Nem’s Notes…
Phase 2 Year 3
MOLECULES, CELLS & TISSUES 2 (page 1 of 2)
DNA Damage
co-author: Sabina Rashid
DNA Damage
Examples of areas in which DNA can be damaged include:
(a) Double/single strand breaks
(b) Base dimerisation/cross links
(c) Chemical crosslinks
(d) DNA adducts
(e) Abasic sites
(f) Hydroxylation of bases
Carcinogens
Carcinogens are substances which can cause cancer via damage to DNA and
include: (a) Motor vehicle emissions
(b) Emissions from other carbon fuel sources
(c) Waste incineration
(d) Medicines and medical techniques
(e) Tobacco and lifestyle
(f) Generated carcinogens in food/cooking/spoilage
Cytochrome
P450s
Cytochrome P450s are a family of haem-containing membrane bound proteins which
are used in the first phase of a two-phase process for eliminating toxic compounds
from the body. There are 27 members of the P450 superfamily which have diversified
by evolution to cope with environmental chemicals, food toxins and drugs.
They catalyse a large number of oxidation, reduction and hydrolysis reactions on
diverse substances including drugs. There are polymorphisms in these large gene
families such as P450 2D6 (CYP2D6) which can cause fast, medium or slow
metabolism of certain drug types.
Repair
Mechanisms
DNA damage occurs naturally under normal circumstances. For normal functioning
there are a number of repair mechanisms which are employed and include those
listed in the table below.
Name
Base Excision Repair
Nucleotide Excision Repair
Recombination Repair
Mismatch Repair
Mechanism
Removal of individual or patches of damaged bases
or mispaired bases
Removal of larger molecules such as pyrimidine
dimers or bulky DNA adducts
Post replication deletion of mutations or
chromosomal abnormalities
Mispaired bases are removed and replaced with
paired bases
The repair mechanisms have the same basic mechanism:
(a) Excision
(b) Resynthesis
(c) Ligation
Cancer & The
Cell Cycle
For a mutation to have any cancerous effect it must be in a critical sequence (such as
one controlling regulatory pathways). These sequences include:
(a) Mitogenic/Mitoinhibitory pathways
(b) Apoptosis pathways
(c) Cell Cycle pathways
Normal cells have two copies of each of the tumour suppressors, so mutation or
deletion of one copy will leave one remaining and prevent clonal expansion of that
cell-line. If both copies are lost then proliferation of the abnormal cell may occur.
more online at http://homepage.virgin.net/nemonique.sam/noteindx.htm page 1 of 2
Nem’s Notes…
Phase 2 Year 3
MOLECULES, CELLS & TISSUES 2 (page 2 of 2)
DNA Damage
p53 Protein
co-author: Sabina Rashid
p53 is a gene regulatory protein which activates transcription of p21, a CDK inhibitor
protein. p53 is activated during DNA damage and causes p21 to bind to cyclin-CDK
complexes which normally drive the cell into S phase. This effectively halts the cell
cycle at the G1 checkpoint and prevents it from entering S phase. This allows time for
repair of the damaged DNA whilst in G1 phase or for apoptosis signalling.
Mutated p53 is associated with many tumours since the cell will enter S phase
whether there are errors or not. If there are errors abnormal cells will proliferate. p53
is normally expressed in low quantities and is regulated by binding of mdm-2, a
molecule which inhibits p53. Active p53 induces mdm-2 expression so an
autoregulatory feedback loop exists.
p53 can become ‘stabilised’ in response to genotoxic stressors which allows repair
mechanisms to come into play. p53 stabilisation pathways depend on the stress the
cell is responding to.
Stressor
DNA Damage
Inappropriate
Proliferation
Pathway
p53 is phosphorylated by the phosphoinositide-3-kinase (PIK)
family which blocks mdm-2 binding
p53 is bound by p19ARF which blocks mdm-2 binding sites
(This is called the ‘oncogene checkpoint’)
p53 causes either G1 arrest (mediated by CDK inhibitors) or apoptosis and is a tumour
suppressor due to its negative effect on cell growth.
Carcinogenesis Carcinogenesis is the process by which a normal cell changes into a cancerous one.
Cancer is the loss of control of normal growth mechanisms leading to uncontrolled cell
proliferation and tumours.
Cancer is a multi-step, multi-pathway process with at least 5 or more steps involved.
Generally speaking there are three main phases.
Phase
Features
Initiation
Irreversible DNA damage that does not kill cell.
Can be caused by mutagens (eg UV irradiation), carcinogens (eg
aflatoxin) or spontaneous mutations.
Promotion
Progression
Mutations
Selective clonal expansion of those cells by some promoting factor.
There is continuous exposure above a certain threshold which
causes selective pressure on the mutant cell-line. It is reversible and
can be caused by drugs (eg phenobarbital, tamoxifen) and
chemicals.
Subsets of the altered cells become capable of independent growth.
This can be caused by autonomous growth, inactivation of tumour
suppresors, activation of oncogenes, metastases or genetic
instability.
Not all initiated cells have the potential to become tumours. Only those with
appropriate DNA damage will cause cancer. The types of mutation that can occur
include: (a) Somatic mutation
Alterations in scattered cells of mature body
(b) Germ line mutation
Changes handed down from germ cells.
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