Origin and Expansion of Trinucleotide Repeats and Neurological Disorders

Unstable expansions of trinucleotide repeats (TNRs) are associated with a growing number of neurological disorders (at least 14), including HD (Huntington's disease), fragile X-syndrome, MD (Myotonic dystrophy) and Freidreich's ataxia. These disorders are often characterized by a tendency of certain pathological alleles to further expand due to biases in the parental origin of mutations, at times, leading to the most severe forms. TNR expression involves changes in the repeat tract length, threshold value, secondary structure formation, interruptions, mismatch repair mechanism, genes and their involved sequences, the product thereof and anticipation. The interactions of each of these factors with the others influence manifestations of the disease. The exact cellular events and/or mechanism of varied expression in all the neurological diseases with similar repeats have not yet been clearly understood. A correlation between trinucleotide expansion, chromosomal fragile sites and neurological diseases has, however, been established. This review deals with the involvement of TNRs in neurological disorders with respect to the type of repeat, level of normal, premutation and expansion of repeats, chromosomal locus and the involved genes along with clinical implications. Possible answers to two basic questions regarding the mechanisms of involvement of repeat expansions and interrelationships between such expansions and fragile sites have been provided based on the available experimental data. Detection of trinucleotide repeat expansion and fragile genetic sites could be among the excellent parameters for screening and diagnosis of human neurodegenerative disorders. An insight into the mechanisms involved may help the clinicians to develop suitable treatments.