Duchenne and Becker Muscular Dystrophy
There are over 40 primary congenital muscle disorders as determined by the defective genes causing the disorders rather than specific clinical descriptions. The most common form of muscular dystrophy is X-linked Duchenne muscular dystrophy (DMD) and the milder form called Becker muscular dystrophy (BMD) is the second most common form of the disease. Together DMD and BMD affect approximately 1 in 3600 newborn males per year worldwide and comprise nearly 80% of all new cases of dystrophy and 56% of all new cases of congenital myopathy of all types. The underlying cause of DMD and BMD is abnormalities of the DMD gene, encoding the protein called dystrophin.
The location of the dystrophin gene is on chromosome Xp21. This has been confirmed by DNA polymorphism linking. This is the largest gene known in the human genome, spanning 2.5 megabases. Most DMD cases are caused by out-of-frame mutations in the dystrophin gene followed by absence of dystrophin. In contrast, most BMD cases result from in-frame mutations that allow the expression of truncated partially active protein.
The first molecular test for DMD, Chamberlain-Beggs Multiplex PCR, detected 98% of large deletions and some duplications in the dystrophin gene. Sometimes when the test detected one or more deleted or duplicated exons, additional adjacent exons not in the hot spot region had to be tested separately in order to find the break points which are important for differentiating BMD from DMD. This test has now been replaced in most laboratories by the Multiplex Ligation-Dependent Probe Amplification (MLPA) test for aberrant copy number in the DMD gene. MLPA tests all 79 exons in two reactions, and detects the exact molecular cause, including break points, for approximately 60% of DMD and BMD cases. Various forms of direct sequencing and exon pre-screening from patient DNA have been used to detect the majority of the remaining 40% of mutations in the DMD gene. These remaining mutations are mostly small (less than one exon) variations collectively known as point mutations including insertions, duplications, deletions, deletions plus insertions (indels) and single or multiple base changes. One of the most common and efficient methods for detecting mutations in the exons and regions of interest in disease genes is PCR amplification of the area of interest resulting in millions of copies of that DNA fragment followed by direct sequencing of the fragment. An automated process using direct sequencing following PCR has been developed for the detection of deletions, duplications/insertions and point mutations in any gene or family of genes and has been applied to ten genes known to bear mutations that cause muscular dystrophy (Bennett et al. 2009).
Among non-PCR methods, an immunoblot assay can detect abnormalities in dystrophin in the absence of detectable PCR deletions. DMD-specific FISH probes are useful for the detection of carriers of DMD gene deletions.
Metabolite ratios as measured by in vivo proton magnetic resonance spectroscopy and muscle function scores are significantly decreased in patients with DMD when compared with normal control subjects. A statistically significant decrease in trimethyl ammonium/total creatine ratio in patients with DMD, as compared with control subjects, was found to correlate with decreased muscle function (Hsieh et al. 2009). miR-206 and other muscle-specific miRNAs in serum are useful for monitoring pathological progression DMD (Hu et al. 2014). Rise of serum levels of matrix metalloproteinase-9 (MMP-9) indicate pathological progression of DMD (Nadarajah et al. 2011).