2010)


Massive recall foot pedal
Mistake in blueprint-level
 DNA


Damage
Has devastating effects
DNA is the starting point of protein
production, thus cell function
DNA Damage Example
 Tumor



 If


suppressor genes
Gene that encodes protein: p53
Recognizes damaged DNA
And prevents mutant cell from replicating
p53 is damaged
Mutated cells grow uncontrollably
And become tumorous!
Another Example
 RNA




polymerase genes
Copy machine
DNA to RNA step is JAMMED!
Cell cannot function
EX) Cockayne’s Syndrome, will be discussed in the
flash video.
DNA is Fragile
 Ionizing
radiation often breaks DNA strands
 Reactive chemicals, many of which are from
the cell itself, cause structural alteration
 UV radiation causes chemical alterations
 Even thermal energy from the cell can cause
spontaneous alterations, or lesions
DNA Repair to the Rescue!
 The
cell has a remarkable ability to repair
DNA
 These DNA repair mechanisms are
responsible for repairing thousands of DNA
alterations per day!
Repair Mechanisms
 Base

Excision Repair
Multi-step process that corrects non-bulky
damage
 Mismatch

Corrects DNA replication errors
 Double

Repair
stranded breakage repair
Repairs complete cuts in the DNA strand
 Nucleotide

excision repair (NER)
Removes bulky lesions in the cell
Nucleotide Excision Repair
 NER
is perhaps the most flexible of the
DNA repair pathways considering the
diversity of DNA lesions it acts upon.



Pyrimidine dimers
Nucleotides with altered chemistry
Thymine dimers
Thymine Dimer
Thymine
H
Ultraviolet
Radiation
O
CH3
N
Thymine
O
H3C
N
H
Covalent Bonds
O
N
H
H
DNA Backbone
N
O
Nucleotide Excision Repair
1. Transcription-Coupled Pathway
2. Global Genomic Pathway
CSA
XPC
CSB
RNA Polymerase
5’
T= T
3’
3’
5’
RNA
Nucleotide Excision Repair
TFIIH
XPB
5’
XPD
3’
T= T
3’
5’
Hydrogen bonds
Nucleotide Excision Repair
XPF
ERCC1
5’
XPB
3’
XPG
T= T
3’
XPD
5’
Nucleotide Excision Repair
5’
3’
XPF
ERCC1
XPB
T= T
XPG
3’
XPD
5’
Nucleotide Excision Repair
DNA
Polymerase
δ/ε
5’
3’
3’
5’
Ligase
Is NER really important?
 If
we want to see whether car tires are important,
 Let someone ride a car without it.
Is NER really important?
 If
we want to see whether NER is important,
 Let see what goes wrong in a cell and the
organism when NER is dysfunctional!
 Two diseases
Xeroderma Pigmentosum [XP]
 Cockayne’s Syndrome [CS]
(Remember the copy machine?)

What goes wrong in XP?
• Patients with Xeroderma
Pigmentosum have
mutated XP genes and
cannot make proper XP
enzymes.
• NER doesn’t function at
normal levels, DNA
damage is not repaired
well, many genes cannot
be used.
XPC
XPB
XPG
XPD
XPF
ERCC1
Symptoms of XP
• Skin- sunburns and
freckles
• Cancer/Carcinomadamaged tumor
suppressor genes
(1000 fold risk)
• Eye abnormalities
• Neurological Disorders
in severe cases
Treatments?
• Avoid sunlight and
wear protective
coverings
• Dimericine® -T4N5
Liposome Lotion
(FDA Fast Track Designation)
• Gene therapy using
recombinant
retroviruses – still
under development
Proper XP genes
T4
N5
VIRUS
XP
affected
Cell
NER restored!
What goes wrong in CS?
• Cockayne’s Syndrome is caused by
mutation CSA and CSB genes
• Only the transcription coupled pathway
is impaired – so active genes cannot be
transcribed.
• Global pathway is
unchanged – normal
risks of cancer
Symptoms of CS
• Mutated CSA and CSB
likely hinder transcription
mechanisms affecting
damaged and undamaged
DNA alike
• Dwarfism, photosensitivity
disorders, premature
aging, and hearing loss
• No cure, treatment
focuses on relief
Key points to take away
• DNA is under constant threat of damage
• Cells repair DNA as its used (transcribed)
• Cells repair DNA in the inactive state
• Malfunction in repair mechanisms lead to
life-threatening diseases
REFERENCE
Karp, Gerald. “Cell Biology” Pg 291
Pescarmona, Gianpiero. “DNA Breaks and Repair”
<http://flipper.diff.org/app/pathways/info/1940>
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