Trinucleotide repeat expansion

Trinucleotide Repeat Expansion is a genetic phenomenon that plays a critical role in the development of certain hereditary disorders. This process involves the lengthening of a specific trinucleotide sequence within a gene beyond a normal range. Trinucleotide repeats are sequences of three nucleotides—the basic building blocks of DNA—that are repeated multiple times in a row. When these repeats expand beyond a certain threshold, they can disrupt normal gene function, leading to various genetic disorders.

Overview

Trinucleotide repeat expansion can occur in any part of a gene, including coding regions (exons), non-coding regions (introns), and untranslated regions (UTRs). The expansion of these repeats is a dynamic mutation, meaning it can increase in length from one generation to the next. This phenomenon is associated with a type of genetic instability and can lead to an increase in severity of the disease or earlier onset in successive generations, a process known as anticipation.

Mechanism

The exact mechanism of trinucleotide repeat expansion is not fully understood, but it is believed to involve errors in DNA replication, DNA repair, and DNA recombination processes. During DNA replication, the DNA polymerase may slip on the template strand, leading to the addition of extra repeats. Similarly, during repair and recombination, the machinery may misalign the repeat sequences, contributing to the expansion.

Associated Disorders

Several genetic disorders are caused by trinucleotide repeat expansions. These include:

• Huntington's Disease: Caused by expansions of the CAG repeat in the HTT gene.
• Fragile X Syndrome: Results from the expansion of the CGG repeat in the FMR1 gene.
• Myotonic Dystrophy: Caused by an expansion of the CTG repeat in the DMPK gene.
• Friedreich's Ataxia: Due to the expansion of the GAA repeat in the FXN gene.
• Spinocerebellar Ataxias: A group of disorders caused by expansions in various genes.

Each of these disorders is characterized by a different repeat sequence and affected gene, leading to a wide range of symptoms and severity.

Diagnosis and Treatment

Diagnosis of disorders caused by trinucleotide repeat expansions typically involves genetic testing to identify the presence and length of the expanded repeats. Treatment options are currently limited and primarily focus on managing symptoms rather than curing the underlying genetic mutation. However, research into gene therapy and molecular approaches to prevent or reduce repeat expansion is ongoing.

Research and Future Directions

The study of trinucleotide repeat expansions has provided insights into the mechanisms of genetic instability and its role in disease. Ongoing research aims to develop targeted therapies that can address the root cause of these disorders. Advances in genetic editing technologies, such as CRISPR-Cas9, offer potential avenues for correcting or mitigating the effects of repeat expansions.