Huntington Disease, (HD), is a hereditary degenarative brain disease. Usually begining in mid-life, cells in the caudate nucleus of the brain begin to die, causing a relentless deterioration of intellectual ability, emotional control, motor skills, balance and speech. Chorea, or involuntary movements, is nearly always a symptom as well. HD is caused by a single dominant gene. Every child of an affected parent has a 50% chance of inheriting the gene that causes HD. The HD gene is found on chromosome 4.
It is inherited as an autosomal dominant condition. Since it is not carried on one of the sex chromosomes, male and female offspring have the same chance of being affected. Presymptomatic testing is a method for identifying persons carrying the HD gene before symptoms appear. This can be useful for couples who have a history of HD in their families and wish to know if they have the HD gene before deciding upon having children. The genetic testing itself involves donating a small sample of blood that is screened in the laboratory for the presence or abscence of the HD mutation. Testing may require a sample of D.N.A from a closely related affected relative, preferably a parent, for the purpose of confirming the diagnosis of HD in the family.
Genetic techniques used to do presymptomatic testing for the HD gene can be Gel Electrophoresis or Polymerase Chain Reaction (P.C.R).
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The Polymerase Chain Reaction is a technique used to amplify a particular fragment or sequence of DNA. Gel Electrophoresis is used to ‘sort out’ specific genes on D.N.A. Restriction enzymes are used to ‘cut’ the D.N.A sequence needed in P.C.R. P.C.R is a technique which is used to amplify the number of copies of a specific reigon of D.N.A to be adequately tested. The first step of P.C.R – Unknown D.N.A is heated, which causes the paired strands to seperate (single strands now accessible to primers).
Second step – Add large excess of primers relative to the amount of D.N.A being amplified, and cool the reaction mixture to allow double strands to form again. Third step – To a mixture of all 4 individual letters (deoxyribonucleotides), add an enzyme which can ‘read’ the opposing strands ‘sentence’ and extend the primers ‘sentence’ by ‘hooking’ letters together in the order in which they can pair across from one another – A:T and C:G. The enzyme used in P.C.R is Taq Polymerase because it can withstand high temperatures without being denatured. Now the enzyme synthesizes new D.N.A in opposite directions. PCR occurs when two primers bind to a DNA template in a proximity and orientation which allows the Taq Polymerase to synthesize the DNA sequence which lies between the two primers. The PCR reaction involves numerous repeated cycles of DNA synthesis. Each cycle uses the products of the previous cycle as a DNA templates.
After 30 – 40 cycles, a single DNA template molecule can produce millions of DNA products containing the sequence found between the two primers. Gel Electrophoresis is a technique used for the seperation of nucleic acids and protiens (D.N.A).
Seperation of large molecules depends on two forces – charge and mass. When a biological sample such as protiens or D.N.A, is mixed in a buffer solution and applied to a gel these two forces act together. The electrical current from one electrode repels the molecules while the other electrode attracts the molecules. The frictional force of the gel material acts as a ‘molecular sieve’, seperating the molecules by size. During electrophoresis, macromolecules are forced to move through the pores when the electrical current is applied.
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If the D.N.A tested is shown to be positive to the HD gene then the ‘CAG’ repeats will be between 42 – 86. In a ‘normal’ unaffected person the sequence ‘CAG’ repeats only 11 – 34 times. In terms of emotional and practical consequences, not only for the individual taking the test but for his/her entire family, testing is enormously complex and has been surrounded by considerable contreversy. For example people with a positive test result may risk losing health and life insurance, suffer loss of employment and other liabilities. Much of the working-through process which an at-risk individual must undergo to accept the illness is often expressed in terms of responses to other family memebers, with statements such as “What have I done to my child!” . Whatever the result of genetic testing, the at-risk individual and family members can expect powerful and complex emotional responses.
The health and happiness of spouses, brothers, sisters, children, parents and grandparents are affected by a positive test result, as are an individuals friends, work associates, neighbours and others. Because recieving the test results may prove to be devestating, testing guidelines call for continued counselling even after the test is complete and the results are known. BIBLIOGRAPHY: Internet: http://www.hdfoundation.org/testread.htmlhttp://ww w.ucl.ac.uk/ucbhjow/bmsi/lec5 Book: Evans, B., Ladiges, P., Mckenzie, J. and Batterham, P. (1999).
Heinemann Biology Two. Video Cassette: V.E.A.
Breaking the code Part Two, Applying Genetic Techniques To Human Disease. 1996..
