FREE ESSAY ON GENETIC SCREENING OF BREAST CANCER

GENETIC SCREENING OF BREAST CANCER

Hereditary breast cancer is a disease caused by mutations on breast cancer suppresser
genes(ACCV Pg.17). Mutations allow normal cells to divide abnormally(ACCV Pg.13).
Resulting cells divide faster as they do not specialise and form useless lumps of cells
called malignant tumours(ACCV Pg.13).
Genetic Screening is the process where Deoxyribonucleic Acid (DNA) fragments are analysed
for a specific gene. The purpose is to identify individuals carrying disease causing
genes so they can change their life style and also help invent a cure(ACCV Pg.20). This
is done by amplifying DNA withdrawn from an individual, then specific gene mutations are
targeted using the Electrophoresis process.
The two genes, BRCA1 and BRCA2 isolated in 1994 and 1995 respectively are breast cancer
suppresser genes(Internet 1). BRCA1 is located on chromosome 17q21 and BRCA2 on
13q(Internet 2). A person that possesses certain mutations to these genes has an
increased risk of up to 80-90% in developing breast cancer(Internet 3).
The cost of genetic screening ranges among several hundred to several thousand dollars
depending on the tests performed and can take several weeks to many months from the
initial blood sample(Internet 4).
Public acceptance of genetic screening for severe disease causing genes in early
childhood is high(New Scientist Pg. 14). Many people argue for less debilitating diseases
that discrimination will occur against individuals carrying those genes(New Scientist Pg.
14).
In human cells there are 22 pairs of autosomal chromosomes and two sex chromosomes. These
chromosomes contain information for protein synthesis. DNA stores this information by a
sequence of nucleotides. There are four different nucleotides that construct DNA. They
all contain a 5 ring carbon sugar (Deoxyribose), a phosphate molecule and one of four
nitrogenous bases. The base names are Adenine (A), Thymine (T), Guanine (G), and Cytosine
(C). Adenine is complementary to Thymine and Guanine to Cytosine. The arrangements of a
series of nucleotides are genes. Hereditary Breast Cancer is an autosomal dominant
disease(Internet 3), meaning only one parent needs to carry the trait expression in the
parents offspring. The disease is cause by mutations found on the BRCA1 or 2 tumour
suppresser genes(Internet 3).
BRCA1 has 24 exons distributed over a genomic region of 81 kilobases long and located on
chromosome17q21(Internet 3)
Exon 11 being the largest that codes for 61% of a protein, 1863 amino acids and 5592
nucleotides long(ACCV Pg. 17). The irrelevant information known as introns found on BRCA1
range in size from 403 base pairs to 9.2 kilobases(Internet 3). Over 100
disease-associated mutations have be identified to this gene(Internet 3) 21 of these
found in exon 11(ACCV Pg. 17). These mutations code for a stop signal causing protein
truncation(ACCV Pg. 17).
BRCA2 has mutations that function the same as BRCA1(ACCV Pg. 18). BRCA2 has been linked
to hereditary breast cancer and increases the risk for male breast cancer. (ACCV Pg. 18).
BRCA2 is located on chromosome 13q12(Internet 2). Little additional detail about this
gene is available. Testing for BRCA2 is not widely available except within the research
laboratory.
There are two distinctive stages in protein synthesis of BRCA1, transcription and
translation. Transcription is the synthesis of messenger Ribonucleic Acid (mRNA). The
enzyme RNA polymerase initiates transcription by separation of DNA strands. RNA
nucleotides then bind to their complementary DNA nucleotides of the BRCA1 gene to form a
mRNA strand. The mRNA is different to the DNA strand of the BRCA1 gene as Uracil (U)
replaces Thymine and is complementary to Adenine.
The resulting mRNA strand detaches from the BRCA1 gene before the DNA Ligase enzyme joins
the DNA strands together. Splicing of the mRNA occurs to remove introns(Raven 440). The
mRNA now only contains exons, that are primary transcripts of the gene. The mRNA strands
leave the nucleus through nuclear pores to undergo Translation the second stage of
protein synthesis.
Translation occurs at the ribosome found in the cytoplasm, where production of the tomour
suppresser protein from mRNA occurs. A ribosomal RNA molecule with in the ribosome binds
to the start sequence of the mRNA strand. The ribosome then moves the mRNA strand through
3 nucleotides adding an amino acid. This process continues until the ribosome encounters
a stop signal at this point it disengages from the mRNA and releases the completed
suppresser protein.
Genetic screening can allow testing DNA to determine if an individual carries mutated
forms of the BRCA1 gene.
DNA collection is the first stage to screen for the BRCA1gene. White blood cells
withdrawn from a blood sample contain the needed DNA. The DNA needs amplification so that
a large volume of DNA is available for analysis. Polymerase Chain Reaction (PCR) is a
process that amplifies DNA(ACCV Pg. 20). Endonuclease restriction enzyme shortens the DNA
for amplification. This enzyme recognises the BRCA1 gene and cuts it from the DNA
fragment(ACCV Pg. 20).
PCR is sensitive and requires a gene that is between known two sequences such as
BRCA1(ACCV Pg. 21). Short complementary sequences to the BRCA1 gene, oligonucleotides,
known as a primer is added to the solution along with a DNA polymerase enzyme(ACCV Pg.
21). The DNA polymerase enzyme, taq polymerase from the bacterium Thermus auquaticus is
able to endure high temperatures(ACCV Pg. 21). High magnification occurs if the
oligonucleotides are preferably 20-24 nucleotides long(ACCV Pg. 21). A PCR waterbath then
incubates the DNA samples at different temperatures(ACCV Pg. 21). The first bath,
denaturing, occurs at 940C(ACCV Pg. 22). The high temperature removes the hydrogen bonds
between the nucleotides thus separating the BRCA1 sense and non-sense strands(ACCV Pg.
21). Annealing occurs at 500C(ACCV Pg. 22), in this stage the primer bonds to its target
sequence(ACCV Pg. 21). The final bath is the polymerisation stage that occurs at
720C(ACCV Pg. 22). In this stage the DNA polymerase uses the primers as a starting point
to replicate the DNA(ACCV Pg. 21). (refer to Fig 2 (ACCV Pg. 21)).
The waterbath cycles repeat 30 cycles resulting in over a billion copies of the desired
sequence(ACCV Pg. 21).
The next technique separates the cloned BRCA1 by electrophoresis on an agarose gel(Raven
Pgs. 439-441). DNA is a negatively charged molecule due to the phosphate molecules in its
makeup. For this reason it is possible to separate the fragments of DNA using an electric
potential across the gel(Raven Pg. 439). Fragments migrate down the gel by size --
smaller fragments move faster (and therefore go further) than larger ones(Raven 439-441).
Blotting of the agarose gel removes the nucleic acid onto a nitrocellulose sheet(Raven
440). This is done by placing a nitrocellulose sheet over the gel and then paper towel.
The paper towel draws a buffer solution through the agarose gel transposing the nucleic
acid on to the nitrocellulose sheet(Raven Pg. 440). A solution is added to the
nitrocellulose sheet containing radioactive probes. The single strand probe hybridises
with the BRCA 1 gene allowing the gene to expose an x-ray film(Raven 440) (Refer to Fig 3
(Raven 439-441)). Analysis of the film determines if BRCA1 is carried by the individual.
Genetic screening indicates if an individual with a family history carries a
susceptibility gene to breast cancer. This information has large benefits to these high
risk individuals.
People carrying the susceptibility genes have an increased risk of up to 80-90% in
developing breast cancer(Internet 3). This information will enable the individual to
change their life style as a consequence. The individual carrying the faulty gene is able
to have children that may never develop the cancer, but any children this individual has
will also fall into the high risk category. Individuals who choose not to have children
will not pass the mutated gene to there offspring and then from generation to generation.
In doing so this will reduce the amount of people carrying a mutated gene from
inheritance. A person with no family history can carry a mutation to these susceptibility
genes. This is possible because mutations occur regularly during cell division.
Individuals in the high risk category deciding to live their live without having children
both increase the survival of the species while altering the gene pool and as a result
altering genetic diversity. This concept will not alter the gene pool dramatically as
only 5-10% of breast cancer occurrences are due to genetic factors(ACCV Pg. 13).
Increased developments in genetic screening have enabled scientists to take a single cell
from an eight celled embryo and screen for mutations on BRCA1 or 2(New Scientist Pg. 14).
The genetically abnormal embryos face abortion.
Issues arise on what we should screen for in embryos, as some diseases occur later in
life like hereditary breast cancer while others such as Down's Syndrome causes severe
suffering in early life. Many professionals believe that this technique should not be
conducted on diseases that increase risk of developing that disease later in life such as
breast cancer(New Scientist Pg. 14). A study result from the general public shows that
less than half of the people surveyed where against genetic screening of genes that
predispose for cancer in their early thirties(New Scientist Pg. 14).
If genetic screening of embryos is available the amount of inherited disease will reduce
and perhaps eliminated occurrences altogether. The information also allows scientists to
develop possible cures for diseases such as breast cancer(Internet 5).
Raven and Johnson, Biology Fourth Edition Revised
Published 1996 by W.m.c Brown Publishing USA Times Mirror High Education Group.
The Anti-Cancer Council of Victoria, Breast Bowels and Mutant DNA
Published by The Anti-Cancer Council of Victoria. The Anti-Cancer Council of Victoria
Copyright 1996.
New Scientist, 28th October 1995
Every child a perfect child? Author Gail Vines. Published weekly by Reed Business
Publishing Pty Ltd
Time, January 17 1994, Diagram by Nigel Holmes Pg. 25
Published by TIME Australia Magazine Pty Ltd Copyright 1994
Internet 1, Breast Cancer Susceptibility Screening, Author Roxy R. Mickelson
Copyright 1997
Internet Address: www.ndsu.nodak.edu/instruct/mcclean/plsc431/students/mickelson.html
Internet 2, Gene Watch: The New Genetics-Consequences for Clinical Practice.
Author Dr. Eric Sidebottom, Oxford. Copyright Bandolier-Last Update: 10-July-98
Internet Address: www.
Internet 3, www3.ncbi.nlm.nih.gov:80
Internet 4, National Action Plan on Breast Cancer. NAPBC Fact Sheet: Genetic
Testing For Breast Cancer Risk: It's Your Choice
Internet Address: www.napbc.org
Internet 5, 'Breast Cancer Gene' May be Useful in Treating the Disease Its Protein
Slows Formation, Growth of Tomours in Lab, Author David Brown
Internet Address: www.