Molecular Basis Of Inherited Bleeding Disorders

Question 1

Haemophilia

Answer 1

X linked
Abnormal bleeding caused by defects in coagulation system
Bleeding in joints and muscles

Question 2

F8 Gene, it’s activation and deficiency causes:

Answer 2

Encodes FVIII protein
Deficiency of FVIII = Haemophilia A
Activated by proteolysis

Question 3

Coding sequence of F8 gene spread over ___ exons

Answer 3

26

Question 4

F9 Gene

Answer 4

Encodes FIX protein
Deficiency of FIX = Haemophilia B
Activated by proteolysis

Question 5

Coding sequence of F9 gene spread over ___ exons

Answer 5

8

Question 6

2 main groups of mutation that cause haemophilia

Answer 6

Point - missense, nonsense, frameshift
Gross - deletions, insertions, inversion

Question 7

Missense Mutation in haemophilia

Answer 7

Most common PM in haemophilia
Conservative: serine -> alanine
Non-conservative: arginine -> threonine

Question 8

Nonsense mutation

Answer 8

Shortened FVIII protein = biologically inactive = severe haemophilia A
Stop codon in exon 14

Question 9

Frameshift mutation in F8 gene

Answer 9

Biological activity of FVIII lost = severe Haemophilia A

Question 10

Deletion or Insertion in haemophilia

Answer 10

Gene function disabled = severe haemophilia

Question 11

How does inversion cause haemophilia?

Answer 11

Rearrangement of intron 22 or 1 of F8 gene = gene function disabled = severe haemophilia

Question 12

How to identify F8 Mutations

Answer 12

Obtain blood sample from male w/ haemophilia
Extract DNA sample
PCR F8 gene
Determine DNA sequence of F8 gene
Compare patient & normal sequence - look for mutation

Question 13

Intron 22 Inversion Detection

Answer 13

PCR amplifies F8 gene
Different products when inversion present & absent
(Or RT-NPCR detects only mutated DNA)
Banding pattern used to identify heterozygous females who are carriers

Question 14

Importance in identifying female carriers:

Answer 14

Genetic counselling
Prenatal diagnosis

Question 15

Measuring FVIII activity level in plasma of females is unreliable due to:

Answer 15

Lyonisation
Influence of female sex hormones
FVIII = acute phase protein

Question 16

Von Willebrand Disease

Answer 16

Most common inherited bleeding disorder, autosomal dominant
Mutation in VWF gene
Variable & mainly mild bleeding tendency

Question 17

Symptoms of VWD

Answer 17

Skin and mucus membranes affected - easy bruising
Nose bleeds 🐽
Menorrhagia
Bleeding from minor wounds, dental extractions, surgery, child birth, oral and GI bleeding

Question 18

Van Willebrand Factor

Answer 18

Large multi domain plasma protein w/ binding sites for FVIII, collagen & platelets
Mediates platelet adhesion & aggregation

Question 19

VWF & Blood clotting

Answer 19

Vessel damage exposes sub endothelium to blood
VWF binds collagen in sub endothelium
Gp1b binding site on VWF becomes exposed & platelets bind
Platelets adhere to damaged area

Question 20

Type 1 VWD

Answer 20

⬇️ amount of VWF in plasma - defect in VWF synthesis or ⬆️ clearance

Question 21

Type 2 VWDs

Answer 21

⬇️ plasma VWF activity

Question 22

Type 2A VWD

Answer 22

Loss of biggest multimers. 2 defects:
- defective assembly & secretion of VWF multimers
- ⬆️ susceptibility of VWF to proteolysis

Question 23

Type 2B VWD

Answer 23

Mutation causes Gp1b platelet binding site to become exposed. VWF binds platelets spontaneously & thrombocytopaenia leads to bleeding

Question 24

Type 2M VWD

Answer 24

Mutation causes defect in Gp1b binding site. Loss of ability to bind platelets. ⬇️ platelet adhesion = bleeding

Question 25

Type 3 VWD

Answer 25

Rare, severe form. Plasma VWF levels ~ 0. Autosomal recessive