CAG Repeat Length ∞ Area

CAG Repeat Length

Meaning

CAG Repeat Length denotes the precise count of consecutive cytosine-adenine-guanine trinucleotide sequences within a specific gene’s DNA. This sequence codes for glutamine, leading to a polyglutamine tract in the resultant protein. The number of these repeated units serves as a critical genetic marker.

Context

This genetic characteristic is central to a class of neurodegenerative conditions termed trinucleotide repeat expansion disorders. An expanded CAG repeat in the Huntingtin gene (HTT) underlies Huntington’s disease. Other conditions like spinocerebellar ataxias and Kennedy’s disease also involve abnormal CAG expansions in distinct genes, impacting neurological function.

Significance

The CAG repeat length directly correlates with the clinical presentation and progression of these inherited disorders. For instance, in Huntington’s disease, more repeats often predict an earlier age of onset and a more severe course. This genetic metric provides a definitive diagnostic tool and offers crucial prognostic information for affected individuals.

Mechanism

When the CAG repeat sequence expands beyond a critical threshold, the gene’s protein product contains an abnormally long stretch of glutamine residues. This expanded polyglutamine tract causes the protein to misfold and aggregate within specific cells, particularly neurons. Accumulation of these misfolded proteins disrupts cellular processes, leading to neuronal dysfunction and degeneration.

Application

Genetic testing for CAG repeat length is standard for individuals with suggestive symptoms or a family history of these conditions. It confirms a clinical diagnosis and offers clarity to patients. This testing also informs genetic counseling, allowing families to understand inheritance patterns and reproductive risks, and supports research aimed at developing therapeutic interventions.

Metric

CAG repeat length is precisely determined via molecular genetic analysis of a patient’s DNA, typically from a blood sample. Specialized laboratory techniques, such as Polymerase Chain Reaction followed by fragment analysis, are employed to amplify and size the specific DNA region. The result is reported as an exact numerical count of the trinucleotide repeats.

Risk

Identifying an expanded CAG repeat length carries significant implications, as many of these conditions currently lack a cure. Predictive testing for late-onset disorders, while providing certainty, can impose substantial psychological burden, including anxiety and distress. Comprehensive genetic counseling is therefore essential to ensure individuals fully comprehend the potential ramifications of their genetic information.

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