A lot of diseases are caused by or associated with mutations in particular genes. One of these diseases is Huntington’s disease (HD), which causes a breakdown of nerve cells in the brain that affects an individual’s ability to think, speak and move.
What Gene or Genes Are Related to Huntington’s Disease?
1. Go to the OMIM database: http://www.omim.org/.
2. Search OMIM for "Huntington’s disease."
3. From the list of results, choose "#143100. HUNTINGTON DISEASE; HD."
4. You will see a "Phenotype-Gene Relationships" table. The "Gene/Locus" column in this table tells you which gene is responsible for the phenotype, or the disease, as well as for any conditions related to the disease.
5. Write down the "Gene/Locus."
6. It is often not enough just to ask which gene is associated with a disease. A researcher may also be interested in what went wrong with that gene. Read the "TEXT" section below the "Phenotype-Gene Relationships" table of Huntington’s disease.
7. a) Which trinucleotide repeat over-expanded in the gene associated with Huntington’s disease?
b) Which amino acid does that trinucleotide encode?
c) What’s the normal range of the repeat number of that trinucleotide?
Research has found that people having fewer than 36 CAG repeats in the HTT gene will not be affected by HD, while people with greater than 39 repeats are almost certainly going to get HD. Higher CAG repeat numbers (over 39) are associated with the onset of HD at an earlier age. The exact molecular mechanism is not completely understood at this time. Note that it is the number of CAG repeats in the "huntingtin gene" (HTT) that causes "Huntington's disease" (HD); the gene ends in "-tin" while the disease ends in "-ton".