Trinucleotide repeat disorders

Trinucleotide repeat disorders are a family of human genetic disorders caused by the repeat of three nucleotides to an extent which exceeds the normal and healthy threshold of such repeats for a given gene (this value varies between genes). The mutation is a subset of unstable microsatellite repeats which occur throughout the genome.

CAG repeat disorders are a common trinucleotide repeat disorder involving a repeat of the CAG codon, causing the addition of multiple glutamine residues in the final protein product; a structure known as a polyglutamine (poly-Q) tract. CAG repeats are responsible for at least ten neurological disorders in humans including, most famously, Huntingdon's Disease. The disease is caused by the hydrophobicity of glutamine, causing gradual aggregation of proteins in the brain. This is the reason that many CAG repeat disorders have slow onset. Another trinucleotide repeat disorder, CGG repeat, causes Fragile X Syndrome.

See also: dynamic mutation, pre-mutation and genetic anticipation