Haemoglobinopathies

Question 1

Haemoglobin

Answer 1

Four globin subunit proteins or ‘chains’

Each with iron-containing haem prosthetic group attached

Question 2

Haemoglobin subunits

Answer 2

Each have slightly differing aa sequences
–> precise folding of each subunit + way the 4 fit together is critical in determining ability of molecules to carry + release O2

Question 3

Adult Haemoglobin

Answer 3

2 alpha

2 beta

Question 4

Alpha subunit

Answer 4

2 genes for subunit on chromosome 16

Therefore overall 4 genes for alpha subunit, 2 maternal + 2 paternal

Question 5

Beta subunit

Answer 5

Five genes for subunit on chromosome 11

• -> epsilon, gamma A, gamma G, delta and beta
• -> each produce slightly different forms of beta globin

Question 6

Different haemoglobins

Answer 6

Can be produced from different gene combinations from chromosomes 11 + 16

Question 7

First form Haemoglobin

Answer 7

In Embryonic Yolk Sac (up to about 6wks)
Zeta 2 Epsilon 2 a.k.a Hb Gower-1
V high O2 affinity

Question 8

After 6 weeks Haemoglobin

Answer 8

Haemoglobin F
2 alpha 2 gamma
Higher affinity than maternal haemoglobin, so O2 passed from maternal to foetal Hb

Question 9

3-6 months after birth

Answer 9

HbA replaces foetal haemoglobin
Some HbA2 (alpha 2 gamma 2) also present in adult

Question 10

Thalassaemia

Answer 10

Genetic defect resulting in inadequate quantities of one or other of subunits that make up haemoglobin
Can be alpha or beta

Question 11

Alpha thalassaemia

Answer 11

One or more of alpha genes on chromosome 16 deleted or faulty

Question 12

Beta thalassaemia

Answer 12

Point mutation on chromosome 11

Question 13

Thalassaemia genetic defect

Answer 13

Mutation in non-coding introns of gene –> inefficient RNA splicing to produce mRNA –> decreased mRNA production
Partial or total deletion of globin gene
Mutation in promoter –> decreased expression
Mutation in termination site –> production of longer, unstable mRNA
Nonsense mutation

Question 14

Alpha thalassaemia epidemiology

Answer 14

Manifests immediately at birth

Severity depends on number of gene alleles defective or missing

Question 15

Alpha thalassaemia- 1 alpha gene defective

Answer 15

Alpha thalassaemia minima
Minimal effect on haemoglobin synthesis
Individuals called silent carriers
May have slightly reduced MCV and MCH
3 alpha globin genes enough to permit normal Hb production 
No clinical symptoms

Question 16

Alpha thalassaemia- 2 alpha genes defective

Answer 16

Alpha thalassaemia minor
2 alpha globin genes permit normal RBC production, but mild microcytic hypochromic anaemia
Can be mistaken for iron deficiency anaemia –> inappropriately treated with iron

Question 17

Alpha thalassaemia- 3 alpha genes defective

Answer 17

Haemoglobin H disease
2 unstable haemoglobins in blood
–> Haemoglobin Barts (4 gamma) and Haemoglobin H (4 beta)
Both unstable
Both have higher O2 affinity than Hb –> poor release of O2 in tissues
Microcytic hypochromic anaemia

Question 18

Alpha thalassaemia- 4 alpha genes defective

Answer 18

Foetus can’t survive outside uterus
May not survive gestation
Most infants stillborm with hydrops fetalis
Oedematous
Little circulating Hb
–> all tetrameric gamma chains (Haemoglobin Barts)

Question 19

Beta thalassaemia

Answer 19

Mutations in Haemoglobin beta gene on Chromosome 11
Autosomal recessive
Heterozygous, homozygous or intermedia

Question 20

Beta thalassaemia- heterozygous

Answer 20

Thalassaemia trait
Beta thalassaemia minor
Decreased MCV, MCH
Normal HbA and HbF
Increased HbA2

Question 21

Beta thalassaemia- homozygous

Answer 21

Beta thalassaemia major
No beta chains
Body’s inability to construct beta-globin leads to underproduction of Haemoglobin A
–> microcytic anaemia

Question 22

Beta thalassaemia- intermedia

Answer 22

Both beta globin genes mutated

But still able to make some beta chains

Question 23

Beta thalassaemia manifestation

Answer 23

When switch from gamma to B chain synthesis occurs several months after birth
May be a compensatory increase in g and d chain synthesis –> increased levels of Hb F and A2

Question 24

Pathological effects B thalassaemia

Answer 24

Excess alpha globins produced in developing erythroblasts in marrow
–> alpha tetramers unstable + precipitate on erythrocyte membrane
Causes intra-medullary destruction of developing erythroblasts, erythroid hyperplasia + ineffective erythropoiesis
–> SEVERE HYPOCHROMIC MICROCYTIC ANAEMIA

Question 25

Untreated B Thalassaemia Minor

Answer 25

``` Hypochromic Microcytic Anaemia Bone marrow expansion, Splenomegaly Bone deformity, extramedullary erythropoietic masses Failure to thrive 6 months HF and death age 3-4 ```

Question 26

Beta thalassaemia facial bone abnormalities

Answer 26

Bossing of skull Hypertrophy of maxilla- exposed upper teeth Depression of nasal bridge Periorbital puffiness

Question 27

Thalassaemia Major treatment

Answer 27

Regular transfusions Iron chelation therapy Splenectomy Allogeneic BM transplant

Question 28

Thalassaemia vs Iron deficiency

Answer 28

In Thalassaemia, although red cells are microcytic, serum iron and ferritin are normal

Question 29

Iron overload

Answer 29

Excess haemolysis --> free iron released --> free iron in blood If hydrogen peroxide present, Fenton reaction can occur --> hydroxyl radicals produces --> oxidise and damage all biological tissues

Question 30

Fenton reaction

Answer 30

Hydroxyl radicals oxidise + damage all biological tissues | Cirrhosis, diabetes, glandular dysfunction (GH deficiency)

Question 31

Iron Chelating compounds

Answer 31

Bind to free iron | Prevent Fenton reaction

Question 32

Desferoxamine | 'Desferal'

Answer 32

``` Iron Chelation therapy 8-12 hour s.c. infusion 5-7 days/week Chelation enhanced with ascorbate Toxicity with higher doses - diarrhoea, vom, fever ```

Question 33

Deferiprone | Exjade

Answer 33

Oral iron chelator 75-100mg/kg/day Agranulocytosis/neutropenia may occur Not suited for pregnancy

Question 34

Deferasirox | Exjade

Answer 34

Once daily iron chelator GI bleeding Kidney or liver failure

Question 35

Sickle cell

Answer 35

Mutant form of one of beta haemoglobin subunits Red cells sickles Obstruct capillaries + restrict blood flow to organ --> ischemia, pain + organ damage

Question 36

Sickle cell signs

Answer 36

Haemolytic anaemia- Hb 6-8g/dL Microvascular occlusion- rigid sickle cells adhere to endothelium, interact with WBC + vessel wall, cause NO depletion Large vessel wall damage

Question 37

Sickle cell Hb

Answer 37

Glutamic acid --> Valine (GAG-GTG) Codon 6 of beta globin chain Beta S produced Produced abnormal Hb S- (2 alpha 2 BetaS)

Question 38

HbS

Answer 38

May precipitate or crystallise | Distorts RBCs--> fragile + easily destroyed

Question 39

Sickled RBCs

Answer 39

Decreased survival time Occlude capillaries Lead to ischaemia + infarction of organs downstream of blockage

Question 40

Clinical consequence Sickle Cell

Answer 40

Anaemia Increased susceptibility to infection Vaso-occlusive crises Chronic tissue damage

Question 41

SCD management

Answer 41

``` Infection prophylaxis Analgesics for crises Education Transfusions Hydroxyurea (increases HbF, reduced painful crises) Bone marrow transplant ```

Question 42

Carrier detection screening

Answer 42

Simple blood analysis

Question 43

Newborn screening

Answer 43

Cord blood | Heel-prick sample

Question 44

HbC

Answer 44

Mutation where abnormal beta subunit Reduced plasticity + flexibility of erythrocytes Red cell destruction

Question 45

HbC homozygous

Answer 45

Mild haemolytic anaemia

Question 46

HbC heterozygous

Answer 46

No anaemia

Question 47

HbE

Answer 47

Single point mutation in Beta chain Get from both parents Increases after 3-6 months --> mild beta thalassaemia

Question 48

G6PD deficiency

Answer 48

X linked recessive Causes haemolysis + jaundice Mediterranean Triggers e.g. Fava beans, oxidative drugs like aspirin or infection