CPT® Code 81189

The CPT® Code 81189 pertains to the molecular genetic testing of the CSTB (cystatin B) gene, specifically focusing on a full gene sequence analysis. This testing is crucial for identifying mutations associated with Unverricht-Lundborg disease, a progressive myoclonic epilepsy that typically manifests between the ages of 6 and 15. The CSTB gene, located on chromosome 21 at position 21q22.3, encodes the cystatin B protein, which plays a significant role in regulating cathepsin enzyme activity within cell lysosomes. The gene features a distinctive DNA allele characterized by a dodecamer repeat pattern of 12 amino acids, which usually occurs 2-3 times in healthy individuals. However, individuals diagnosed with Unverricht-Lundborg disease often exhibit more than 30 repeats in both copies of the CSTB gene. In some cases, one copy may have an expanded allele while the other may present with one of nine identified mutations, such as amino acid substitutions or truncated proteins that are dysfunctional. The clinical presentation of this condition includes involuntary muscle jerking (myoclonus), which can be aggravated by various stimuli, and may progress to more severe symptoms like ataxia, dysarthria, and cognitive decline. The full gene sequence analysis provided by CPT® Code 81189 allows for a comprehensive examination of the CSTB gene, facilitating the identification of functional variants and genetic markers that may contribute to the disease's pathology.