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Anticipation and Huntington’s Disease

Anticipation and Huntington’s Disease. William Fergus 5/9/00. What is Huntington’s Disease?. A degenerative brain disease Usually beginning in mid-life, cells in the brain (i.e., basal ganglia) die, causing deterioration of intellectual ability, emotional control, balance and speech.

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Anticipation and Huntington’s Disease

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  1. Anticipation and Huntington’s Disease William Fergus 5/9/00

  2. What is Huntington’s Disease? • A degenerative brain disease • Usually beginning in mid-life, cells in the brain (i.e., basal ganglia) die, causing deterioration of intellectual ability, emotional control, balance and speech. • Chorea, which means involuntary movements, is nearly always a symptom as well.

  3. What is Huntington’s Disease? • Neurotransmitters in the basal ganglia are largely responsible for motor skills and balance • When one pathway is damaged, neural signals travel through other pathways, causing symptoms Huntington’s Unaffected

  4. How is it Inherited? • Autosomal Dominant • Closely related to the IT15 gene located on the chromosome 4p16.3 which codes for a protein named huntingtin.

  5. The Genetic Imperfection • “Triplets” consist of 3 nucleotides consecutively repeated (e.g., CAG CAG CAG CAG) a region of DNA. • All possible combinations of nucleotides are known to exist as triplet repeats, although some are more common than others. • In unaffected people, the gene IT15 will contain between 11 and 34 repeats of the triplet CAG. The CAG triplet encodes the amino acid glutamine. Therefore, the number of CAG triplets in gene IT15 equals the number of glutamine amino acids in the huntingtin protein.

  6. The Genetic Imperfection • In affected people, there are between 37 and 121 repeats of the CAG triplet. • This is commonly called a “stutter.” • A person who has between 37 and 41 repeats may develop some of the symptoms of HD, but not necessarily the full-blown disease. • People who have over 41 repeats almost always develop the disease. • The more repeats they have, the earlier the onset, and the worse the symptoms are.

  7. Anticipation • Most repeats that are long enough to cause disease are unstable and have a tendency to get longer with each successive generation. • The longer the expansions are, the earlier symptoms arise or the more severe these symptoms become.

  8. Anticipation • In some disorders, the expansion produces more of a toxic gene product. This is likely to occur in Huntington’s disease. • For other repeat disorders, the expansion results in less transcription from the affected gene, and as a consequence, less of the functional protein is produced. Fragile X is an illness that exemplifies this type of mechanism.

  9. Pedigree

  10. Sources • http://www.macalester.edu/~psych/whathap/UBNRP/Huntington/neuro.html • http://info.med.yale.edu/chldstdy/plomdevelop/genetics/99decgen.htm • http://www.interlog.com/%7Erlaycock/what.html • http://www.geneclinics.org/profiles/huntington/details.html

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