The Haemoglobin Molecule and Thalassemia Flashcards
What are the different types of globin chain proteins?
Alpha, Beta, Gamma, Delta (and embryonic- epsilon and zeta)
State the three different haemoglobins that are present in the human body.
HbA – alpha + beta = 95%
HbA2 – alpha + delta = 1-3.5%
HbF – alpha + gamma = trace .7%
Describe how affinity of haemoglobin changes with oxygen binding and how this helps its role of oxygen transport.
The more oxygen binds, the greater the affinity of the haemoglobin for oxygen.
This is good because if deoxyhaemoglobin has a low affinity for oxygen(as no oxygen is already bound), it will only pick up oxygen if the oxygen saturation is very high (i.e. in the lungs) so it will not take up oxygen in the metabolically active tissues where the oxygen saturation is low and where the tissues need oxygen.
Similarly, oxyhaemoglobin has a high affinity for oxygen so it will only give up oxygen in environments where the oxygen saturation is very low (i.e. respiring tissues that need oxygen)
State 3 factors that can shift the oxygen dissociation curve to the right.
Increase in 2,3-DPG
Increase in [H+]
CO2 (hypercapnia)
What effect do HbS and HbF have on the oxygen dissociation curve?
HbS has a lower affinity for oxygen than HbA so it shifts the ODC to the right
HbF has a higher affinity for oxygen than HbA so it shifts the ODC to the left
What is special about alpha globin genes?
There are TWO alpha globin genes from each parent so there are FOUR alpha globin genes in total.
Which globin chains are present in early embryonic life but are switched off after about 3 months gestation?
Zeta and epsilon
Which globins are present in foetal haemoglobin?
Alpha
Gamma
When are the genes coding for the globin in foetal haemoglobin switched off?
It is decreased towards birth and in the first year after birth.
After 1 year of life, the normal adult pattern of haemoglobin synthesis would have been established.
Which globin chains are present in HbA2?
Alpha
Delta
On which chromosomes are the two globin gene clusters and which genes are present in each cluster?
Chromosome 16 – ALPHA cluster TWO alpha genes Zeta gene Chromosome 11 – BETA cluster Beta gene Gamma gene Delta gene Epsilon gene
What is thalassemia?
Disorders in which there is a reduced production of one of the two types of globin chains in haemoglobin leading to imbalanced globin chain synthesis
How is thalassemia classified in terms of clinical severity?
Minor: Thalassemia trait = asymptomatic carrier common variant with little clinical significance
Intermediate: Thalassemia intermedia
somewhere in between
can require transfusions or not
Major: Transfusion dependent – Thalassemia Major = fatal without transfusion
What is the outcome of alpha thalassemia major?
Fatal in utero because alpha globin is needed to make HbF (alpha + gamma)
Clinical presentation of thalassemia major?
Clinical features
a. Severe anaemia presenting at 4 months leading to Chronic fatigue
b. Failure to thrive
c. Jaundice
d. Hyperactive bone marrow
e. Delay in growth and puberty
f. Extramedullary haematopoiesis
g. Skeletal deformity (bone marrow expansion due to extramed haematopoiesis causing the distinctive facial features)
h. Hepatosplenomegaly
i. Iron overload (patients with thalassaemia absorb more iron in GI tract)
Which 2 types of abnormal haemoglobin can result due to Alpha thalassaemia
Haemoglobin H and Hb barts
Lack of Alpha chain forms tetramers of Beta chains instead of 2 alpha and beta= HbH
Same but tetramer of gamma chain= Hb barts
What is beta thalassemia major? Describe how the disease progresses.
Severe defect in both beta globin chains
The foetus will have no problem in utero because they have normal functioning HbF (which doesn’t need beta globin)
At around 2-3 month after birth, you get a transition from HbF to HbA
At this time the baby will become profoundly anaemic.
They will need life-long transfusions from this point onwards.
State some features of thalassemia trait.
This the carrier state of thalassemia.
They may be mildly anaemic but they can also be normal.
Usually have a LOW MCH and LOW MCV
Summarise the laboratory diagnosis of Thalassemia
Start simple: - FBC - Film Complex tests: - Hb electrophoresis/ HPLC - DNA studies eg PCR to look at genes for globin chains
What types of mutation cause alpha thalassemia and beta thalassemia?
both can be caused by deletion or point mutation in the respective alpha/beta globin gene
Which ethnic groups have a high prevalence of thalassemia?
Chinese South-east Asian Greek Turkish Cypriot
Laboratory diagnosis: What features would you see when looking at FBC of thalassemia?
- Poikilocytosis (no anisocytosis)
- Target cells
- Hypochromasia
- Nucleated RBC
What are the consequences of regular blood transfusions used to treat beta thalassemia major?
Iron overload: Chronic transfusion has been associated with iron accumulation. There’s not enough transferrin to bind to all the iron, the unbound iron can catalyse production of reactive oxygen species which can damage the liver (cirrhosis), heart (cardiac failure), endocrine organs (hypopituitarism, hypothyroidism, diabetes) etc.
Also increases chance of infection.
Name three iron chelator drugs that can be used to treat iron overload.
Desferrioxamine (DFO/desferal)
Deferiprone
Deferasirox
What are the key indicators when monitoring the iron overload in the management of thalassemia patients
- Serum ferritin (but not that accurate as it is also an ACP that rises in inflammation etc)
- Cardiac and hepatic MRI to measure iron levels in heart and liver
Liver biopsy used to be done but not anymore
Laboratory diagnosis: What features would you see when looking at Blood film of thalassemia?
target cells
poikilocytosis (different shape)
NO anisocytosis(different size)
Microcytic and hypochromic (relate this to the FBC card earlier)
Laboratory diagnosis:
What is the most definitive test for thalassemia
DNA studies - PCR, sanger sequencing etc
Most alpha thalassemias are diagnosed this way because they are quite hard to diagnose (HPLC gives normal HbA2 and HbF)
Complications of thalassemia
Cholelithiasis (gallstones) Biliary sepsis Cardiac failure endocrinopathies liver failure
Treatments for thalassemia major (minor usually asymtomatic)
Main treatments:
- Regular blood transfusions
- Iron chelation therapy (remove XS iron)
Other treatments:
- Splenectomy (patients may have splenomegaly due to extramedullary hematopoiesis)
- Supportive medical care for cardiac, liver problems
- Hormone therapy, iron overload can cause hypogonadism
- Hydroxyurea to boost HbF production
- Bone marrow transplant
Why are thalassemia patients prone to infection? What can be done to reduce the chances?
They tend to have iron overload and there are pathogens that thrive on iron especially gram - bacteria like YERSINIA
Also, thalassemia patients could have splenectomy due to splenomegaly via extramedullar haematopoiesis which is an important immune organ.
to prevent this, immunisation in splenectomised patients is good. (prophylaxis)
Main concern for management of thalassemia patients
infection and monitor iron overload
What are:
Sickle b thalassemia
HbE b thalassemia
Sickle b thal:
inherits one sickle allele from one parent and b thal allele from the other parent
NB sickle allele is also a mutation on the beta globin gene. So this disease is basically inheriting two different faulty beta globin alleles from the 2 parents
HbE b thal:
- Single point Mutation in beta chain, forming HbE which is an abnormal haemoglobin
- Reduced synthesis of functional beta chain leads to beta thalassemia
Mature rbc do not have
nuclues or mitochondria
Synthesis of haemoglobin : how much in eryhthroblast stage and reticulocyte stage
65% erythroblast stage
35% reticulocyte stage
Strucgure of haemolgobin a
4 chains– 2 a, 2 b– type of chains may alter with different haemoglobin
Haem group in centre with central iron ion– ferrous form fe2+
Structure of haemolgobin a
4 chains– 2 a, 2 b– type of chains may alter with different haemoglobin
Haem group in centre with central iron ion– ferrous form fe2+
Where is haem synthesised
Mitochondria
Where is globin synthesised
Ribosomes
How is iron transported into the cell
Bound to transferrin, transferrin iron complex is endocytosed by vesciles and transported to mitochondria.
Enzyme that regulates synthesis of haem and where is it located
Delta ALA
– Negative feedback step
Located in mitochondria
List 5 proteins in which haem can also be in apart from haemoglobin
Myoglobin, cytochromes, peroxidases, catalases, tryptophan
What makes up haem component
Protoporphyrin ring with central iron atom– ferroportoporphyrin
Synthesis of globin-8 FUNCTIONAL GLOBIN CHAINS ARRANGED IN A AND B CLUSTER-
list B cluster– which chromosome and which arm
and a cluster– which chromosome and which arm
B cluster- B, Gamma, Delta, Epislon globin genes located on short arm of chromosome 11
A cluster- A1 , a2 and zeta globin genes on the short arm of chromosome 16
which gene will
Foetus and adult
Which stage of life will the chromosome 16 genes be responsible for
Embryo
Which globin from the alpha cluster is the first to be produced
Zeta globin chains– produced after conception, but also production stopped 7 weeks of gestation– embroygenic
a globin chain is produced when and until
what happens if this isnt functioning properly
Pregestation and even through adult life
embryonic death
Which globin from the beta cluster is the first to be produced
form what type of haemoglobin
gamma globin chain
foetal haemoglobin
When does gamma globin chain begin to tail off and which globin chaintakes over its place
Gamma globin starts to tail off few weeks postpartum and B globin chain starts to increase
When does gamma globin chain begin to tail off and which globin chaintakes over its place
Gamma globin starts to tail off few weeks postpartum and B globin chain starts to increase
Primary globin structure
a 141 AA
Non a 146 AA
Secondary globin structure
75% a and B chains- helical arrangement
Tertiary globin structure
Approximate sphere
Hydrophilic surface (charged polar side chains), hydrophobic core
Haem pocket
How many globin chains in haemoglobin and ho many oxygens can haemoglobin bind to
4 globin chains
4 oxygens
Phenomenon in which oxygen binding becomes easier after first
Postive cooperativity
p50
2,3 DPG role
which cells can high levels be found
when Hb is half saturated with O2
Promote deoxygenated state– high levels found in metabolic cells
What 4 factors does normal position of curve depend on
Concentration of 2,3-DPG
H+ ion concentration (pH)
CO2 in red blood cells
Structure of Hb
Left shift means? Vv
Greater affinity, less readily donating
REDUCED CONCENTRATIONOF 2,3 DPG
INCREASED pH
Right shift– less affinity, more readily donating oxygen
INCREASED 2,3 DPB
DECREASED pH
Haemoglobinopathies– 2 types are?
Structural variants of haemoglobin
Defects in globin chain synthesis – thalassaemia
2 different ways of Classifying thalassaemia
Globin type affected
Clinical severity-
Minor trait– asymptomatic
Intermediate– can require or not require transfusions
Major– require transfusions
B Thalassaemia caused by
Deletion or mutation in B globin gene
Results in reduced or asbsent porduction of B globin chain
Prevalence of B thalassaemia
Mainly in Mediterranean countries, Arabina peninusla, Iran, Indian subcontinent, Africa, Southern China, South East Asia
Inheriting pattern of B thalassaemia
Autosomal Recessive mendelian
BB^0= Trait BB=Normal B^0B^0= Major B+ (Able to produce some betaglobin) BB+-- tRIAT B+B^0--BThal Intermedia
Thalassaemia major cause
Carry 2 abnormal copies of the beta globin gene
Thalassaemia Requires
regular blood transfusions Iron chelation therapy Splenectomy Supportive medical care Hormone therapy Hydroxyurea to boost HbF Bone marrow transplant
When do you present
Clinical presentation usually after 4-6 months of life
Other complications of B thalassaemia
Cholelithiasis and biliary sepsis
Cardiac failure
Endocrinopathies
Liver failure
How are transfusions carried out
Phenotyped red cells
Regular transfusion 2-4 weekly
If high requirement consider splenectomy
Mangement of infection
Yersinia
Other Gram negative sepsis
Prophylaxis in splenectomised patients – immunisation and antibiotics
Iron chelation therapy
ALL 3 CAN BE USED IN COMBINATION
Start after 10-2 transfusions or when serum ferritin >1000 mcg/l
Audiology and ophthalmology screening prior to starting
Deferasirox (Exjade)
Oral
SE: rash, GI symptoms, hepatitis, renal impairment
DFO Long-established SE: vertebral dysplasia, pseudo-rickets, genu valgum, retinopathy, high tone sensorineural loss, increased risk of Klebsiella and Yersinia infection Compliance-- parenteral adminsitration Vitamin C
Deferiprone (Ferriprox)
Oral
Effective in reducing myocardial iron
SE: GI disturbance, hepatic impairment, neutropenia, agranulocytosis, arthropathy
Monitoring of iron overload
Serum ferritin
-Acute phase protein
Liver Biopsy- Rare
T2 cardiac and hepatic MRI
T2* cardiac and hepatic MRI
<20ms – increased risk of impaired LF function
Check annually or 3-6monthly if cardiac dysfunction
-Ferriscan – R2 MRI
Non-invasive quantitation of LIC
Not affected by inflammation or cirrhosis
HbE B Thalassaemia common where
Common combination in South East Asia
Clinical Variation>
Clinically variable in expression can be as severe as B thalassaemia major
Alpha Thalassaemia
Deletion or mutation in a globin gene(s)
Reduced or absent production of a globin chains
Affects both foetus and adult
Excess Band Gamma chains form tetramers of HbH and Hb Barts respectively
Severity depends on number of a globin genes affected
Thalassaemia carrier
Thalassaemia minor / trait
Carry a single abnormal copy of the beta globin gene
Usually asymptomatic
Mild anaemia
Problems Associated with Treatment in Developing Countries
Lack of awareness of the problems
Lack of experience of health care providers
Availability of blood
Cost and compliance with iron chelation therapy
Availability of and very high cost of bone marrow transplant
Screening and Prevention
Counselling and health education for thalassaemics, family members and general public Extended family screening Pre-marital screening? Discourage marriage between relatives? Antenatal testing Pre-natal diagnosis (CVS)
Haemoglobinopathies offer protection against
Malaria– make it hard for the parasite to enter rbc
Lab diagnosis of B thalassaemia
FBC– Full blood count– Microcytic Hypochromic indices
Increased RBCs relative to Hb
Film- Target cells, poikilocytosis but no anisocytosis
Hb EPS / HPLC
A-thal- Normal HbA2 & HbF, +/- HbH (LOSS OF 3 A GLOBIN GENES YOU MAY SEE ABRNOMAL HAEMOGLOBIN VARIANCE.
IF YOU LOSE ALL 4- INCOMPATIBLE FOR LYF)
B-thal- Raised HbA2 & raised HbF
Globin Chain synthesis/ DNA studies
Genetic analysis for β-thalassaemia mutations and XmnI polymorphism (in β-thalassaemias) and α-thalassaemia genotype (in all cases)
