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HelpTest ID SBULB Spinobulbar Muscular Atrophy (Kennedy Disease), Molecular Analysis
Useful For
Molecular confirmation of clinically suspected cases of sporadic or familial spinobulbar muscular atrophy (SBMA)
Presymptomatic testing for individuals with a family history of SBMA and a documented expansion in the androgen receptor (AR) gene
Special Instructions
Method Name
A Polymerase Chain Reaction (PCR)-based assay is utilized to
detect expansion-type mutations (CAG repeats) within the androgen
receptor gene.
(PCR is utilized pursuant to a license agreement with Roche
Molecular Systems, Inc.)
Reporting Name
Spinobulbar Musc Atrophy, Kennedy'sSpecimen Type
VariesSpecimen Type: Whole blood
Container/Tube:
Preferred: Lavender top (EDTA) or yellow top (ACD)
Acceptable: Any anticoagulant
Specimen Volume: 3 mL
Collection Instructions:
1. Invert several times to mix blood.
2. Send specimen in original tube.
Additional Information: Specimen preferred to arrive within 96 hours of draw.
Forms:
1. Molecular Genetics: Congenital Inherited Diseases Patient Information (T521) in Special Instructions
2. New York Clients-Informed consent is required. Please document on the request form or electronic order that a copy is on file. An Informed Consent for Genetic Testing (T576) is available in Special Instructions.
3. If not ordering electronically, complete, print, and send a Neurology Test Request Form (T732) with the specimen (http://www.mayomedicallaboratories.com/it-mmfiles/neurology-request-form.pdf)
Specimen Minimum Volume
0.5 mL
Specimen Stability Information
| Specimen Type | Temperature | Time |
|---|---|---|
| Varies | Ambient (preferred) | |
| Frozen | ||
| Refrigerated | ||
Clinical Information
X-linked spinal and bulbar muscular atrophy (spinobulbar muscular atrophy: SBMA; or Kennedy disease) is characterized by onset of progressive muscle weakness, atrophy, and fasciculations typically in the fourth or fifth decade of life. Affected patients also have signs of androgen insensitivity such as gynecomastia, reduced fertility, and testicular atrophy. The clinical severity and age at onset can be quite variable, even within families. Because this is an X-linked disease, males manifest this disorder and females are generally asymptomatic carriers. However, there have been reports of female carriers who exhibit symptoms such as muscle weakness and cramping.
SBMA is caused by an expansion of the CAG trinucleotide repeat in exon 1 of the human androgen receptor (AR) gene. This trinucleotide repeat is polymorphic in the general population, with the number of repeats ranging from 11 to 34. The number of repeats found in affected individuals can range from 38 to 62. There is no consensus as to the clinical significance of alleles of 35 CAG repeats and literature suggests that alleles of 36 to 37 CAG repeats may be associated with reduced penetrance. As with other trinucleotide repeat disorders, anticipation is frequently observed and larger CAG expansions are associated with earlier onset and a more rapid clinical progression.
Reference Values
Normal alleles: 11-34 CAG repeats
Abnormal alleles: 36-62 CAG repeats
An interpretive report will be provided.
Cautions
For predictive testing, it is important to first document the presence of a CAG-repeat amplification in the AR gene in an affected family member to confirm that molecular expansion is the underlying mechanism of disease in the family.
We strongly recommend that patients undergoing predictive testing receive genetic counseling both prior to testing and after results are available.
Predictive testing of an asymptomatic child is not recommended.
Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Errors in our interpretation of results may occur if information given is inaccurate or incomplete.
A previous bone marrow transplant from an allogenic donor will interfere with testing. Call Mayo Medical Laboratories for instructions for testing patients who have received a bone marrow transplant.
Current evidence suggests that the majority of individuals with SBMA have a CAG-repeat expansion. However, we cannot eliminate the possibility that another type of mutation not detected by our assay is present within the AR gene.
Day(s) Performed
Tuesday; 10 a.m.
Report Available
5 daysPerforming Laboratory
Mayo Medical Laboratories in Rochester
Test Classification
This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.CPT Code Information
81401-AR (androgen receptor) (eg, spinal and bulbar muscular atrophy, Kennedy disease, X chromosome inactivation), characterization of alleles (eg, expanded size or methylation status)