haemoglobinopathies Flashcards
describe the structure of Hb
- Made up of 4 globin subunits, each with a haem prosthetic group
- Different subunits have slightly differing amino acid sequences
how does the amino acid sequence affect the globin subunits?
Differences in the AA sequence affect how they fold and fit together
what is the structure of adult haemoglobin?
2 alpha globin subunits and 2 beta subunits (a2b2)
how many genes are there for the alpha subunit and what chromosome are they found on?
2 genes on chromosome 16
what genes are there for the beta subunit and what chromosome are they found on?
5 genes for the beta subunit on chromosome 11 – epsilon, gamma A, gamma G, delta and beta
which can bind O2 more efficiently; HbA or HbF?
HbF
when and where is Hb Gower-1 formed?
First form of HbF produced in the embryonic yolk sac (up to about 6 weeks)
what is the structure of Hb Gower-1?
zeta 2 epsilon 2
what is the function of Hb Gower 1?
allows for angiogenesis in the fetus
what does the difference in affinity for O2 between HbA and HbF allow for?
allows the transfer of O2 from the mother to the fetus as the blood passes through the placenta
how is O2 stored in fetal muscle?
In the fetus, the myoglobin of the fetus has a higher affinity for O2 so O2 molecules pass from HbF to be stored in fetal muscle
what form of Hb replaces Hb Gower 1 and when, how and where does this happen?
After 6 weeks gestation, the zeta gene is switched off and HbF made of 2 alpha and 2 gamma subunits is made in the liver and spleen and replaces Hb Gower 1
which has a higher affinity for oxygen; HbF or Hb Gower 1?
Hb Gower 1
when does HbA replace HbF?
3-6 months after birth
define thalassaemia
a genetic defect resulting in inadequate quantities of one or other of the subunits that make up Hb
define alpha thalassaemia
results when one or more of the alpha genes on chromosome 16 is deleted or faulty
define beta thalassaemia
results when there’s a point mutation on chromosome 11
what can cause a genetic defect?
- A mutation in the noncoding introns of the gene
- The partial or total deletion of a globin gene
- A mutation in the promoter leading to decreased expression
- A mutation at the termination site leading to production of unstable mRNA
- A nonsense mutation
when does alpha thalassaemia manifest?
immediately at birth
what does the severity of alpha thalassaemia depend on?
depends on the number of gene alleles defective or missing
what cause alpha thalassaemia minima?
- 1 alpha gene is defective
how does alpha thalassaemia present?
minimal effect on Hb synthesis
• 3 alpha globin genes are enough to permit normal Hb production
• No clinical symptoms - Silent carriers
• May have a slightly reduced mean corpuscular volume and mean corpuscular Hb
what is alpha thalassaemia minor?
2 alpha genes are defective or missing
how does alpha thalassaemia minor present?
• 2 alpha genes permit nearly normal production of RBCs but there’s a mild microcytic hypochromic anaemia
what can alpha thalassaemia minor be confused with?
Can be mistaken for iron deficiency anaemia – treated inappropriately with iron
what is HbH disease?
3 alpha genes are defective or missing
• 2 unstable Hbs are present in the blood – Hb Barts and HbH
o Have a higher affinity for oxygen than normal Hb poor release of oxygen in tissues
which form of HbH is more severe?
• HbH caused by deletion of 3 genes is less severe than cases in which 2 genes are deleted and the 3rd gene has a point mutation
what’s the most common form of non-deletional HbH?
HbH Constant Spring (HCS)
how do 4 defective/missing alpha genes present?
- Fetus can’t live outside the uterus and may not survive gestation
- Born as stillborn with hydrops fetalis
- Those born alive die shortly after birth – edematous with little circulating Hb and the Hb that is present is all tetrameric y chains (Hb Barts)
what causes beta thalassaemia?
Caused by mutations in the haemoglobin B gene on chromosome 11
how is beta thalassaemia inherited?
autosomal recessive
what are the 2 main genotypes of beta thalassaemia?
heterozygous (thalassemia trait, beta thalassemia minor) and homozygous (beta thalassemia major)
what is beta thalassaemia intermedia?
occurs when both of the beta globin genes are mutated, but still able to make some beta chains
when does beta thalassaemia manifest?
Disease manifests when the switch from gamma to beta chain synthesis occurs several months after birth
why are there increased levels of HbF and HbA2 in beta thalassaemia?
May be a compensatory increase in g and d chain synthesis resulting in increased levels of Hb F and A2
why is there underproduction of HbA in beta thalassaemia?
In BT major the body makes less beta globins – underproduction of HbA microcytic anaemia
why does hypochromic microcytic anaemia occur in beta thalassaemia?
- Loss of beta globins means excess alpha globins are produced in developing RBCs in the bone marrow
- Alpha tetramers are unstable and precipitate on the RBC membrane
- Causes intra-medullary destruction of developing RBCs, erythroid hyperplasia and ineffective erythropoiesis
what are the levels of; - Hb - MCV - MCH - HbA - HbA2 - HbF in heterozygous beta thalassaemia?
Hb = low MCV = v low MCH = v low HbA = normal HbA2 = high HbF = normal
what are the levels of; - Hb - MCV - MCH - HbA - HbA2 - HbF in homozygous beta thalassaemia?
Hb = v low MCV = v low MCH = v low HbA = v low or absent HbA2 = high HbF = v high
what are the effects of untreated beta thalassaemia major?
- Hypochromic, Microcytic Anaemia.
- Bone marrow expansion, splenomegaly
- Bone deformity, extramedullary erythropoietic masses
- Failure to thrive from about 6 months of age
- Heart failure and death by age 3-4
what are treatments of thalassaemia?
• Regular blood transfusions (but can cause iron overload)
• Iron chelation therapy
• Splenectomy (not usually necessary now)
• Allogeneic bone marrow transplant (for young children if sibling donor available)
- • In some patients, supplementation of iron or folic acid may be useful
what are alternative treatments of thalassaemia?
HbF modulating agents
what is the future for thalassaemia?
gene therapy
how are people with mild forms of alpha thalassaemia treated?
may not require specific treatment except as needed for management of low Hb levels
what is the difference between iron deficiency anaemia and thalassaemia?
In thalassaemia, although RBCs are microcytic, serum iron and ferritin are normal
why is iron overload bad?
• RBCs are broken down in the spleen
• Haem group is separated from the globin units
• Iron removed from haem and attached to ferritin which binds the iron securely
• With excess haemolysis some free Fe may be released from haem and enter blood
• If hydrogen peroxide is present, the Fenton reaction can occur – produces hydroxyl radicals – oxidise and damage all biological tissues
o FR is responsible for cirrhosis, diabetes, glandular dysfunction (especially growth hormone deficiency) and other effects of chronic iron overload
what does the fenton reaction cause?
o FR is responsible for cirrhosis, diabetes, glandular dysfunction (especially growth hormone deficiency) and other effects of chronic iron overload
name iron chelation therapies
- desferoxamine
- deferiprone, ferriprox
- deferasirox, exjade
what is desferoxamine?
- 8-12 hr s.c. infusion 5-7 days per week
* Chelation enhanced with ascorbate
what are the adverse effects of desferoxamine?
•Toxicity with higher doses eg diarrhoea, vomiting, fever, hearing loss, and eye problems
what is deferiprone?
oral iron chelator
what are the adverse effects of deferiprone? when shouldnt it be used?
Agranulocytosis/neutropenia may occur: Not to be used in pregnancy
what is deferasirox?
once daily oral iron chelator
what are the adverse effects of deferasirox?
can cause GI bleeding and kidney or liver failure
what is sickle cell disease?
Mutant form of one of the beta Hb subunits causes the RBCs to become sickle-shaped
what are the signs of sickle cell disease?
- Haemolytic anaemia: haemoglobin levels in the range of 6–8 g/dL
- Microvascular occlusion: rigid sickle cells adhere to endothelium, interact with white cells and vessel wall, cause nitric oxide depletion
- Large vessel damage
what mutation causes sickle haemoglobin?
valine substitution at codon 6 of the beta globin chain
• Abnormal beta chain is produced abnormal Hb called HbS
how does sickle cell disease lead to anaemia?
- In deoxygenated blood, HbS may precipitate/crystallise, distorting RBCs into a sickle shape – fragile and easily destroyed anaemia
- Sickled RBCs have decreased survival time anaemia
how does sickle Hb lead to ischaemia?
•SRBCs can occlude capillaries ischaemia and infarction of organs downstream of the blockage
what are diagnostic tests for confirming sickle cell disease?
- Complete blood count & blood film
- Sickle solubility test (A mixture of HbS in a reducing solution gives a turbid appearance, whereas normal Hb gives a clear solution)
- Hb electrophoresis
- Hb HPLC (high performance liquid chromatography)
what are the major clinical consequences of sickle cell disease
- Anaemia
- Increased susceptibility to infection (particularly encapsulated bacteria)
- Vaso-occlusive crises (commonest is painful crisis)
- Chronic tissue damage (eg stroke, avascular necrosis of hip, retinopathy)
how should sickle cell disease be managed?
- Infection prophylaxis
- Analgesics for painful crises
- Education, life style, avoidance of precipitants
- Transfusions for specific acute and chronic complications
- Hydroxyurea (Increases HbF, reduces painful crises)
- Bone marrow transplantation (few have been done in UK)
why is carrier screening good?
allows identification of couples at risk of producing affected offspring – can then get counselling about the genetic risk, consequences of having an affected child and the options for avoiding an affected pregnancy
results interpretation isnt usually problematic
how is carrier screening done?
easily done with a simple blood analysis (FBC and haemoglobinopathy screen)
when is carrier screening done?
early in pregnancy
or before pregnancy
what time is a high risk time for sickle cell complications?
6 months - 2 years
how is sickle cell diagnosed?
Cord blood or heel-prick sample can be used
why is early diagnosis of sickle cell important?
Enables treatment with antibiotic prophylaxis, parental education, and early intervention for complications
what is haemoglobin C?
mutation where there’s an abnormal beta subunit –> reduced plasticity and flexibility of the RBC –> excess haemolysis
what is haemoglobin E?
abnormal Hb with a single point mutation in the beta chain
when does HbE occur?
• HbE occurs when the offspring inherits HbE gene from parents
how do babies homozygous for HbE present and why?
- Babies homozygous for HbE allele don’t present with symptoms bc of HbF
- After 3-6 months, HbF disappears and HbE increases mild B-thalassaemia
what is the inheritance pattern of G-6-P dehydrogenase deficiency?
X-linked recessive
when does jaundice occur in someone with G6P dehydrogenase?
Resultant jaundice in response to a number of triggers e.g. certain foods e.g. fava beans or oxidative drugs
what are carriers of the G6P deficiency gene protected against?
malaria
when is a diagnosis of G6P deficiency suspected?
Diagnosis is suspected when patients from certain ethnic groups develop anemia, jaundice and symptoms of hemolysis after exposure to triggers such oxidative drugs
