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Presymptomatic Testing For Huntington Disease

Uploaded by LoUiE on Aug 28, 2002

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). 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...

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Uploaded by:   LoUiE

Date:   08/28/2002

Category:   Biology

Length:   4 pages (820 words)

Views:   2391

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