20.04.11 Blood disorders Flashcards
1
Q
What are inherited bleeding disorders?
A
- group of disorders that share the inability to form a proper blood clot
- Caused by defects in blood components (usually platelets or 1 of the 13 clotting factors)
- Can be mild, moderate or severe
- Majority are inherited (but can be caused by other factors - anaemia, vit K deficiency, HIV etc
-
2
Q
Name 3 common bleeding disorders
A
1) von Willebrand disease
2) Haemophila A
3) Haemophilia B
- But can get deficiencies in any of the clotting factors in the cascade
3
Q
How are bleeding disorders currently tested?
A
- NGS is becoming more common - Blood disorders with a typical phenotype undergo single gene testing - But if phenotype is less specific, then a large NMG panel approach is taken - PanelApp panel is now live for the new genomic hubs to use from April 2020
4
Q
Briefly describe the clotting cascade
A
- get seperate extrinsic and intrinsic pathways which join to form common pathway
1) extrinsic pathway - Damaged endothelial cells express tissue factor (TF)
- TF activates factor 7 which then goes on to activate factors 10 and 11
- Activated factor 10 produces prothrombinase
2) Intrinsic pathway - blood cells get exposed to collagen due to damage
- activates factor 12, then binds to calcium to activate factor 9
- Factor 9 then binds to factor 8 to create factor 10 activator complex - this then activates factor 10 which stimulates prothrombinase production
3) Common pathway - Prothrombinase converts prothrombin (factor 7) to thrombin (activated factor 7)
- Thrombin does 3 things
1) converts firbinogen to fibrin
2) activates factor 5 to produce more thrombin (positive feedbck loop)
3) activates factor 13 which helps to bind fibrin strands together to produce cross-linking fibrin
5
Q
von Willebrand disease (vWD)
A
- most common, usually mild
- due to low plasma concentration of vWF or structurally abnormal vWF with reduced activity
- Affects males and females
- vWF (found at 12p) normally has two jobs:
1) carrier protein for factor 8, which protects F8 from degradation
2) acts as an adhesison protein between platelets and subendothelial collagen fibres (so vWF helps stick platelets to fibres during primary haemostatis to from platelet plug) - vWF formed as a polypeptide and then dimises and from a multimer with high molecular weight
- Multimers are functionally more effective in promoting platelet adhesion and aggregation
- low levels of normal VWF causes low levels of factor 8, as its half life is reduced with no carrier protein
- Clinical features - defective primary haemostasis (bruising, epitaxis, prolonged bleeding, menorrhagia)
6
Q
What are the subtypes of vWD?
A
1) Type 1 - low levels of normal VWF
2) Type 2 - structurally abnormal VWF
- 2A - low levels of high MW multimers
- 2B - VWF with increased affinity for platelet glycoprotein 1B
- 2M - Reduced platelet dependent function (so normal high MW multimers levels but don’t stick platelets together well)
- 2N - reduced affinity for F8
3) Type 3 - Complete loss of VWF
7
Q
Treatment for VWD
A
- Depends on severity of bleeding
1) Hormone injection of Desmopression - stimuates production of F8
2) Tranexamic acid tablets - stops blody from breaking down clots
3) F8 or vWF concentrate - purified from human plasma, only given in more severe cases
8
Q
Diagnosis fo VWD
A
1) quantify VWF antigen levels in plasma (ELISA-based immunoassay)
2) quantify efficacy of plasma VWF in its ability to bind platelets (VWF ristocetin cofactor assay - works as ristocetin promotes platelet aggregation in presence of VWF)
3) Can also measure high MW multimer size
4)
9
Q
Haemophilia A
A
- Most common severe bleeding disorder
- F8 deficiency
- Sex-linked - almost only affects males - female carriers can be asymptomatic, or have mild symptoms (depending on skewed X inactivation). HOM femalea are rare but would have similar phenotype to affected males
- Clinical severeity depends on factor levels:
1) mild - 5-20 IU/dl - bleeds after major trauma or post-op
2) moderate - 1-5 IU/dl - bleeds after mino truama
3) severe - <1 IU/dl - Spontaneous bleeds (mostly into joints or muscles) - leads to chronic synovitis - Factor levels checked during diagnosis
- F8 gene is very large (26 exons)
- See whole range of SNVs and CNVs, milder cases caused by milder variants (missense) etc
- 50% of severe cases caused by flip-tip inversion in intron 22 due to NAHR between CpG repeat regions during meiosis (introns 1-22 moved away from normal location and orientation is reversed)
- 33% are de novo variants
- All affected children should be screened for mutations:
1) if severe screen for intron 22 inversion first, then intron 1 inversion, then seq whole gene
2) Carrier testing can also be carried out - Inversions are detected by long PCR method (multiplex PCR) or inverse PCR which both use primers that span breakpoints of common inversions
- Sequencing of the whole gene is currenlty classed as the gold standard for testing
- MLPA can also be used to detect rarer deletions or insertions
10
Q
Haemophilia B
A
- X-linked recessive (Xq27.1) - also caused Christmas disease
- similar phenotype to A (joint and muscle haemoarthroses)
- Deficiency of F9
Highler risk fo bleeding during delivery, as F9 levels don’t rise during pregnancy (like they do with F8)
- Less common than haemophilia A (only 15-20% to total cases)
- Normally caused by missense variants
- There are no common repeat mutations (although 20-30% of cases are cause dby founder mutations)
- Testing strategy is to seq F9 genes, the MLPA if required for rare deletions/insertions
11
Q
Treatment for Haemophilia
A
1) Prophylaxis treatment
2) On demand treatment
- both involve injection of clotting factor
For A you can develop F8 inhibitors, so needs to routinely check for these
- If there is a family history, then screen for variants
- If male pregnancy, can check F8 and 9 levls at birth
12
Q
Other bleeding disorders
A
- There are also lost of rare bleeding disorders:
1) Congenital afibrinogenemia (F1 deficiency) - Complete absence of fibrinogen (severe)
- Get uncontrolled bleeding from umbilical cord at birth
- 3 genes (fibrogen alpha, beta or gamma chain genes at 4q28)
- AR/AD, but most common is 11kb deletion in FGA
2) F7 deficiency - HOMs can be asymptomatic or severely affected (can cause death)
- CNS, GI and joint bleeds
- Missense are most common, very heterogeneous
- Treatment with recombinant F7
13
Q
Inherited thrombophilias
A
- increased risk of venous or arterial thrombosis
- Thrombophilia = form clots inappropriately
- most common are FVL and prothrombin thrombophilia
14
Q
Factor V leiden mutation
A
- Activated protein C resistance
- After injury, F5 is activated and binds to activated F10 - Together they convert protrhombin to thrombin
- APC normally cleaves F5 at R306, R506 or R679 (making it inactive) and stopping coagulation, to controll size of a clot
- FVL is a c.1691G>A p.(Arg506Gln) variant that stops cleavage at R506 - very common - 8.8% of caucasians
- Mutation makes F5 less susceptible to APC cleavage, so get reduced -ve feedback and increased risk of venous thrombosis (het = 5 fold, hom = 50 fold increase)
- Diagnosed by APC resistance assay =, coagulation screen or by mutation screening
15
Q
F2 prothrombin thrombophilia
A
- Found in 2-3% of caucasians
- can be HET or HOM for c.20210G>A variant - glycine to alanine sub in 3’UTR of F2 gene
- Causes over expression of F2 gene which leads to high plasma prothrombin levels and a 2-3 fold increased risk of thrombosis (DVT) in HETs, and 20 fold increase in HOMs
- prothrombin promotes production of blood clots through conversion to thrombin and then fibrinogen to fibrin
