15 - Hemoglobin Disorders Flashcards
What are the 2 classes of hemoglobin disorders?
- structural variants
(abnormal globin chain structure due to globin gene mutation; varied clinical effect depending on location and nature of mutation) - thalassemias
(under-production of structurally normal globin chains; generally microcytic/hypochromic anemais of varying severity)
What are the two main globin gene clusters?
- alpha cluster on chromosome 11
(2 alpha, 1 theta, 2 psi, and 2 zeta genes) - beta cluster on chromosome 16
(1 beta, 1 delta, 2 gamma, 1 epsilon gene)
What are the 3 normal hemoglobin species? What are the percentages in a normal adult?
- Hb A (2alpha2beta)
*96% in adults - Hb A2 (2alpha2delta)
*3% in adults - Hb F (2alpha2gamma)
*dominates during fetal life, 1% in adults
What is the incidence of abnormal hemoglobin? What are the usual clinical symptoms?
- 500+ structural hemoglobin variants (mostly single amino acid replacements, occasionally deletions/insertions)
- mostly clinically silent with no functional consequences
What are some examples of clinical consequences caused by abnormal hemoglobin?
- sickling
- Hb instability
- altered oxygen affinity (increased or decreased)
- increased susceptibility to oxidation (to methemoglobin)
- under-production of globin chains
- combinations of the above
What are the 2 main laboratory techniques for diagnosing abnormal hemoglobin?
- electrophoresis (gel or capillary)
- HPLC (high performance liquid chromatography)
Describe hemoglobin electrophoresis
- typically performed in parallel with alkaline and acid buffers
- HbA has isoelectric point of 6.8
- neg charge in alkaline buffers, migrates towards anode (+)
- pos charge in acid buffers, migrates towards cathode (-)
Describe HPLC
- fully automated cation exchange chromatography method
- whole blood method:
- Hb adsorbed onto resin particles
- different species differentially eluted based on affinity for resin by gradually changing ionic strength of elution buffer
- some correlation with migration on alkaline electrophoresis
Describe sickle cell disease
- homozygous abnormality of the beta globin chain
- more common in african americans (1 in 600 homozygous)
- Glu -> Val substitution at AA 6 of the beta chain (beta6val)
- heterozygous HbS “S trait” confers protection against malaria
Describe the pathophysiology of sickle cell disease
- deoxygenated HbS forms long polymers that distort the shape of the cell into an elongated, sickled form
- extend of polymerization is time and concentration dependent
- initially reversible but after multiple sickling/unsickling cycles, membrane damage produces irreversibly rigid sickled cell
- RBC lifespan decreased to 20 days
What affects the concentration of HbS?
- percentage of HbS of total Hb:
- homozygous or heterozygous
- presence of other Hb species (e.g. Hb F)
- total Hb concentration in the red cells (MCHC; Mean corpuscular hemoglobin concentration)
- concentration increased in cellular dehydration
- concentration decreased when co-existent thalassemia
What factors influence the time dependence of sickling?
- transit time of red cells through low oxygen tension microvasculature
- sickling enhanced in anatomic sites with sluggish flow (e.g. spleen and bone marrow)
- blood flow thorugh microvasculature retarded in certain pathologic states (e.g. inflammation)
What clinical settings predispose a patient to sickling?
- hypoxia
- acidosis (shift dissociation curve to the right causing increased deoxygenation of HbS)
- dehydration (hypertonicity causing RBC dehydration)
- cold temperatures (sluggish blood flow)
- infections
What are 2 major effects of RBC sickling?
- chronic hemolysis (correlates with the number of irreversibly sickled cells)
- microvascular occlusion with resultant tissue hypoxia and infarction (related to increased “stickiness” of SS red cells because of membrane damage)
When do symptoms usually begin for a patient with sickle cell?
- newborns clinically fine because of high HbF
- hematologic manifestations begin by 10-12 weeks of age
- clinical severity variable from patient to patient
What are 6 common clinical manifestations of sickle cell disease?
- severe anemia
- acute pain crises (from vaso-occlusion)
- auto-splenectomy (from repeat splenic infarction)
*increases infection risk - acute chest syndrome (major cause of death from pulmonary infections or fat emboli)
- strokes (first usually when 2-8 years old)
- aplastic crisis (acute decrease in RBC production usually from parvovirus B19 infection “fifths disease”)
What are the laboratory findings in sickle cell disease?
- chronic anemia (Hb 5-11 g/dl)
- increased bilirubin
- sickled cells, target cells, polychromasia
- increased reticulocytes
- normal MCV
What does this blood smear show?
**sickle cell disease
- target cell
- sickled cell
What would the gel electrophoresis results look like for sickle cell disease?
In class she stressed to KNOW THIS!
**bands for S and F hemoglobin (along with the normal A and A2 bands)
What would the HPLC results look like for sickle cell disease?
Large spikes for Hb S and F (note lack of A spike)
What is Hb SC disease?
- compound heterozygous state
- Hb C from glu6lys substitution of the beta globin
- generally milder than SS but highly variable
What is Hb S/Beta thalassemia?
- heterozygous Hb S with trans beta thalassemia allele, resulting in decreased or absent production of normal beta chains
- asymptomatic to nearly identical to SS
- lab findings= Hb S > Hb A
How do you manage a patient with sickle cell disease?
- newborn screening
- infection prophylaxis
- supportive care for acute manifestations
- hydroxyurea (chemo to reduce RBC counts and increase HbF levels)
- regular red cell transfusions
- allogeneic stem cell transplant (the only curative therapy but it’s only used in very sick patients)
What is the outcome for a patient with sickle cell disease?
- median age of death= 45 for males, 48 for females
- although much better than <20 year expectancy in the 1970’s
- gains due to decreased child mortality (infection prophylaxis and comprehensive care)
- major causes of death:
- liver dysfunction
- pulmonary HTN
- stroke/vaso-occlusive crisis
- acute chest syndrome
What is S-trait?
- clinically benign carrying of sickle cell gene (in 8% of african americans)
- no anemia/normal RBC survival
- NO sickling
- may be mild, sub-clinical kidney damage
- lab values= 60% Hb A, 40% HbS
What would the HPLC results look like for S-trait?
Large Hb A and S spikes (small F spike, unlike sickle cell disease)
What is HbC disease?
- homozygous for Hb C
- glu6lys substitution of the beta globin
- cells abnormally rigid/dehydrated
- NOT a sickling disorder
- mild to moderate hemolytic anemia
- RBC lifespan shortened to 30-35 days
- often asymptomatic
- splenomegaly (occasional abdominal pain)
- 1/6000 african americans
What are the lab findings for HbC disease?
- Hb levels 8-12 g/dl
- numerous target cells
- mild microcytosis
- spherocytes
- occasional C crystals
What would the HPLC results look like for HbC disease?
- >90% Hb C
- NO Hb A!
- <7% Hb F
Describe Hb C trait
- 2% of african americans
- NO anemia (no effect on patient but important for genetic counseling)
- few target cells
- 50-60% HbA, 30-40% HbC
What are thalassemias?
A group of inherited disorders characterized by decreased production of structurally normal globin chains
(highly heterogeneous both clinically and genetically)
What are the 2 main types of thalassemias? Where are they commonly seen?
- beta-thal (wide distribution in the mediterranean, middle east, parts of india/pakistan, and southeast asia)
- alpha-thal (occurs throughout africa, mediterranean, middle east, and southeast asia)
Describe the main features of thalassemias
- typically microcytic/hypochromic anemias (due to decreased Hb) of varying severity
- severity directly related to the degree of chain imbalance
- excess normally produced globin chains accumulate and cause intramedullary cell death and/or decreased RBC survival
Describe beta thalassemia. What are the 3 main types?
- decreased beta globin chain production from affected alleles
- 250+ known mutations
- most common= splicing errors
- gene deletions=rare
- clinically classified:
- beta-thal major (Cooley’s anemia)
- beta-thal intermedia
- beta-thal minor
Describe beta-thal major
- absence or marked decrease in beta chain production on BOTH beta alleles
- excess of normal alpha chains (unable to form tetramers and precipitate in normboblasts and erythrocytes)
- intramedullary cell death and decreased RBC lifespan
What are the symptoms of beta-thal major?
- infants well at birth (anemia develops over the first few months of life)
- severe anemia (Hb 2-3 g/dl; recall normal is 12-15)
- virtually all Hb F
- bizarre red cell morphology (hypochormia, targeting)
- transfusion dependent
- severity dependent on adequacy of transfusion program and efficacy of iron chelation
What are some symptoms of an inadequately transfused patient with beta-thal major?
- stunted growth and bony abnormalities
- frontal bossing (mongoloid facies)
- increased skin pigmentation
- fever
- wasting
- hyperuricemia
- spontaneous fractures (expanded marrow spaces)
- hepatosplenomegaly
- infections
- folate deficiency
- death in childhood
What are some features of an adequately transfused patient with beta-thal major without adequate iron chelation therapy?
- essentially normal development (avoidance of classic complications)
-
without adequate iron chelation therapy:
- absence of pubertal growth spurt/menarche
- endocrine disturbances (DM, adrenal insufficiency)
- death from cardiac iron deposition by 30s
What are some features of an adequately transfused patient with beta-thal major with adequate iron chelation therapy?
Essentially normal development (avoidance of classic complications)
with aggressive iron chelation therapy:
- less severe cardiac disease/endocrine disturbances
- significantly improved life span
Describe beta-thal minor
- heterozygous form
- mild or no anemia, Hb > 10 g/dl
- microcytosis, scattered target cells
- basophilic stippling
- elevated HbA2 (3.5-7%) **KNOW
- discovered incidentally on CBC (usually asymptomatic)
- incidence:
- common in mediterranean and asian populations
- 1.5% of african americans
Describe beta-thal intermedia
Heterogeneous group (everything between the extremes of major and minor)
What are the clinical subtypes of alpha thalassemia?
**usually a result of gene deletion (in contrast to beta-thal), recall we have 4 alpha genes; 2 on each chromosome 11
- silent carrier (1 gene deleted; asymptomatic)
- alpha-thal trait (2 genes deleted)
- Hb H disease (3 genes deleted)
- hydrops fetalis (4 genes deleted; infants stillborn or die within hours)
Describe alpha-thal trait
- 2 genes deleted
- mild microcytic anemia (hematologically similar to beta-thal minor)
- discovered incidentally
Describe Hb H disease
- 3 genes deleted
- mild to moderate; chronic hemolytic anemia
- Hb H= beta tetramer that doesn’t effectively transfer oxygen (recall alphas cannot combine in beta thalassemias so this doesn’t happen)
- Hb H is soluble (doesn’t initially precipitate in normoblasts) but becomes unstable over time, precipitating in circulating RBCs **causes hemolysis
