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Lec.4 Regulation of gene expression
Gene expression includes mechanisms that are used by cells to
increase or decrease the production of specific gene products
(protein or RNA), and is termed gene regulation, Only certain
genes are expressed at certain times in certain places in certain
amounts .DNA for all genes is in all cells, but only
EXPRESSED in certain cells.
Abnormalities in gene expression can lead to diseases
including cancer
Regulation of gene expression is to determine how much and
when particular gene products made. It is the bases for cellular
differentiation and adaptability.
Gene expression is regulated at many stages. transcription is
the primary site of regulation , regulation of gene expression
can occur at the posttranscriptional and posttranslational level.
Not all genes are regulated for example gene that encode
product required for basic cellular functions are continually
expressed (known as constitutively expressed or housekeeping
genes) they may be expressed in all cells or in some cells as in
hepatocytes
Regulated stages of gene expression
Any step of gene expression may be modulated, from the
translational
-
post
step to
transcription
RNA
-
DNA
of a protein. The following is a list of stages
modification
where gene expression is regulated, the most extensively
utilized point is Transcription Initiation:
Modification of DNA
Transcription
Post-transcriptional modification
RNA transport
Translation
mRNA degradation
Modification of DNA:
gene expression can be affected by:
A. The availability of DNA to the transcriptional apparatus,
B. The amount of DNA.
C. The arrangement of DNA.
A. The availability of DNA:
DNA is found complexed with histone and non histone protein
to form chromatin, Chromatin-modifying enzymes provide
initial control of gene expression by making a region of DNA
either more available or less available for transcription to be
transcriptionally active the chromatin must be decondensed
while the transcriptionally inactive is highly condensed. in the
active chromatin the histone protein is modified by
acetylation or phosphorylation these modification will
decrease the strength of association of protein with DNA and
this will allow transcription factors to access specific region
on the DNA.
Another modification is by changing the methylation
level,DNA methylation reduces gene expression. ;some
enzymes methylate histone proteins, other enzymes methylate
certain bases in DNA itself. Inactive DNA is generally highly
methylated compared to DNA that is actively transcribed.
Methylation of DNA is a common method of gene silencing.
B. Amount of DNA: A change in the number of copies of a gene
can affect the amount of gene product produced.
Regulation of gene expression at the level of transcription:
The transcription of active gene is regulated by controlling
the assembly of the basal transcriptional complex containing
RNA polymerase and its binding to TATA box of the
promoter .the basal transcriptional complex contains the
TATA binding protein and other proteins called general
transcription factors that form a complex with RNA
polymerase II.
The control region of a gene also contain DNA regulatory
sequences (also called enhancers) that are specific for that
gene and may increase its transcription 1ooo fold or more.
transactivators or activators are gene –specific
transcription factors that binds to DNA regulatory
sequences and interact with another protein called co-
activator .
Co-activator interacts with basal transcriptional complex and
can activate its assembly at the initiation site on the promoter.
Silencers are regions of DNA sequences that, when bound by
particular transcription factors, can silence expression of the
genes.change in the binding of any of these can change the
expression of the gene.
Post-transcriptional regulation:
After the DNA is transcribed and mRNA is formed, there must
be some sort of regulation on how much the mRNA is
translated into proteins. Cells do this by modulating the
capping, splicing, addition of a Poly(A) Tail, the sequence-
specific nuclear export rates, and, in several contexts,
sequestration of the RNA transcript.
Regulation of translation
Translation of the mRNAs may be regulated by the activation
or inactivation of the protein factors required to initiate
translation.
Transport & stability of mRNA:
Stability of mRNA plays a role in regulating gene expression
because mRNA with long half life can generate more protein
than those with shorter halflives. mRNA can be degraded by
nuclease enzyme during transport from the nucleus to
cytoplasm ,but this is prevented by a protein bound to it &
poly A tail, when mRNA ages its poly A tail become shorter .
Biochemistry of Cancer
The term cancer applies to a group of diseases in which cells
do not respond to normal restraints on growth,normal cells in
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body respond to signals that direct them to stop
proliferating,cancer cells donot. in addition they don’t require
growth stimulatory signals and resist growth inhibitory
signals,they have infinite proliferative capacity,immortalized
and can grow without structural support.
It is a long term multistage genetic process. first the DNA is
damaged (base changes or strand breaks) by some form of
carcinogen: physical, chemical, and biologic agents (e.g.
smoking,UV light, radiation, chemicals, and virus), or
replication errors ,mutation results from the damaged DNA if
it is not repaired properly or if not repaired before replication
occurs. These agents damage or alter DNA, so that cancer is
truly a disease of the genome.
Normal cells contain proto-oncogenes whose function is to
promote proliferation & cell survival. Oncogenes are mutated
derivatives of normal genes, tumor suppressor genes(normal
growth suppressor genes).
when a cell with one mutation proliferate it results in a group
of cells with this one mutation,from which one cell may have
second mutation and with each proliferation the chance of
more mutation increase, until the cells have multiple mutation
(4-7) necessary for full transformation to cancer.
proto-oncongen(normal gene) are converted to oncogen by
many mechanisms:
Radiation and chemical carcinogens cause mutation in the
regulatory region of the gene increasing protein production
or mutation occurs in coding region of gene results in the
synthesis of new protein which is able of changing the cell
to malignant.
or mutation may move the proto-oncogen from one
position to another in the genome and in the new location it
is expressed differently this allows the gene to be
expressed in tissue where it is normally not expressed .
the proto-oncogene may be amplified-so more copy of the
gene is produced in a single cells-so more protein will be
produced,increasing the growth rate of the cells.
Mutation may occur in the repair enzymes which are the
first line of defense preventing conversion of chemical
damage in DNA to mutation.
Tumor suppressor genes normally inhibit proliferation of
cells in response to signal as DNA damage. they contribute
to cancer when it is inactivated by a mutation.
Tumor Markers
Tumor markers are substances, usually proteins that are
produced by the body in response to cancer growth or by the
cancer tissue itself Tumor markers can be:
Enzymes, glycoproteins, hormones and hormone-like
substances, hormone receptors, oncogenes, and oncogene
receptors.
Some tumor markers are specific, while others are seen in
several cancer types. Many of the well-known markers are
also seen in non-cancerous conditions. so, these tumor
markers are not diagnostic for cancer.
The goal is to be able to screen for and diagnose cancer early,
when it is the most treatable and before it has had a chance to
grow and spread.
Some people are at a higher risk for particular cancers because
they have inherited a genetic mutation. While not considered
tumor makers, there are tests that look for these mutations in
order to estimate the risk of developing a particular type of
cancer. BRCA1 and BRCA2 are examples of gene mutations
related to an inherited risk of breast cancer and ovarian cancer.
Ideal tumor markers
Be specific to the tumor
Level should change in response to tumor size
An abnormal level should be obtained in the presence of
micrometastases
The level should not have large fluctuations that are
Independent of changes in tumor size
Levels in healthy individuals are at much lower
concentrations than those found in cancer patients
Predict recurrences before they are clinically detectable
Test should be cost effective.
Why are they done?
Tumor markers are not diagnostic in themselves. A definitive
diagnosis of cancer is made by looking at biopsy specimens
(e.g., of tissue) under a microscope. However, tumor markers
provide information that can be used to:
1) Screen: some may be used in those with a strong family
history of a particular cancer. In the case of genetic markers,
they may be used to help predict risk in family members.
2) Help diagnose: In a patient that has symptoms, tumor markers
may be used to help identify the source of the cancer, such as
CA-125 for ovarian cancer, and to help differentiate it from
other conditions.
3) Stage: If a patient does have cancer, tumor marker elevations
can be used to help determine how far the cancer has spread
into other tissues and organs.
4) Determine prognosis. Some tumor markers can be used to
determine how aggressive a cancer is likely to be.
5) Guide Treatment. Some tumor markers will give information
about what treatments the patients respond to.
6) Monitor Treatment. Tumor markers can be used to monitor
the effectiveness of treatment, especially in advanced cancers.
If the marker level drops, the treatment is working; if it stays
elevated, adjustments are needed.
7) Determine recurrence. Currently, one of the biggest uses for
tumor markers is to monitor for cancer recurrence. If a tumor
marker is elevated before treatment, low after treatment, and
then begins to rise over time, then it is likely that the cancer is
returning. (If it remains elevated after surgery, then chances
are that not all of the cancer was removed.)