Lecture 34 - Other Muscular Dystrophies Flashcards
How are muscular dystophies classified?
- Age of onset
- Pattern of weakness
• Muscular dystrophies have typical patterns of muscle involvement (despite same genes in the muscles)
• Thorough physical examination is performed - Pattern of inheritance
• e.g. X-linked inheritance immediately narrows it down - Involvement of other systems
• e.g. GIT - Specific abnormalities on muscle biopsies
• Muscle biopsy not always needed - Cause gene (where identified)
• Sometimes gene mutations have more than one clinical phenotype and thus also presentation
Why is it important to make a diagnosis?
- Information about disease
• Life expectancy
• Prognosis - Allows monitoring for disease specific complications
• Cardiac
• Respiratory etc. - Appropriate treatment
• and to avoid inappropriate treatment - Genetic counselling
• Always of benefit
• Assessment of risk in siblings
• Identification of carriers (because carriers can have health problems)
Compare the following in different forms of muscular dystrophy: • Age of onset • Pattern of weakness • Pattern of inheritance • Involvement of other systems • Muscle biopsy
Analysis of all these features differentiate between the various muscular dystrophies
Age of onset:
• Infantile onset: Congenital muscular dystrophy
• Adult onset: Limb-girdle muscular dystrophy
Pattern of weakness:
• Generalised: all over the body including face
• Focal: e.g. Rigid-spine syndrome
Inheritance pattern:
Look for:
• Male to male transmission
• Successive generations affected
Involvement of other systems:
• Brain: structural abnormalities, cognitive abnormalities
• Musculoskeletal: spinal rigidity; scoliosis; joint contractures
• Endocrine systems
• Eye: cataracts; structural abnormalities
Muscle biopsy
• Normal: uniform muscle fibres etc.
• Dystrophic muscle: connective tissue, necrosis, variation in fibres size, fat etc.
• This tells us that it is a muscle dystrophy, but can’t really differentiate between them
What are the features of muscular dystrophy due to FKRP gene mutation?
- Large head
- Early onset joint contractures
- Significant cognitive abnormalities (mental retardation)
- Facial muscle weakness
- CK in the 5000’s: muscular degeneration
- Abnormal myelination (dye-myelination)
- Cerebellar cysts
This is a very characteristic pattern of features that indicate FKRP gene mutation
Describe the process of muscle biopsy, and how this can lead to diagnosis
- Local anaesthetic
- Needle extraction of muscle about the size of a die
- Sent off for:
• Histology
• Electron microscopy
• Specific testing (based on what one thinks is going on)
e.g.
• Western blot
• Mutation analysis
• Immunohistochmistry
Diagnosis:
There is a whole battery of tests that is performed
It is through weighing up of the results of all these different tests that a specific diagnosis can be made.
A single test won’t be definitive
- Invisible stitches, and goes home the same day
Not a major procedure
Describe the use of immunohistochemistry in muscle biopsy
Fluorescent mAbs against specific proteins that might be associated with the disease process:
- Membrane protein
• Missing in dystrophic muscle
• Present in normal muscle
• Decreased staining in some dystrophies (could be primary or secondary…) - α-dystroglycan
(Anchors many proteins in the sarcolemmal membrane)
• Normal staining → normal muscle
• Abnormal staining → doesn’t give a specific diagnosis, just indicates that there is a problem in this protein
• α-dystroglycan is absent both in DMD and Limb girdle muscular dystrophy type IC
Give some features of Myotonic Dystrophy (DM1):
• Mode of inheritance
• Prevalence
• Systems involved
- Most common dystrophy seen in adults
- Chromosome 19
- 1/8000
- Autosomal dominant inheritance
- Variable severity
- Anticipation
- Muscle biopsy not very helpful
A multi-system disorder
• Proximal as well as distal muscle affected
• Smooth muscle affected (GIT and uterine problems)
• Cognitive deficits
• Excessive somnolence
• Personality changes
• Cataracts
• Endocrine dysfunction (diabetes, infertility)
What is ‘Anticipation’?
Disease becomes worse in successive generations
What are the clinical presentations of myotonic dystrophy?
Three phenotypes:
- Congenital
• Very severe
• Presentation in first 4 weeks of life - Classic DM1
• Presents in adolescence of adulthood
• Most common
3. Mild DM1 • Adult onset • Mild phenotype • May only be cataracts and mild myotonia • Can be missed
When does congenital myotonic dystrophy present?
First 4 weeks of life
Give the symptoms of congenital myotonic dystrophy
What is the prognosis for these babies?
- Hypotonia (floppy)
- Facial and proximal muscle weakness
- Delayed motor development
- Feeding difficulties
- Severe intellectual deficits
Prognosis:
Very commonly death within first four weeks due to respiratory failure
Describe the features of classical myotonic dystrophy
- Immobility of facial muscle (no facial expression)
- Frontal balding
- Cataracts
- Wasting of sternocleidomastoid muscle
- Gynecomastia (benign enlargement of breast tissue in males)
What is myotonia?
When is it seen?
“Delayed relaxation of muscles after contraction”
- Uncomfortable, but not extremely debilitating
- Useful for diagnosis
Symptom observed not solely in myotonic dystrophy, but in a number of conditions
• Not seen in babies with DM1
• Present in affected parents
Describe the importance of muscle biopsy in DM1
(Myotonic dystrophy)
Not very helpful because the muscle:
Does not look particular dystrophic
Observations:
• Not very dystrophic, however:
• Many central nuclei
• Ringbinden (aberrant myofibrils wrapping themselves around others)
Often not necessary, because there is a good genetic test for DM1
Describe the genetic basis of myotonic dystrophy (DM1)
Is there a good genetic test?
What is the pattern of inheritance?
DMPK gene
Expanded CTP trinucleotide repeats
Normal: 5-35 Premutation: 35-49 • Slight increase in n° of repeats • Asymptomatic • Increased chance of passing disease on to offspring Full penetrance: >50 repeats • DM1
Genetic test:
• 100% sensitive
Inheritance:
• Autosomal dominant
• Exhibits anticipation
Describe the mechanism of anticipation in DM1
Transmission from which parent shows anticipation?
Anticipation: increase disease severity and decreasing age of onset in successive generations
- DMPK CTP alleles of >35 are unstable
- Repeats can expand in meiosis
- Offspring can inherit repeat sequences that are much longer that the parents’
• Anticipation usually occurs with maternal transmission
Describe a case of anticipation
In DM1
Grandmother
• Cataracts
Mother:
• Decreased facial expressions
Son:
• Congenital muscular dystrophy
• Very severe
• Severe cognitive impairment
Describe the molecular pathogenesis of DM1
- RNA transcribed from expanded DMPK gene
- RNA form hairpin secondary structures
- Hairpins bind specific proteins and sequester them in the nucleus
• MBNL1: Muscleblind-like 1 - Loss of function of MBNL1
• Because it has been sequestered - Up-regulation of CUGBP1
• Because it is no longer being inhibited by MBNL1
→
• Disrupted regulation of alternative splicing
• mRNA translation disruption
• Decrease mRNA stability - Normally in the embryo:
• Normally increased CUGBP1 and decreased MBNL1
• During development, CUGBP1 levels drop and MBNL1 levels increase
• In DM1, the decreased levels of MBNL1 lead to: - Enhanced embryonic isoform expression in adults
• CUGBP1 splices RNA to bring about the expression of different isoforms
• Over-expression of embryonic isoforms leads multiple disease symptoms: - Symtoms due to disrupted splicing
• Insulin-receptor like 1 gene → insulin resistance
• Chloride channel → myotonia
• Troponin T in heart → cardiac defects, arrhythmia
Which two proteins does the toxic RNA from the BMPK gene bind?
Describe the relationship between these proteins
MBNL1
CUGBP1
Reciprocal inhibition
What are the possible therapies for DM1?
RNA-based inhibition of toxic CUG-expanded RNA species
- Small molecule inhibitors
- RNA-interference-mediated suppression of mutated DMPK transcripts
- Antisense oligonucleotides mediated knockout of DMPK
List the main features of Limb girdle muscular dystrophies
• Age of onset
• Who is affected
• Clinical presentation
• Pathology
• Differentiation between the various subtypes
- Progressive muscle disorders
- Collection of disorders
- Onset: 10-60 years
- Males and female equally affected
Presentation: • Muscle weakness and hypertrophy • First in pelvic girdle • Then shoulder • Respiratory and cardiac involvement • No central nervous system involvement
Pathology:
• Cytoskeletal
• (rather than contractile)
Various subtypes
• Differentiated based on:
- Inheritance
- Pattern of weakness
Inheritance:
LGMD type 2: Autosomal recessive
LGMD type 1: autosomal dominant
Which MDs have X-linked inheritance?
DMD
BMD
How is LGMD diagnosed?
Only get a specific diagnosis in about 75% of cases
- Clinical presentation
• Pattern of weakness
• Pattern of inheritance
• Family history - Creatine kinase levels
- Muscle histology
• Immunohistochemistry - Genetic testing
Describe an example of how pattern of weakness can guide diagnosis?
Early onset contractures
→ Emery-Dreifuss Muscular Dystrophy
(an X-linked LGMD)
