Trinucleotide repeat disorders

Trinucleotide Repeat Disorders are a group of genetic disorders caused by the expansion of a trinucleotide sequence within a gene beyond the normal, stable threshold. These expansions can lead to a variety of hereditary diseases, each characterized by different symptoms and modes of inheritance. The trinucleotide repeats can be of various sequences such as CAG, CGG, GAA, and CTG, and their expansion disrupts the normal function of the affected gene, leading to disease.

Causes and Mechanism

The cause of trinucleotide repeat disorders is a mutation that involves the expansion of a specific trinucleotide sequence within a gene. This expansion occurs during DNA replication and can increase in size from generation to generation, a phenomenon known as anticipation. The larger the expansion, the earlier the onset and the more severe the symptoms of the disease tend to be. The mechanism by which these expansions cause disease varies among different trinucleotide repeat disorders but can include loss of function of the affected gene, toxic gain of function of the gene product, or toxic gain of function of the RNA containing the repeat.

Types of Trinucleotide Repeat Disorders

There are several types of trinucleotide repeat disorders, each associated with expansions of different trinucleotide sequences in different genes. Some of the most well-known include:

• Huntington's Disease (HD) - Caused by CAG repeat expansion in the HTT gene.
• Fragile X Syndrome (FXS) - Caused by CGG repeat expansion in the FMR1 gene.
• Myotonic Dystrophy (DM) - Caused by CTG repeat expansion in the DMPK gene for type 1 and CCTG repeat expansion in the CNBP gene for type 2.
• Friedreich's Ataxia (FRDA) - Caused by GAA repeat expansion in the FXN gene.
• Spinocerebellar Ataxias (SCAs) - A group of disorders caused by CAG repeat expansions in various genes.

Symptoms

The symptoms of trinucleotide repeat disorders vary widely depending on the specific disorder and the size of the trinucleotide repeat expansion. Common symptoms across many of these disorders include neurological and motor impairments, cognitive deficits, and psychiatric symptoms. For example, Huntington's Disease is characterized by movement disorders, cognitive decline, and emotional disturbances, while Fragile X Syndrome primarily affects learning and behavior.

Diagnosis

Diagnosis of trinucleotide repeat disorders typically involves genetic testing to identify the presence and size of the trinucleotide repeat expansion. This testing can confirm a diagnosis and help predict the severity and onset of the disease. Prenatal testing is also available for some trinucleotide repeat disorders.

Treatment

Currently, there is no cure for trinucleotide repeat disorders, and treatment focuses on managing symptoms and improving quality of life. This may include medications to manage symptoms such as muscle stiffness, seizures, or psychiatric conditions, as well as physical, occupational, and speech therapy. Research is ongoing to find more effective treatments and potentially curative therapies, including gene therapy and small molecule drugs that target the underlying genetic mutations.

Research Directions

Research into trinucleotide repeat disorders is focused on understanding the mechanisms by which trinucleotide expansions cause disease, developing animal and cell models of these disorders, and finding therapeutic strategies to prevent or reverse the expansions. Gene editing technologies, such as CRISPR/Cas9, offer promising avenues for correcting the genetic mutations underlying these disorders.