Triplet Repeat Disease

Trinucleotide, or triplet repeats, consist of three consecutive nucleotides that are repeated within a region of DNA (for example, CCG CCG CCG CCG CCG). Expansion of a triplet repeat gene segment leads to increased disease severity and decreased age of onset in proportion to the degree of expansion. Repeats that are translated into protein alter the structure of the protein, conferring new functions. These are found in the genome of humans and many other species. All possible combinations of nucleotides are known to exist as triplet repeats, though some, including CGG and CAG, are more common than others. The repeats may be within genes or between genes. In genes, they may be in regions that specify proteins (coding regions called exons) or in noncoding regions (introns). If present within exons, they may be present in translated regions and hence encode a series of identical amino acids, or they may occur in regions not translated into protein. Triplet repeats are frequently found in genes that encode transcription factors and those involved in regulating development.