10 - Hemoglobinopathies and Thalassemias Flashcards
Normal Hemoglobin
Post-embryonic hemoglobins:
contain two pairs of polypeptide
chains
* Alpha
* Non-alpha
* Beta (α2β2: HbA) – makes up
most of adult Hb
* Gamma (α2γ2: HbF) – fetal Hb
* Delta (α2δ2: HbA2) – minor
adult Hb; 2.5%
- Makes up a four subunit,
allosteric molecule - Conformation and O2 affinity
changes as each successive O2
molecule is bound
States of normal hemoglobin
proportion of globin chains from prenatal to 1 yo
Chromosomal location of globin chains
O2 hemoglobin dissociation curve
Hemoglobin disorder types
Quantitative disorders (Thalassemias)
* Result from the decreased and imbalanced production of structurally
normal globins
* Imbalances in the production of α and β chains leads to damage of RBCs
and RBC precursors in the bone marrow
Qualitative disorders
* Arise from point mutations that change the amino acid sequence of the
globin, thereby leading to functional changes in the hemoglobin
* Decreased solubility (HbS)
* Instability
* Altered O2 affinity
* Altered maintenance of the oxidation state of heme iron
The Evolution of Hemoglobin Disorders
- Tend to occur in higher frequency in geographic
regions endemic for falciparum malaria – survival
advantage
Alpha thalassemias – Genetic Nomenclature
QUANTITATIVE DISORDERS OF HEMOGLOBIN
- SE Asia and W Africa
- Chromosome 16 – 2 genes present; more heterogenous
presentations - (α α/ α-): Silent carrier; no anemia, no morphologic abnormalities
- (α α/–) or (α -/ α -): α thalassemia trait; mild microcytic anemia
- Tetrameric Hemoglobins
- (α -/–): α thalassemia major
- (–/–): Hydrops fetalis
- 4 Υ Hgb Bart
- 4 β Hgb H – precipitates into Heinz bodies
- May not see changes on hemoglobin electrophoresis
- Generally you will need mutation analysis for diagnosis
Alpha thalassemia
QUANTITATIVE DISORDERS OF HEMOGLOBIN
- Genotypes:
- (αα/–)
- (α-/α-)
- Trans v cis
- Lifelong microcytosis with mild anemia
- Target cells on peripheral smear
- Importance of genetic counseling
Alpha thalassemia (intermedia / major)
QUANTITATIVE DISORDERS OF HEMOGLOBIN
- Genotype: (α -/–) leads to four beta chains (HbH)
- Hg H is unstable and precipitates as the RBC ages, forming Heinz bodies,
which causes bite cells and a hemolytic anemia - Variable presentation
- Some can present like thal-intermedia – occasionally need transfusions,
moderate anemia - Some can present like thal-major
- Splenomegaly
- Hb 7-11 g/dl
- Low MCV and MCH, high RDW
Alpha thalassemia major
QUANTITATIVE DISORDERS OF HEMOGLOBIN
- Genotype: (–/–) leads to four
gamma chains (Hb Bart;
Hydrops fetalis) - All four genes affected – no alpha chains
form - Severe anemia, high output heart failure
→ hydrops fetalis (generalized edema,
“water laden fetus”) - Marked hepatosplenomegaly
- Anistocytosis, poikilocytosis
- Intrauterine death, followed by stillbirth
at 25-40 weeks - Can be treated in utero by exchange
transfusions
Beta Thalassemias – Genetic Nomenclature
QUANTITATIVE DISORDERS OF HEMOGLOBIN
- Southern Europe, SE Asia, Africa, Middle East
- β thalassemias result from >200 mutations in the beta
globin gene - Chromosome 11
- Normal beta gene synthesis: β/ β
- Complete absence of beta gene synthesis: β0
- β0/β0: Cooley’s anemia – β Thalassemia major
- Decreased but not absent beta gene synthesis: β+
- See changes on hemoglobin electrophoresis
Beta thalassemia major
QUANTITATIVE DISORDERS OF HEMOGLOBIN
- Absence or severe deficiency of beta globin synthesis
- Symptoms usually evident at 6-12 mos
- Genotypes:
- (β0/ β0)
- (β0/ β+)
- Leads to four alpha chains – very unstable
- Moderate to severe lifelong anemia requiring transfusions
- Increased HbF → impaired oxygen delivery to tissue
- Iron overload
- Transfusions
- Iron hyperabsorption from the gut
- Growth retardation
- Ineffective erythropoiesis → extramedullary erythropoiesis
- Leads to facial abnormalities: frontal bossing, maxillary prominence
- Hepatosplenomegaly
- Prone to infection
Beta thalassemia major - Lab findings
- Microcytic, hypochromic RBCs
- Marked anistocytosis, poikilocytosis
- Target cells - Nucleated
red blood cells
Poikilocytosis generally refers to an increase in abnormal-shaped red blood cells that make up 10% or more of the total red blood cells
Beta thalassemia major - Complications
QUALITATIVE DISORDERS OF HEMOGLOBIN
Hemoglobin S (HbS):
Mutation: HbS results from a mutation in the beta-globin gene, leading to the substitution of glutamic acid with valine at the sixth position of the beta-globin chain.
Clinical Significance: Individuals with two copies of the HbS gene have sickle cell anemia, a genetic disorder characterized by misshaped (sickle-shaped) red blood cells. This can lead to various complications, including pain, anemia, and organ damage.
Hemoglobin E (HbE):
Mutation: HbE is caused by a mutation in the beta-globin gene, resulting in the substitution of glutamic acid with lysine at the 26th position of the beta-globin chain.
Clinical Significance: HbE is usually found in individuals of Southeast Asian descent. While having one copy of the HbE gene may not cause significant health problems, having two copies (HbE/HbE) can lead to a mild form of beta-thalassemia, a blood disorder characterized by reduced production of hemoglobin.
Hemoglobin C (HbC):
Mutation: HbC is the result of a mutation in the beta-globin gene, leading to the substitution of glutamic acid with lysine at the sixth position of the beta-globin chain.
Clinical Significance: HbC trait (heterozygous) individuals usually do not experience severe health problems. However, individuals with two copies of the HbC gene may have hemoglobin C disease, a condition characterized by mild hemolytic anemia and enlarged spleen.
Phenotypes caused by Hgb S,E,C variants
QUALITATIVE DISORDERS OF HEMOGLOBIN
- More severe
phenotype
– HbSS: sickle cell
anemia
– HbSC: SC disease
– Sickle β thalassemia - HbS β0
- HbS β+
- Less severe phenotype
– HbAS: sickle cell trait
– HbC disease
– HbE disease
Hemoglobin S
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Occurs due to a point
mutation: Glu 6Val - Sixth amino acid in β chain is changed from glutamine (hydrophilic) to valine (hydrophobic)
- Leads to different electrophoretic mobility
- Deoxy-HbS: 50 times less soluble than deoxy-HbA
- Homozygous: HbSS
- Heterozygous: HbAS
(sickle trait) - Chromosome 11
Sickle Cell trait demographics
QUALITATIVE DISORDERS OF HEMOGLOBIN
- 1 in 12 African-Americans carry sickle cell trait (heterozygous)
- 1 in 500 African-American births have a homozygous mutation
- 1 in 300 African-American births are SS,
SC, or S/Bthal - 1 in 1000-5000 Hispanic-American births
- Also found in middle East, Mediterranean, Southern India
Hemoglobin E
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Second most common Hb variant (after HbS)
- Substitution of glutamic acid by lysine at the 26th
position of the beta chain - Single-base substitution creates a cryptic splicing site → abnormal mRNA processing and reduction of mRNA that can be translated
- Sometimes referred to as a “thalassemic hemoglobinopathy” – decreased beta chain production
- Highest frequency in South and SE Asia about 10% ofthe population.
- Can have co-inheritance with HbS or genes involved in beta thalassemia.
Hemoglobin E syndromes
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Heterozygotes: HbAE (HbE trait)
- Asymptomatic; may or may not have mild anemia
- May have normal smear or may have hypochromia, microcytosis,
target cells, basophilic stippling - Homozygotes: HbEE
- Asymptomatic with no overt hemolysis or splenomegaly
- Mild anemia, microcytosis (MCV 65-70), reduced MCH
- Peripheral smear with hypochromia, microcytosis, variable target
cells, irregularly contracted cells - HbE and HbA2 makes up 85-99% of total hemoglobin
Hemoglobin C
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Third most common mutant Hb
- Substitution of glutamic acid with lysine at the sixth
position on the beta globin chain - Decreased solubility in the deoxy and oxy forms – undergoes
intraerythrocytic aggregation and crystal formation - Dehydrated and poorly deformable RBCs – get trapped in the spleen
- Occurs in 2-3% of African-Americans
- Important: on Hb electrophoresis, comigrates with
HbA2, HbE and HbOarab - Need citrate gel electrophoresis or high-performance liquid
chromatography (HPLC) - Can have co-inheritance with HbS, beta thalassemia
genes
Hemoglobin C syndromes
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Heterozygotes: HbAC (Hb C trait)
- Clinically normal
- Normal hemoglobin, may have microcytosis
- Peripheral smear may be normal or show microcytosis and target cells
- Homozygotes: HbCC
- Mild hemolytic anemia, microcytosis, slight reticulocytosis
- Elevated MCHC
- On peripheral smear, see prominent target cells, microcytosis, irregularly
contracted RBCs - Usually not very symptomatic
Hgb Constant Spring (hgb CS)
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Mutation at the termination codon of α2-globin
- This leads to synthesis of unstable and elongated α2-globin chains with 172 instead of 141 amino acid
residues. - Homozygotes present clinically like α-thal intermedia
- Found almost exclusively in people from Southeast
Asia and China - Typing:
- CS trait: CS 1%
- CS homozygous: CS 5%
- CS/αthal: looks clinically like Hgb H (3 gene deletions), CS 3-5%
Sickle cell disease syndromes
QUALITATIVE DISORDERS OF HEMOGLOBIN
Sickle cell disease can be due to 3 genotypes:
* HbSS (Sickle cell anemia)
* HbSC (SC disease)
* Sickle β Thalassemia
* HbSβ0: do not make any normal HbA – clinically
indistinguishable from SS
* HbSβ+: can make some HbA – clinically more mild
than SS
Sickle cell anemia
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Hematologic manifestations
- Anemia
- Typically NOT microcytic
- If microcytosis, think about concomitant iron deficiency or thalassemia
- Leukocytosis, thrombocytosis
- Howell-Jolly bodies (asplenia)
- Reticulocytosis
- Anisocytosis, poikilocytosis
- Increased risk for thrombosis
- 12% of HbSS pts have a VTE by age 40
- Other lab features
- Increased LDH
- Decreased haptoglobin
- Increased reticulocytes, increased indirect bilirubin
Sickle cell crises
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Painful vaso-occlusive crisis
- Aplastic crisis
- Parvovirus infection of RBC precursors → low reticulocyte count with
rapidly decreasing Hb - Acute Chest Syndrome
- Hyperhemolytic crisis
- Delayed hemolytic transfusion reaction
- Transfused cells and the patient’s own cells are hemolyzed (“bystander
hemolysis”) - Splenic or hepatic sequestration crisis
- Can occur in children prior to splenic infarction
- Can occur in adults with SC disease
- Profound anemia, massive splenomegaly, hypovolemic shock
Sickle cell anemia – organ involvement
QUALITATIVE DISORDERS OF HEMOGLOBIN
Acute chest syndrome
QUALITATIVE DISORDERS OF HEMOGLOBIN
Acute chest syndrome
* Most common cause of death
* Second most common cause of hospitalization
* CLINICAL diagnosis
* Fever * Cough * Chest pain * Hypoxia * New infiltrate on CXR * Treatment * RBC exchange transfusion * O2, antibiotics, prevention
(incentive spirometry)
Sickle cell anemia – long-term management
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Vaccinations: pneumococcal, meningococcal, H influenza
- Folate 1 mg daily (unless deficiency documented) – may be on examinations but no longer SOC
- Annual eye exams
- Hydroxyurea
- Increases HbF, decreases WBC, increases cell size
- Decreases stroke, pain, acute chest and increases survival
- Crizanlizumab (Adakveo)
- Inhibits P-selectin adhesion
- FDA approved to decrease pain VOCs by 50% but not in the EU
- Voxelotor (Oxbryta)
- Stabilizes the deoxygenated Hgb molecule and prevents polymerization
- FDA approved to improve anemia
Sickle cell anemia – long-term management
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Simple Transfusions
- To optimize before surgery and before delivery in pregnancy
- Exchange transfusions
- Acute chest syndrome
- Acute stroke
- Priapism
- Chelation therapy for iron overload
- Allogeneic bone marrow transplant
- Gene therapy
- Various
- Lyfgenia - Autologous transplant of stem and progenitor cells transduced with
BB305 lentiviral vector encoding a modified β globin gene (lovotibeglogene
autotemcel)] - Casgevy – CRISPR targets BCL11A which allows increased production of the non-
alpha globin Υ and therefore Hgb F(exagamglogene autotemcel)
SC disease
QUALITATIVE DISORDERS OF HEMOGLOBIN
- Hb electrophoresis: 50% HbS, 50% HbC (runs as HbA2)
- Less anemic than HbSS patients – 20% normal HCT
- Presence of HbC leads to more intracellular
dehydration, worsened sickling and hyperviscosity - Clinical features:
- Splenomegaly
- More ocular and bone complications
- Typically live longer than patients with HbSS
Sickle Cell - Beta Thalassemia
QUALITATIVE DISORDERS OF HEMOGLOBIN
- HbSβ0: do not make any normal HbA –
clinically indistinguishable from SS - HbSβ+: can make some HbA – clinically
more mild than SS
Summary of QUALITATIVE DISORDERS OF HEMOGLOBIN
Sickle cell trait
- 8-9% of the African-American population
- Generally symptomatic
- Rare complications
- Renal papillary necrosis
- Hematuria
- Renal medullary carcinoma
- HbA to HbS ratio of 60%:40%
- Altered gene has decreased function
Conclusions for hemoglobinopathies and thalassemias
- Normal structure and function of hemoglobin is
imperative for gas exchange and delivery of O2 to
tissues - Changes in hemoglobin structure and production leads to various pathology.
- Thalassemias are due to imbalanced globin chain synthesis (quantitative problem)
- Can be transfusion dependent and transfusion independent
- Can involve genes responsible for the alpha or beta globin chains
- Hemoglobinopathies are usually due to mutations in the beta globin chain which leads to altered hemoglobin structure and function.
(qualitative problem) - Genes can be co-inherited (ie compound heterozygous) which can
lead to different phenotypes!
