Delta-thalassemia (δ-thalassemia)

Article written and reviewed by Dr Luca Moretti

Delta-thalassemia (δ-thalassemia) is a rare inherited hemoglobin disorder caused by reduced or absent synthesis of the delta chain of hemoglobin, which is responsible for formation of the adult HbA₂ fraction (α₂δ₂). Although HbA₂ accounts for only 2–3% of total hemoglobin, its measurement plays a fundamental role in the differential diagnosis of microcytosis, particularly for identifying carriers of beta-thalassemia. Delta-thalassemia is generally clinically silent but may substantially complicate laboratory and genetic interpretation of thalassemic patterns, especially in family settings or investigations of at-risk populations.

Etiology, pathogenesis and pathophysiology

Delta-thalassemia is caused by point mutations or, less commonly, small deletions in the HBD gene (delta-globin) on chromosome 11. These abnormalities may lead to reduced (δ⁺-thalassemia) or completely absent (δ⁰-thalassemia) delta-chain production.

At the molecular level, the HBD gene has a structure similar to that of the beta-globin gene, but its expression is physiologically much more limited. Mutations causing delta-thalassemia may involve:

• Splice sites, causing abnormal mRNA production and reduced or absent delta-chain synthesis
• Promoter regions, which reduce gene transcription
• Nonsense or frameshift mutations, with formation of truncated and unstable chains

Delta-thalassemia variants are inherited in an autosomal manner and may produce a laboratory phenotype even in heterozygotes; the degree of HbA₂ reduction depends on the specific variant, zygosity and any association with other hemoglobin disorders.

The pathophysiology of delta-thalassemia is related to HbA₂ deficiency. Hemoglobin A₂ (α₂δ₂) consists of two alpha chains and two delta chains and is normally present in low amounts in adults. Its oxygen-carrying function is similar to that of HbA (α₂β₂), and its absence does not cause relevant direct clinical effects. However, deficient delta-chain production prevents formation of HbA₂, resulting in reduced HbA₂ values to a degree that varies with the variant and zygosity and is not necessarily below 2%.

In heterozygous carriers, HbA₂ synthesis may simply be low or at the lower limit of normal; in individuals homozygous for delta-thalassemia, HbA₂ levels may be markedly reduced or virtually absent. In double heterozygosity with beta-thalassemia, delta-thalassemia may instead reduce or normalize the increase in HbA₂ expected in beta-thalassemia trait, making diagnosis more difficult and potentially affecting genetic counseling.

Significant abnormalities of erythrocyte production or tissue oxygenation are generally not described, nor are systemic complications observed, given the quantitatively modest role of HbA₂ in adult hemoglobin physiology. However, coexistence with other hemoglobin disorders (beta-thalassemia, structural variants, alpha-thalassemia) may result in distinctive clinical and laboratory patterns.

Risk factors and epidemiology

Delta-thalassemia is a rare and probably underdiagnosed condition; its frequency varies among populations and no reliable global estimate is available. The responsible mutations are reported more often in some regions of the Mediterranean basin, South Asia and some Latin American populations, but never reach the frequencies seen in beta- or alpha-thalassemia.

The main risk factors are:

• Family history of reduced or atypical HbA₂ values
• Consanguinity in areas where thalassemia is endemic
• A parent carrying a pathogenic HBD gene variant

Diagnosis is often incidental during thalassemia screening or following atypical clinical findings in relatives of patients with beta-thalassemia.

Clinical features

In the great majority of cases, delta-thalassemia is clinically asymptomatic. Affected individuals do not have significant anemia or hematologic abnormalities attributable to the isolated HBD variant. The complete blood count is generally normal; the presence of microcytosis or hypochromia requires investigation for iron deficiency or coinherited alpha- or beta-thalassemia.

In double heterozygosity with beta-thalassemia or other hemoglobin disorders, the picture may become more complex, with marked microcytosis, more significant anemia and an atypical electrophoretic pattern.

It is important to emphasize that isolated delta-thalassemia causes neither clinical symptoms nor long-term complications. Its relevance is therefore essentially laboratory and genetic, particularly in prenatal counseling and in the differential diagnosis with other conditions that alter HbA2 and with associated hemoglobin disorders.

Diagnosis and investigations

The diagnosis of delta-thalassemia is based on a combination of hematologic, biochemical and molecular findings. Clinical suspicion typically arises in individuals with:

• Persistently low HbA₂ or presence of an HbA₂ variant
• Reduced or low-normal HbA₂ values on electrophoresis or HPLC, not necessarily below 2%
• Suggestive family history or atypical thalassemia screening

The diagnostic work-up includes:

• Complete blood count and red blood cell indices: generally normal in isolated delta-thalassemia; any microcytosis suggests a concomitant condition
• Hemoglobin electrophoresis or HPLC: reduced or absent HbA₂, normal HbA, and no increase in HbF
• Exclusion of other causes of low HbA₂: iron deficiency, concomitant globin abnormalities and possible analytical interference, interpreted in the clinical context
• Molecular analysis of the HBD gene: PCR or sequencing for point variants and small sequence variants; MLPA or other copy-number techniques when deletions or duplications are suspected

In more complex cases, the differential diagnosis should include combined defects (delta/beta-thalassemia, unstable HbA₂ variants), rare hemoglobin variants and, above all, conditions that may cause an apparent reduction in HbA₂, such as iron deficiency. When iron deficiency is present, it is useful to reassess HbA₂ after iron deficiency has been corrected; any change in the value should nevertheless be interpreted together with the hematologic picture and, when indicated, molecular analysis, because a concomitant HBD variant may keep HbA₂ reduced.

Particular attention should be paid to combined carriers of delta- and beta-thalassemia: in these individuals, the classic increase in HbA₂ expected in beta-thalassemia may be masked, creating a risk of diagnostic errors in genetic counseling, especially in the prenatal setting.

Treatment and prognosis

No specific treatment is required for isolated delta-thalassemia because the condition causes neither symptoms nor pathologic progression. The prognosis is excellent and the condition does not affect life expectancy.

The main management issue is correct identification in screening programs, genetic counseling for family members and the differential diagnosis with beta-thalassemia, in order to avoid interpretive errors in prevention pathways for thalassemia major.

Complications

Delta-thalassemia is not associated with clinical complications of its own or with a risk of progression. Problems arise only when it coexists with other hemoglobin disorders, especially beta-thalassemia, because diagnostic masking may occur and affect genetic counseling.

A missed diagnosis may lead to:

• Misclassification of beta-thalassemia carriers
• Inadequate assessment of reproductive risk in at-risk couples
• Errors in prenatal management of pregnancies at risk of thalassemia

For these reasons, awareness of delta-thalassemia and its correct identification in screening pathways are key components of modern laboratory medicine.

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