• Huntington's Disease
• Background Information
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• Gene therapy
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  Chain Reaction
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Module 1 Huntington's Disease (HD)
----a genetic disease that attacks our nervous system

Related topics: diseases, nervous coordination in humans, molecular genetics, applied genetics, techniques in modern biotechnology, biotechnology in medicine, bioethics Guiding questions: What type of disease is it? What are the symptoms and signs of a disease? Which part of our body is being malfunctioned? How can a disease gene cause the disease? (Tips: How can a gene become a protein?) How can I know the gene responsible for the disease? Which technique supports the success of Human Genome Project? What are the treatments of Huntington’s disease (HD)?

Introduction

“I started spluttering my words a lot and having to repeat things.  Two years later, I started tripping over my feet and falling a lot.  In the following years I lost my ability to speak and walk……” 

Try to imagine what this person looks like.  Is this person like an old man with shaking hands or a teenager with learning difficulty?

Actually, this is the story of Terry, a boy who was diagnosed with Juvenile Huntington’s Disease 亨汀頓氏舞蹈症 at the age of five.  His daddy also found to have Huntington’s disease when Terry was three years old. 

Terry’s story has revealed an important truth that this disease is affecting people of all ages, with an onset as early as a little child.  What’s more, this is a genetic disease which can pass to the offspring.

In this module, you are going to learn more about Huntington’s disease, including the genetic base and treatment.

What is Huntington’s disease (HD)?

Huntington’s disease (HD) is a genetically programmed degeneration 退化 of neurons where patients will suffer from uncontrolled movements, loss of intellectual faculties 智能 and emotional disturbance 情緒困擾.

Symptoms include mood swings 情緒波動, passive, emotionally depressed or angry.

Signs include difficulty in learning new things, driving, remembering a fact, making decision or even answering questions.

Genetic base of Huntington’s disease (HD)

Huntington’s disease is a genetic disease where there is a mutation in a small sequence of DNA on chromosome 4 which is the Huntingtin (HTT) gene.  This is the first gene being mapped to a human chromosome in 1993.  A normal person has 6 to 35 CAG repeats at that gene but patients of Huntington’s disease (HD) have 40 or more repeats of CAG.

With more repeats of CAG, the disease manifests 表露 earlier and becomes more devastating 破壞性的.

The gene responsible for Huntington’s disease (HD) is dominant; this means children can inherit this disease in both homozygous 純合子 and heterozygous 雜合子 conditions. For any one parent having an Huntington's Disease (HTT) gene, his/her children will have 50% chance of inheriting this disease.

The identification of all the approximately 20,000 – 25,000 genes in human DNA is completed in 2003 by the Human Genome Project (HGP).  This was a 13 year project coordinated by the U.S. Department of Energy and the National Institutes of Health. Aims of this project include determining the sequences of the 3 billion chemical base pairs that make up human DNA, storing the information in databases, transferring the related technologies to the private sector and addressing the ethical, legal and social issues that may arise from the project.

The success of the HGP relies on the invention of polymerase chain reaction (PCR) 聚合酶鏈式反應. PCR is invented by Kary B. Mullis in 1983 where a small amount of DNA can be amplified by million of times.  To learn more about this technology, please go to the   Virtual lab  .

Mechanism of Huntington’s disease (HD)

The product of HTT gene is huntingtin, which has the highest activity level in the brain. Although the function of this protein is still not known well, it may be involved in cell signaling 細胞訊號傳遞, transporting materials, protein binding and protecting the cell from self-destruction.

HD patients have increase repeats of 3 DNA nucleotides (CAG) in the HTT gene.  This leads to the production of defective huntingtin and accumulation of clumps of toxic protein.  This causes neural dysfunction 神經功能障礙.

Treatment of Huntington’s disease (HD)

In principle, gene therapy 基因治療 can be one way of treating HD. Gene therapy normally targets on the disease gene and replaces it with a healthy gene. In the case of huntington’s disease, this is done by defective gene silencing 基因靜默 which reduces the expression of defective protein. Shutting down the production of all huntingtin proteins will cause brain death 腦死亡. Fortunately, in most cases, only one HHT gene is defective in HD patients.

To know more about gene therapy and its ethical concerns, please click   here  .