20.04.10 Haemoglobinopathies - sickle cell, thalassemia's

Question 1

What are haemoglobinopathies

Answer 1

Disorders caused by pathogenic sequence variants which affect the genes that synthesize globin chains of haemoglobin.

Question 2

What is haemoglobin

Answer 2

Iron containing oxygen-transport metalloprotein in the red blood cells of all vertebrates.

Question 3

Normal red blood cells contain 3 types of haemoglobin

Answer 3

1. HbA (86-98%) α2beta2
2. HbA2 (1.5-3.2%). α2delta2
3. HbF (0.5-0.8%). α2gamma2

Question 4

What encodes the 2 α globin genes

Answer 4

• HBA1 and HBA2 (chr 16)
• Different expression levels: α2:α1 is 3:1
• Expression is controlled by locus control region (LCR)
• earliest embryonic forms are closest to LCR, so HBA2 is closer to LCR

Question 5

What is alpha-thalassemia

Answer 5

• Includes all conditions with a deficit in alpha globin chain production
• Most prevalent in regions where malaria is endemic as it confers some protection.
• Frequency of alpha-thalassemia alleles in Mediterranean basin= 5-10%, West Africa= 20-30%, Saudi Arabia, Papua New guinea, Thailand and India= as high as 60-80%

Question 6

What is the most common cause of α thalassemia

Answer 6

• Deletion of 1 or both alpha genes (90% cases)
• 10% cases caused by point mutations in HBA2 (α+α) or HBA1 (αα+), from promoter to polyadenylation site
• α0 indicates no HBA expression from the allele
• α+ indicates some α-globin is produced from affected allele.

Question 7

Is there are correlation between phenotype and degree of α-globin chain deficiency

Answer 7

Yes

Question 8

What causes the majority of deletion type α-thalassemia

Answer 8

Reciprocal recombination between the two chromosome 16. Due to the high degree of sequence homology within α globin gene cluster.

Question 9

What produces -α3.7

Answer 9

• Rightward crossover due to mispairing of Z boxes during meiosis.
• Fusion protein has 3 α globin genes, reciprocal is an allele with 1 α globin gene

Question 10

What produces -α4.2

Answer 10

• Leftward crossover due to mispairing of X boxes during meiosis.
• Fusion protein has 3 α globin genes, reciprocal is an allele with 1 α globin gene

Question 11

When are α globin genes most active

Answer 11

Early in fetal life. Therefore alpha-thalassemia is an issue for an unborn child

Question 12

What are the 4 α -thalassemia syndromes

Answer 12

1. Hemoglobin Bart hydrops fetalis (Hb Bart) syndrome. Most severe
2. Hemoglobin H (HbH) disease.
3. α-thalassemia trait.
4. α-thalassemia silent carrier.

Question 13

What is Hemoglobin Bart hydrops fetalis

Answer 13

• Most severe α -thalassemia syndrome.
• Due to loss of all 4 α-globin genes
• Usually results in still birth or neonatal death. Only HbH formed.

Question 14

What is Hemoglobin H (HbH) disease

Answer 14

• Loss of 3 alpha-globin genes.
• Body produces HbH (beta-globin tetramers instead to compensate but unstable and forms Heinz bodies (red blood cell inclusions)
• Patients also have microcytic anaemia, moderate extravascular hemolysis, splenomegaly

Question 15

What is α-thalassemia trait

Answer 15

Loss of 2 alpha globin genes either in cis (–/αα, α0 carrier) or in trans (-α/-α). Asymptomatic but may have microcytic anaemia. Important in antenatal setting to know which to assess risk to unborn child.

Question 16

What is an α-thalassemia silent carrier

Answer 16

Loss of 1 α-globin gene (-α/αα, α+ carrier). Asymptomatic,

Question 17

Example of other alpha-thalassemia syndromes

Answer 17

• Alpha-thalassemia retardation 16 (ATR-16) syndrome. Since alpha-globin genes are located at tip of 16p, they are vulnerable to translocation. ART-16 occurs when other adjacent genes are involved. Short stature, reduced IQ, hypospadias and cryptorchidism.
• Alpha-thalassemia retardation X (ART-X), Due to mutations in ATRX gene. Appearance of het alpha-thalassemia but normal alpha-globins. Patients also have dysmorphic features and profound dev delay

Question 18

What is beta thalassemia

Answer 18

• Blood disorder where reduced/absent beta globin chain synthesis results in reduction/absence of adult haemoglobin (HbA).
• 1.5% global population are thought to be carriers.
• Prevalent in countries where falciparum malaria is endemic.

Question 19

What encodes beta globin

Answer 19

HBB gene. Chr 11

Question 20

What kind of mutations are common in beta-thalassemia

Answer 20

-Single nucleotide substitutions, small deletions, insertions or frameshift mutations. Large gene deletions are rare.

Question 21

What two types of mutant alleles are there in beta thalassemia

Answer 21

• Beta zero: no output from affected allele. i.e. caused by nonsense, frameshift, deletion, splice variant
• Beta plus: variable output

Question 22

What can alter disease severity in beta thalassemia

Answer 22

• Type of mutation in HBB
• Genetic modifiers that reduce alpha/ non alpha chain imbalance. i.e. co-inheritance of alpha thalassemia lessens imbalance by reducing alpha-globin gene output. Conversly duplicated alpha globin genes may cause beta-thalassemia intermedia
• modifiers that increase gamma globin production or to continue production of gamma globins in adulthood (persistence of fetal Hb).

Question 23

What are the different sub types of beta thalassemia

Answer 23

1. Beta thalassemia major (most severe)
2. Beta thalassemia intermedia
3. Beta thallassemia trait

Question 24

What is beta thalassemia major

Answer 24

• Patients have severe anaemia, hepatosplenomegaly, diarrhoea, fever.
• Patients will have increased HbF (alpha2 gamma2) and no HbA.
• Treatment: regular transfusions.

Question 25

What is beta thalassemia intermedia

Answer 25

• Later onset, mild-to-moderate anaemia. Rarely splenomegaly.
• Higher amount of HbA compared to beta thalassemia major
• Need occasional transfusion (during illness or pregnancy)

Question 26

What is beta thalassemia trait/minor

Answer 26

-Heterozygotes. Asymptomatic but may have microcytic RBCs and elevated HBA2 (delta globin genes)

Question 27

What is dominant beta thalassemia

Answer 27

• Due to rare mutations resulting in the synthesis of unstable beta globin, leading to ineffective erythropoiesis
• Heterozygotes with beta thalassemia

Question 28

What is Sickle cell disease

Answer 28

• Autosomal recessive disorder characterised by RBCs with an abnormal rigid sickle shape.
• Caused by HBB c.20A>T p.(Glu6Val)
• Highest prevalence in West Africa (1 in 4 are carriers) as hets have a protective advantage from malaria
• In low oxygen (hypoxia), acid conditions, polymerisation of HbS leads to sickle shape.
• Vascular occlusions cause a sickle crisis (bone pain, chest disease, damage to major organs, anaemia, stroke.

Question 29

Common type of sickle cell disease

Answer 29

• Sickle cell anaemia
• Homozygous for c.20A.T p.(Glu6Val) in HBB gene, aka HbSS
• Het carriers are usually asymptomatic with no anaemia

Question 30

Other types of sickle cell disease

Answer 30

-Inheritance of HbS (c.20A>T p.(Glu6Val) with a second HBB variant

Question 31

Treatment of sickle cell disease

Answer 31

Blood transfusion, prophylactic antibiotics, bone marrow transplant

Question 32

What methods are used to test for haemoglobinopathies

Answer 32

• Haematology: full blood count, microscopic analysis. Hb pattern analysis using HPLC (high performance liquid chromatography), isoelectric focussing.
• Genetics: Used for more complicated cases, if there is likely to be a prenatal, or to get a definitive diagnosis of alpha thalassemia.

Question 33

What antenatal screening is done for haemoglobinopathies

Answer 33

• If a woman is identified as a carrier of significant haemoglobinopathy then baby’s father is also tested
• Screening differs in high and low prevalence areas.
• Aim is to identify high risk couples early enough so prenatal testing can be offered by the end of 13 weeks pregnancy.

Question 34

Why is bone marrow transplantation a high risk treatment

Answer 34

5-10% mortality rate and limited success due to graft vs host disease. Used for only the most severe cases or where there is difficulty finding HLA matched donors.

Question 35

Novel therapies for haemoglobinopathies

Answer 35

• Gene therapy using lentiviral vectors to introduce wild type exogenous beta or gamma globin, epigenetic reactivation of fetal Hb expression.
• Activin receptor ligand traps: mitigates disease complications of ineffective erythropoiesis.
• Macrophage targeting. Macrophages found to have a negative effect on haematopoiesis in thalassaemia.