Thalassemia

Slide 1 · Thalassemia

Inherited disorders of globin-chain synthesis

α-thalassemia • β-thalassemia

Molecular basis | Pathogenesis | Morphology | Clinical correlation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Distinguish α- and β-thalassemia at the gene and chain levels

Explain ineffective erythropoiesis, hemolysis, and tissue injury

Connect phenotype and morphology to molecular genotype

Interpret characteristic laboratory patterns and clinical complications

Slide 3 · Thalassemia: A quantitative globin defect

Globin synthesis ↓

Chain imbalance

Ineffective erythropoiesis + hemolysis

Anemia / tissue hypoxia

Germline variants reduce production of one major globin chain

Deficient hemoglobin synthesis produces microcytic, hypochromic cells

Excess unmatched chains damage erythroid precursors or circulating RBCs

Slide 4 · Hemoglobin A contains two paired chains

α

β

β

α

Hemoglobin A: α₂β₂

The tetramer is conceptual, not a molecular-structure rendering.

Slide 5 · α-globin genes are duplicated on each chromosome 16

Two homologous chromosomes 16

α

α

Maternal

α

α

Paternal

Two α-globin genes occur on each chromosome 16

A diploid genome normally contains four functional α-globin genes

Gene deletions form the principal molecular basis of α-thalassemia

Slide 6 · β-globin is encoded at chromosome 11

Two homologous chromosomes 11

β

Maternal

β

Paternal

One β-globin gene is present on each chromosome 11

A diploid genome normally has two β-globin alleles

β-thalassemia usually results from mutations that impair β-globin gene expression

Slide 7 · Two distinct mechanisms produce thalassemic anemia

Reduced synthesis

Hypochromic microcytic erythrocytes

Low oxygen transport

Reduced cell survival

Precursor loss in the marrow

Peripheral hemolysis

The relative importance of these mechanisms differs by thalassemia type

Slide 8 · The geographic distribution reflects genetic selection

Thalassemias are common in the Mediterranean, Middle East, Africa, India, and Asia

Together, they are among the most frequent inherited disorders worldwide

Carrier prevalence is linked to protection against malaria

Slide 9 · α and β defects have a shared starting point

β-thalassemia

β chains ↓

Free α chains ↑

Insoluble intracellular aggregates

α-thalassemia

α chains ↓

Free γ (fetus) or β (adult) ↑

Relatively stable homotetramers

Free α is especially insoluble and damaging to erythroblasts

Slide 10 · β⁰ versus β⁺: Residual synthesis matters

β⁰ allele

Functional β output: Absent

Often terminator or severe splice defects

β⁺ allele

Functional β output: Reduced

Often promoter or partially preserved splice defects

Phenotype also depends on the balance of other globin chains

Slide 11 · Three molecular classes cause β-thalassemia

Splicing

Normal mRNA formation impaired

Promoter

Transcription reduced

Terminator

Protein translation interrupted

RNA-splicing variants alter maturation of β-globin mRNA

Promoter variants impair transcription

Premature termination or frameshift variants block functional translation

Most reported β-thalassemia variants are small sequence changes

Slide 12 · Disrupted normal splicing may create β⁰-thalassemia

Splice junction mutation

Normal mRNA absent

β-globin synthesis absent

β⁰ phenotype

Normal splice junctions are necessary to make mature β-globin mRNA

A severe splice-junction mutation prevents formation of normal mRNA

The affected allele produces no functional β chain

Slide 13 · An ectopic splice site may preserve partial expression

Ectopic splice site

Mixed RNA processing

Some normal mRNA

β⁺ phenotype

A mutation can create an abnormal splice site inside an intron

Both normal and abnormal splicing may then occur

Some normal β-globin mRNA is retained, producing a β⁺ phenotype

Slide 14 · Promoter variants reduce β-globin transcription

Promoter mutation

β-globin transcription ↓

Functional β chain ↓

β⁺ phenotype

A promoter alteration decreases the amount of β-globin mRNA made

The supplied Robbins section describes a roughly 75–80% transcription reduction

Residual normal globin synthesis corresponds to β⁺-thalassemia

Slide 15 · Premature stops and frameshifts favor β⁰-thalassemia

Nonsense variant

Premature stop codon

Truncated or absent functional β

Frameshift variant

Changed translation reading frame

No functional β chain from allele

Functional β-globin translation may be completely blocked

Slide 16 · The first β-thalassemia consequence is HbA deficiency

β output ↓

HbA formation ↓

Microcytosis + hypochromia

Oxygen carriage ↓

A shortage of β chains limits assembly of HbA (α₂β₂)

Affected red cells are poorly hemoglobinized

Hypochromia and microcytosis are direct consequences

Slide 17 · Free α chains are the chief cytotoxic lesion

β chain deficit

Unpaired α excess

α aggregates

Membrane injury

β production falls while α-chain synthesis continues

Unpaired α chains precipitate as insoluble aggregates

These aggregates initiate oxidative and membrane injury

Slide 18 · Membrane injury kills developing erythroblasts

Marrow response

Erythroblast proliferation ↑

Large erythroid precursor pool

Functional result

Membrane injury / apoptosis

Few mature circulating cells per precursor

Marrow expansion therefore does not guarantee effective RBC output

Slide 19 · Surviving RBCs remain vulnerable to the spleen

RBC membrane damage

Splenic recognition

Sequestration / phagocytosis

Peripheral hemolysis

Some damaged erythrocytes are released from marrow despite inclusions

Altered membranes promote splenic sequestration

Phagocytic removal shortens circulating RBC lifespan

Slide 20 · Ineffective erythropoiesis and hemolysis coexist

Inside marrow

Precursor apoptosis

Ineffective erythropoiesis

In circulation

Damaged circulating cells

Splenic hemolysis

Together with reduced Hb synthesis, these sustain chronic anemia

Slide 21 · Reticulocytosis does not fully compensate

Profound anemia

Erythropoietic stimulus ↑

Precursor expansion + apoptosis

Inadequate effective reticulocyte output

The marrow is stimulated by severe anemia

Reticulocyte numbers may rise, yet remain inappropriately low for anemia severity

The discrepancy is explained by erythroblast destruction before maturation

Slide 22 · Anemia drives marked erythroid marrow expansion

Anemia / tissue hypoxia

Erythropoietic drive

Erythroid marrow hyperplasia

Bone remodeling

Oxygen deprivation amplifies erythropoietic drive

Erythroid precursors massively expand in active marrow

Marrow expansion can erode cortical bone and deform the skeleton

Slide 23 · Marrow hyperplasia alters the skull and face

Expanded marrow cavities thin and remodel cortical bone

Facial prominences can enlarge or become distorted

Skull radiographs may show a characteristic “crewcut” pattern

These changes are most conspicuous when severe anemia is untreated

Slide 24 · Extramedullary hematopoiesis enlarges organs

Liver

Hematopoiesis outside marrow

Spleen

Hematopoiesis + phagocytic activity

Lymph nodes / masses

Additional hematopoietic sites

Severe erythropoietic demand recruits liver, spleen, and lymph nodes

Hepatosplenomegaly is common in severe forms

In extensive disease, extraosseous hematopoietic masses can develop

Slide 25 · Expanded erythropoiesis can consume tissue resources

Massive erythroid expansion

Nutrient demand ↑

Hypoxic tissues compete

Growth failure / cachexia

Large populations of metabolically active erythroid precursors require nutrients

Other tissues simultaneously experience reduced oxygen delivery

Untreated severe disease may therefore lead to growth impairment and cachexia

Slide 26 · Ineffective erythropoiesis suppresses hepcidin

Erythroid expansion

Erythroferrone ↑

Hepcidin ↓

Gut iron uptake ↑

Expanded erythroid progenitors release erythroferrone

Erythroferrone inhibits hepatic hepcidin production

Reduced hepcidin permits increased intestinal iron absorption

Slide 27 · Iron absorption rises despite ineffective use

Iron entry

Absorption from gut ↑

Hepcidin restraint ↓

Iron use

Effective RBC output ↓

Iron gradually accumulates

The mismatch between uptake and utilization contributes to iron overload

Slide 28 · Transfusions create a second route to iron excess

Endogenous pathway

Erythroferrone ↑

Hepcidin ↓ → absorption ↑

Treatment-associated pathway

Repeated RBC transfusions

Cumulative transfusional iron

Without iron-control measures, both sources can drive systemic overload

Slide 29 · Chronic iron overload damages parenchymal organs

Heart

Cardiac injury

Liver

Hepatic iron toxicity

Pancreas

Pancreatic iron deposition

Iron deposition can progress to secondary hemochromatosis

Heart, liver, and pancreas are particularly vulnerable

Cardiac disease due to iron overload is a major complication of β-thalassemia major

Slide 30 · β-thalassemia spans three clinical phenotypes

Major

Severe; Regular transfusions

Intermedia

Moderate; Variable genotype

Minor / trait

Carrier phenotype

β-thalassemia major: Severe, usually transfusion-dependent anemia

β-thalassemia intermedia: Intermediate clinical severity, usually without regular transfusion requirement

β-thalassemia minor (trait): Asymptomatic or mild microcytic anemia

These are clinical categories; Genotype alone does not determine every case

Slide 31 · β-thalassemia major generally involves two affected alleles

Two homologous chromosomes 11

β⁰

Maternal

β⁺

Paternal

Common genotypes include β⁰/β⁰, β⁰/β⁺, or β⁺/β⁺

Severe reduction in β-chain supply produces major chain imbalance

The phenotype is usually transfusion-dependent

Slide 32 · β-thalassemia major emerges after infancy

Fetal HbF predominates

HbA synthesis increases with age

β-chain deficiency emerges

Anemia becomes clinically evident

Neonatal hemoglobin contains a large fetal component, HbF

The switch toward HbA exposes inadequate β-chain synthesis

Onset around 6–9 months after birth

Slide 33 · Hemoglobin fractions help explain severe β disease

β⁰/β⁰

HbA may be absent

HbF predominates

β⁺-containing genotype

Some HbA may persist

HbF remains substantially elevated

An untransfused major phenotype may have hemoglobin around 3–6 g/dL

Slide 34 · β-thalassemia major has a complex smear pattern

Illustrative RBC morphology — schematic, not a blood specimen

Prominent anisocytosis and poikilocytosis

Marked microcytosis and hypochromia

Target cells, basophilic stippling, and fragmented RBCs

Circulating nucleated RBCs may reflect stress hematopoiesis

Slide 35 · Microcytosis and hypochromia indicate deficient Hb

Illustrative RBC morphology — schematic, not a blood specimen

Reduced globin synthesis lowers hemoglobin available per erythrocyte

Small red cells and increased central pallor follow

These signs are not specific to thalassemia and require a differential diagnosis

Slide 36 · Target cells and stippling support, but do not prove, thalassemia

Illustrative RBC morphology — schematic, not a blood specimen

Target cells show a central hemoglobinized area surrounded by pallor

Basophilic stippling can occur in affected cells

Neither feature uniquely identifies a thalassemia genotype

Slide 37 · Nucleated RBCs signal severe erythropoietic stress

Illustrative RBC morphology — schematic, not a blood specimen

Poorly hemoglobinized normoblasts may be present in peripheral blood

Marrow stress and extramedullary hematopoiesis contribute to their release

Their presence should be interpreted with overall severity and clinical context

Slide 38 · Spleen and marrow changes reflect chronic turnover

Marrow

Expanded erythroblasts

Bone cortical remodeling

Spleen

Extravascular RBC clearance

Extramedullary hematopoiesis

Extramedullary hematopoiesis further enlarges the spleen and liver

Slide 39 · Untreated major disease has severe systemic consequences

Chronic severe anemia

Marrow / organ expansion

Growth + skeletal effects

Severe clinical course

Growth retardation and skeletal deformities arise from longstanding anemia

Hepatosplenomegaly reflects extramedullary hematopoiesis

Early mortality can result from uncompensated severe anemia

Slide 40 · RBC transfusion reduces erythropoietic stress

Donor RBC transfusion

Oxygen carriage ↑

Erythropoietic drive ↓

Marrow expansion ↓

Transfusion raises circulating functional hemoglobin

Improvement of anemia reduces hypoxic stimulation

Suppressing excess erythropoiesis can limit marrow and skeletal complications

Slide 41 · Transfusion is beneficial but adds iron

Therapeutic benefit

Anemia less severe

Marrow expansion reduced

Complication

Transfusional iron loading

Risk of secondary hemochromatosis

The balance makes iron monitoring and chelation essential in treated patients

Slide 42 · Chelation is central to preventing iron toxicity

Chronic transfusion ± uptake

Iron overload

Iron chelation

Reduced organ injury risk

Iron deposition occurs unless overload is actively prevented

Chelation reduces risk of injury to heart and other organs

The source emphasizes combining appropriate transfusions with iron chelation

Slide 43 · Curative approaches target hematopoietic stem cells

Hematopoietic stem cell transplantation can be curative

Gene-addition therapy introduces functional β-globin into stem cells

The source also mentions CRISPR-based therapeutic strategies

Access and very high cost constrain use in resource-limited settings

Slide 44 · Prenatal molecular diagnosis is possible

Family / carrier history

Molecular variant identification

Prenatal DNA analysis

Genetic counseling

Pathogenic β-globin variants can be identified by DNA analysis

Genetic characterization can support reproductive counseling

This is especially relevant when both parents carry affected alleles

Slide 45 · β-thalassemia trait usually causes little morbidity

Two homologous chromosomes 11

β

Maternal

β⁺

Paternal

Typically one β⁰ or β⁺ allele and one functional allele

Individuals are generally asymptomatic

Mild anemia or isolated microcytosis may be detected incidentally

Slide 46 · β-thalassemia minor has recognizable red-cell changes

Illustrative RBC morphology — schematic, not a blood specimen

Microcytosis and hypochromia commonly persist

Target cells and basophilic stippling may be seen

Marrow erythroid hyperplasia is usually mild

Morphology must be interpreted alongside iron studies and Hb fractions

Slide 47 · Elevated HbA₂ is a clue to β-thalassemia trait

Hemoglobin analysis typically shows increased HbA₂

Robbins gives an illustrative range of 4–8% of total hemoglobin

HbF is usually normal or only slightly increased

HbA₂ interpretation belongs with overall laboratory context

Slide 48 · Differentiate β-thalassemia trait from iron deficiency

Finding

β-thalassemia trait

Iron deficiency

Microcytosis

Typical

Typical

HbA₂

Often increased

Not diagnostic

Iron studies

Assess to exclude deficiency

Demonstrate iron deficiency

Context

Inherited carrier state

Acquired iron depletion

Slide 49 · β-thalassemia intermedia has variable genotypes

Genes

Variable defective alleles

Hemoglobin

Moderately reduced effective output

Clinical course

Intermediate severity

Clinical severity is intermediate between major and trait

Biallelic mild defects or selected heterozygous configurations may produce it

It is often not dependent on regular blood transfusions

Modifiers that alter globin-chain balance help explain variability

Slide 50 · Coinherited α-thalassemia can ameliorate β disease

β deficiency

α synthesis also ↓

Free α imbalance ↓

Precursor survival ↑

Decreasing α synthesis reduces excess unpaired α chains

This can lessen erythroid precursor damage

Some patients with two β-thalassemia alleles therefore have a milder phenotype

Slide 51 · Extra α-globin genes can worsen β-chain imbalance

α-gene duplication

α chains in excess

More α without β partners

Clinical severity ↑

An α-gene duplication increases α-chain output

With one defective β allele, the extra α supply can worsen chain imbalance

The example demonstrates that phenotype depends on relative chain abundance

Slide 52 · β-thalassemia phenotype is not a simple allele count

Underlying determinants

β⁰ / β⁺ expression

α-chain gene dosage

Observed phenotype

Mild trait

Intermedia or major

Clinical classification captures the net physiological effect

Slide 53 · α-thalassemia severity follows four-gene dosage

Two homologous chromosomes 16

α

α

Maternal

α

α

Paternal

The phenotype depends on how many of these four gene copies are deleted.

Slide 54 · Deficient α chains leave γ or β chains unmatched

Fetal period

Unpaired γ chains

γ₄ = Hb Barts

After birth

Unpaired β chains

β₄ = HbH

Neither tetramer delivers oxygen normally

Slide 55 · α-thalassemia generally causes less precursor toxicity

Excess α (β defect)

Insoluble aggregates

Prominent precursor injury

Excess non-α (α defect)

β₄ / γ₄ tetramers

High oxygen affinity / instability

Severe α-gene loss nevertheless has major physiological consequences

Slide 56 · One α-gene deletion: Silent carrier

Two homologous chromosomes 16

—

α

Maternal

α

α

Paternal

Three of four α-globin genes remain

No clinically important anemia or physical signs are expected

Very slight microcytosis may be detectable

Slide 57 · Two α-gene deletions: α-thalassemia trait

Two homologous chromosomes 16

—

—

Maternal

α

α

Paternal

α-chain output is lower, causing microcytosis

Anemia is absent or minimal; Physical signs are usually lacking

HbA₂ is typically normal or low

Two deletions can occur in cis or in trans

Slide 58 · The arrangement of two deletions matters

Cis deletion

-- / αα

One chromosome lacks both genes

Trans deletion

-α / -α

Each chromosome has one gene

Their reproductive risks differ substantially

Slide 59 · Cis α-gene deletions change fetal risk

α-thalassemia trait

Identify cis / trans phase

Assess partner genotype

Counsel on fetal risk

A parental chromosome lacking both α genes can transmit a two-gene deletion

The partner’s α-globin genotype determines risk of HbH disease or α-thalassemia major

Genotyping is therefore more informative than red-cell indices alone for reproductive counseling

Slide 60 · Three α-gene deletions cause HbH disease

Two homologous chromosomes 16

—

—

Maternal

—

α

Paternal

Only one α-globin gene remains functional

β-chain excess forms β₄ tetramers

The resulting anemia resembles β-thalassemia intermedia in severity

Slide 61 · HbH delivers oxygen poorly to tissues

HbH (β₄) formation

Oxygen affinity ↑

Tissue unloading ↓

Disproportionate hypoxia

β₄ tetramers have unusually high oxygen affinity

Oxygen unloading is inefficient

Tissue hypoxia may exceed what the measured hemoglobin level suggests

Slide 62 · HbH instability promotes splenic destruction

β₄ tetramers

Oxidative instability

RBC inclusions / damage

Splenic clearance

HbH is susceptible to oxidation

Oxidation leads to intracellular precipitation and inclusions

Abnormal RBCs undergo sequestration and phagocytosis in the spleen

Slide 63 · Loss of all four α genes causes α-thalassemia major

Two homologous chromosomes 16

—

—

Maternal

—

—

Paternal

No α-globin synthesis occurs from the normal four-gene cluster

Fetal excess γ chains form Hb Barts (γ₄)

High-affinity Hb Barts does not release oxygen effectively to tissues

Slide 64 · α-thalassemia major threatens fetal oxygenation

α chains absent

Hb Barts (γ₄)

Severe fetal hypoxia

Hydrops / organomegaly

Severe tissue anoxia becomes evident during pregnancy

Typical findings include pallor, generalized edema, and massive hepatosplenomegaly

Severe disease historically caused intrauterine or early neonatal death

Slide 65 · Fetal survival and intervention can change outcome

Embryonic ζ chains initially pair with γ chains to support oxygen transfer

Signs of severe disease typically appear later in gestation

Intrauterine transfusions can allow survival in some affected fetuses

Survivors may require ongoing transfusion; Stem cell transplant can be curative

Slide 66 · α-thalassemia phenotypes form a gene-dose sequence

1 deleted

Silent carrier

2 deleted

Trait

3 deleted

HbH disease

4 deleted

α major / Hb Barts

Increasing loss of α-globin gene function → generally increasing severity

Slide 67 · β and α thalassemia differ in their toxic partner chains

Feature

β-thalassemia

α-thalassemia

Principal mutation

Often point mutation

Usually gene deletion

Excess chain

α

β or γ

Key mechanism

α precipitation + apoptosis

Tetramers + impaired O₂ unloading

Severe form

β major

α major / Hb Barts

Slide 68 · Clinical workup integrates morphology, fractions, and genes

CBC + smear

Iron studies

Hemoglobin analysis

Targeted molecular analysis

Recognize microcytosis and hypochromia

Evaluate iron deficiency rather than assuming the cause

Analyze hemoglobin fractions when appropriate

Slide 69 · Case: Microcytosis with elevated HbA₂

A clinically well adult has persistent microcytosis

Hemoglobin analysis shows elevated HbA₂

Iron status does not support iron deficiency

Most compatible textbook classification: β-thalassemia trait

Which finding most strongly supports the carrier diagnosis?

Answer and reasoning in presenter notes

Slide 70 · Case: Severe anemia emerging late in infancy

An infant becomes markedly anemic after the HbF-to-HbA transition

Smear shows target cells, hypochromia, and marked size variation

HbF is predominant; HbA is greatly reduced

Pattern is compatible with β-thalassemia major

Why does clinical disease emerge months after birth?

Answer and reasoning in presenter notes

Slide 71 · Case: Hypoxia disproportionate to hemoglobin

A patient has an α-globin deletion syndrome with HbH (β₄)

The anemia is moderately severe

Tissue oxygen delivery is poorer than the hemoglobin level alone implies

High-affinity HbH accounts for the discrepancy

What molecular property explains poor oxygen delivery?

Answer and reasoning in presenter notes

Slide 72 · Case: α-thalassemia trait and family planning

Two individuals have similar mild microcytosis

One carries --/αα; The other -α/-α

Blood indices alone cannot distinguish these arrangements

DNA-level phase informs offspring risk

Why distinguish cis from trans two-gene deletions?

Answer and reasoning in presenter notes

Slide 73 · Integrating thalassemic anemia from gene to organ

Globin gene variant

Hb synthesis / chain imbalance

Marrow + RBC pathology

Clinical / organ consequences

Gene-level deficit causes reduced hemoglobin synthesis

Chain excess injures erythroblasts or disrupts oxygen unloading

Anemia stimulates marrow and extramedullary hematopoiesis

Slide 74 · Core distinctions for diagnosis and teaching

Thalassemia is a defect in globin quantity, not simply an abnormal Hb structure

β-thalassemia severity is driven strongly by unmatched α chains

α-thalassemia severity correlates with deletion count and gene arrangement

Always consider iron deficiency in microcytic anemia

Slide 75 · Primary reference

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th edition (2026).

Chapter 14: Red Blood Cell and Bleeding Disorders.

Section: Thalassemia (printed pp. 585–589).

Source-derived clinical classifications and numerical values retain the textbook framing.