20.04.10 Haemoglobinopathies - sickle cell, thalassemia's Flashcards
What are haemoglobinopathies
Disorders caused by pathogenic sequence variants which affect the genes that synthesize globin chains of haemoglobin.
What is haemoglobin
Iron containing oxygen-transport metalloprotein in the red blood cells of all vertebrates.
Normal red blood cells contain 3 types of haemoglobin
- HbA (86-98%) α2beta2
- HbA2 (1.5-3.2%). α2delta2
- HbF (0.5-0.8%). α2gamma2
What encodes the 2 α globin genes
- 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
What is alpha-thalassemia
- 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%
What is the most common cause of α thalassemia
- 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.
Is there are correlation between phenotype and degree of α-globin chain deficiency
Yes
What causes the majority of deletion type α-thalassemia
Reciprocal recombination between the two chromosome 16. Due to the high degree of sequence homology within α globin gene cluster.
What produces -α3.7
- 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
What produces -α4.2
- 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
When are α globin genes most active
Early in fetal life. Therefore alpha-thalassemia is an issue for an unborn child
What are the 4 α -thalassemia syndromes
- Hemoglobin Bart hydrops fetalis (Hb Bart) syndrome. Most severe
- Hemoglobin H (HbH) disease.
- α-thalassemia trait.
- α-thalassemia silent carrier.
What is Hemoglobin Bart hydrops fetalis
- 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.
What is Hemoglobin H (HbH) disease
- 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
What is α-thalassemia trait
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.
What is an α-thalassemia silent carrier
Loss of 1 α-globin gene (-α/αα, α+ carrier). Asymptomatic,
Example of other alpha-thalassemia syndromes
- 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
What is beta thalassemia
- 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.
What encodes beta globin
HBB gene. Chr 11
What kind of mutations are common in beta-thalassemia
-Single nucleotide substitutions, small deletions, insertions or frameshift mutations. Large gene deletions are rare.
What two types of mutant alleles are there in beta thalassemia
- Beta zero: no output from affected allele. i.e. caused by nonsense, frameshift, deletion, splice variant
- Beta plus: variable output
What can alter disease severity in beta thalassemia
- 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).
What are the different sub types of beta thalassemia
- Beta thalassemia major (most severe)
- Beta thalassemia intermedia
- Beta thallassemia trait
What is beta thalassemia major
- Patients have severe anaemia, hepatosplenomegaly, diarrhoea, fever.
- Patients will have increased HbF (alpha2 gamma2) and no HbA.
- Treatment: regular transfusions.
What is beta thalassemia intermedia
- Later onset, mild-to-moderate anaemia. Rarely splenomegaly.
- Higher amount of HbA compared to beta thalassemia major
- Need occasional transfusion (during illness or pregnancy)
What is beta thalassemia trait/minor
-Heterozygotes. Asymptomatic but may have microcytic RBCs and elevated HBA2 (delta globin genes)
What is dominant beta thalassemia
- Due to rare mutations resulting in the synthesis of unstable beta globin, leading to ineffective erythropoiesis
- Heterozygotes with beta thalassemia
What is Sickle cell disease
- 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.
Common type of sickle cell disease
- Sickle cell anaemia
- Homozygous for c.20A.T p.(Glu6Val) in HBB gene, aka HbSS
- Het carriers are usually asymptomatic with no anaemia
Other types of sickle cell disease
-Inheritance of HbS (c.20A>T p.(Glu6Val) with a second HBB variant
Treatment of sickle cell disease
Blood transfusion, prophylactic antibiotics, bone marrow transplant
What methods are used to test for haemoglobinopathies
- 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.
What antenatal screening is done for haemoglobinopathies
- 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.
Why is bone marrow transplantation a high risk treatment
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.
Novel therapies for haemoglobinopathies
- 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.
