Lecture 9 - Muscular dystrophies Flashcards
iMuscular dystrophies
A group of diseases showing progressive muscle weakness and breakdown
There is no cure for MD
How does the muscle basement membrane attach to the skeletal muscle?
Two main ways:
Laminins - α7/β1D forming a transmembrane receptor binding α2 laminin which bins to actin inside the cells
Dystroglycan complex - contains a whole lot of components to this complex, mainly made of dystroglycan, a protein containing lots of glycan
Muscle cells basement membranes: what does it contain and what laminin isoforms are present?
Contains specific laminin isoforms
Main muscle isoform - a2 laminin chain (α2β1γ1) is expressed in muscle BM, at myotendinous junctions, and at the neuromuscular junction, NMJ also contains the laminins α4β2γ1and α5β2γ1isoforms
Name two types of muscular dystrophies
Duchenne: complete loss of dystrophin - develops in childhood, wheelchair usually by end of 1st decade. Death by mid 30’s
Becker: Partial loss of dystrophin - develops in childhood, but may not need a wheelchair until 40’s. May have a normal lifespan.
What parts of the muscle basement membranes can be mutated and what conditions can they cause?
- Laminin (Congenital MD)
- Integrin
- Dystroglycan
- Dystrophin (Duchenne and Becker)
- Collagen (Bethlem myopathy)
Congenital muscular dystrophy: what is it, what causes it most often, how severe is it, what symptoms are present, and what mutations have a less severe phenotype
Caused by a range of mutations affecting muscle BM
50% have Laminin α2 chain mutations
Can be very severe:
* Manifest at birth or in the first few years
* Progressive degeneration of muscles
* Complete loss of expression associated with the severe phenotype
Symptoms:
* Peripheral and central nervous system defects
* Muscle weakness
* Delayed motor development
* Joint problems
Mutation leading to partially functional/truncated Laminin α2 shows less severe, perhaps due retention of some activity
Why is lamin mutation so bad in muscular dystrophies
Laminin removal - neither MBM attachment method can be used whereas if dystroglycan/integrin have mutations the other system can be used and the individual is mostly fine
Laminin: what is the main muscle BM laminin, what is the major laminin-binding integrin in muscles?
Laminin 211 is the main muscle BM laminin
α7β1 is the major laminin-binding integrin on the muscle cell surface - α7β1 links laminin to the actomyosin cytoskeleton, loss of α7 integrin in mice results in a progressive form of muscular dystrophy
Dystrophin: what is it, how big is it, and where are mutations most often found in DMDs?
A protein involved in muscle-MBM attachment
- One of the longest genes in the human genome ~ 2.5 megabases
- The Primary transcript is 2100 kilobases with 79 exons
- The mature mRNA is 14 kilobases
- The protein is 3685 amino acids in muscle
Mutations common between exons 45 - 50, placing exon 51 out of frame and resulting in premature stop, thus no functional dystrophin
DMD: what is it, what is the timeline of this disease, and what are its symptoms?
Duchenne muscular dystrophy - an MD disease involving the complete loss of dystrophin
Symptoms first show when the child starts walking, wheelchair by the early teens, and death by the early 20’s, although survival is increasing due to better patient management
- Progressive muscular wasting
- Poor balance
- Drooping eyelids
- Atrophy
- Curvature of the spine and the back)
- Inability to walk
- Frequent falls
- Waddling gait
- Calf deformation
- Limited range of movement
- Respiratory difficulty
- Joint contractures
- Cardiomyopathy
- Arrhythmias
Why is a retroviral approach like that used for laminin in JEB not possible for DMD?
Dystrophin too large - unable to make it fit in the retrovirus, can still use smaller minigenes, though
Adeno-associated virus: what is the process used to treat MDs, and how is the different from the JEB therapy? Why is this used in this case?
Adeno-associated virus used to infect cells with Cas9 and target RNA
AAV does not integrate into the host cell genome - non-pathogenic
Watch leccy back
Dystroglycan: what is it, where is it expressed, and what known mutations are there?
A transmembrane laminin receptor independent of integrins
Ubiquitously expressed, but roles in tissues other than muscle are not clear
Mice with dystroglycan deletion are not viable, no known human mutations of dystroglycan, presumably because they are also lethal
Dystroglycan highlights how diseases may result from mutations that are not in the immediate gene of interest
Watch leccy
The LG4 and 5 domains of laminin 211: what do they do and how do mutations in N-linked/O-linked sugars affect laminin binding?
Bind to glycosylated moieties on dystroglycan
- Removal of N-linked sugars does not affect laminin binding
- Removal of O-linked sugars inhibits laminin binding
