Lecture 22: X-linked disorders and Mitochondrial inheritance

Question 1

Overview X-linked recessive disorders;

Answer 1

• Males mostly affected
• All daughters of affected males are carriers
• No male to male transmission
• Carrier females have 50% chance transmitting trait to son and 50% giving carrier status to daughter

Question 2

What are examples of X-linked recessive disorders?

Answer 2

Haemophilia
Fragile x syndrome
Duchennes muscular dystrophy

Question 3

Give a brief overview of haemophilia and its clinical features

Answer 3

• Clotting disorder due to deficiency of either factor VIII or factor XI
• 1 in 5000 males

Clinical features

• Recurrent and spontaneous bleeds into joints, muscles, soft tissue
• Long term damage to affected joint

Question 4

Give a brief overview of heamophilia management;

Answer 4

• Infusions of coagulation factor
• Pain relief
• Rest of affected joint
  BUT
• Risk of infection from blood products
• Time off

Question 5

When it comes to genetic technology what can be analysed in heamophilia for management?

Answer 5

• Pedigree analysis
• Phenotype analysis
• DNA analysis
  Direct mutations
  Indirect analysis
• Therapeutic stratagies
  recombinant clotting factor
  gene therapy

Question 6

What is the function of pedigree analysis in heamophilia?

Answer 6

To determine which females are carries and which offspring / potential offspring are at risk

Question 7

In a phenotype analysis of haemophilia why do female carriers have reduced clotting factor 8 levels?

Answer 7

May have lyonisation (one x is deactivated) therefore can present with some of the phenotype

Question 8

What are the pitalls of a phenotypic study in heamophilia?

Answer 8

• Coagulation levels influenced by exercise, infection and oestrogens.
• Technical difficulties with fetal blood sampling

DNA analysis is more accurate, timely and definitive

Question 9

What cells can be used for DNA analysis in haemophilia?

Answer 9

• Blood
• Skin, oral mucosae
• CVS
• Amniotic fluid cells

Question 10

How can the DNA be analysed?

Answer 10

• Direct mutation analysis - Where the mutation responsible is known
• Indirect analysis i.e linkage or RFPL analysis where the mutation is not known

Question 11

Is heamophilia caused by a single gene mutation?

Answer 11

No, haemophilia is a heterogenous disorder with several types of mutations i.e
- Point mutations
- Structural changes;
Deletions
Inversions

Question 12

Whats the implication of genetic heterogeneity in heamophilia?

Answer 12

• Severity of condition (i.e the phenotype) may vary with different mutations
• For genetic diagnosis, need to know the mutation in each family

Question 13

How can the genes be examined?

Answer 13

• Southern analysis looking for large structural changes i.e deletions
• Mutation scanning strategies i.e direct sequencing of the gene
• New mutation detected, needs to be confirmed to be pathogenic

i.e direct or indirect analysis

Question 14

What is a direct mutation analysis approach?

Answer 14

Usually PCR based strategy

• Point mutation may result in gain or loss of a restriction enzyme site
• Allele-specific oligionucleotides
• Sequencing of PCR fragment

Question 15

What is an indirect (linkage) analysis?

Answer 15

• Used where gene note cloned or pathogenic mutation unknown
• Uses linked markers i.e restriction fragment length polymorphism (RFLP) or microsatellite repeats to track the disease within a family
• Mostly PCR

Question 16

What are the problems with indirect (linkage) analysis?

Answer 16

• Need to access several family members
• Need an informative marker gene
• Potential for recombination if marker gene distant from disease gene
• Non-paternity

Question 17

What are the new therapeutic strategies for heamophilia?

Answer 17

• Recombinant factor 8 and 10 now available; potentially safer and free of infective risk.

10-20% factor 8 or 10 dramatically improves quality of life (down at 1% in some)

Question 18

What is mitochondrial DNA?

Answer 18

• Double stranded circular DNA
• Maternally inherited
• 37 genes encode 13 proteins, distinct lack of non-coding intron sequences
• Mutation rate is 5-10x that of nuclear DNA

Question 19

How does mtDNA become impacted by disease?

Answer 19

mtDNA mutations result in respiratory chain deficiency

• ATP
• Apoptosis
• ROS
• Cellular oxidation and reduction

Question 20

What do mtDNA diseases generally result in?

Answer 20

Impacts high energy requiring organs i.e

• Brain
• Skeletal and heart muscle