lecture 34 Flashcards
How do are muscular dystrophies classified?
- 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?)
Why make a diagnosis?
- 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
What is age of onset?
- infantile: congenital muscular dystropy
- e.g. in poor tone in baby, normal response is flex
- adult onset: limb girdle muscular dystrophy
What are patterns of weakness?
- generalised, whole body
- focal: rigid spine syndrome
- very important in making a diagnosis
What are inheritence patterns?
- autosomal
- recessive
- dominat
- sex linked
How are other systems involved?
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
What is FKRP?
- gene
- mutations cause congenital muscular dystrophy, mental retardation and cerebellar cysts
What abnormalities might be seen on a muscle biopsy?
- marked variation in fibre size
- increase in central nuclei
- fatty infiltration
- increase in connective tissue
How are muscle biopsies done?
- 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
What is immunohistochemistry?
- 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
What is absent alpha-dystroglycan staining in DMD and LGMD1C?
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
What is myotonic dystrophy (DM1)?
- 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
What is pattern of weakness in DM1?
- quite patchy
- often distal muscles
- some muscles in the face
- foot drop
- smooth muscle: bowel, uterus
What are clinical findings re: myotonic dystrophy?
- 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
What is congenital myotonic dystrophy?
- 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
What do you see in adults with myotonic dystrophy?
- characteristic pattern of facial weakness
- immobility of facial expression
- multisystem disorder
What is myotonia?
- = 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
What do you see in muscle biopsy of DM1?
- 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
What is the molecular pathogenesis os myotonic dystrophy?
- 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
What is anticipation in DM1?
- 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
What is RNA gain of function in DM1?
- 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
What are possible therapeutic strategies for DM1?
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
What are limb-girdle muscular dystrophies?
- 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
What is the classification of LGMDS?
- 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
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
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
What is muscle pathology in the LGMBs?
- can be very variable
- some helpful
- vaculoses in LGMD1A
- myotilin aggregates on myotilin staining in LGMB1A
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
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
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
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
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
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
Can FSHD be due to new mutations?
yes
de novo mutation: one-off event
- germline mosaicism - possibly more than 1 event
