Trinucleotide repeat disorders

Simple sequence repeats occur throughout the human genome and expansions of such repeats cause several diseases, many of which are related to the nervous system. Sequence repeats exist in various lengths up to 12 nucleotides (so-called dodecanucleotide repeats). In this chapter, only the trinucleotide repeats will be discussed. Trinucleotide repeat disorders (TRDs) are defined as a group of genetic diseases caused by intronic or exonic trinucleotide repeat expansions, exceeding the normal, stable threshold, which differs per gene. In this chapter, general principles of TRDs will be discussed. For more details of individual diseases the reader is referred to specific chapters, except for Huntington's disease (HD) and dystrophia myotonica (DM, also known as myotonic dystrophy), which will be discussed here.