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CBL2 - DMD.docx

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Dr Kingswell

Foundations Unit CBL2 Duchenne Muscular Dystrophy - property of Melissa Pasqua Objectives 1.Describe the chemical nature, structure (primary, secondary, tertiary and quaternary), and properties of proteins using myosin and actin as examples. - Primary polypeptide chain - Secondary hydrogen bonding - Tertiary the 3D structure of each subunit - for actin, its globular - for myosin, its head, neck and tail - Quaternary protein complexes combining together to make one unit. - for actin = multimer (actin proteins coming together to make microtubules) - for myosin, its a dimer 2.Recognize how different types of gene mutations result in abnormal protein production. 3.Describe the relevant and appropriate history, physical examination, diagnostic tools and investigations for a patient with a myopathy/muscular dystrophy. Clinical Diagnosis Duchenne muscular dystrophy (DMD) - Progressive symmetric muscular weakness (proximal greater than distal) often with calf hypertrophy - Symptoms present before age five years - Wheelchair dependency before age 13 years Becker muscular dystrophy (BMD) - Progressive symmetric muscle weakness and atrophy (proximal greater than distal) often with calf hypertrophy; weakness of quadriceps femoris may be the only sign. - Activity-induced cramping (present in some individuals) - Flexion contractures of the elbows (if present, late in the course) - Wheelchair dependency (if present, after age 16 years) - Preservation of neck flexor muscle strength (differentiates BMD from DMD) - Note: The presence of fasciculations or loss of sensory modalities excludes the d iagnosis of a dystrophinopathy. Individuals with an intermediate phenotype (outliers) have symptoms of intermediate severity and become wheelchair dependent between ages 13 and 16 years. DMD-associated dilated cardiomyopathy (DCM) - Dilated cardiomyopathy (DCM) with congestive heart failure, with males typically presenting between ages 20 and 40 years and females presenting later in life - Usually no clinical evidence of skeletal muscle disease; may be classified as "subclinical" BMD - Rapid progression to death in several years in males and slower progression over a decade or more in females [1] Testing Serum creatine phosphokinase (CK) concentration % of Affected Serum CK Phenotype Individuals Concentration DMD 100% 1 >10X normal 1 Males BMD 100% >5X normal DMD-associated 2 DCM Most individuals "Increased" DMD ~50% 3,4 2-10X normal Female 3,4 Carriers BMD ~30% 2-10X normal Electromyography (EMG) - useful in distinguishing myopathic vs neurogenic disorder - done by demonstrating short-duraiton, low-amplitude, polyphasic, rapidly recruited motor unit potentials - used only rarely in diagnosis Skeletal muscle biopsy - muscle histology: early in disease shows non-specific dystrophic changes, including variation in fiber size, foci of necrosis and regeneration, hyalinization, and, later in the disease, deposition of fat and connective tissue. - Western blot and immunohistochemistry: Western Blot Dystrophin 1 Phenotype Molecular Dystrophin3 Immunohistochemistry 2 Quantity Weight Nondetectabl Complete / almost DMD e 0%-5% complete absence Intermediat Normal / 5%-20% Males e abnormal Normal appearing or Normal 20%-50% BMD Abnormal 20%-100% reduced intensity patchy staining Normal or minor changes DMD Femal random Normal / >60% (70%9%) or mosaic pattern 4 abnormal Dystrophin-negative fibers e XCI (9%2%) Carrie rs DMD Normal / <30% on average Mosaic pattern skewed Dystrophin-negative fibers XCI 6 abnormal (29%25%) (44%33%) Clinical testing - Deletion/duplication analysis can detect either deletions or duplications of DMD in probands and carrier females: - Multiplex ligation probe amplification (MLPA) [Gatta et al 2005] - Array genomic hybridization (aGH) (also called chromosome microarray (CMA) [Bovolenta et al 2008, del Gaudio et al 2008, Hegde et al 2008] Note: (1) This technology offers an advantage over MLPA because it can detect intronic rearrangements. (2) Up to 7% of persons with a dystrophinopathy do not have coding region mutations [Dent et al 2005]; the fraction of these that have intronic rearrangements of clinical significance is unknown. (3) This type of array does not detect intronic point mutations. Note: MLPA and aGH have largely supplanted the following methods used in the past to detect deletions or duplications: Multiplex PCR [Multicenter Study Group 1992] Southern blotting [Darras et al 1988] FISH (with probes covering DMD exons 3-6, 8, 12, 13, 17, 19, 32-34, 43-48, 50, 51, and 60) - Sequence analysis or mutation scanning detects point mutations (i.e., small deletions or insertions, single-base changes, and splicing mutations): Sequencing of the entire DMD gene can be performed by traditional PCR and Sanger sequencing, or by more automated methods such as universal long PCR combined with massive pyrosequencing [Bonnal et al 2010]. Sequencing of the entire gene is often necessary to detect rare or private mutations. Mutation scanning methods such as denaturing high performance liquid chromatography (dHPLC) or newer methods such as single-condition amplification internal primer sequencing (SCAIP) [Flanigan et al 2003, Flanigan et al 2009] and high-resolution melting curve analysis [Almomani et al 2009] can detect single-base changes at reasonable cost, especially compared to the previously high costs of sequencing. Note: Because the costs of direct sequencing have decreased, only a few clinical laboratories perform mutation scanning for individuals in whom a deletion or duplication has not been identified. Genetic Counseling - The dystrophinopathies are inherited in an X-linked manner. - The risk to the sibs of a proband depends on the carrier status of the mother. - Carrier females have a 50% chance of transmitting the DMD mutation in each pregnancy. - Sons who inherit the mutation will be affected; daughters who inherit the mutation are carriers and may or may not develop cardiomyopathy. - Males with DMD do not reproduce. - Males with BMD or DMD-associated DCM may reproduce: all of their daughters are carriers; none of their sons inherit their father's DMD mutation. - Carrier testing for at-risk females and prenatal testing for pregnancies at increased risk are possible if the DMD disease-causing mutation in the family is known or if informative linked markers have been identified. [1] The usual procedure:
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