Test Code: 6520

Duchenne/Becker Muscular Dystrophy: DMD Gene Deletion/Duplication
Test Code: 6520
Turnaround time: 3 weeks

CONDITION DESCRIPTION

CEN4GEN offers a line of tests that together provide the highest detection of DMD mutations available. Testing includes a high resolution array CGH to detect deletions and duplications (test code 6520) and full gene sequence analysis (test code 5484). CEN4GEN’s comprehensive DMD testing can provide confirmation of a clinical diagnosis, characterization of the DMD gene mutation, and carrier testing for female family members and prenatal testing. The combined DMD copy number analysis and DMD full gene sequence analysis detects ~98-99% of mutations in both males and females, thereby providing the most comprehensive and robust analysis of the DMD gene.

Deletion/Duplication by DMD copy number analysis
Testing begins (test code 6520), a targeted CGH array which consists of overlapping probes covering the entire 2.2MB of the DMD gene. Deletion and duplication mutations account for 65% of mutations in the DMD gene, and this test will detect deletions and duplications in both males and females. This testing is indicated for individuals suspected to carry a DMD gene mutation who have not yet had testing or for individuals with previous deletion/duplication test results that do not clearly identify the breakpoints and size of the deletion or duplication. This test can also be performed for female carrier testing even when an affected male is not available for testing.

Methods other than CGH array that have been used to test for DMD deletions and duplications have inherent drawbacks. These methods include multiplex PCR, Southern blotting, and MLPA. Drawbacks of these methods include difficulties in detecting small deletions, difficulties in detecting most duplications, difficulties in detecting female carriers, and the inability to determine precise boundaries of deletions and duplications. Use of these other methodologies will fail to identify a mutation in ~5-10% of individuals tested (Prior and Bridgeman, 2005). This test can be performed when an individual has had previous negative deletion/duplication results by one of these other methods.

Full Gene Sequence Analysis
If no deletion or duplication is identified by the DMD copy number analysis, testing can continue with full gene sequence analysis (test code 5484). The remaining 35% of mutations in the DMD gene are point mutations and small deletions and duplications that can be detected by direct sequencing. Sequence analysis interrogates the 14kb coding region, 1.4kb of intronic sequence flanking the exon/intron boundaries, 8 DMD promoters, and 5 cryptic deep intronic mutations. This test is indicated for individuals suspected to carry a DMD mutation in whom previous testing did not identify a deletion or duplication. Rarely, novel missense changes or changes in introns other than at standard splice consensus sites are discovered. Testing of additional family members may be necessary for further interpretation in these cases. This test follows the ACMG recommendations for interpretation and reporting of sequence variations (Richards et al., 2008).

Advantages of Using CEN4GEN’s Comprehensive DMD Testing

  • Equal sensitivity and detection for males and females
  • Deletions and duplications mapped to the exact nucleotide breakpoint using CGH array
  • Enhanced detection of duplications that may be missed by other methods
  • Rapid turn-around time
  • Improved access to carrier and prenatal testing
  • Carrier risk adjustment using Bayesian analysis can be performed if appropriate family history information is provided

References:

  • Hegde, M. et al. Microarray-based mutation detection in the dystrophin gene. Human Mutation 2003, 29(9):1091-1099.
  • Prior, T. and Bridgeman, S. Experience and strategy for the molecular testing of Duchenne muscular dystrophy. J Mol Diag 2005, 7(3):317-326.
  • Richards, C., et al. ACMG recommendations for standards for interpretation and reporting of sequence variations: Revisions 2007. Genetic Med 2008, 10(4):294-300.

GENES
DMD

INDICATIONS
This test is indicated for:

  • Males with a clinical diagnosis or symptoms of Duchenne or Becker muscular dystrophy
  • Females who are at risk to be a carrier or have a family history of Duchenne or Becker muscular dystrophy
  • Individuals with previous deletion/duplication test results that do not clearly identify the breakpoints and size of the deletion or duplication Prenatal testing is available to females who carry an identified DMD mutation

METHODOLOGY

DNA isolated from peripheral blood is hybridized to a CGH array to detect deletions and duplications. The targeted CGH array has 385,000+ overlapping probes which covers the entire 2.2MB of the DMD gene.

Please note that a “backbone” of probes across the entire genome are included on the array for analytical and quality control purposes. Rarely, off- target copy number variants causative of disease may be identified that may or may not be related to the patient’s phenotype. Only known pathogenic off-target copy number variants will be reported. Off-target copy number variants of unknown clinical significance will not be reported.

DETECTION

Clinical Sensitivity: Deletion and duplication mutations account for approximately 65% of mutations to the DMD gene in Duchenne muscular dystrophy and 85% of mutations in Becker muscular dystrophy, and are detectable by the CGH array. Detection is limited to duplications and deletions. Array CGH will not detect point mutations or intronic mutations (refer to DMD full gene sequencing).

REFERENCE RANGE
Ratio of 1.2 for duplication.

SPECIMEN REQUIREMENTS

Submit only 1 of the following specimen types
* Preferred specimen type: Whole Blood

Type: Whole Blood
Specimen Requirements:
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.

Type: Saliva
Specimen Requirements:
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.

RELATED TESTS
• Full gene sequence analysis of the DMD gene is available to test for point mutations and mutations in the DMD promoter and intronic regions.