The haemoglobin molecule and thalassaemia

Question 1

What are the two gross components of haemoglobin and where are they synthesised?

Answer 1

haem - mitochondria

globin - ribosomes

Question 2

Schematic of synthesis of haemoglobin.

Answer 2

(not in sofia)

Question 3

Outline the structure of haem

Answer 3

Combination of protoporphyrin ring with central iron atom.

Iron usually ferrous (Fe2+) - binds reversibly with oxygen.

Question 4

Which enzyme in mitochondria is responsible for haem synthesis?

Answer 4

ALAS.

Question 5

How many functional globin chains are there?

Answer 5

8

Question 6

How are globin chains arranged? Where are these located? Which functional globin chains does each contain?

Answer 6

Clusters.

b- cluster (b, g, d and e globin genes) on the short arm of chromosome 11

a- cluster (a (a1 and a2) and z globin genes) on the short arm of chromosome 16

Question 7

How are different globin chains expressed over the course of life?

Answer 7

zeta and eta expressed very early, drop to zero after 6 weeks.

alpha and gamma rise as zeta and eta fall and then predominate during foetal development.

gamma drops as birth approaches and drops to zero by 42 weeks (post natal)

beta rises and as gamma drops.

alpa and beta predominate throughout adult life.

Question 8

Where is globin synthesised over the course of life.

Answer 8

Yolk sac –> liver and spleen (liver predominates) –> bone marrow for adult life.

Question 9

What are the normal adult haemoglobin phenotypes? Which chains do they contain? How common are they?

Answer 9

Hb A - a2b2. 96-98%

HbA2 - a2d2. 1.5-3.2%

Hb F - a2g2. 0.5-0.8%

Question 10

Explain the primary, secondary and tertiary structure of globin.

Answer 10

Primary - 141AA for alpha. 146AA for non alpha.

Secondary - 75% alpha and beta chains in helical arrangement.

Tertiary - approximate sphere, hydrophilic surface, hydrophobic core, haem pocket.

Question 11

What effect does 2,3-DPG have on oxygen binding?

Answer 11

Favours oxygen disassociation - haem less able to access oxygen

Question 12

explain the sigmoid shape of the oxygen dissociaiton curve.

Answer 12

Cooperative binding - binding of one molecule facilitates the second molecule binding.

Question 13

How is P50 defined and what is its value?

Answer 13

PP of O2 at which half of Hb is saturated with O2 - 22.6mmHg

Question 14

Oxygen dissociation curve.

What shifts do changes in pH, 2,3-DPG, HbF, HbS cause?

Answer 14

Question 15

What does the normal position of the Hb dissociation curve depend on?

Answer 15

• Concentration of 2,3-DPG
• H+ion concentration (pH)
• CO2in red blood cells
• Structure of Hb

Question 16

How might you shift the Hb dissociation curve to the left or right?

Answer 16

Right -

• High 2,3-DPG
• High H+
• High CO2
• HbS

Left -

• Low 2,3-DPG
• HbF

Question 17

What causes haemoglobinopathies?

Answer 17

Structural variants of haemoglobin,

defects in globin chain synthesis.

Question 18

How is thalassaemia classified?

Answer 18

According to the globin type affected and the clinical severity -

minor trait

intermedia

major

Question 19

How does beta thalassaemia arise?

Answer 19

Deletion of mutation in beta globin gene.

Reduced/absent production of beta globin chains

Question 20

How is beta thalassaemia inherited?

Answer 20

recessive mendelian.

Question 21

How can thalassaemia be diagnosed?

Answer 21

Abnormalities in FBC - Microcytic Hypochromic indices and Increased RBCs relative to Hb

Blood film - target cells present, poikilocytosis (high RBC variation) but no anisocytosis (little variation in size)

Hb electrophysiology study/HPLC -

alpha thalassaemia - normal HbA2 and HbF, variable HbH

beta thalassaemia - raised HbA2, raised HbF.

Globin chaun synthesis/DNA studies -

beta thalassaemia - genetic analysis for B thalassaemia mutations and Xmnl polymorphism

alpha thalassaemia - genetic analysis for alpa thalassaemia genotype.

Question 22

What causes thalassaemia major?

Answer 22

•Carry 2 abnormal copies of the beta globin gene

Question 23

What are the consequences of thalassaemia major? When does it present clinically?

Answer 23

• Severe anaemia, incompatible with life without regular blood transfusions
• Clinical presentation usually after 4-6 months of life

Question 24

What characteristics might RBCs in a beta thalassaemia patient have?

Answer 24

alpha chain precipitates.

Pappenheimer bodies.

Question 25

Outline the clinical presentation of thalassaemia major.

Answer 25

* Severe anaemiausually presenting after 4 months * Hepatosplenomegaly * Blood film shows gross hypochromia, poikilocytosis and many NRBCs * Bone marrow - erythroid hyperplasia * Extra-medullary haematopoiesis

Question 26

Outline some clinical features of beta thalassaemia major.

Answer 26

* Chronic fatigue * Failure to thrive * Jaundice * Delay in growth and puberty * Skeletal deformity * Splenomegaly * Iron overload

Question 27

What complications can arise from beta thalassaemia major?

Answer 27

* Cholelithiasis and biliary sepsis * Cardiac failure * Endocrinopathies * Liver failure

Question 28

What are the major causes of death in beta thalassaemia patients?

Answer 28

–Cardiac disease - due to iron overload 71% –Infections 12% –Liver disease 6% –Other causes 11% (out of 240 thalassaemia major patients born in italy between 1960 and 1984)

Question 29

How is beta thalassaemia major treated?

Answer 29

* Regular blood transfusions * Iron chelation therapy * Splenectomy * Supportive medical care * Hormone therapy * Hydroxyurea to boost HbF * Bone marrow transplant

Question 30

How frequently are transfusions given? If transfusion requirement is high what treatment option is considered?

Answer 30

2-4 weekly. Splenectomy.

Question 31

How is infection risk managed? e.g. for Yersinia and other gram- species.

Answer 31

prophylaxis in splenectomised patients - immunisation and antibiotics.

Question 32

When is iron chelation therapy given?

Answer 32

Started after 10-12 transfusions of when serum ferritin \>1000mcg/l. Audiology and opthalmology screening prior to starting.

Question 33

Table comparing iron chelators.

Answer 33

Question 34

How is deferasirox administered? How large is its dose? What are its side effects?

Answer 34

–Oral –Dose 20-40mg/kg –SE: rash, GI symptoms, hepatitis, renal impairment

Question 35

How is desferrioxamine administered? How large is the dose given? What are the side effects?

Answer 35

–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

Question 36

How is deferiprone administered? How large is the dose? What are the side effects?

Answer 36

–Oral –Dose 5-100 mg/kg/day –Effective in reducing myocardial iron –SE: GI disturbance, hepatic impairment, neutropenia, agranulocytosis, arthropathy

Question 37

Table comparing iron chelators.

Answer 37

Question 38

How is iron overload monitored?

Answer 38

•Serum ferritin –\>2500 associated with significantly increased complications –Acute phase protein –Check 3 monthyif transfused otherwise annually * Liver biopsy (rarely) * T2\* cardiac and hepatic MRI –\<20ms – increased risk of impaired LF function –Check annually or 3-6monthly if cardiac dysfunction •Ferriscan– R2 MRI –Non-invasive quantitation of LIC –Not affected by inflammation or cirrhosis –\<3mg/g normal –\>15mg/g associated with cardicdisease –Check annually or 6monthly if result \>20

Question 39

What are the features of alpa thalassaemia?

Answer 39

* Deletion or mutation in aglobin gene(s) * Reduced or absent production of aglobin chains * Affects both foetusand adult * Excess bandgchains form tetramers of HbHand Hb Bartsrespectively * Severity depends on number of aglobin genes affected

Question 40

How is alpa thalassaemia classified?

Answer 40

Alpha+ Alpha⁰ HbH Hb Barts as more alpha chains are lost.