Haemolytic anaemias + Haemoglobinopathies Flashcards
what are haemoglobinopathies
inherited disorders where expression of one or more of the globin chains of haemoglobin is abnormal
two main categories of haemoglobinopathies
abnormal haemoglobin variants
- result of mutations in genes for α or β chains
- alter stability and/or function of haemoglobin
- eg sickle cell disease
thalassaemias
- reduced or absent expression of normal globin chains
- imbalance in composition of haemoglobin tetramer
- reduced level of haemoglobin
inheritance pattern of haemoglobinopathies
- typically autosomal recessive
- heterozygotes show mild or no symptoms
- homozygotes show symptoms of disease
structure of haemoglobin
- tetramer of 4 globin polypeptide chains
-
2 alpha (α) chains and 2 non-alpha chains (β
, δ or γ) held together by noncovalent interactions - each globin chain complexed with an oxygen binding haem group
changes in haemoglobin types
- different haemoglobins expressed during developent as an adaptive response to variations in oxygen requirements
- several embryonic forms expressed early in development
- fetal haemoglobin (HbF) main form just before birth
- HbA commences before birth and steadily increases to become dominant after birth
3 main types of haemoglobin in adults
- HbA 2α+2β ~95%
- HbA2 2α+2δ ~3%
- HbF 2α+2γ <1%
where are the globin gene clusters located
- α globin genes on chromosome 16
- γ, δ, β globin genes on chromosome 11
- humans have 4 α genes and 2 β genes
expression of globin genes
- expression of these genes under tight control to ensure 1:1 ratio of α:non-α chains
- defects in regulation of expression of globin genes can result in abnormalities in absolute and relative amounts of globin chains resulting in thalassaemia
- defects in coding regions result in abnormal variants with structural defects that alter stability and/or function of haemoglobin
thalassaemias
- heterogeneous group of genetic disorders
- more prevalent in South Asian, Mediterranean, Middle east and Far east
- result from decreased or absent α or β globin chain production resulting in an imbalance in composition of a2b2 tetramer
α thalassaemia
deletion or loss of function of one or more of the four α globin genes
1- silent carrier state
- asymptomatic
- carrier of disease
2- α thalassaemia trait
- minimal or no anaemia
- microcytosis and hypochromia
- resembles β thalassaemia minor
3- Haemoglobin H disease (HbH)
- moderately severe anaemia
- tetramers of β globin (HbH) form
- microcytic, hypochromic anaemia
- target cells and Heinz bodies
- resembles β thalassaemia intermedia
4- hydrops fetalis
- severe anaemia, in utero death
- excess γ globin forms tetramers in foetus (Hb Bart) that can’t deliver oxygen to tissues
β thalassaemia
mutation in one or both β globin genes leading to reduction or absence of β globin
β thalassaemia minor/trait
- asymptomatic with mild anaemia
- microcytic and hypochromic RBC
- resembles α thalassaemia trait
- heterozygous
β thalassaemia intermedia
- severe anaemia
- mild variants of homozygous (reduction)
- some compound heterozygous states
β thalassaemia major
- severe transfusion dependent anaemia
- manifests 6-9 months after birth when synthesis switches from HbF to HbA
- homozygous
thalassaemia phenotypes
- thalassaemia major transfusion dependent
- thalassaemia intermedia require transfusions intermittently
- thalassaemia minor require no transfusions
peripheral blood smear results of thalassaemia
- hypochromic and microcytic RBC
- anisopoikilocytosis
- frequent target cells
- nucleated RBC
- Heinz bodies
how is thalassaemia a form of haemolytic anaemia
- relative excess of the unaffected globin chain contributes to defective nature of RBC eg. insoluble aggregates of α chains
- haemoglobin aggregates get oxidised
- premature death of erythroid precursors within bone marrow leading to ineffective eryhtropoiesis
- excessive destruction of mature red cells in spleen leading to shortened RBC survival
consequences of thalassaemia
- extramedullary haemopoiesis to compensate but results in splenomegaly, hepatomegaly and expansion of haemopoiesis into bone cortex imparing growth and skeletal abnormalities
- reduced oxygen delivery leads to stimulation of EPO which further contributes to drive to make more defective RBC
- iron overload due to repeated blood transfusions to treat anaemia and the excessive absorption of dietary iron due to ineffective haemopoiesis
- reduced life expectancy
treatment for thalassaemias
- red cell transfusion from childhood
- iron chelation to delay iron overload
- folic acid to support erythropoiesis
- immunisation
- holistic care - cardiology, endocrine, psychological, ophthalmology input to manage complications
- stem cell transplantation to replace defective red cell production
- pre-conception counselling for at risk couples and antenatal screening
what is sickle cell disease
- autosomal recessive disease due to mutation of β globin gene causing HbS variant
- GAG changed to GTG so uncharged valine instead of charged glutamic acid at position 6 of β globin
- HbS variant common in black africans, arab, mediterranean and south asian
- heterozygous HbS (sickle cell trait) causes mild anaemia and resistance to malaria due to changes in RBC making it hard for Falciparum parasite to grow
- homozygous HbSS develop sickle cell disease
- HbS can be co-inherited with another abnormal Hb e.g. HbC or β thalassaemia
mechanism of sickle cell anaemia
- HbS forms** tetramers under normal oxygen tension** so readily gives up oxygen compared to HbA
- HbS forms polymers under low oxygen tension causing cell to form sickle shape
- repeated episodes of sickling causes red cell membrane to lose its elasticity causing irreversibly sickled red blood cells
consequences of sickle cell formation
vaso-occlusive episodes due to occlusion of small capillaries from trapped sickle cells
- recurrent acute pain
- painful bone crises
- stroke
- acute chest syndrome
- chronic kidney disease
- avascular encrosis - joint damage
anaemia
- sickle cells undergoing haemolysis
- shortened erythrocyte lifespan from ~120 days to ~20-30 days
jaundice and gallstones
- increased bilirubin from chronic haemolysis
splenic atrophy
- splenic infarction
- susceptibility to infection by encapsulated bacteria (streptococcus pneumoniae, streptococcus meningitidis)
aplastic crises
- often triggered by parvovirus
treatment for sickle cell anaemia
haemopoietic stem cell transplantation is only cure but rarely performed
treatment concentrated on reducing symptoms with regular medical care to prevent complications:
- folic acid
- penicilin
- vaccinations
- hydroxycarbamide - increases HbF levels
- red cell exchange
what is haemolytic anaemia
- results from abnormal breakdown of RBC in blood vessels (intravascular haemolysis) or spleen (extravascular haemolysis)
- normal RBC lifespan ~120 days
- bone marrow can compensate for decrease in lifespan by increasing production up to a point (5-6 fold)
- if haemolysis exceeds capacity of marrow then rate of destruction exceeds rate of production and anaemia develops
consequences of haemolytic anaemia
- severity of anaemia typically worse than chronic disease if Hb very low or sudden fall in Hb
- accumulation of bilirubin leading to jaundice and pigment gallstones
- overworking of red pulp leading to splenomegaly
- massive sudden haemolysis can cause cardiac arrest due to lack of oxygen delivery to tissues & hyperkalaemia due to release of intracellular contents
key laboratory findings in haemolytic anaemias
- raised reticulocytes as marrow tries to compensate
- raised bilirubin due to breakdown of Haem
- raised LDH as red cells rich in this enzyme
types of haemolytic anaemis
inherited defective gene
- glycolysis defect (pyruvate kinase deficiency)
- pentose phosphate pathway (G6PDH deficiency)
- membrane protein (eg hereditary spherocytosis)
- haemoglobin defect (eg sickle cell)
acquired damage to cells
- mechanical damage (eg microangiopathic anaemia)
- antibody damage (autoimmune haemolytic damage)
- oxidant damage (exposure to chemicals or oxidants)
- heat damage (severe burns)
- enzymatic damage (snake venom)
microangiopathic haemolytic anaemias (MAHA)
mechanical damage of red cells
- shear stress as cells pass through defective heart valve eg. in aortic valve stenosis
- cells snagging on fibrin strands in small vessels where there’s increased activation of clotting cascade eg. in DIC
heat damage from severe burns
osmotic damage
what is disseminated intravascular coagulation (DIC)
bleeding and clotting occur at the same time in the patient eg. malignancy, obstetric complications, trauma, sepsis
what is thrombotic thrombocytopenic purpura
syndrome where small thrombi form within the microvasculature
what is haemolytic uraemic syndrome (HUS)
clots in vessels of kidney common in children after developing E coli diarrhoea
what are schistocytes
- fragments of RBC resulting from mechanical damage
- good indicator that some form of pathology is present
autoimmune haemolytic anaemias
- caused by autoantibodies binding to red cell membrane proteins
- result from infections, lymphoproliferative disorders and reactions to drugs (eg. cephalosporins)
- classified as warm (IgG) or cold (IgM) based on the temperature the autoantibodies react best under laboratory conditions
- macrophages in the spleen recognises antibody bound cells as abnormal and destroys them
- red cell lifespan reduced resulting in anaemia
warm (IgG) autoimmune haemolytic anaemia
- IgG antibodies recognise epitopes on red cell membrane
- leads to macrophages in spleen recognising antibody-coated red cells and destroying cell by phagocytosis or nibbling a bit off
- some membrane is lost so red cells form a spherocyte
- splenomegaly often occurs as spleen is doing extra work
cold (IgM) autoimmune haemolytic anaemia
- IgM autoantibodies recognise red cell epitopes and complement fixed to patient’s red cells
- bind best at cooler temps so bind in distal parts of body like fingertips
- IgM autoantibodies span several red cells creating large agglutinates that block small capillaries
- creates ischaemic conditions in peripheral body parts causing numb fingertips, pallor, blue discolouration, gangrene
- IgM falls off in warmer parts of body and agglutination disappears
- complement binding to RBCs directly create holes in membrane and cause macrophages in spleen to recognise and destroy cells
what is the direct Coombs test
- used to detect antibodies or complement bound to surface of RBCs
- patient’s red cells mixed with anti-human globulin antibody that will attach to antibodies on red cell making them clump together
- suggests patient’s haemolysis is immune related
how does pyruvate kinase deficiency cause haemolytic anaemia
- mutations in PKLR gene causing deficiency in pyruvate kinase which catalyses final step in glycolysis and produces ATP
- red blood cells lack mitochondria so deficiency inhibits their only metabolic pathway that supplies ATP for cellular processes
- sodium potassium ATPase pump activity inhibited from insufficient ATP so red cells lose potassium to plasma
- water moves down concentration gradient causing cells to shrink resulting in cellular death and haemolytic anaemia
- mild deficiency doesn’t require treatment, severe deficiency require regular blood transfusions
how does G6PDH deficiency cause haemolytic anaemia
- G6PDH is rate limiting enzyme of pentose phosphate pathway which supplies reducing energy by maintaining NADPH levels
- NADPH protects against oxidative stress by maintaining level of reduced glutathione
- pentose phosphate pathway only source of reduced glutathione in RBCs
- risk of haemolytic anaemia in states of oxidative stress such as infection or certain chemicals and medications
- damaged red cells are phagocytosed in spleen and metabolism of excess Hb to bilirubin can lead to jaundice
hereditary spherocytosis
- inherited autosomal dominant disease
- abnormalities in erythrocyte membrane proteins causing cells to be spherical which make cells less flexible and more easily damaged
- ankyrin, spectrin, protein 4.2 or Band 3 defects disrupt vertical interactions between cytoskeleton and plasma membrane
- poor deformability of spherocytes causes them to be trapped and damaged as they pass through spleen resulting in reduced lifespan and haemolytic anaemia
- symptoms include anaemia, jaundice, splenomegaly and Howell-Jolly bodies in blood
- severe symptoms improved with partial or full splenectomy
hereditary eliptocytosis
- many cells elliptical rather than biconcave disc shape
- spectrin defect most common
- also defects in band 4.1, band 3 and glycophorin C proteins
hereditary pyropoikilocytosis
- spectrin defect
- severe form of hereditary elliptocytosis
- abnormal sensitivity of red cells to heat
- similar morphology to thermal burns
