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Trinucleotide Repeat Disorders
Descriptions of other diseases that involve codon repeat expansions.
When the cause of a disease can be traced to having too many copies of a certain nucleotide triplet in the DNA, the disease is said to be a trinucleotide repeat disorder. Today, there are 14 documented trinucleotide repeat disorders that affect human beings**. Huntington’s Disease is part of this group.
Some of these 14 trinucleotide repeat disorders are more alike than others. While the symptoms and the affected body parts vary by disease, scientists consider two illnesses to be similar if they share the same repeated codon as their cause. Six of the 14 trinucleotide repeat disorders have little or no apparent similarity to each other, or to the 8 remaining diseases. These 6 are described in brief at the end of this section. The 8 remaining disorders, one of which is Huntington’s Disease, all share the same repeated codon as their cause: CAG. Since CAG codes for an amino acid called glutamine, these 8 trinucleotide repeat disorders are collectively known as polyglutamine diseases (“poly” being the Greek word for “many”). (For background information on codons and amino acids click here.)
Polyglutamine diseases have much in common: Each of them is characterized by a progressive degeneration of nerve cells in certain parts of the body (for background info on nerve cells, click here.) In each disease, this degeneration first disrupts the function of certain group(s) of nerve cells. After 10-20 years, many of the affected nerve cells die. The major symptoms of these diseases are similar to one another and they usually affect people around the same time, in mid-life (although childhood cases have also been reported, as in the case of juvenile HD).
It deserves to be reiterated that while the polyglutamine diseases are similar to each other, they are not identical. Although they share the same repeated codon (CAG), the repeats for the different polyglutamine diseases occur on different chromosomes, and thus on entirely different segments of DNA. (For more info on chromosomes, click here.) Despite this fact, scientists are excited about research in any of the polyglutamine diseases because finding a way to stop the CAG repeat from occurring in one disease may help lead to a cure for the other 7 as well. While this is by no means a certainty, the possibility offers wonderful incentive to be persistent in research; eight for the price of one would certainly be a great deal!
Below you will find detailed descriptions for each of the polyglutamine disease, as well as a general description of all the non-polyglutamine diseases.
**Although only 14 trinucleotide repeat disorders are well-documented in medicine, genetic analysis has led researchers to believe that others exist as well. These disorders are even less common than the well-documented disorders and so have been more difficult to study, which leaves much of their story untold. They will be omitted here.
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