Genetic Basis of Thalassemia

Normal Globin Gene Organization

Globin gene clusters located at terminal ends of short arms of chromosomes 11 and 16.
Normal hemoglobin pattern dictates developmental stage-specific expression.

Chromosome	Gene Cluster	Specific Genes
Chromosome 16	$α$ -globin cluster	Zeta ( $ζ$ ), alpha 1 ( $α 1$ ), alpha 2 ( $α 2$ ). Four genes total (two per chromosome).
Chromosome 11	$β$ -globin cluster	Epsilon ( $ε$ ), gamma 1 ( $γ 1$ ), gamma 2 ( $γ 2$ ), delta ( $δ$ ), beta ( $β$ ).

Autosomal recessive inheritance pattern.
Rarely autosomal dominant inheritance.
Over 300 pathogenic variants characterized.
High prevalence in Mediterranean, African, Asian, and Southeast Asian ancestries.
Pathogenesis primarily involves point mutations in $β$ -globin genes.
Mutations disrupt multiple steps of $β$ -globin expression: transcription initiation, messenger RNA (mRNA) synthesis, translation, posttranslational modification.

Variant	Molecular Defect	Consequence
$β^{0}$ -Thalassemia	Point mutations or deletions.	Absent $β$ -chain mRNA. No detectable $β$ -chain synthesis.
$β^{+}$ -Thalassemia	Point mutations or deletions.	Reduced or nonfunctional $β$ -chain mRNA. Decreased $β$ -chain synthesis.
$δ β$ -Thalassemia	Deletion mutation.	Deletion of both $δ$ and $β$ globin genes. Compensatory increase in $γ$ -globin (HbF).
HbE/ $β$ -Thalassemia	Point mutation creating alternate splice site.	Structural variant (lysine replacing glutamic acid at codon 26) acts as thalassemic mutation. Decreased production of abnormal globin chain.
Hb Lepore	Unequal crossover.	Fusion globin gene ( $δ$ - $β$ ). Low production due to regulation by $δ$ -globin promoter.

Characterized by reduction or absence of $α$ -globin production.
Pathogenesis primarily involves gene deletions, unlike $β$ -thalassemia.
Less commonly, nondeletional point mutations occur (e.g., Hb Constant Spring), causing more severe phenotypes than deletional counterparts.
Disease severity directly correlates with number of deleted $α$ -globin genes.

Gene Deletions	Genotype	Clinical Syndrome	Pathophysiology & Features
1 Deletion	$- α / α α$	Silent Carrier ( $α^{+}$ -thalassemia)	Slightly reduced $α$ -globin. No anemia, no microcytosis.
2 Deletions	$- α / - α$ (Trans) $- - / α α$ (Cis)	$α$ -Thalassemia Trait	Trans common in African descent. Cis common in Asian/Mediterranean descent. Microcytosis, hypochromia, mild anemia.
3 Deletions	$- - / - α$	Hemoglobin H (HbH) Disease	Excess $β$ -chains form $β_{4}$ tetramers (HbH) postnatally. Hemolytic anemia, extravascular hemolysis.
4 Deletions	$- - / - -$	Hydrops Fetalis ( $α^{0}$ -thalassemia)	Excess $γ$ -chains form $γ_{4}$ tetramers (Hb Barts) in utero. No normal adult or fetal hemoglobin. Death in utero without intervention.

Primary pathology stems from quantity of globin produced (imbalance).
Excess unaffected chains accumulate, aggregate, and precipitate.
Unmatched $α$ -chains in $β$ -thalassemia form highly unstable tetramers ( $α_{4}$ ).
Results in severe membrane damage, apoptosis of red cell precursors (ineffective erythropoiesis), and shortened red cell survival (hemolysis).

$α$ -gene Deletions: Co-inheritance of $α$ -thalassemia in a $β$ -thalassemia patient decreases disease severity. Improves $α : β$ chain imbalance.
$α$ -gene Triplication: Co-inheritance of extra $α$ -genes in a $β$ -thalassemia trait patient worsens disease severity. Exacerbates chain imbalance, causing non-transfusion-dependent thalassemia (intermedia) phenotype.
- Promoter Polymorphisms: Polymorphisms in regulatory genes (e.g., BCL11A transcription factor) increase HbF production. Lessens disease severity by replacing missing $β$ -chains with $γ$ -chains.