Download X-linked myopathies

Nov 22, 2016
Sandra Chan
Allen Chiu
Joseph Correia
Tam Myngoc Nguyen
X-Linked Muscular
Dystrophies:
Duchenne’s Disorder
PHM142 Fall 2016
Instructor: Dr. Jeffrey Henderson
X-linked Muscular Dystrophies
• X-linked recessive disorders: affect mostly males
• Genetic conditions that cause muscle weakness and
eventually atrophy
• Effect skeletal and cardiac muscle
X-linked Muscular Dystrophies
Types:
• X-linked Emery-Dreifuss (EDMD): FHL1 gene
• Joint rigidity, atrophy of scapuloperoneal muscles, cardiovascular
effects
• Duchenne: DMD gene
• Becker: DMD gene
• Later onset, less severe, slower progression
Duchenne Muscular Dystrophy
• Motor symptoms:
• Delayed motor skills
• Gait abnormalities
• Frequent falling
• Difficulty standing up
• Muscle weakness starting from the lower limbs
• Raised creatine kinase levels
• Scoliosis
• Dilated cardiomyopathy in adolescence
Duchenne Muscular Dystrophy
• Associated risks:
• Language and developmental delays
• Lower IQ
• Risk of ADHD, Autism
• Restrictive lung disease
Duchenne Muscular Dystrophy
• Incidence: 1 in 5000 boys
• Cause: mutation in DMD gene
• DMD  a large gene that codes for dystrophin protein
• Dystrophin: in the sarcolemma of muscle
Dystrophin
Structure
• hydrophilic1
• rod shaped
• 4 domains
• N-terminal actin-binding domain
• C terminal complex-binding domain
• Central triple-helical rod domain
• Cysteine-rich domain
Function
• Integral part of Dystrophin associated protein complex (DAPC)2
• Protein complex attaches cytoskeletal actin to ECM to maintain muscle
integrity/tensile strength
• Protection of sarcolemma (ex. shock absorption)
• Cell signaling (ex. docking site)
Mechanism
Mechanism
ECM
DAPC
Receptor
Dystrophin
Actin
Sarcolemma
Mechanism
ECM
DAPC
Receptor
nNOS
Actin
Sarcolemma
Mechanism
ECM
Ach
DAPC
Receptor
nNOS
Actin
V-gated
channels on
SR (Ca2+ in)
Sarcolemma
Mechanism
ECM
Ach
DAPC
Receptor
nNOS
Actin
V-gated
channels on
SR (Ca2+ in)
Calcium
binding to
troponin
Sarcolemma
Mechanism
ECM
Ach
Sarcolemma
DAPC
Receptor
nNOS
Actin
V-gated
channels on
SR (Ca2+ in)
Calcium
binding to
troponin
Actin and
myosin
interaction
Mechanism
ECM
Ach
Sarcolemma
DAPC
Receptor
nNOS
Actin
Creatine Kinase
+
V-gated
channels on
SR (Ca2+ in)
Calcium
binding to
troponin
Actin and
myosin
interaction
ATP  ADP Power
Stroke
Mechanism
Ach
Sarcolemma
Receptor
Creatine Kinase
+
V-gated
channels on
SR (Ca2+ in)
Calcium
binding to
troponin
Actin and
myosin
interaction
ATP  ADP Power
Stroke
Mechanism
Ach
Sarcolemma
Receptor
Creatine Kinase
+
V-gated
channels on
SR (Ca2+ in)
Calcium
binding to
troponin
Actin and
myosin
interaction
ATP  ADP Power
X
Stroke
Mechanism
Ach
Ca2+
Sarcolemma
Receptor
Ca2+ Dependent Proteases
-
V-gated
channels on
SR (Ca2+ in)
Calcium
binding to
troponin
Creatine Kinase
+
Actin and
myosin
interaction
X
ATP  ADP Power
X
Stroke
Mechanism
Mechanical damage to membrane
• Protein leakage (ex. CK) 3
• Calcium leaks in  disturb electrochemical gradient + proteolysis
Disturbances in signal transduction
• Nitric oxide synthase (nNOS) is associated with dystrophin (via spectrin
repeats)4
• NO oxide production activates heme-containing proteins
• Proteins regulate muscle contraction, glucose metabolism, and calcium
dynamics
• Dilates blood vessels
Diagnosis of DMD
• DMD should be suspected in patients with:
• Family history of DMD: any abnormal muscle function
• No family history: Gower’s sign, delayed walking (>16-18 months), frequent
falls
• Increased transaminases
• Blood Sample:
• Creatine kinase concentrations increased
• Muscle Biopsy:
• Lack of dystrophin expression
• Genetic Tests
Types of Genetic Testing
• Multiplex Ligation-Dependent Probe Amplification (MLPA)
• Modified PCR
• Can detect exon deletion or duplications
• Most cost effective
• Direct Sequencing of Gene
• Used to detect small mutations if MLPA unsuccessful
• May be more cost efficient in the future
Source: Bushby et al., (2010)
Treatment
• No cure available
• Treatment focuses on managing the progression of symptoms
i) Drug Therapy
ii) Physical Therapy
iii) Novel Therapies
Drug Therapy (Corticosteroid Therapy)
• Improve muscle strength, prolong ambulation, reduce decline of
cardiorespiratory function, reduce risk of scoliosis, improve life
expectancy
• Most common are prednisone and prednisolone
Prednisone
• Mechanism of action is unclear
• Initiation of Treatment: between plateau of motor skill development and
decline of motor skills (e.g. 4-6 years of age)
• Duration of Treatment: Continue even after loss of ambulation to delay
symptoms
• Side Effects: weight gain, high BP, behavioural changes, delayed growth
• Deflazacort
Physical Therapy
• Promotes mobility and prevents contractures
• Stretching (e.g. Achilles tendon, hip flexors)
• Surgery may be necessary
• Use of braces, splints or wheelchairs
Novel Therapies
• Gene therapy, stem cell therapy, dystrophin restoration
• E.g. Stop Codon Suppression Agents (i.e. Ataluren): ribosome reads through
stop codons, allowing for the full translation and production of functional
protein
• Only works for patients with nonsense mutations in gene
• E.g. Exon skipping: allows exons to be omitted to restore dystrophin reading
frame
• At pre-mRNA level
• ~13% of DMD patients have mutations that can be corrected by skipping exon 51
• Challenges: clinical trial design, systemic delivery, safety/tolerability, host
immune responses
Summary Slide
• DMD is an X-linked recessive disorder that causes muscle weakness/atrophy in skeletal and cardiac
muscle
• Lack of dystrophin expression results in membrane damage causing protein leakage, calcium influx and
disturbances in cellular signalling
• These processes result in muscle atrophy and eventually cell death
• A variety of physical, blood, and tissue testing is available for DMD; however, genetic testing is needed
for definitive diagnosis
• Multiplex Ligation-Dependent Probe Amplification (MLPA) is the most common and most cost effective
method to determine type and location of mutation.
• No cure available
• Treatment focuses on managing the progression of symptoms
• Drug Therapy: Prednisone, Prednisolone, Deflazacort
• Physical Therapy: Stretching, Surgery, Assistive Devices (E.g. splints, wheelchairs)
• Novel Therapies: Stop Codon Suppression Agents (e.g. Ataluren), Exon Skipping
References
1) "The DMD Mutations Database." The dystrophin associated complex, . Accessed 19 Nov. 2016.
www.umd.be/DMD/W_DMD/protein.html
2) Goldstein, Jeffery A., and Elizabeth M. McNally. "Mechanisms of muscle weakness in muscular
dystrophy." The Journal of General Physiology, vol. 136, no. 1, 2008, pp. 29-34.
3) Rahimov, Fedik, and Louis M. Kunkel. "Cellular and molecular mechanisms underlying muscular
dystrophy." The Journal of Cell Biology, vol. 201, no. 4, 2013, pp. 499-510.
4) Guiraud, Simon, Annemieke Aartsma-Rus, Natassia M. Vieira, Kay E. Davies, and Gert-Jan B. Ommen. "The
pathogenesis and therapy of muscular dystrophies." Genomics and Human Genetics, vol. 16, 2015, pp. 281308.
5) Yiu, Eppie M., and Andrew J. Kornberg. "Duchenne muscular dystrophy." Neurology India 56.3 (2008): 236.
6) Ziat, Esma, and Anne T. Bertrand. "FHL1 protein isoforms in Emery-Dreifuss muscular dystrophy." Orphanet
Journal of Rare Diseases 10.2 (2015): 1.
7) Duchenne and Becker muscular dystrophy: National Library of Medicine (US). Genetics Home Reference
[Internet]. Bethesda (MD): The Library; 2016 Nov 15. Duchenne and Becker muscular dystrophy; [reviewed
2016 June; cited 2016 Nov 20]; [about 3 screens]. Available from:
https://ghr.nlm.nih.gov/condition/duchenne-and-becker-muscular-dystrophy
References
8) Abbs, S., Tuffery-Giraud, S., Bakker, E., Ferlini, A., Sejersen, T., & Mueller, C. R. (2010). Best practice
guidelines on molecular diagnostics in Duchenne/Becker muscular dystrophies. Neuromuscular Disorders,
20(6), 422-427.
9) Bushby, K., Finkel, R., Birnkrant, D. J., Case, L. E., Clemens, P. R., Cripe, L., ... & Poysky, J. (2010).
Diagnosis and management of Duchenne muscular dystrophy, part 1: diagnosis, and pharmacological and
psychosocial management. The Lancet Neurology, 9(1), 77-93.
10) Falzarano, M. S., Scotton, C., Passarelli, C., & Ferlini, A. (2015). Duchenne muscular dystrophy: from
diagnosis to therapy. Molecules, 20(10), 18168-18184.
11)Young, Courtney S., and April D. Pyle. "Exon Skipping Therapy." Cell 167.5 (2016): 1144.
12) Angelini, Corrado, and Enrico Peterle. "Old and new therapeutic developments in steroid treatment in
Duchenne muscular dystrophy." Acta Myol 31.1 (2012): 9-15.
13) Merlini, Luciano, et al. "Early prednisone treatment in Duchenne muscular dystrophy." Muscle & nerve 27.2
(2003): 222-227.