Flashcards in Week 4- Haemoglobinopathies Deck (42):
How much Hb is in the average adult?
7 x 10^20 Hb molecules.
(each RBC has 280 million and we have 5000 million RBCs/ml of blood)
Describe the HB molecule?
Two alpha and two beta polypetide chains. Each chain contains a ferrous iron.
Molecular weight= 64500 Da
Steps of haemoblobin development? Each type at each stage?
Gower (3 weeks)- Zeta2E2
Portland (5 weeks)- Zeta2Gamma2
Gower (5 weeks)- Alpha2E2
Fetus- F (10 weeks)- Alpha2Gamma2
F (1%) A2 (2-3%)=alpha2delta2 A (97%)=alpha2beta2
With regards to Hb development, why do thalassemias have big consequences?
Thalassemias affect alpha and beta chains. Alpha and beta make up 97% of adult Hb so big consequences!
Especially alpha, as its found 5 weeks onwards.
However alpha has 4 genes, 2 on each chromosome. Beta only has 1. In alpha, 4 things have to go wrong for disease.
Describe the chromosomes with Hb genes on?
Beta globin gene cluster- chromosome 11.
epsilon, 2 gammas (G+A), delta and beta.
The 2 gamma genes are active in fetal growth and produce Hb F. The adult gene, beta, takes over after birth.
Alpha globin gene cluster- chromosome 16
zeta 2, zeta 1, alpha 2, alpha 1. Each of the 4 alpha genes contribute to alpha globin synthesis.
(all functional globin genes have 3 exons and 2 introns)
What is always at the first part of Hb?
Alpha and zeta are on the same chromosome so first part of HB always either zeta or alpha
e.g Gower1- zeta2E2
F- Alpha 2 gamma2
How many types of gamma are there?
Two. differ by a glycine residue.
What's the difference between Hbopathies and thalassemia?
Haemoglobinopathies- synthesis of mutant Hb chains. e.g. sickle cell anaemia.
Thalassemia- decreased synthesis of WT Hb but has normal structure.
(can overlap-some abnormal globins are made at a reduced rate. These disorders of Hb lead to anaemia)
What Hb do sickle patients have?
Sickle Hb (HbS) is different from normal Hb (HbA).
Single point mutation in beta chains.
How do you get sickle cell anaemia?
Both beta genes have to have the mutation. (one on each chromosome). If only one is mutated you have sickle cell trait.
Substitution of valine for glutamic acid on position 6 in the beta chain of Hb.
How much sickle Hb do sickle cell trait and sickle cell anaemia sufferers have?
Trait-> 20-45% HbS.
Disease-> 80% HbS
What happens with sickle cells?
When sickle Hb gives up its oxygen to tissues, it sticks together to form long rods inside RBCs, making RBCs rigid and sickle shaped.
Sickle cells form crystal structure and get stuck, aren't flexible. Usually 7 Hb molecules stick together to make a crystal.
HbS is insoluble and forms crystals when exposed t low oxygen tension.
How can sickle cell anaemia lead to different fingers adn toes?
Painful crises caused by occlusion of small blood vessels. Can occur in bones, lungs and spleen. Most serious events occur in the brain. In children the hand-foot syndrome is common when occlusions occur in small bones-> can lead to digits of varying lengths.
What problems does sickly cell anaemia cause?
Patients suffer from sever anaemia and severe pain. Suffer crises:
Visceral- when it affects your organs, can get pooling in organs.
Aplastic- no cell formation. (can be caused by parvovirus. Treated with transfusion.)
Haemolytic- fall in Hb and reticulocytosis.
Macrophages in spleen try phagocytose weird RBCs, causes enlargement and stress on spleen. Body compensates for lost RBCs, lots of precursors.
How does the sickle cell gene protect from malaria?
Have protection from plasmodium-malaria. 2011
Soares and team from Portugal have found that low levels of circulating haem in blood of sickle-cell gene carrying mice down, releasing small amounts of carbon monoxide. This limits the accumulation of haem after infection with the malaria parasite. This appears to prevent the disease.
What are the types of sickle cell crises?
Most common, can damage bones, lungs, CNS
Sickle cells adhere to neutrophils and endothelial
Sicklign within organs and pooling of blood can make anaemia worse
caused by infection with parvovirus. Reduction in haemoglobin. Take folic acid to help cell production.
Increased rate of haemolysis. Fall in Hb.
What were the findings of the sickle Hb? difference from normal Hb
1949 pauling showed that HbS had a higher isolectric point than Hba. (The ph at which it has no net charge.)
1957 INgram discovered that the HbS had a valine instead of a glutamic acid. Valine has no charge, glutamic acid has negative. makes valine stick out of Hb molecule. makes it sticky too.
Because valine has no charge, HbS 50 times less soluble. So crystallises
What are sickle cells like in high and low tensions?
When oxygen is bound to th Hb it looks noral, when it loses oxygen at peripheral tissues it polymerises into long fibres of & Hbs long.
At low oxygen tension (pressure), HbS is insoluble and forms crystals.
Sickle cell trait
carrier of sickle Hb gene.
One HbS and one HbA gene
Enough Hba-? RBCs are flexible and no anaemia.
RBCs look normal.
How do you diagnose sickle cell?
Low Hb: 6-9 g/dL
Sickle cells and target cells in blood.
Blood is deoxygenated
Electrophoresis- no HbA is detected HbF levels 5-15%
What are sickle cell tests and treatments?
1. In pregnancy samples obtained by chionic villus biopsy
amniotic fluid cells can be used
PCR amplifies DNA.
2. Treatment= prophylactic- folic acid, 5mg/week to increase cell production. vaccination- reduce infection rates.
Treat crises= rest, hydrate, antibiotics
Transfusions= suppressed HbS production. Stem cells.
Hydroxyurea (15-20 mg/kg) can increase Hb levels and improve outcome.
Give 5 Hb variants that are clinically significant? (some extra reading)
HbS- sickle cell. mutation in beta chain
HbC- west africa. lysine instead of valine. Rhomboidal crystal (instead of 7 long)
HbD- 2 groups D^punjab + D^non-punjab. If D punjab, inherited with sickle-> anaemia. Non punjab not severe.
HbE- mutation of glutamic acid-> lysine at position 26. Subjects get mild beta-thalassemia (homozygous)
HbH- form of alpha thalassemia, reduced formation of alpha chains. HbH disease.
What is thalassemia?
Reduction in alpha or beta synthesis.
How is thalassemia classified?
Which globin effected: alpha or beta.
rate of synthesis: alpha+ reduced synthesis. alpha0 no synthesis.
Genotype: e.g. heterozygous alpha+
How many copies of the genes are there and how do they affect protein production- in thalassemia
Alpha globin genes- 4 copies- chromosome 16
Beta- 2 copies- chromosome 11
All copies are used to the same extent. i.e. if one copy of beta gene deleted then 50% protein made.
What causes thalassemias?
Gene deletions. Also single base substitutions leading to mis-splicing of premRNA.
Where are most alpha and beta thals most prominent?
Alpha= far east
What's needed to make alpha globulin chain?
4 genes needed to make alpha globin chains ( 2 from each parent)
What are the silent carriers and alpha thalassemia trait caused by?
Silent carrier- heterozygous alpha+ thalassemia. If one alpha gene deleted, then no signs of disease.
Alpha thalassemia trait-alpha+ homozygous or alpha0 heterozygous. 2 alpha genes deleted. Not enough alpha globulin produced. Suffer hypochromic anaemia. Microcytic cells.
Dedcribe HbH disease
3 alpha genes deleted. Anaemia (moderate to severe) and hepatosplenomegaly.
Microcytosis, polychromasia, poikilocytosis, hypochromasia.
Hb Barts Hydrops Fetalis
4 genes deleted. After 10 weeks, no functional Hbs. Haemoglobin Barts (gamma4) and Hbh (beta4) are synthesised. Incompatible with life.
What happens when alpha globulin not made?
Accumulation of other globin chains. Insoluble aggregate. Damages erythroblast.
Beta thalassemia- when are they a carrier and when anaemic?
Why is beta not as bad as alpha?
When only one affected- carrier.
If two affected you have anaemia.
Not as bad as alpha because HbB only needed for HbA. So HbF will rise to compensate.
What are most beta thalassemias caused by?
Point mutations in beta globin gene or close to b globin gene.
Symptoms and clinical findings of B thal. major?
Enlarged maxilla. Prominent frontal and parietal bones. Hyperplasia- bones trying to make too many cells.
Siderosis- deposits of iron in liver
Secondary iron overload *treated with iron chelation)
What are genetics of beta thal. major?
Genes tend to be mutated rather than deleted. Mainly point mutations. Mutations spread through the gene and in promoter and enhancer regions.
Whereas alpha- mainly deletions, beta can be splicing, frameshift etc.
B thalassemia intermedia
Thallassemia= moderately severe.
Patients don't need regular transfusions.
Caused by: Homozygous B thal. where HbF is made more than usual. Mild defects in B chain synthesis.
Dominant B thal. B trait which is unusally severe. Survival rate is good nowadays.
What are the clinical features of B-thalassemia intermedia?
enlarged spleen or liver
B thalassemia minor (trait)
Abnormal trait inherited from one parent. Mild anaemia-> asymptomatic.
Usually don't know you have it.
What are thalassemia treatments?
2-3 units every 4-6 weeks.
Iron chelation therapy
Comon drug= deferoxamin,
deferipone- 3 x daily, iron excreted in urine.
deferasirox- new iron chelator, once daily
Folic Acid- if diet is poor
Bone marrow transplant- 80% success but very intense, not common
Beta thalassemia diagnosis
Hypochromic, microcytic anaemia
Raised reticulocyte %
Electrophoresis-? increased levels of HbA and HbF
Dna analysis for mutations
Iron overload tests-> serum ferritin liver biopsy.