Lecture 21- X-linked disorders and Mitochondrial inheritance

Question 1

Haemophilia clinical symptoms and management

Answer 1

deficiency in clotting factor.
recurrent and spontaneous bleeds into joints, muscles and soft tissue. long term damage to affected joints.

infusions of coagulation factors
pain relief
rest of affected joint

Physical suffering
Ongoing need for therapy 
Risks and complications of therapy
Social, education and work implications
Long term complications eg hepatitis B and C, HIV, possibly variant CJD, chronic joint damage

Question 2

diagnostic challenge in X linked recessive

Answer 2

Diagnosis of asymptomatic female carriers in a family with an X linked recessive disorder

Offering strategies for a woman who is known carrier and wishes to have children

Question 3

strategies for diagnosing X linked recessive inheritence

Answer 3

pedigree analysis; examine family tree and provide probabilities
phenotypic analysis of clotting factors in the blood

Question 4

problems with phenotypic analysis

Answer 4

Variation in normal clotting factor levels
eg with exercise, stress etc
Requires fetal blood sample for prenatal diagnosis, therefore late and difficult

Question 5

Direct mutation analysis for haemophilia

Answer 5

Analyze DNA from affected patient – identify mutation

Potential female carriers eg sister of a haemophilia patient – can accurately confirm carrier status

Question 6

Indirect Genetic Analysis for haemophilia with unknown gene

Answer 6

If mutation in family not known can use linked polymorphic markers / SNP’s to track the abnormal gene in a family

Question 7

Direct mutation analysis

Answer 7

preferred approach
-accurate
-does not require additional family members
usually PCR based strategy
-point mutation may result in gain or loss of a restriction enzyme site
-allele-specifc oligonucleotides
-sequencing of PCR fragment

Question 8

Indirect analysis

Answer 8

used where gene not clones or pathogenic mutation unknown
uses linked markers; eg. restriction fragment length polymorphism or micro satellite repeats to track the disease gene within a family
most are PCR based

problems include

• need access to several family members
• need informative marker gene
• potential for recombination if marker gene distant from disease gene
• non paternity

Question 9

Options for women confirmed as carrier and wanting children

Answer 9

Mutation analysis of fetal DNA obtained by chorionic villous biopsy or amniocentesis
In vitro fertilization followed by analysis of 6-8 cell blastocyst:
Select non male embryos
Select embryos with normal factor VIII gene

Question 10

Mitchondrial DNA

Answer 10

Double stranded circular DNA
Maternally inherited
Each mitochondria has 2 to 10 DNA molecules
Each cell contains multiple mitochondria
Mutation rate is 10x that of nuclear DNA, does not have protective histones or effective repair mechanisms

Question 11

Mitonchondrial DNA and disease

Answer 11

Mutations in mtDNA produce effect by deficiency in respiratory chain (5 enzyme complexes within inner mitochondrial membrane)
		production of ATP
		apoptosis
		production of reactive oxygen species
		cellular oxidation and reduction
Diseases associated with mtDNA mutations affect organs with high energy requirements
		brain
		skeletal and heart muscle

Question 12

Point mutations of mtDNA

Answer 12

MERRF (myoclonic epilepsy with ragged red fibres)
irregular contour of muscle fibres due to subsarcolemmal collections of mitochondria that appear red with trichrome stain

Question 13

Deletion/insertions of mtDNA

Answer 13

chronic progressive external ophthalmoplegia
slowly progressive paralysis of extraocular muscles
commences with ptosis