lecture 34 Flashcards
1
Q
How do are muscular dystrophies classified?
A
- age of onset
- pattern of weakness
- pattern of inheritence
- involvement of other systems
- specific abornmalities on muscle biopsy
- causative gene where identified (but sometimes genes have more than one clinical phenotype?)
2
Q
Why make a diagnosis?
A
- to know what the disease course is likely to be
→ life expectancy, independence etc - to enable monitoring for disease complications
→ cardiac, respiratory, endocrine, ocular etc - to ensure treatment is appropritate
→ and to avoid inappropriate treatments - to enable genetic counselling (always of benefit)
→ recurrent risk in siblings
→ counselling of other family members
→ where carrier status carriers its own risk
3
Q
What is age of onset?
A
- infantile: congenital muscular dystropy
- e.g. in poor tone in baby, normal response is flex
- adult onset: limb girdle muscular dystrophy
4
Q
What are patterns of weakness?
A
- generalised, whole body
- focal: rigid spine syndrome
- very important in making a diagnosis
5
Q
What are inheritence patterns?
A
- autosomal
- recessive
- dominat
- sex linked
6
Q
How are other systems involved?
A
brain
- abnormalities of brain development or maturation
- cognitive abnormalities
musculoskeletal
- spinal rigidity, scoliosis
- joint contractures (Achilles, ITB, elbow and wrist)
- weakness
endocrine systems
eye
- stuctural or retinal abnormalities
- cataracts
7
Q
What is FKRP?
A
- gene
- mutations cause congenital muscular dystrophy, mental retardation and cerebellar cysts
8
Q
What abnormalities might be seen on a muscle biopsy?
A
- marked variation in fibre size
- increase in central nuclei
- fatty infiltration
- increase in connective tissue
9
Q
How are muscle biopsies done?
A
- in children put them under GA for 15 minutes
- need to check which type of anaesthetic to use
- take a small piece of muscle about the size of a dice
- gets sent off in three different pieces: histology
EM block
diagnostic screen
→ biochemical analysis?
→ research?
→ mutation analysis
→ western blot
→ immunohistochemistry
not a major procedure
needle in adults
10
Q
What is immunohistochemistry?
A
- fluorescent antibody staining
- looking for specific proteins
- e.g. membrane protiens
- always compare to a normal
- might show up as absent or decreased/incomplete staining
- look for specific patterns of change
11
Q
What is absent alpha-dystroglycan staining in DMD and LGMD1C?
A
A) characterisation of the alpha-dystroglycan antibody. By western blotting, the antibdoy stains a 156 kDa band in muscle (M) and a 120 kDa band in brain
B) transverse sections of control mouse: alpha-dystroglycan shows homogenous staining around the muscle fibre surface. no signal detected on cyrosections of mdx mice.
B) on 6 µm cyrosections, near complete loss of alpha-dystroglycan expression observed in a LGMD 1C patient in contast to a normal expression of beta-dystroglycan and the laminin alpha-2-chain. Muscle tissue from a DMD patient serves as a negative control. Bar, 50µm
12
Q
What is myotonic dystrophy (DM1)?
A
- autosomal dominant inheritence (1/8,000)
- chromosome 19
- a multisystem disorder
→ proximal and distal weakness and wasting
→ smooth muscle involvment: constipation, uterine
→ cognitive deficits
→ excessive somnolence, personality changes
→ cataracts
→ endocrine dysfunction: diabetes, infertility - shows anticipation (worse in successive generations)
- muscle biopsy findings very non-specific
- most common seen in adults
- lots of people have it and don’t know
13
Q
What is pattern of weakness in DM1?
A
- quite patchy
- often distal muscles
- some muscles in the face
- foot drop
- smooth muscle: bowel, uterus
14
Q
What are clinical findings re: myotonic dystrophy?
A
- three phenotypes: classic, congenital and mild
- congenital:
→ most severe, presents in first 4 weeks of life
→ respiratory failure, feeding difficulties and early death common - classic DM1:
→ most common
→ presents in adolescence or adulthood with muscle weakness - mild DM1
→ cataract and mild myotonia in adulthood, can be missed
15
Q
What is congenital myotonic dystrophy?
A
- presents at birth or in neonatal period
- combination of:
→ hypotonia (‘floppy’ bay)
→ facial and proximal muscle weakness
→ delayed motor development
→ respiratory insufficiency
– babies often die of respiratory failure <4 weeks of age
→ feeding difficulties
→ severe intellectual deficits
16
Q
What do you see in adults with myotonic dystrophy?
A
- characteristic pattern of facial weakness
- immobility of facial expression
- multisystem disorder
17
Q
What is myotonia?
A
- = delayed relaxtion of muscles after contraction
- seen in a number of muscle disorders
- can be uncomfortable
- not usually a big issue in myotonic dystrophy but useful for diagnosis
- not seen in babies with DM1 but is present in affected parent
- sometimes needs medical treatment if very uncomfortable
→ quinidine, mexilitine, carbamazepine
18
Q
What do you see in muscle biopsy of DM1?
A
- lots of central nuclei
- not particularly specific
- ringbinden: aberrant myofibrils that wrap themselves around an existing muscle fibre in a tight spiral
- occur most commonly in muscles affected by neurogenic atrophy
- not specific
19
Q
What is the molecular pathogenesis os myotonic dystrophy?
A
- expanded CTG trinucleotide repeat in the gene DMPK
- normal alleles contain 5-35 CTG repeats
- pre-mutation alleles: 35-49 repeats
→ individuals with pre-mutations: asymptomatic
→ offspring: risk of inheriting a larger repeat → symptoms - fully penetrant alleles: > 50 CTG repeats → DM1
- genetic testing positive 100%
- DM1 is inherited in an AD manner
20
Q
What is anticipation in DM1?
A
- DMPK CTG alleles of >35 repeats are unstable and can expand in length during meisosis
- offspring can inherit repeat lengths much long than those int he transmitting parent
- anticipation = increasing disease severity and decreasing age of onset in successive generations
- anticipation usually in maternal transmission of DM1
- most often babies with severe congenital DM1 have inherited the expanded DMPK allele from the mother
21
Q
What is RNA gain of function in DM1?
A
- mutant RNA transcribed from the expanded allele induce symptoms of the disease
- RNA CUG expansions fold into hairpin-like secondary structures which sequester specific proteins, resulting in depletion below a functional threshold
- two important proteins bind to CUG repeats: MBNL1 (muscleblind-like 1) and CUGBP1 (CUG-binding protein 1)
- in DM1, MBNL1 is sequestered on CUG repeat-containing RNA resulting in loss-of-function
- CUGBP1 us up-regulated through a signalling pathway, causing downstream effects such as disrupted regulation of alternative splicing, mRNA translation and mRNA stability, which contribute to the multiple features of DM1
- usually MBNL1 nuclear levels increase during development while CUGBP1 nuclear levels decrease: the level and localisation of these two proteins control a fetal to adult splicing tansition. This is reversed in DM1 tissues
- embryonic stage: MBNL1 nuclear levels low, CUGBP1 levels high
- during development MBNL1 nuclear levels increase while CUGBP1 levels decrease, inducing an embryonic-to-adult transition of downstream splice targets
→ IR exon 11, CIC-1 exons containing stop codons, and cTNT exon 5 etc - in DM1, MBNL1 is sequestered to CUG repeats, while CUGBP1 levels are increased due to phosphorylation and stablisation
- this enhances expression of embryonic isoforms in adults, resulting in multople disease symptoms
22
Q
What are possible therapeutic strategies for DM1?
A
RNA-based mechanisms to inhibit the toxic CUG-expanded RNA species in DM1
- small molecule inhibitors such as pentamidine-like compounds
- RNA interference (RNAi)-mediated suppression of mutated DMPK transcripts
- antisense oligonucleotide (AO)-mediated knowckdown of DMPK
23
Q
What are limb-girdle muscular dystrophies?
A
- generally progressive muscle disorders
- onset 2nd to 6th decade, M=F
- present with muscle weakness and hypertrophy
→ usually pelvic girdle first, then shoulder - respiratory and cardiac involvement common
- generally no central nervous system involvement
- patholgy: generally cytoskeletal rather than contractile
- i.e. generally associated with the sarcolemmal membrane
some nuclear proteins or contractile apparatus
DAPC
24
Q
What is the classification of LGMDS?
A
- later onset (differences within this group)
- pattern of weakness
- inheritence:
→ autosomal recessive: LGMD type 2 (most common)
→ autosomal dominant: LGMD type 1, FSHD
→ x-linked: DMD/BMD, EDMD
25
What are clinical cues to the LGMDs?
patient ? LGMD
```
clinical presentation
- pattern of muscles involved, additional clinical features? family history
+
creatine kinase levels
+
muscle histology
+
muscle immunoanalyiss
```
```
→→
genetic testing
→
precise diagnosis ~75%
→
genetic counselling
recognition of risk of cardiac/respiratory complications
surveillance
proactive management
```
26
What are main areas of muscle weakness in different types of dystrophy?
- DMD/BMD: proximal
- Emery-dreifuss type: proximal upper limb
- limb-girdle type: proximal
- FSHD: face, proximal upper limp
- oculopharyngeal: ocular, proximal
specific patterns are indicative of specific dystrophies
distribution of weakness gives clues to diagnosis
- e.g. early contractures are universal in boys with Emery-Dreifuss MD
27
What is muscle pathology in the LGMBs?
- can be very variable
- some helpful
- vaculoses in LGMD1A
- myotilin aggregates on myotilin staining in LGMB1A
28
What are clinical clues to the LGMDs?
- typical picture of LGMD caused by lamin A/C mutations with a prominent contractural phenotype involving the achilles tendons, elbows and spine predominantly, together with humeroperoneal muscle weakness
- dominant forms are type 1
- recessive are type 2: much more common, often associated with cardiomyopathy, respiratory involvment
29
What is the presentation of FSHD?
- dominantly inherited myopathy
- affects 1/20,000 people
- most symptomatic by age 20
- facial weakness
- scapular winging
- proximal arm weakness
- leg weakness usually less prominent
→ peroneal but not proximal, causes foot drop
don't close their eyes when asleep
30
What are signs of FSHD on examination?
- weakness of eye closure
→ most never able to whistle
→ always sleep with eyes open
- high riding scapulae
- poorly developed pectoral and scapular muscles
- pectus carinatum ('pigeon chest')
- weakness of anterior compartment of leg
→ foot frop - weakness of peroneal muscles
31
What is the muscle involvement in FSHD?
- selective muscle involvement
- weakness often patchy
- often asymmetric
- scapular and pectoral muscles affected early
- lower 1/3 of abdomen affected → Beevor sign
- heart unaffected
- respiratory muscles usually unaffected
- distributions poorly understood
→ ? why some muscles and not others
32
What is the genetic basis of FSHD?
- the gene for FSHD is not known
- inheritance is autosomal dominant
→ 90% cases map to chromosome 4q35
→ 10-30% cases sporadic
- penetrance incomplete
→ 30% all inherited cases are asymptomatic
→ symptoms more common in males than females
- germline mosaicism is occasionally seen
33
What is the D4Z4 repeat sequence?
- Deletion of D4Z4 repeat sequence near telomere chr 4
→ most people: 12-96 copies of the repeat sequence
→ FSH patients have no more than 8 copies
→ smaller the fragment, the more severe the FSHD
→ smaller the fragment the earlier the age of presentation
→→ infantile FHSD: 1- 3 repeats, very severe weakness
- problem: the gene prove detects changes in homologous (very similar areas) on chromosomes 4 + 10 (some difficulties differentiating between the two)
- about 5% of patients have a negative gene test
→ technical problems with the gene test
→ genetic heterogeneity: ? more than 1 gene causes FSHD
- 4q repeat arrays can translocate to chromosome 10
more complex gene probes enable distinction between changes on 4q and 10q
34
Can FSHD be due to new mutations?
yes
de novo mutation: one-off event
- germline mosaicism - possibly more than 1 event
