Limb-Girdle Muscular Dystrophy (LGMD) Type 2G: TCAP Gene Sequencing
Test Code: 8262
Turnaround time: 5 weeks
Limb-girdle muscular dystrophy (LGMD) is a descriptive term applied to a clinically and genetically heterogeneous group of childhood- or adult-onset muscular dystrophies. LGMD is characterized by weakness and wasting restricted to the limb musculature, proximal greater than distal. Most individuals with LGMD show relative sparing of the heart and bulbar muscles, although exceptions occur, depending on the genetic subtype. Onset, progression, and distribution of the weakness and wasting vary considerably among individuals and genetic subtypes. Serum creatine kinase (CK) levels in individuals with LGMD are usually elevated, and muscle biopsy reveals dystrophic changes. Immunohistochemistry (IHC) testing of a muscle biopsy sample can be used to determine the presence or absence of specific proteins, and confirmatory genetic testing is available in some cases. LGMDs are distinct from the much more common X-linked dystrophinopathies, which include Duchenne and Becker muscular dystrophy (DMD/BMD).
LGMD 2G, also referred to as telethoninopathy, is caused by mutations in the TCAP gene (17q12). Significant clinical variability has been seen among fourteen affected individuals in four families. Characteristics include proximal and distal lower limb weakness, proximal upper limb weakness, difficulty walking and running, and foot drop. Some individuals showed distal atrophy while others exhibited calf hypertrophy. Cardiac involvement occurred in about half of affected individuals, and females seemed to be less severely affected than males. Average age of onset is 9-15 years, with progression to wheelchair by 18 years after onset. Serum CK levels are 3-17 times normal and IHC shows an absence of telethonin.
For patients with suspected LGMD 2G, sequence analysis is recommended as the first step in mutation identification. For patients in whom mutations are not identified by full gene sequencing, deletion/duplication analysis is appropriate.
- Bonnemann, C. Limb-girdle muscular dystrophy in childhood. Pediatric Annals. 2005; 34(7):569-577.
- GeneTests: Limb-Girdle Muscular Dystrophy Overview: http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=lgmd-overview.
- Guglieri, M. et al. Limb-girdle muscular dystrophies. Curr Opin Neurol. 2008; 21:576-584.
This test is indicated for:
- Confirmation of a clinical diagnosis of LGMD 2G
- Carrier testing in adults with a family history of LGMD 2G
PCR amplification of 2 exons contained in the TCAP gene is performed on the patient’s genomic DNA. Direct sequencing of amplification products is performed in both forward and reverse directions, using automated fluorescence dideoxy sequencing methods. The patient’s gene sequences are then compared to a normal reference sequence. Sequence variations are classified as mutations, benign variants unrelated to disease, or variations of unknown clinical significance. Variants of unknown clinical significance may require further studies of the patient and/or family members. This assay does not interrogate the promoter region, deep intronic regions, or other regulatory elements, and does not detect large deletions.
Clinical Sensitivity: Unknown. Mutations in the promoter region, some mutations in the introns and other regulatory element mutations cannot be detected by this analysis. Large deletions will not be detected by this analysis. Results of molecular analysis should be interpreted in the context of the patient’s biochemical phenotype.
Analytical Sensitivity: ~99%
Submit only 1 of the following specimen types
* Preferred specimen type: Whole Blood
Type: Whole Blood
In EDTA (purple top) or ACD (yellow top) tube: Infants (2 years): 3-5 ml
Older Children & Adults: 5-10 ml
Specimen Collection and Shipping: Refrigerate until time of shipment. Ship sample within 5 days of collection at room temperature with overnight delivery.
OrageneTM Saliva Collection kit (available through CEN4GEN) used according to manufacturer instructions.
Specimen Collection and Shipping: Store sample at room temperature. Ship sample within 5 days of collection at room temperature with overnight delivery.
Submit copies of diagnostic biochemical test results with the sample, if appropriate.
Sequence analysis is required before deletion/duplication analysis by targeted CGH array. If sequencing is performed by another third party provider, please submit a copy of the sequencing report with the test requisition.
• Deletion/duplication analysis of the TCAP gene by CGH array is available for those individuals in whom sequence analysis is negative.
• An LGMD sequencing panel that includes 11 LGMD genes is also available.
• Custom diagnostic mutation analysis (test code: 6875) is available to family members if mutations are identified by targeted mutation testing or sequencing analysis.