The Haemoglobin molecule and thallaseamia Flashcards
Summarise the key features of RBCs
Carry oxygen from lungs to tissues
Transfer CO2 from tissues to lungs
3.5-5 x 1012 /L (WBC ^9- so much more RBCs)
Contain haemoglobin (Hb)
Each red cell contains approximately 640 million molecules of Hb
Do not have nucleus or mitochondria
Why is it important that Hb is packaged into RBCs
Hb is very toxic- strong oxidant properties
Hence in haemolytic anaemias, when RBCs lyse and release free Hb into the circulaiton- this can cause lots of damage to tissues.
Summarise the key features of Haemoglobin
Found exclusively in RBCs
MW 64-64.5 kDa
Normal concentration in adults:120-165g/L
Approximately 90 mg/kg produced and destroyed in the body every day
Each gram of Hb contains 3.4mg Fe
What is important to remember about the range of Hb concentration for females
Slightly less than that for males
110-165 g/L
When does the synthesis of haemoglobin take place and why is it important that it occurs at these stages
Synthesis occurs during development of RBC and begins in pro-erythroblast:
65% erythroblast stage
35% reticulocyte stage
Needs to be synthesises before nucleus is lost (mature erythrocytes have no nucleus).
Describe the basic structure of a haemoglobin molecule
Haemoglobin (Hb) is a protein molecule found in red blood cells. Each molecule of haemoglobin consists of 2 pairs of globin protein chains together with 4 haem groups. Each haem group consists of a protoporphyrin ring with an iron atom at its centre and a single haem group sits in a pocket formed by a single globin chain:
Haem (synthesised in mitochondria)
Globin (synthesised in ribosomes)
Outline the synthesis of haemoglobin
Transferrin endocytosed in vesicle with receptor it binds to
Iron released from veiscle
Receptor and transferrin recycled in transferrin cycle
Fe can be stored as ferritin in the cell (or Fe released from ferritin)
Fe enters mitochondria
b. Glycine, B6 and Succinyl CoA create delta-ALA which then undergoes a few moderations outside the mitochondria and then passes back in as proto-porphyrin.
Proto-porphrin and Fe combine to form four haem molecules
Globin: 2. Globin: a. Amino acids are used in ribosomes to create the globin chains. ( alpha and beta) Alpha and beta chains associate (A2B2) 3. Haemoglobin: a. Globins and haem associate.
Describe the regulation of the synthesis of haem
If we have too much haem
Negative feedback on delta-ALA enzyme
Hence no production of porphobilinogen
No production of uroporphrinogen
No production of Coproporphyrinogen
No production of proo-porphryin
No production of haem
What does delta-ALA stand for
5-Aminolevulinic acid
Where else is Haem found
Not exclusive to haemoglobin
Also contained in other proteins e.g. myoglobin, cytochromes, peroxidases, catalases, tryptophan
Summarise the key features of haem
Same in all types of Hb
Combination of protoporphyrin ring with central iron atom (ferroprotoporphyrin)
Iron usually in ferrous form (Fe2+)
Able to combine reversibly with oxygen
Synthesised mainly in mitochondria which contain the enzyme ALAS
Regulation
Summarise the key features of globin
Various types which combine with haem to form different haemoglobin molecules
Eight functional globin chains, arranged in two clusters:
b- cluster (b, g, d and e globin genes) on the short arm of chromosome 11
a- cluster (a and z globin genes) on the short arm of chromosome 16
Describe the different genes in which globin chains are encoded
Several different types of globin proteins exist each encoded by their own gene (s). The globin genes are located in two clusters. The alpha cluster is found on chromosome 16 and contains the genes for α globin (adult variety) and ζ globin (zeta globin – an embryonic variant). The alpha genes are duplicated so that there are two functional alpha genes within an alpha cluster. (alpha 1 and 2)
The beta cluster is found on chromosome 11 and contains the genes for β globin, and δ globin (adult varieties), γ globin and ε globin (fetal and embryonic variants respectively).
What are the embryonic variants of haemoglobin
Gower 1- ( 2 z and 2e)
Portland ( 2 z and 2 gamma)
Gower 2 (2 alpha and 2 e)
What is the main fetal haemoglobin variant
HbF
(2 alpha and 2 gamma)
(also found in large amounts at birth)
What are the main adult haemoglobin variants
HbF
HbA2- 2 alpha and 2 delta
HbA- 2 alpha and 2 beta
What is special about the alpha globin genes
There are TWO alpha globin genes from each parent so there are FOUR alpha globin genes in total
Describe globin gene expression and switching
§ a - is made relatively early and stays high throughout.
§ b - is equal and opposite to g and becomes dominant after birth.
§ g - is equal and opposite to b and is dominant pre-natal.
§ d - production begins mid-natal and remains low forever.
§ e and z - is equal and opposite to a and levels drop ~0 after 8 weeks.
- When are the genes coding for the globin in foetal haemoglobin switched off?
It is decreased towards birth and in the first year after birth.
After 1 year of life, the normal adult pattern of haemoglobin synthesis would have been established.
Explain why alpha thallasaemia majors are not compatible with life
Due to gene expression of alpha
Expressed highly in embryonic, fetal and adult life
Therefore absence of this globin chain would mean that the fetus nor the embryo could make haemoglobin- no delivery of O2- death before birth.
Explain why beta thalassameias are compatible with life
Beta globin only becomes dominant with alpha after birth (gamma before birth in embryo)
Will see symptoms 3 months after birth
Describe the distribution of the different adult haemogloobins found in the blood
HbA (22) is the most common – 96-98%.
HbA2 (22) is the second most common – 1.5-3.2%.
HbF (22) is the least common – 0.5-0.8%.
How can we determine the different proportions of each haemoglobin in an adult
High performance liquid chromatography can seperate haemoglobins based on their electrophoretic charge and molecular mass
Different peaks= different Hb
HbF- to the left of HbA
HbA2- to the right of HbA
Each of these have a glycated fraction however only glycated HbA is normally present in sufficient quantity to be visible on a HPLC chromatogram and then only up to approximately 5%
Describe the structure of globin
Primary
α 141 AA
Non- α 146 AA
Secondary
75% α and b chains-helical arrangement
Tertiary
Approximate sphere
Hydrophilic surface (charged polar side chains), hydrophobic core
Haem pocket (where haem component sits once four chains have assembled)
Compare the structures of oxygenated and deoxygenated haemoglobins
§ Haemoglobin has the highest affinity to oxygen when the binding is loose (cooperativity) – more O2 means greater binding of O2.- O2 can access haem pocket
§ 2, 3-DPG is made by muscle cells (and other highly metabolising cells) to increase dissociation of oxygen
Configuration changes so that O2 cannot access haem pocket- squeezed out and delivered to tissues
Ultimately, what does the oxygen-haemoglobin dissociation curve describe
O2 carrying capacity of Hb at different pO2
What are the key features of the oxygen-haemoglobin dissociation curve
Sigmoid shape
Binding of one molecule facilitate the second molecule binding (cooperativity)- equally losing one O2 molecule makes it easier for other oxygen molecules to dissociate
P 50 (partial pressure of O2 at which Hb is half saturated with O2) 26.6mmHg- for HbA- approx 4kPa
kPa of O2 on the x-axis
HbO2 (%) on y-axis
What does the P50 allow us to compare
The different affinities of different haemoglobin molecules for oxygen.
What does the normal position of the dissociation curve depend on
Concentration of 2,3-DPG
H+ ion concentration (pH)
CO2 in red blood cells
Structure of Hb
Describe the factors that lead to a right shift
Right shift (easy oxygen delivery)
High 2,3-DPG
High H+
High CO2
HbS
Describe the factors that lead to a left shift
Left shift (give up oxygen less readily) Low 2,3-DPG HbF (important for fetus to have Hb with a greather affinity for oxygen than adult haemoglobin).
What is meant by the T and R configurations of haemoglobin
Deoxyhaemoglobin exists in a tight (T) configuration and has a relatively low affinity for oxygen. Oxygen molecules are taken up sequentially by the 4 haem groups and at some point the partially liganded Hb molecule switches to a relaxed (R) configuration which has a markedly higher affinity for oxygen.
Describe the axis of the dissociaiton curve
On the Y axis is the oxygen saturation which is defined as the fractional occupancy of the oxygen binding sites, and on the X axis is concentration of oxygen which is expressed as its partial pressure:
Describe how affinity of haemoglobin changes with oxygen binding and how this helps its role of oxygen transport.
The more oxygen binds, the greater the affinity of the haemoglobin for oxygen.
This is good because if deoxyhaemoglobin has a low affinity for oxygen (as no oxygen is already bound), it will only pick up oxygen if the oxygen saturation is very high (i.e. in the lungs) so it will not take up oxygen in the metabolically active tissues where the oxygen saturation is low and where the tissues need oxygen.
Similarly, oxyhaemoglobin has a high affinity for oxygen so it will only give up oxygen in environments where the oxygen saturation is very low (i.e. respiring tissues that need oxygen)
Describe the different parallel shifts of the ODC
The binding of oxygen by haemoglobin is regulated by specific molecules in its environment. H+ ions, CO2 and 2,3- diphosphoglycerate (2,3-DPG) all stabilize the T form of the oxygen molecule by forming H bonds and thus decrease the oxygen affinity of the molecule. This is represented on the oxygen dissociation curve as a shift to the right i.e a higher concentration of O2 is needed for maximum O2 saturation if the concentration of CO2, H+ ions or 2,3-DPG are high. Thus in metabolically active tissues where the concentration of H+ ions and CO2 are high, oxyhaemoglobin will assume the T configuration and give up oxygen readily.
Conversely in the lungs where CO2 is exhaled, oxygen affinity is higher. This effect of CO2 on the affinity of Hb for oxygen is called the Bohr effect.
What effect does 2,3-DPG have on oxygen delivery
It facilitates oxygen delivery by making the haemoglobin molecule less flexible and pushing out the oxygen.
How can we categerosie haemoglobinopathies
Structural variants of haemoglobin - mutation resulting in change in structure of haemoglobin- HbS
or
Defects in globin chain synthesis (thalassaemia)- quantitative defect in production of globin and thus haemoglobin
Describe the key features of thalassaemia
Genetic disorders characterized by a defect of globin chain synthesis
Most common inherited single gene disorder worldwide
“thalassa” - sea“haimia” – blood disorder
Prevalent where malaria is also endemic- Hb variants offer protection against malaria
Describe the classification of thalassaemias
§ Classification of thalassaemia:
o Globin type affected (alpha or beta gene cluster)
o Clinical severity:
§ Minor or “trait”.
§ Intermedia.
§ Major.
Major disorders- transfusion dependent for survival
Trait- carrier states- asymptomatic- but can pass on gene
Intermediate- wide spectrum of phenotype- dependent or non-dependent on transfusions
Better to classify as transfusion or non-transfusion dependent
What is thalassaemia
Disorders in which there is a reduced production of one of the two types of globin chains in haemoglobin leading to imbalanced globin chain synthesis
The thalassaemias are disorders in which there is underproduction of one of the types of globin chains of adult haemoglobin and are called alpha or beta thalassaemia according to the chains affected.
What can cause an underproduction of a globin chain
Globin genes are transcribed into messenger RNA which is processed before translation into protein. Underproduction of a globin chain may therefore result from deletion of part (or all) of the gene, or else genetic mutations which lead to defects in transcription, mRNA processing, translation or stability of the final protein product.
Different sets of mutations develop in different parts of the world. They probably arose independently and were expanded by selection, possibly in relation to malaria.
Describe the psuedo genes found on the alpha cluster
There are 3 pseudo genes (w) and a fourth which although theoretically functional does not produce a detectable protein
Summarise beta thalassaemia
Deletion or mutation in b globin gene(s)
Reduced or absent production of b globin chains
Prevalence – mainly Mediterranean countries (Greece, Cyprus, Southern Italy), Arabian peninsula, Iran, Indian subcontinent, Africa, Southern China, South-East Asia
Ultimately, how is beta thalassaemia inherited
Beta Thalassaemia is inherited in a recessive mendelian fashion. The Carriers are asymptomatic save for the microcytic hypochromic indices. The clinical classification of Minor, Intermedia and Major shows how the mutations vary in severity. This can be explained by the degree of supression of globin chain sunthesis, some mutations result in no globin production (b0) where as others have decreased levels of production (b+). Inheritance of 2 b0 genes will give rise to a Major where as inheritance of 2 b+ genes will give rise to an intermedia (a clinically milder form)
Give an example of how beta thalassaemia can be inherited
§ Inheritance:
o When 2 beta trait have a child, there is a 25% chance of a beta-major offspring.
o Beta Thal Intermedia can also come about when one partner has a beta+ mutation which is less severe.
§ b0 = deletion of one beta globin-encoding gene.
§ b+ = mutation of one beta-globin encoding gene.
Describe the blood film findings of thalassaemia major
The main feature of thalassaemia is a microcytic hypochromic blood picture in the absence of iron deficiency. In addition to the microcytosis the RBC count is relatively high when compared to the haemoglobin.
The peripheral film shows hypochromia and target cells some poikilocytosis but no anisocytosis. (RDW is normal)
Describe the findings of Hb electrophoretic studies in thalassaemia major
§ Hb EPS (electrophysiology studies)/HPLC (high performance liquid chromatography):
o a-thal – normal HbA2 and HbF, ±HbH.- makes it hard to diagnose alpha- thalassaemia using only Hb studies
o b-thal – raised HbA2 and HbF.
In beta thalassaemia the HbA2 is increased in relation to the severity of the mutation but is rarely above 7%.
Describe the genetic studies for thalassaemia major
Globin Chain synthesis/ DNA studies
Genetic analysis for β-thalassaemia mutations and XmnI polymorphism (in β-thalassaemias) and α-thalassaemia genotype (in all cases)
How can we diagnose the alpha thalassaemia trait
There is no simple diagnostic process to diagnose alpha thalassaemia trait
a presumptive diagnosis of a-thalassaemia trait can be made if microcytosis and hypochromia is seen in the absence of iron deficiency.
What would a blood film with beta thalassaemia trait show and what are the features of beta thalassaemia trait
The blood film of a Beta thalassaemia trait will show microcytosis, hypochromia and occasional cells showing basophillic stippling
is a carrier trait and often asymptomatic.
§ Diagnosis usually made by blood film showing hypochromic microcytic blood cells and raised HbA2 and HbF
HbA still predominate (still have one functioning beta gene which can compensate)
Summarise beta thalassaemia major
Carry 2 abnormal copies of the beta globin gene
Severe anaemia, incompatible with life without regular blood transfusions
Clinical presentation usually after 4-6 months of life
What will the blood film of a patient with beta thalassaemia major show
The peripheral blood film will show extreme hypochromia, microcytosis and poikilocytosis.
Often Howell Jolly bodies and nucleated RBC’s will be present as a result of splenectophy and a hyper plastic bone marrow
Anaemia
In Beta thalassaemia major two forms of inclusion bodies may be seen, Alpha globin precipitates and pappenheimer bodies. If stained with Perls stain the inky blue granules aare demonstrated showing the papenheimer bodies to be haemosiderin granules.
Alpha globin chains are unstable and readily precipitate. These cause the cells to be more rigid and ineffect give alpha thalassaemia major its haemolytic component.
Describe the clinical features of beta thalassaemia major
Beta Thalassaemia Major presents at 6-12 months of age as the synthesis of g globin chains is reduced and the b globin chains would be expected to be increasing toward normal adult levels but in the case of beta thalassaemia major the beta genes are supressed.
As a result of the gross anaemia the marrow becomes hyperplastic in an attempt to redress the anaemia. Should this persist the bome marrow expands giving rise to characteristic facial changes, frontal bossing and thikening og the maxillary bones
The patient will show splenomegally as extramedullary haemopoesis (erythropoesis outside the bone marrow) develops –reverts back to embryonic sites of haematopoiesis
Haemoglobin electrophoresis will show little or no HbA and increased levels of HbA2.
List the clinical features of beta thalassaemia
Chronic fatigue Failure to thrive Jaundice- unstable Hb- chronic haemolysis Delay in growth and puberty Skeletal deformity Splenomegaly Iron overload
List the complications of beta thalassaemia
Cholelithiasis and biliary sepsis (due to increased BR as result of jaundice) Cardiac failure (iron deposits) Endocrinopathies (iron deposits) Liver failure (iron deposits)
Describe the importance of detecting beta thalassaemia major early
Patients with thalassaemia major usually present within the first year of life with failure to thrive, and general malaise. Splenomegaly and bony deformities of the skull are characteristic and bone changes in the long bones may be associated with recurrent fractures. Without transfusion the children usually die by the age of 7. If blood transfusions are commenced in infancy, however, then early growth and development may be normal. The blood transfusions are themselves associated with considerable morbidity due predominantly to iron overload but also as a result of the transmission of blood borne viruses (e.g. hepatitis B and C and HIV). Each unit of blood contains 200 mg iron and this accumulates in the liver, heart and endocrine glands. The effects of this start to appear by the end of the first decade.
Why does the bone marrow enlarge in patients with beta thalassaemia
Cells which do manage to mature and enter the circulation contain β-chain inclusions and are removed by the spleen which subsequently enlarges. The anaemia stimulates erythropoietin production and this causes expansion of the bone marrow in the skull and long bones.
Describe the endocrinopathies associated with beta thalassaemia major
Secondary sexual development may be delayed or absent, the normal adolescent growth spurt fails to occur and diabetes, hypoparathyroidism and adrenal insufficiency may become apparent. In addition, progressive liver and cardiac damage occur and liver damage from the iron overload may be exacerbated further by infectious hepatitis. Death usually occurs before the age of 25. Removal of iron is difficult.
Describe the pathogenesis of beta thalassaemia
There is death of red cells in the marrow because of ineffective erythropoiesis
The removal of red cells by the spleen causes:
· Large spleen
· Anaemia
· Increased EPO
· Expansion of bone marrow
What are the most common causes of deaths in patients with beta thalassaemia
240 Thalassaemia major patients born in Italy between 1960 and 1984 Cardiac disease 71% Infections 12% Liver disease 6% Other causes 11%
US cohort (Cooley’s Anemia Foundation) 11% of 724 registered patients died from Jan 1999 to July 2008 Cardiac 42 (54%) Infections 5
List the treatments for beta thalassaemia major
Regular blood transfusions
Iron chelation therapy
Splenectomy (very symptomatic or high transfusion requirement)
Supportive medical care
Hormone therapy
Hydroxyurea to boost HbF
Bone marrow transplant (only curative measure)
Explain the need for iron chelation therapy
Toxicity of iron overload- organ toxicity etc
Iron chelation- stop iron overload
Transfusion dependent- major cause is blood transfusions (iron burden)
Non-transfusion dependent- due to increased gut absorption- iron dysregulation- hepcidin (negative regulatory effect on iron)- downregulated- increased uptake of iron from G.I tract
Describe the features of transfusions to treat beta thalassaemia major
Phenotyped red cells (match blood groups to prevent hallo-immunisation)
Aim for pre-transfusion Hb 95-100g/L (to reduce risk of extra-medullary haematopoiesis)
Regular transfusion 2-4 weekly
If high requirement, consider splenectomy
Describe infection management in patients with beta thalassaemia major
Yersinia
Other Gram negative sepsis (Klebsiella)
Prophylaxis in splenectomised patients – immunisation and antibiotics
Splenectomy- at risk of infections from encapsulated bacteria
Infections from organisms which thrive on Fe
Summarise the features or iron chelation in patients with beta thalassaemia major
Start after 10-2 transfusions or when serum ferritin >1000 mcg/l
Audiology and ophthalmology screening prior to starting
What have studies demonstrated about iron chelation therapy
Many years of clinical and research experience have improved knowledge of management of iron overload and how to achieve effective chelation therapy
Achievement of continuous chelation coverage is an essential characteristic of effective chelation therapy. Constant, 24-hour control of NTBI/LPI protects against the harmful effects of toxic iron and helps to prevent further tissue damage
Good treatment compliance is important in achieving chelation coverage has been shown to be a key factor in attaining therapeutic efficacy and improving morbidity and survival
What determines survival in patients with iron chelation therapy
Compliance with desferrioxamine(DFO) therapy determines survival
Compliance with this drug an issue- infusion period of 12 hours- stigma and burden on life
Deferiprone and deferasirox are administered orally, which eliminates a major hindrance to chelation therapy with desferrioxamine – the necessity for delivering the agent subcutaneously or intravenously over an extended period of time using specialized pumps. To date the indication of deferiprone is restricted to second line in Europe.
Describe Deferasirox (Exjade)
Oral (once daily) Dose 20-40mg/kg SE: rash, GI symptoms, hepatitis, renal impairment Faecal exretion Half-life of 12-16 hours
Limited clinical experience
Describe Desferrioxamine (Desferal)
Long-established
Sc infusion 8-12 hours 5-7 days per week (or IV in cardiac iron overload)
Dose 20-50 mg/kg/day
SE: vertebral dysplasia, pseudo-rickets, genu valgum, retinopathy, high tone sensorineural loss, increased risk of Klebsiella and Yersinia infection
Compliance
Vitamin C
Urinary and faecal excretion
Half-life of 20-30 minutes
Describe Deferiprone (Ferriprox)
Licensed 1999 Oral (3 times daily) Dose 5-100 mg/kg/day Effective in reducing myocardial iron SE: GI disturbance, hepatic impairment, neutropenia, agranulocytosis ( which can lead to sepsis), arthropathy
Need weekly blood counts to monitor neutropenia
Urinary excretion
Half-life of 3-4 hours
What are the advantages and disadvantages of Desferrioxamine
Advantages: Three decades experience Survival benefit Heart failure prevented and reversed
Disadvantages: Paraenteral administration- limits compliance Toxicity (dose dependent) Ocular Skeletal Auditory
What are the pros and cons of Deferiprone
Advantages:
Oral
Cardioprotective
Disadvantages:
3 x/day 7days/week
Short plasma t 1/2
Unpredictable control of
body iron
Toxicity
- Agranulocytosis(~0.5%)
- Arthropathy
- Zinc deficiency
What are the pros and cons of Deferasirox
Pros: oral control of body iron specific once daily
cons:
Cardiac protection uncertain
Limited clinical experience
Toxicity limited but long term data lacking
Describe the benefits of combination therapy in iron chelation
To limit side effects and toxicity- can reduce dose
When is bone marrow transplantation an option
Bone marrow transplantation has the potential to cure thalassaemia major and should be considered in transfusiondependent thalassaemics under the age of 16 years who have an HLA-identical sibling greater than 18 months of age.
Outline how we can monitor for iron overload
Serum ferritin
>2500 associated with significantly increased complications
Acute phase protein
Check 3 monthy if transfused otherwise annually
Liver biopsy
Rarely performed
T2* cardiac and hepatic MRI
<20ms – increased risk of impaired LF function
Check annually or 3-6monthly if cardiac dysfunction
Describe the ferriscan for monitoring iron overload
Ferriscan – R2 MRI Non-invasive quantitation of LIC Not affected by inflammation or cirrhosis <3mg/g normal >15mg/g associated with cardic disease Check annually or 6monthly if result >20
Describe sickle beta-thalassaemia
§ Thalassaemia mutations can be co-inherited with other complications – such as SCD and beta-thal.
§ Co-inherited Beta-Thal:
o Sickle Beta Thalassaemia.
o HbE Beta Thalassaemia – very common in SE Asia and can be as severe as beta-thal major.
The blood film shows all the features of both sickle and beta thalassaemia, sickled cells, target cells, microcytosis and hypochromia. As little or no HbA is being produced in these patients HbS will be the dominant haemoglobin and will precipitate as it does in homozygote sickle cell patients
Describe HbE beta-thalassaemia
HbE b-thalassaemia: globin chain of HbE unstable, so similar to b-thalassaemia intermedia
Very common combination in South East Asia
Clinically variable in expression can be as severe as thalassaemia major
Explain the features of HbE beta-thalassaemia
In south east asia where both beta thalassaemia and HbE are found, HbE Beta thalassaemia is common.. The globin chain of HbE is unstable and hence at a molecular level this can be classed as a mild form of beta thalassaemia. Hence coinheritance of Beta Thalassaemia Trait. In terms of clinical severity it is classed as a Beta Thalassaemia intermedia. However the degree of clinical support required by such individuals varies quiet markedly.
What is the outcome of alpha thalassaemia major
Fatal in utero because alpha globin is needed to make HbF (alpha + gamma
This is why only up to 3 alpha genes can be affected in adult
Describe the features of alpha thalassaemia
o Due to a deletion or mutation in alpha globin genes – reduced or absent alpha globins.
o Affects both foetus and adult (alpha is in ALL globin variants).
o Severity depends upon number of chains affected.
o Excess beta and gamma chains will form tetramers of HbH (beta excess) and HbBarts (gamma excess).
What is meant by a thalassaemia carrier
Also known as Thalassaemia minor / trait Carry a single abnormal copy of the beta globin gene Usually asymptomatic Mild anaemia Usually low MCH and low MCV
Describe HbH disease
Loss of 3 alpha chains
Severe anaemia
Non-transfusion dependent
Cells still well haemoglobinised
The peripheral blood film of HbH disease has a haemolytic element to it with microcytosis, anisoocytosis poikilocytosis and “puddling” of haemoglobin within the RBC.
Hb Electrophoresis shows a fast band which is probably more easily seen shortly after the strip has started to run.
The amount of Hb Barts seen in the neonate does not directly correleat with the amount of HbH present in the adult
Describe the different types of alpha thalassaemia
Alpha chains are found in HbA and HbF so alpha thalassaemia may present clinically in utero. Alpha thalassaemia is usually (>80% cases) due to a deletion of one or more alpha genes and since each alpha cluster (one on each chromosome) has two alpha genes, four syndromes are possible as follows each with an increasing degree of anaemia and associated morbidity: α+ trait where one locus fails to function, α0 trait where two loci on the same chromosome are dysfunctional, HbH disease with three loci affected and Hb Bart’s hydrops fetalis where all four loci are defective and death in utero is the norm. α+ thalassaemia is particularly common in Africa and in those of African descent and α0 thalassaemia is particularly common in SE Asia.
Describe the features of heterozygotes for A+ and Ao thalassaemias and explain how we can distinguish the two
Someone who is heterozygous for alpha+ thalassemia will still have 3 functioning alpha globin genes so they will only be mildly anaemic
n the heterozygous state there are still 2 functioning alpha globin genes so they will also only experience mild anaemia
Most people with alpha 0 thalassemia have a MCH < 25 pg
What could be the potentially devastating consequences for someone with mild anaemia caused by alpha 0 thalassemia?
Someone with alpha 0 thalassemia may not experience any symptoms themselves, but if they try and have a child with someone who also has alpha 0 thalassemia then there is a chance that their child may not have any functioning alpha genes (haemoglobin barts).
What are the problems associated with treatment in developing countries
Lack of awareness of the problems
Lack of experience of health care providers
Availability of blood
Cost and compliance with iron chelation therapy
Availability of and very high cost of bone marrow transplant
Describe screening and prevention techniques
Counselling and health education for thalassaemics, family members and general public Extended family screening Pre-marital screening? Discourage marriage between relatives? Antenatal testing Pre-natal diagnosis (CVS
The success of prevention programs in Cyprus and Italy, for example, resulted by the mid-1980s in the dramatic reduction of the number of affected births, plummeting to almost zero in Cyprus by 1986
What is the main cause of the pathophysiology of beta thalassamia
The surplus of alpha globin chains form tetramers.
These alpha globin tetramers precipitate in the bone marrow, which leads to ineffective erythropoiesis and haemolysis in the peripheral circulation
Summarise the features of the beta thalassaemia trait
Heterozygotes for a beta thalassaemia gene are said to have β thalassaemia trait. These carrier states are usually clinically silent, and can be referred to as thalassaemia minor. They can, however, be identified in the laboratory on the basis of abnormal red cell indices. Typically, patients with β thalassaemia trait have smaller red cells than usual (microcytosis) and a reduced mean cell haemoglobin (MCH) with a normal mean cell haemoglobin concentration (MCHC).
The red cell count is usually raised and the haemoglobin level is normal or slightly reduced. If β thalassaemia trait is suspected then the level of HbA2 should be measured and is typically raised. If levels are equivocal even on repeat testing, and there is no evidence for coexisting iron deficiency then DNA analysis could be considered.
Why is it important to identify patients with the beta thalassaemia trait
There are two situations in which identifying patients as having β thalassaemia trait is of value. Firstly, the microcytosis may be misinterpreted as iron deficiency if the raised red cell count and normal MCHC are not noted.
If these patients are then put on long term iron they can become iron overloaded. Secondly, it is important to identify pregnant patients with thalassaemia trait so that their partners can be tested and the couple can be counselled about their chance of having a baby with clinically significant thalassaemia and can be offered further testing.
Describe the genetic background of beta thalassaemia
Genetic background: b-thalassaemia is recessive autosomal, with asymptomatic carriers
Mutations: can be b0 where no globin is produced or b+ where production is decreased
Possible combinations: bb0/bb+ thalassaemia trait; b0b0 thalassaemia major; b0b+ thalassaemia intermedia
Describe the mutations responsible for alpha and beta thalassaemias
Alpha thalassemia – deletion
Beta thalassemia – point mutation (over 100 described)
