THALASSEMIA

Thalassemia — the world's most common single-gene disorder, where the full globin genotype determines whether someone is an asymptomatic carrier, has a treatable chronic condition, or has a transfusion-dependent disease.

Whole genome sequencing reads the complete alpha-globin (HBA1/HBA2) and beta-globin (HBB) gene clusters, resolving every deletion, point mutation, and compound heterozygous combination — the full architecture that standard carrier screens do not characterize.

HBA1
HBA2
HBB

About this condition

Thalassemia

Thalassemia syndromes are autosomal recessive hemoglobinopathies caused by reduced or absent production of alpha- or beta-globin chains, leading to imbalanced globin chain synthesis, ineffective erythropoiesis, and chronic hemolytic anemia. Collectively, thalassemias are the most common single-gene disorders worldwide — an estimated 270 million people carry a thalassemia variant, and approximately 60,000 severely affected children are born annually. Thalassemia carrier frequencies are highest in malaria-endemic regions: the Mediterranean, sub-Saharan Africa, the Middle East, South and Southeast Asia, and southern China.

Alpha-thalassemia is caused by deletions or mutations affecting the HBA1 and HBA2 genes on chromosome 16p13.3. Because there are four alpha-globin gene copies (two HBA1 and two HBA2 on each chromosome 16), alpha-thalassemia severity follows a dosage-dependent gradient: one gene deleted (alpha-thalassemia silent carrier), two genes deleted (alpha-thalassemia trait), three genes deleted (HbH disease, moderate anemia), four genes deleted (Hb Bart's hydrops fetalis, uniformly fatal in utero without intervention). Beta-thalassemia is caused by point mutations or small insertions/deletions in the HBB gene. Beta-thalassemia major (Cooley's anemia, beta⁰/beta⁰) requires lifelong transfusion; beta-thalassemia intermedia has variable severity; beta-thalassemia trait is asymptomatic.

The genetic architecture of thalassemia is among the most complex of any Mendelian disorder. Over 300 beta-globin variants and dozens of alpha-globin deletion types have been characterized. Compound heterozygosity — inheriting different thalassemia variants from each parent — creates phenotypes not predictable from either variant alone. Co-inheritance of alpha- and beta-thalassemia modifies disease severity: concurrent alpha-thalassemia reduces globin chain imbalance in beta-thalassemia, paradoxically ameliorating the condition. Gene therapy (betibeglogene autotemcel, approved 2022) and gene editing (exagamglogene autotemcel/exa-cel, approved 2023) offer curative potential for transfusion-dependent patients, making definitive genotyping directly treatment-enabling.

Alpha- and beta-thalassemia involve different genes and inheritance patterns. Compound heterozygous combinations create distinct clinical phenotypes not predictable from individual variant testing alone.

Gene locus: HBA1/HBA2 (16p13.3), HBB (11p15.4)

Standard carrier screening tests for common regional thalassemia variants. It cannot resolve the full alpha-globin deletion genotype, compound heterozygous states, or modifier loci that determine clinical severity and reproductive risk.

Alpha-globin gene copy number requires genome-level resolution — SNP panels cannot determine it

Alpha-thalassemia is predominantly caused by large deletions that remove one or both alpha-globin genes from a chromosome. Determining the number of residual functional alpha-globin gene copies — the primary determinant of clinical severity — requires deletion mapping with breakpoint characterization. SNP-based carrier screening panels do not detect gene deletions. Even targeted deletion testing (MLPA, Gap-PCR) evaluates only the most common regional deletion types and may miss rare or atypical deletions. Whole genome sequencing maps all alpha-globin deletions, determines precise gene copy number, and characterizes breakpoints — the information required to distinguish alpha-thalassemia trait ('cis' vs 'trans' deletion configurations) and predict hydrops fetalis risk.

Gene therapy eligibility requires the definitive genotype — not a screening result

The approval of betibeglogene autotemcel (Zynteglo, 2022) and exa-cel (Casgevy, 2023) for transfusion-dependent beta-thalassemia has made definitive HBB genotyping directly treatment-enabling. Gene therapy eligibility criteria specify the beta-thalassemia genotype and, in some protocols, exclude certain variant types. For patients being evaluated for curative gene therapy or gene editing, the complete globin genotype — including identification of any co-inherited alpha-thalassemia that may affect the treatment response — must be established with certainty. Whole genome sequencing provides this definitive genotype in a single, comprehensive test.

Thalassemia — the world's most common single-gene disorder, where the full globin genotype determines whether someone is an asymptomatic carrier, has a treatable chronic condition, or has a transfusion-dependent disease.

Thalassemia

Standard carrier screening tests for common regional thalassemia variants. It cannot resolve the full alpha-globin deletion genotype, compound heterozygous states, or modifier loci that determine clinical severity and reproductive risk.

Alpha-globin gene copy number requires genome-level resolution — SNP panels cannot determine it

Gene therapy eligibility requires the definitive genotype — not a screening result

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