Haem 4 - The Hb molecule and Thalassaemia

Question 1

Describe Hb synthesis

Answer 1

Occurs during RBC development - begins in pro-erythroblast stage

• 65% in erythroblast stage (still contains nucleus and mitochondria at this point)
• 35% reticulocyte stage

Question 2

Where is haem synthesised

Answer 2

In mitochondria

Question 3

Where is globin synthesised (alpha and beta globin chains)

Answer 3

Ribosomes

Question 4

What is the transporter the body for free iron?

Answer 4

Transferrin

Question 5

Describe the structure and synthesis of haem

Answer 5

The same haem is used in all type of haemoglobin

Haem is a combination of a protoporphyrin ring with central iron atom - ferroprotoporyphyrin

Iron usually in Fe2+ form (ferrous form)

-Haem synthesised mainly in mitochondria (which contain delta-ALAS enzyme)

Question 6

Haem is able to combine …….. with oxygen

Answer 6

Reversibly

Question 7

Various forms of globin can combine with haem - forming different Hb molecules. There are 8 functional globin chains - how are they arranged?

Answer 7

8 functional globin chains in 2 clusters

1. Beta cluster - Beta, gama, delta, epsilon globin genes - short arm of Chr 11
2. Alpha cluster - alpha and zeta globin genes - short arm of Chr 16

Question 8

When does zeta and epsilon production stop?

Answer 8

In foetal life - taken over by a-globin production.

Hence a-globin defects manifest earlier.

Question 9

From birth, where is most of the alpha and beta globin made?

Answer 9

In the bone marrow - this means beta-thalassaemia usually develops after birth

Question 10

Describe the tertiary globin structure

Answer 10

1. Approximate sphere
2. Hydrophilic surface, hydrophobic core (accounting for solubility properties)
3. Haem pocket

Question 11

Describe deoxyhaemoglobin

Answer 11

It has 2,3-DPG attached - stops O2 binding, has a less stable structure

Question 12

Describe the primary and secondary globin structure

Answer 12

Primary structure = alpha chains have 141 amino acids

Secondary structure has 146 non amino acids

Question 13

What is the p50 of the Hb-O2 dissociation curve?

Answer 13

26.6mmHg

Question 14

What does the position of the Hb-O2 Dissociation curve depend on

Answer 14

1. 2,3-DPG. If high - shifts curve right
2. H+ conc - pH. High H+ = shifts curve right (decreased pH)
3. CO2 conc in RBCs. High CO2 shifts right
4. Hb structure. HbS shifts right, HbF shifts left

Question 15

What are the 2 categories of haemoglobinopathies

Answer 15

1. Hb structural variants

2. Defects in globin chain synthesis (thalassaemia)

Question 16

What is the most common inherited single gene disorder worldwide

Answer 16

Thalassaemia

Question 17

How can thalassaemias be classified

Answer 17

1. Globin type affected - most commonly alpha and beta

2. Clinical severity - minor (trait), intermediate, major

Question 18

Describe Beta-Thalassaemia

Answer 18

Autosomal recessive (Chr 11)

1. Deletion/mutation in beta-globin chain –> reduced production of B-globin
2. Mainly occurs in Mediterranean countries
3. Carriers = asymptomatic (except microcytic hypo chromic indices)
4. Variation in severity as varying degree of interference with globin chain synthesis (some mutations = no globin production, some mutations = decreased production)

• 2x beta(0) genes = major, 2x beta(1) genes = intermediate

Question 19

How are thalassaemias diagnosed

Answer 19

Lab diagnoses:

1. FBC - microcytic hypochromic indices + increased RBCs relative to Hb
2. Film - increased no. of target cells, poikilocytosis, but no anisocytosis

Hb EPS/HPLC (electrophoresis):

alpha thalassaemia - normal HbA2 and HbF, +/- HbH, not definitive

beta thalassaemia - raised HbA2 and HbF - more straightforward/easy to diagnose

Question 20

What is the gold standard for a-thalassaemia diagnosis

Answer 20

Globin chain synthesis/DNA studies

Genetic analysis for B-thalassaemia mutations + Xmnl polymorphism (in B-thalassaemias) _ a-thalassaemia genotype (in all cases)

Question 21

Describe B-thalassaemia major

Answer 21

1. 2 Abnormal copies of B-globin gene
2. Clinical presentation = after 4-6 months of life
3. Abnormally shaped RBC
4. RBC inclusions - alpha chain precipitates
5. Pappenheimer bodies = excess iron precipitating in granules due to ineffective erythropoiesis - seen using Perls stain

Question 22

What is the clinical presentation of Alpha and beta thalassaemia

Answer 22

1. Extra-medullary haematopoiesis (main cause of presentations)
2. Severe anaemia (fatal) - presents after 4 months usually
3. Hepatosplenomegaly - due to extramedullary haematopoiesis
4. Blood fil shows gross hypochromia - poikilocytosis and many nucleated RBCs
5. Bone marrow shows erythroid hyperplasia
6. Bone marrow shows erythroid hyperplasia

Question 23

What are some features of beta thalassaemia

Answer 23

1. Chronic fatigue
2. Skeletal deformity
3. Jaundice
4. Delay in growth an puberty
5. Splenomegaly
6. Iron overload

Other complications include - gallstones (cholelithiasis) & biliary sepsis (due to chronic haemolysis), cardiac failure - most deaths from b-thalassaemia (iron overload linked), endocrinopathies, liver failure (iron overload)

Question 24

What are the features of beta-thalassaemia

Answer 24

1. Microcytosis
2. Hypochromia

(also some cells with basophilic stripping)

Question 25

Which thalassaemia is lethal in utero

Answer 25

Alpha-thalassaemia major

Question 26

What is the treatment for thalassaemia major

Answer 26

1. Regular blood transfusions | 2. Iron chelation therapy

Question 27

Describe how transfusions are given to treat thalassaemia (major)

Answer 27

1. RBCs are phenotypes/matched 2. Try to get pre-transfusion conc of 95-100g/L --> this helps to reduce extra-medullary haematopoiesis 3. Transfusions regularly every 2-4 weeks 4. If a high requirement, consider splenectomy (+ if splenomegaly). But splenectomy makes body more susceptible to infection - e.g. Yersinia (loves iron, thrives in this case), other gram negative sepsis, splenectomised patients should have immunisation + antibiotics

Question 28

When is iron chelation therapy done?

Answer 28

After 10-12 transfusions or when serum ferritin > 1000mcg/l AUDIOLOGY and OPHTHALMOLOGY screening done prior to starting (due to retinal side effects)

Question 29

What are the types of iron chelation therapy

Answer 29

1. Desferrioxamine - DFO (deferral). Sc infusion, 5-7 days a week. SEs include: vertebral dysplasia, pseudo-rickets, retinopathy. DON'T use in children. Issues with compliance. 2. Deferiprone - oral dose. Most effective in reducing myocardial iron. SEs include: hepatic impairment, neutropenia, agranulocytosis. Patients need weekly blood tests to monitor WBCs 3. Deferasirox - generally well tolerated. 1/day oral dose (20-40mg/kg). SEs include: renal impairment

Question 30

How is iron overload monitored

Answer 30

1. Measure serum ferritin. If over 2500mcg/L, less accurate (associated w greater complications). Ferritin is also an acute phase protein so may be increased during infection/inflammation. Check 3 monthly if transfused patient. 2. T2 Cardiac and hepatic MRI. Cardiac MRI = relies on contractility of cardiac fibres. If heart relaxation time < 20ms --> stiff fibres --> iron overload ---> increased risk of impaired LF function 3. Liver ferriscan - R2 MRI. Has replaced liver biopsy. Non-invasive quantification of Liver Iron Conc. Unaffected by inflammation or cirrhosis --> more accurate estimation. LIC < 3mg/g normal, >15mg/g associated with cardiac disease. Checked annually or 6-monthly

Question 31

What type of disorder is sickle-beta thalassaemia

Answer 31

Sickling disorder - NOT thalassaemia

Question 32

Describe HbE-beta thalassaemia

Answer 32

HbE Common in south east Asia - leads to globin chain production. It has clinically variable expression - but can be as severe as B-thalassaemia major

Question 33

Describe Alpha-thalassaemia and who it affects

Answer 33

Deletion/mutation in alpha-globin chains --> reduced/absent production of a-globin chains. Affects foetus and adult Severity depends on number of a-globin chains deleted - if all 4 alpha globin chains deleted --> death of foetus

Question 34

What do excess beta and gamma chains form?

Answer 34

Tetramers of HbH

Question 35

Describe thalassaemia carrier/minor/trait

Answer 35

When person carries single abnormal copy (e.g. of beta-globin chain) Usually asymptomatic + diagnosed with milid anaemia + microcytosis + hyperchromia

Question 36

Describe Haemoglobin H (HbH) disease

Answer 36

3 a-globin chains affected - shows poikilocytosis

Question 37

What are the problems associated with treatment in the developing world

Answer 37

1. Blood availability 2. Cost and compliance with iron chelation therapy 3. Availability/cost of bone marrow transplant