Haemoglobinopathies

Question 1

Structure of adult Hb?

Answer 1

Consists of globin:
• 2 α globin like chains
• 2 β globin like chains

AND

Haem - 1 haem group is attached to each globin chain, i.e: 4 haem groups

Question 2

Purpose of globin in Hb?

Answer 2

Keeps the haem soluble and protects it from oxidation

Question 3

Major forms of Hb and their structures?

Answer 3

1. HbA - 2 alpha and 2 beta chains (α2β2)
2. HbA2 - 2 alpha and 2 delta chains (α2δ2)
3. HbF - 2 alpha and 2 gamma chains (α2γ2)

Question 4

In adults, how much of each Hb type is present?

Answer 4

HbA is the major form (97%)

HbA2 (2.5%)

HbF (0.5%)

Question 5

Locations of the genes controlling globin chain production?

Answer 5

α-like genes are on chromosome 16; there are 2 α genes per chromosome, i.e: 4 per cell

β-like genes are on chromosome 11; there is β gene per chromosome (2 per cell)

Question 6

How does expression of globin genes change throughout embryonic life and childhood?

Answer 6

β globin chain production increases following birth and γ globin chain decreases

α globin chain production increases prenatally and remains stable throughout the rest of life

Within a few months, HbF disappears

By the 6-12 months of age, adult levels of the globin chains are achieved

Question 7

What are haemoglobinopathies?

Answer 7

Hereditary conditions affecting globin chain synthesis; there are 100s of known mutations, although they generally are AUTOSOMAL RECESSIVE conditions

Question 8

2 main groups of haemoglobinopathies?

Answer 8

Thalassaemias

Structural haemoglobin variants - normal production of STRUCTURALLY ABNORMAL globin chain, leading to variant Hb, e.g HbS

Question 9

What are thalassaemias?

Answer 9

DECREASED RATE of globin chain synthesis, resulting in impaired Hb production

2 types:
• Alpha thalassaemia - α chains are affected
• Beta thalassaemia - β chains are affected

Question 10

Results of thalassaemias?

Answer 10

Inadequate Hb production leads to a MICROCYTIC HYPOCHROMIC ANAEMIA

There is also unbalanced accumulation of globin chains, as only 1 chain is affected; this causes toxicity and the result is ineffective erythropoiesis and haemolysis

Question 11

Occurrence of thalassaemias?

Answer 11

Most common monogenic disorders and a major cause of morbidity worldwide

Less common in the UK (becoming increasing common due to changing population demographics), more common in other parts of the world

Question 12

Aetiology of thalasssaemias?

Answer 12

Carrying this mutation protects against malaria, so there is elective pressure in malaria endemic areas, allowing these mutations to flourish

Question 13

Mutations causing alpha thalassaemia?

Answer 13

Mutations affecting α globin chain synthesis

Unaffected individuals have 4 normal α genes (αα/αα)

Reduced synthesis - α+ (results from deletion of one alpha gene from chromosome 16)

Absent synthesis - α0 (results from deletion of both alpha genes from chromosome 16)

Question 14

Types of Hb affected by alpha thalassaemia?

Answer 14

α chains are present in all adult forms of Hb, so HbA, HbA2 and HbF are all affected

Question 15

Classifications of alpha thalassaemia?

Answer 15

Unaffected - 4 normal α genes (αα/αα)

α thal trait - one or two genes missing:
• α+/α (-α/αα)
• α0/α (–/αα)
• α+/α+ (-α/-α)

HbH disease - only 1 alpha gene is left, α0/α+ (–/-α)

Hb Barts hydrops fetalis - not functional α genes left, α0/α0 (–/–)

NOTE - 2 α from mother and 2 from father

Question 16

Symptoms of alpha thalassaemia trait?

Answer 16

Asymptomatic, no treatment required

Question 17

Results with alpha thalassaemia trait?

Answer 17

Microcytic, hypochromic red cells with mild anaemia

Question 18

What is alpha thalassaemia often confused with?

Answer 18

Distinguish from iron deficiency anaemia

With alpha thalassaemia, ferritin is normal and rbc count is raised

Question 19

What is HbH?

Answer 19

Severe form of alpha thalassaemia; patients have only one α gene per cell, α0/α+ (–/α)

The excess β chains form tetramers (β4), called HbH which cannot carry oxygen

Question 20

Results with HbH?

Answer 20

Anaemia with a very low MCV and MCH

Red cell inclusions of HbH can be seen with special stains

Question 21

Clinical features of HbH disease?

Answer 21

Moderate anaemia TO transfusion-dependent anaemia

Splenomegaly due to extramedullary haematopoiesis

Jaundice due to ineffective erythropoiesis and haemolysis

May have growth retardation, gallstones and iron overload

Question 22

Treatment of HbH disease?

Answer 22

Severe cases require a splenectomy +/- transfusion

Question 23

Occurrence of HbH disease?

Answer 23

Most common in southeast Asia, Middle East and the Mediterranean, where α0 is prevalent

Question 24

What is Hb Barts hydrops fetalis syndrome?

Answer 24

Most severe form of α thalassaemia; these patients inherit no α genes from either parent, α0/α0 (–/–)

So, they have minimal or not α chain production and HbA CANNOT BE MADE

Hb Barts (γ4) and HbH (β4) form the majority of Hb at birth, in these patients

Question 25

Clinical features of Hb Barts hydrops fetalis syndrome?

Answer 25

Severe anaemia Cardiac failure Growth retardation Severe hepatosplenomegaly Skeletal and CV abnormalities ALMOST ALL DIE IN UTERO

Question 26

Blood film in patients with Hb Barts hydrops fetalis syndrome?

Answer 26

Shows numerous nucleated rbcs in peripheral blood

Question 27

Why does beta thalassaemia occur?

Answer 27

Disordered beta chain synthesis, usually caused by point mutations (200 identified so far) There is reduced (β+) or absent (β0) beta chain production from 1 beta globin gene, depending on the mutation It affects only β chains and so only HbA (α2β2) is affected NOTE - alpha thalassaemia is usually caused by gene deletions whereas beta thalassaemia usually occurs due to point mutations

Question 28

Classification of beta thalassaemia?

Answer 28

β thalassaemia trait (β+/β or β0/β): • Asymptomatic • No/mild anaemia, low MCV/MCH, raised HbA2 β thalassaemia trait intermedia (β+/β+ or β0/β+) • Moderate severity requiring occasional transfusion β thalassaemia major (β0/β0): • Severe, lifelong transfusion dependency

Question 29

PC of beta thalassaemia major?

Answer 29

Presents aged 6-24 months, as HbF disappears Pallor, failure to thrive Extramedullary haematopoiesis causes: • Hepatosplenomegaly • Skeletal changes • Organ damage

Question 30

Hb analysis of beta thalassaemia major?

Answer 30

Mainly HbF and minimal HbA

Question 31

Complications of extramedullary haematopoiesis?

Answer 31

Cord compression Hair on end sign - marrow expansion leads to this appearance of the skull

Question 32

Mx of β thalassaemia major?

Answer 32

Regular transfusion programme to maintain a Hb of 95-105 g/l: • Suppress ineffective erythropoiesis • Inhibit over-absorption of iron Bone marrow transplant may be an option, if carried out prior to development of complications

Question 33

Main causes of mortality with β thalassaemia major?

Answer 33

Maintaining the Hb allows for relatively normal growth and development Iron overload then becomes the main cause of mortality

Question 34

Consequences of iron overload?

Answer 34

Endocrine dysfunction: • Impaired growth and pubertal development • Diabetes • Osteoporosis Cardiac disease: • Cardiomyopathy • Arrhythmias Liver disease: • Cirrhosis • Hepatocellcular cancer Increased risk of sepsis, as bacteria like iron NOTE - similar to haemochromatosis but it has a more severe, early onset

Question 35

Mx of iron overload?

Answer 35

Venesection is not feasible, as they are already anaemic Iron chelating drug, e.g: desferrioxamine, necessary; these form complexes that are excreted in urine or stool NOTE - there are 250mg of iron per unit of red cells and chronic anaemia drives increased iron absorption (body's natural response)

Question 36

Transfusion-related complications?

Answer 36

Viral infection Allo-antibodies (difficult to cross-match blood) Tranfusion reactions

Question 37

Pathophysiology of sickling disorders?

Answer 37

Point mutation in codon 6 of the β globin gene, which substitutes glutamine to valine, producing βS This alters the structure of the resulting Hb, leading to HbS formation (α2βs2) HbS polymerises when exposed to low oxygen levels for a prolonged period; this distorts the red cells, damaging the rbc membrane NOTE - HbS is a variant Hb

Question 38

Characteristic appearance of a sickling rbc?

Answer 38

....

Question 39

Cause of sickle cell trait?

Answer 39

HbAS One normal and one abnormal β gene (β/βs)

Question 40

PC of sickle trait?

Answer 40

Asymptomatic carrier state Few clinical features as the HbS level is too low to polymerise May sickle if severely hypoxic, e.g: high altitude, under anaesthesia

Question 41

Blood results with sickle trait?

Answer 41

Blood film normal Mainly HbA with <50% HbS

Question 42

Cause of sickle cell anaemia?

Answer 42

2 abnormal β genes (βs/βs); it is autosomal recessive The HbS is >80% and they have no HbA

Question 43

PC of sickle cell anaemia?

Answer 43

Sickle crisis - episodes of tissue infarction due to vascular occlusion; symptoms depend on site and severity: • Digits (dactylitis), bone marrow, lung, spleen, CNS • Pain may be extremely severe

Question 44

Other effects of sickle cell anaemia?

Answer 44

Chronic haemolysis leads to a shortened RBC lifespan Sequestration of sickled RBCs in the liver and spleen Hyposplenism occurs due to repeated splenic infarcts

Question 45

What is sickle cell DISEASE?

Answer 45

Patient has compound heterozygosity for HbS and another β chain mutation, e.g: • HbS/β thalassaemia; it is mild if β+ and severe is β0 • HbSC disease; it is milder but there is an increased risk of thrombosis

Question 46

Consequences of sickle cell vaso-occlusion?

Answer 46

Abnormal RBCs become stuck in vessels, leading to tissue ischaemia and severe pain

Question 47

Precipitants of sickle cell crisis?

Answer 47

Hypoxia Dehydration Infection Cold exposure Stress, fatigue NOTE - education on these precipitants can reduce the risk of crisis

Question 48

Treatment of painful crisis?

Answer 48

Opiate analgesia and rest Hydration and oxygen If evidence of infection, antibiotics In severe crises, e.g: chest crisus or neuro symptoms, red cell exchange transfusion, i.e: venesect, then transfuse and repeat (decreases conc. of HbS and improves tissue perfusion)

Question 49

Long-term effects of sickling disorders?

Answer 49

Impaired growth ``` Risk of end-organ damage, e.g: • Pulmonary hypertension • Renal disease • Avascular necrosis • Leg ulcers • Stroke ```

Question 50

Long-term Mx of sickling disorders?

Answer 50

Hyposplenism - must reduce the risk of infection: • Prophylactic penicillin • Vaccination: pneumococcus, meningococcus, haemophilus (encapsulated bacteria) Folic acid supplementation, as there is increased rbc turnover and so increased demand Hydroxycarbamide can reduce the severity of disease by inducing HbF production

Question 51

Diagnosis of haemoglobinopathy?

Answer 51

Consider ethnic origin FBC, Hb, red cell indices Blood film High Performance Liquid Chromatography (HPLC) or gel electrophoresis - to quantify Hbs present: • Identifies abnormal Hb, e.g: HbS in sickle cell disease • Raised HbA2 (diagnostic of beta thalassaemia trait) • HPLC normal in alpha thalassaemia trait so DNA testing required

Question 52

Interpret these results of HPLC: • HbA >80% • HbF <1% • HbA2 1.5-3.5% ?

Answer 52

Normal patient

Question 53

``` Interpret these results: • Italian origin • Hb 109, MCV 64, MCH 19 • Ferritin 38.0 micrograms/l • HbA2 = 5.3% (HPLC ``` ?

Answer 53

Beta thalassaemia trait

Question 54

Interpret these results: • African origin • Hb 140, MCH 29 • -HbS = 37.3% (HPLC) ?

Answer 54

A/S (sickle cell trait)

Question 55

Screening for haemoglobinopathies?

Answer 55

Antenatal screening to identify carrier parents is not standard in the UK; inv. a family origin questionnaire and FBC Further testing if from a high-risk area or abnormal RBC indices NOTE - high risk of obstetric complications with Barts, if undiagnosed At risk couple are counselled and prenatal diagnosis is offered, where appropriate Newborn screening programme also in place (heel prick)