Trinucleotide repeat disorders consist of a group of human diseases, which are a result of an abnormal expansion of repetitive sequences and primarily affect the nervous system. These occur during various stages of human development. Repetitive sequences, scattered in the microsatellite regions, usually account for about 30% of the human genome. In a normal person, the main purpose of various lengths of repetitive DNA is to allow for evolutionary plasticity. However, when these repeats extend beyond the code for a viable physiological protein, the expression of this aberrant segment is suppressed. After a certain threshold number, this suppression is lost, and an aberrant protein is coded for, which gives rise to either a functional or a non-functional protein, thereby giving rise to a 'gain of function' or 'loss of function' mutation. With every generation, the number of repeats increases drastically, and the age at which the patient presents is inversely related to the number of expansions. The severity, on the other hand, worsens with every generation due to a larger repeat sequence. Thus, the inheritance pattern of the repeat expansion diseases is evidence of the dynamic nature of these mutations and is termed as 'anticipation'. Myotonic dystrophy (DM), Huntington disease, spinocerebellar ataxia, Friedreich ataxia, and fragile X syndrome fall under the spectrum of trinucleotide repeat disorders. This article will study the various parameters of trinucleotide repeat disorders by reviewing in detail the five most commonly studied disorders, as listed above.
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