Musculaar Dystrophy And OI Flashcards
Contrast the mutations leading to duchenne muscular dystrophy and Becker muscular dystrophy
DMD- Veru large deletions, frame shift mutations or frame shift due to exon deletion often results in premature termination codons and truncated proteins
Frameshift May result from splicing mutations, deletions or duplications
BMD- Deletions or duplications however result in in-frame changes to exons (no change in reading frame)
Contrast the effects of DMD and BMD
DMD- Severe functional impairment of dystrophin protein
BMD- creates a protein that is partially functional, or has reduced function
Contrast phenotypes of DMD and BMD
DMD- Severemuscle degeneration
Female carrier patients at risk for adult insert cardiology events
BMD- less severe muscle degeneration compared to duchenne
Age of onset is usually later
What is the effect of DMD on reproductive fitness?
Affected boys are are typically too sick to reproduce: genetic fitness=0.
So in every generation, 1/3 of all DMD causing mutations are removed from the gene pool
How do we know when a DMD mutation has occurred?
Mutation could have occurred in an earlier generation (such as inn father’s sperm formation), and have been passed on to a daughter
Mutation could gave occurred during oogenesis in the mother
Identification of the mutation in the affected boy would allow testing of the mother, and other family members for carrier status
What is the function of dystrophin?
Role in muscle fiber integrity
Summarize the functional role of dystrophin
Dystrophin links the actin cytoskeleton with protein complexes that are associated on the sacrolemma membrane. This provides integrity to muscle fiber. Big protein, big gene! 3 domains:
- Actin binding
- Repeat domain (spacer)
- Domain that binds to stuff on plasma membrane
How much 9f DMD causing mutations are due to deletions?
Majority (about 2/3) are due to relatively large deletions
What is the biggest gene in the genome?
Dystrophin is by far the biggest gene in the genome. It has 79 exons, and is about 2.3 Mb
How can DMD be diagnosed ?
For diagnosis many sets of multiplex CR assays are done in tandem (about 20) which allows coverage of all 79 exons)
- detects most (most up to 98%) of all known DMD deletion mutations
- so this method detects about 2/3 of all DMD mutations
Deletion correlates with severity of Muscular dystrophy
- Mutations that lead to Becker tend to less severe
- If a deletion mutation leads to Becker, it typically keeps genetic code in frame
- A small deletion may lead you DMD if it creates frameshift, or stop codon
What are some other types of mutations that cause DMD?
After normal splicing, the splice junction created by this duplication event may cause genetic code to go out of frame
-May lead to severe Duchenne muscular dystrophy
These other DMD causing mutations in dystrophin are difficult to detect:
- Duplication events
- Single base pair mutations (missense, nonsense)
- Mutation of a small number of base pairs
- Small insertions or deletions (frameshift)
Require direct sequencing and analysis, so the test is more expensive
Why BMD more inherited compared to DMD?
Becker phenotype is less severe, so about 90% of mutations are inherited compared to 2/3 of DMD
BMD: Dystrophin has reduced abundance, may appear normal sized, or reduced size
DMD: Almost no dystrophin is present, or it’s abundance is severely reduced
Describe gross pathology of skeletal muscle in fatal duchenne muscular dystrophy
Yellowish-white fat replaces normally reddish-brown skeletal muscle
Explain DMD having alleilic heterogeneity
Maybe it is known that a patient has a DMD, but the mutation cannot be identified
Patient DNA may be obtained by amino, CVS or via pre-implantation diagnosis of embryo
-It also allows carrier females to be identified in family lineages
Sometimes a new mutation may be difficult to identify, or mutation is NOT a deletion. This means that the mutation would not be detected by multiplex PCR
How can a female gave DMD symptoms?
Functional Mosaicism due to skewed X inactivation May lead to female patients with DMD symptoms
Dystrophin indirect immunofluroescenve: white circles around muscle sacrolemma
In the manifesting Heterozygote female there are areas of tissue that are healthy while others are badly affected
What are the genetic principles illustrated by DMD & BMD?
X-linked recessive
High mutation rate due to large gene
Variable expressivity
- Severe- duchenne (genetic fitness near zero)
- Less severe- Becker (affected men may still be fathers)
Alleilic heterogeneity
Manifesting heterozygote due to X-inactivation
-Carrier females at risk for adult onset cardiomyopathy
What is the repeating unit of collagen?
(G-X-X)n
Describe COL1A1 & COL1A2 at a normal state
The collagen molecule is a triple helix
Normally the collagen triple helix has two collagen 1A1 peptides and one collagen 1A2 peptide
The COLA1 gene is expressed twice as much as COL1A2
How does haploinsufficiency explain OI
Loss of expression from one allele of COL1A1: OI
The 2:1 ratio must be maintained
Loss of expression of one allele of either COL1A1 or COL1A2 will lead ton1/2 the amount of collagen formation
Haploinsufficiency explains classic nom-deforming OI with blue sclera (Type 1 OI)
How does dominant negative explain OI?
A missense mutation (point mutation that changes the encoded amino acid) might produce a collagen chain with altered structure/properties.
A missense mutation in either COL1A1 or COL1A2 will lead to formation of collagen with aberrantbproperties
Abundant collagen is produced, but most of it has structural abnormalities
Dominant negative explains perinatal lethal OI (TYPE 2)
Describe factors influencing severity of OI
A missense mutation (point mutation that changes the encoded amino acid ) might produce a collagen chain with altered properties
Most severe is perinatal lethal OI; also known a type 2
Depending on the mutation, the severity of the OI might be different. This genotype/phenotype correlation is difficult to predict on mutation alone
What would be part of the differential diagnosis for a child with multiple fractures in an infant with minimal trauma?
OI
Whaat causes the gray-blue sclera in OI?
The thinner collagen in the colloid matrix of the eye allows increased light scattering and the appearance of a gray Sclera
What are the genetic principles illustrated by OI?
Autosomal dominant
- haploinsufficiency
- dominant negative
Variable expressivity
-from increased risk of broken bones-progressive forms- to lethality
-Environmental influence- consider two siblings each with OI, same mutation:
1st sib is careful & reserved- less likely to have broken bones
2nd sib seeks adventure and risk-more likely to break bones
Locus heterogeneity-different genes may be involved
-COL1A1, COL1A2
Allelic heterogenity
-often times, mutation is different in different families
