LabPLUS is part of Auckland District Health Board
and provides medical laboratory testing
| Services |
| MOLECULAR GENETICS TESTS - DUCHENNE MUSCULAR DYSTROPHY |
| Background: Duchenne Muscular Dystrophy (DMD) is an X-linked disorder with affected gene mapping to the p21 region of the X-chromosome. This gene is one of the largest described in humans, occupying approximately 1% of the X chromosome. It extends over 2300 Kb and comprises 79 exons. The prevalence of DMD is around 1 in 3500 live born males. The clinically less severe form, Becker muscular dystrophy (BMD) has a lower prevalence of around 1 in 18,500 live born males with onset usually in the second to third decade of life. Approximately 60 - 70% of Duchenne/Becker patients have a deletion in the dystrophin gene. The majority of deletions occur in two "deletion prone" regions located 500 and 1200 Kb downstream of the 5' end of the gene. Consequently, only a subset of the 79 exons needs to be screened in order to detect the majority of the deletions. Deletion detection is undertaken by multiplex PCR using oligonucleotide primers flanking these specific deletion-prone exons. The remaining 30% of DMD/BMD cases, which exhibit no detectable deletions, are assumed to result from micro deletions, point mutations or partial duplications. Since the dystrophin gene is too large to sequence in its entirety, diagnosis in these remaining 30% of cases involves a novel genetic strategy whereby overlapping regions of messenger RNA are analysed for mutational events. Using this combined approach, the majority of DMD/BMD mutations can now be characterised precisely. This is a major advance over traditional methods, which failed to give a definitive answer in many patients, particularly in female relatives of the DMD/BMD patients who often request genetic counseling and carrier testing. Carrier detection may not be straightforward as the normal chromosome usually masks a deletion on the other. Traditional methodology (Southern blotting) is labour intensive and furthermore, cannot detect point mutations and micro deletions. An alternative approach was to measure serum creatine kinase (CK) levels in close female relatives and do linkage analysis with intragenic and flanking DNA markers in an attempt to determine carrier risks and prenatal diagnosis. These measures were unhelpful in most cases. However in our laboratory the partial deletion of one copy of the DMD gene in females can be detected by investigating the dosage of the area of the gene of interest. This allows evaluation of the number of copies of the gene that are present. Analysis:
Blood: 3 x 4.0mL whole blood into EDTA tubes . Invert several times to mix. Forward within 24-48 hours at ambient temperature. Paediatric Samples: Minimum of 3mL whole blood in EDTA tubes. Prenatal Samples:
200 mg of tissue. Specimen must be snap frozen within one hour of collection. Send specimen frozen on dry ice. Paraffin blocks of tissue are also acceptable. The tissue sample should not have had prolonged immersion in formalin before embedding. Usual test turnaround time: 6 weeks once we received patient sample/s. Cost of test: DMD - $348 Note: Prices excludes GST (12.5%) and may change without notice. Please click on a link below to view information about another specific test.
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