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 specialize and form
useless lumps of cells called malignant tumours (ACCV Pg.13). Genetic Screening
is the process where Deoxyribonucleic Acid (DNA) fragments are analyzed 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 This is done by amplifying DNA withdrawn from an individual, then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 autonomic 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).
Bibliography
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