the Hb molecule and thalassaemia

Question 1

what is the number of red cells in an average sized human

Answer 1

3.5-5 x10(power 12)

Question 2

how many moleules of Hb does each red cell contain *

Answer 2

640 million molecules

Question 3

what is the normal conc of Hb in blood

Answer 3

110 - 165g/L

Question 4

what is the rate of production of Hb *

Answer 4

90mg/kg produced and destroyed in the body every day

Question 5

where is Hb found and why *

Answer 5

exclusively in RBC

toxic if free - potent oxidative properties

Question 6

how much iron is in a g of Hb

Answer 6

3.4mg

Question 7

when does synthesis of Hb occur *

Answer 7

development of erythroblast

65% in erythroblast stage

35% in reticulocyte stage

all before RBC enter blood - lost nucleus and ribosomes so couldnt make any more

Question 8

what is the structure of normal adult HB *

Answer 8

2 a and 2 B chains

heme molecule associated with Fe atom at centre

Question 9

describe the synthesis of Haem *

Answer 9

in mt

Fe introduced to cell bound to transferrin - endocytosed - transported to mt

under regulatory control of δ-Aminolevulinic acid (δ-ALA) - excess haem = -ve feedback to ALA

proto-porphyrin is formed by pathway involving ALA and added to haem

Question 10

describe the structure of haem *

Answer 10

also in other proteins eg myoglobin, cytochromes, peroxidases, catalases, tryptophan

same in all types of Hb

combination of protoporpyrin ring with central Fe atom (ferroprotoporphyrin)

iron in ferrous form (Fe2+) - able to bind reversibly to O2

Question 11

summarise the synstesis of globin *

Answer 11

various types of globin

8 functional globin genes arranged in 2 clusters:

a cluster on chromosome 16: a and zeta cain synth

B cluster on chromosome 11: B Y d and E globin genes

zeta gene involved in making Hb in embryo

Question 12

describe the production of globin chains from a cluster through development *

Answer 12

zeta first to be made - stop in early embryo eg 7wks

a takes over - predominant chain from a cluster - if defect in production of a chain = loss of embryo early

Question 13

describe the production of globin chains from B cluster in development *

Answer 13

gamma - most important from b cluster - foetal Hb have 2 a and 2 gamma

still make foetal Hb for firts 3-6 months of life - then B chain takes over

if defect oin B chain production - not seen until 3 months

still gave a small amount of HbF in adults

Question 14

what are the normal adult Hb *

Answer 14

table

Question 15

how can you identify the normal adult Hb *

Answer 15

high performance liquid chromatography

separates hb on electophoretic charge and molecular mass

get different peaks - from L to R - foetal, HbA, HbA2

Question 16

describe the structure of globin *

Answer 16

priamry structure - a has 141 AA, non-a has 146 AA
secondary - 75% a and B chains in a helical arrangement

tertiary - approx sphere, hydrophilic surface (charged polar side chains), hydrophobic core, haem pocket (when 4 chains assemble - area where haem compartment sits

Question 17

describe the structures of oxy and deoxyhaemoglobin *

Answer 17

oxy - allow ox to bind

deox - globin chain has diff config = oxygen dissociation

Question 18

what does the oxygen dissociation curve show and why is it a sigmoid shape *

Answer 18

Ox carrying capacity of Hb at different ppO2

sigmoid - because of coopertaive binding

Question 19

what is p50 *

Answer 19

pp of O2 at whic Hb is half saturated with O2

for HbA this is at 26.6mmHg

it is an estimate of different Hb affinity for O2

Question 20

what causes the ox dissociation curve to shift L *

Answer 20

decrease in 2,3-DPG and H+ conc

foetal Hb

Question 21

what is the effect of the ox dissociation curve shifting L *

Answer 21

ox bind more readily, and released less

eg so HbF can get O2 from placenta - higher affinty than maternal Hb

Question 22

what shifts O2 dissociation curve to R *

Answer 22

elevated 2, 3 DPG (in metbolising cell), and H+ conc, CO2 conc

sickle Hb

Question 23

what is the effect of ox dissociation curve shifted to R *

Answer 23

favour deoxyfied state - and dissociation

give up O2 more readily

lower affinity

Question 24

what are the 2 generic causes of haemoglobinopathies *

Answer 24

structural varients of Hb - abnormal varients are being produced because of mutation eg HbS

genetic defects in globin chain synth - reduced production of a/B chains

Question 25

thalassaemia epi

Answer 25

overlap with endemic malaria - protective widespread distribution where there is a dense population - mainly mediterranian, arabian peninsula, iran, indian subcontinent, africa, southern china, SE Asia

Question 26

how do you classify thalassamia \*

Answer 26

by the globin chain affected - a and B clinical severity: minor 'trait' - carrier, intermedia (non-transfusion dependant), major (transfusion dependant)

Question 27

describe the pathophysiology of thalassamia \*

Answer 27

deletion/mutation in B globin genes = reduced or absent B chains autosomal recessive - see pedigree can have differnet mutations BB(superscript o), more severe than BB+ B+ can make some chains - just reduced

Question 28

how do you diagnose b thalassaemia \*

Answer 28

full blood count - microcytic hypochromic, increased RBC relative to Hb blood film - target cells, poikilocytosis, no anisocytosis Hb EPS/HPLC - a = normal bA2 and HbF ( +/- HbH when have damage to 3 out of 4 a genes) B = raised HbA2 and HbF globin chain synth/DNA studies - genetic analysis for B thalassaemia mutations and Xmnl polymorphism, look for common mutations or whole genes, (in B thal when not sure about mutation/diagnosis/clinical effect) and a thalassamia genotype in all cases

Question 29

blood film for B thalassamia trait \*

Question 30

describe B-thalassaemia major \*

Answer 30

carry 2 abnormal copies of B globin gene = loss of production of B globin severe anaemia, transfusion dependant clinical presentation in 3-6 months

Question 31

blood film for B thal major \*

Answer 31

more anaemic - red cells sparse more variation in shape hypocromic

Question 32

describe the red cell occlusions in B thalassaemia \*

Answer 32

nucleated red cells a chain precipitates - difficult to pick up pappenhiemer bodies from iron deposits

Question 33

what is the clinical presentation of thalassaemia major \*

Answer 33

severe anaemia after 4 months hepatosplenomegaly - extramedullary haematopoeisis to produce red cells film - hypochromia, poikiloctosis, and nucleated RBCs bone marrow - erythroid hyperplasia

Question 34

what are the clincial features of B thalassaemia \*

Answer 34

chronic fatigue failure to thrive jaundice - due to chronic haemolysis a chains are unstable, more likely to go through haemolysis delay in growth and puberty skeletal deformity splenomegaly iron overload extra-medullary haematopoiesis leading to expansion of frontal and maxillary bones

Question 35

what are complications of B thalassaemia \*

Answer 35

cholelithiasis (gall stone disease) and bilary sepsis because of chronic haemolysis = increased serum BR cardiac failure, endocrinopathies, liver failure due to Fe overload - iron deposits here cardiac disease = main cause of death

Question 36

treatment of thalassaemia major \*

Answer 36

regular blood transfusions iron celation therapy - because of iron overload caused by transfusions/increased gut absorption (in thal minor) and downregulation of hepsidin splenectomy when have hhig transfusion requirements - risk of tromboembolic dusease or risk of clotting hormone therpay to treat the endocrinopathies hydroxyurea to boost HbF bone marrow transplant - curative

Question 37

describe transfusions for thalassamia \*

Answer 37

phenotyped red cells aim for pre-transfusion Hb 95-100g/L - reduce risl of extramedullary haematopoiesis regular transfusion 2-4 weekly match donor cells to blood gp - reduce risk of alloantibodies/ alloimmunisation

Question 38

describe management of infection in thalassaemia \*

Answer 38

infectiuon from encapsulated bacteria - yersinia, other gram -ve sepsis (thrive in Fe rich env) propylaxis in splenectomised pts - immunisation and Ab

Question 39

describe iron chelation therapy

Answer 39

start after 10-12? transfusions or wen serum ferritin \>1000mcg/L audiology/opthalmology screening prior to starting improved survival - reduce cardiac failure however poor compliance because SC for 12 hours - burden with social life/stigma

Question 40

describe deferasirox for iron chelation \*

Answer 40

newer than desferrioxamine oral once daily dose 20-40mg/kg SE - rash, GI symptoms, hepatitis, renal impairment half life 12-16hrs excretion - feacal adv - oral = better compliance, control of body iron, specific disadv - short clinical experience (new drug), cardiac protection uncertain, toxicity limited but long term

Question 41

describe desferrioxamine for iron chelation \*

Answer 41

long established SC for 8-12 hrs, 5-7 days a week dose - 20-50mg/kg a day SE - vertebral dysplasia, pseudo-rickets, genu valgum, retinopathy, high tone, sensorineural loss, increased risk of klebsiella and yersinna infection compliance is a prolem half life is 20-30 mins adv - long clinical experience, survival benefit, HF prevented and reversed disadv - SC, dose dependant toxicity of ocular, auditory and skeletal exretion - urinary or feacal advantage -

Question 42

describe deferiprone for iron chelation \*

Answer 42

oral 5-100mg/kg/day effective in reducing myocardial iron SE - GI disturbance, hepatic impairment, neutropenia, agranulocytosis (sudden drop in white cell count - risk of infection), athropathy (Zn deficiency) adv - oral, cardiac protection disadv - 3x a day, short plasma t1/2, unpredictable control of body iron, toxicity - agranulocytosis, arthropathy, zinc deficiency

Question 43

describe the process of administering chelation therapy \*

Answer 43

combination therapy - limit toxicity of any one drug because you can use a lower dose of it

Question 44

how do you monitor iron overload

Question 45

describe HbE thalassaemia \*

Answer 45

common in SE asia clinical variable - can be as severe as B thal major

Question 46

describe a thalassaemia \*

Answer 46

deletion/mutation in a globin chains = reduced/absent production affects foetus/adult express B and Y chains - form tetramers of HbH and Hb Barts resepctively severity depends on number of a genes affected

Question 47

describe thalassaemia B minor/trait \*

Answer 47

carry single abnormal copy of B globin gene usually asymptomatic mild anaemia

Question 48

describe HbH disease \*

Answer 48

it is a form of thalassaemia intermedia with the loss of function of 3a chains - therefore is alpha thalassaemia there is more poikocytosis than in thalassaemia trait red cells well haemoglobinised on the HPLC - a chains are still being produced, there are 2 abnormal peaks at the start of the spectrum: made of complexes of B globin chains HbH and Hb Barts (Hb that consists of 4 gamma Hb) - fast moving hb

Question 49

describe problems of treating thalassaemia in developing countries \*

Answer 49

lack of awareness lack of experience lack of blood for transfusions cost and compliance to iron chelation lack of availability of marrow transplant cost of marrow transplant

Question 50

describe screening and prevention for thalassaemia \*

Answer 50

counselling xtended family screening pre-marital screening discourage marriage between relatives antenatal testing pre-natal diagnosis - CVS

Question 51

if both parents are carriers of thalassamia major - what is the chance of child having thalassaemia major \*

Answer 51

25%

Question 52

what are the axis for an O dissocialtion curve \*

Answer 52

y - hb saturation x- pp of o2