Musculaar Dystrophy And OI Flashcards

1
Q

Contrast the mutations leading to duchenne muscular dystrophy and Becker muscular dystrophy

A

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)

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2
Q

Contrast the effects of DMD and BMD

A

DMD- Severe functional impairment of dystrophin protein

BMD- creates a protein that is partially functional, or has reduced function

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3
Q

Contrast phenotypes of DMD and BMD

A

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

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4
Q

What is the effect of DMD on reproductive fitness?

A

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

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5
Q

How do we know when a DMD mutation has occurred?

A

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

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6
Q

What is the function of dystrophin?

A

Role in muscle fiber integrity

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7
Q

Summarize the functional role of dystrophin

A

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:

  1. Actin binding
  2. Repeat domain (spacer)
  3. Domain that binds to stuff on plasma membrane
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8
Q

How much 9f DMD causing mutations are due to deletions?

A

Majority (about 2/3) are due to relatively large deletions

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9
Q

What is the biggest gene in the genome?

A

Dystrophin is by far the biggest gene in the genome. It has 79 exons, and is about 2.3 Mb

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10
Q

How can DMD be diagnosed ?

A

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
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11
Q

Deletion correlates with severity of Muscular dystrophy

A
  • 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
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12
Q

What are some other types of mutations that cause DMD?

A

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:

  1. Duplication events
  2. Single base pair mutations (missense, nonsense)
  3. Mutation of a small number of base pairs
  4. Small insertions or deletions (frameshift)

Require direct sequencing and analysis, so the test is more expensive

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13
Q

Why BMD more inherited compared to DMD?

A

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

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14
Q

Describe gross pathology of skeletal muscle in fatal duchenne muscular dystrophy

A

Yellowish-white fat replaces normally reddish-brown skeletal muscle

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15
Q

Explain DMD having alleilic heterogeneity

A

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

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16
Q

How can a female gave DMD symptoms?

A

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

17
Q

What are the genetic principles illustrated by DMD & BMD?

A

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

18
Q

What is the repeating unit of collagen?

A

(G-X-X)n

19
Q

Describe COL1A1 & COL1A2 at a normal state

A

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

20
Q

How does haploinsufficiency explain OI

A

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)

21
Q

How does dominant negative explain OI?

A

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)

22
Q

Describe factors influencing severity of OI

A

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

23
Q

What would be part of the differential diagnosis for a child with multiple fractures in an infant with minimal trauma?

A

OI

24
Q

Whaat causes the gray-blue sclera in OI?

A

The thinner collagen in the colloid matrix of the eye allows increased light scattering and the appearance of a gray Sclera

25
Q

What are the genetic principles illustrated by OI?

A

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