Alpha (α) and Beta (β) Thalassemia Variants

1. Basic Overview of Thalassemia Inheritance

1.1 Beta-Thalassemia

• We normally have two beta-globin (β) genes, one from each parent.
• Beta-thalassemia trait (carrier) means one defective (or partially functional) β gene and one normal β gene.
  • This is commonly referred to as β-thalassemia minor or β-thalassemia trait.
• Beta-thalassemia major (or Cooley’s anemia) means both β genes are severely defective (β^0/β^0) or significantly dysfunctional.

Notation tips (commonly used in genetics):

• β = normal beta-globin gene
• β^0 = beta-globin gene with no production of β chains
• β^+ = beta-globin gene with reduced production of β chains

When each parent is a beta-thalassemia carrier (β-thalassemia trait), the typical risk for each child is:

1. 25% (1 in 4) chance of normal (β/β)
2. 50% (2 in 4) chance of beta-thalassemia trait (β^0/β or β^+/β)
3. 25% (1 in 4) chance of beta-thalassemia major (β^0/β^0 or β^+/β^+)

(Note: exact genotypes depend on whether the mutation is β^0 or β^+. In Thailand, both types exist.)

1.2 Alpha-Thalassemia

Humans typically have four alpha-globin (α) genes in total, with two inherited from each parent. We can represent a normal genotype as αα/αα. Different severities of alpha-thalassemia arise depending on how many α genes are deleted or mutated.

Common Thai Genotypes

1. 1-gene deletion (α-thal-2 trait)
   • Genotype: -α/αα
   • Usually causes mild or no anemia; hemoglobin electrophoresis is often normal.
2. 2-gene deletion
   • Can occur as cis (two deletions on the same chromosome) or trans (one deletion on each chromosome):
     • Cis: --/αα (often called “alpha-thal-1 trait”)
       • Common in Southeast Asians, including Thai.
     • Trans: -α/-α
       • More common in Africans, but can also be found in Asia.
3. 3-gene deletion
   • Genotype: --/-α
   • Leads to Hemoglobin H (HbH) disease, which typically shows moderate to severe anemia.
4. 4-gene deletion
   • Genotype: --/--
   • Results in Hemoglobin Bart’s hydrops fetalis, usually fatal in utero or shortly after birth.

Risk Calculation: Parents with Cis 2-Gene Deletion

If both parents have the cis 2-gene deletion (--/αα), each child has:

1. 25% chance (1 in 4) of normal (αα/αα).
2. 50% chance (2 in 4) of alpha-thal-1 trait (--/αα).
3. 25% chance (1 in 4) of Bart’s hydrops fetalis (--/--), a lethal condition.

Explanation

• Normal (αα/αα): Child inherits the healthy chromosome (αα) from each parent.
• Alpha-thal-1 trait (--/αα): Child inherits the deleted chromosome (--) from one parent and the normal (αα) from the other.
• Bart’s hydrops fetalis (--/--): Child inherits the deleted chromosome (--) from both parents.

This simple Mendelian inheritance pattern highlights why Thai couples who both carry the cis 2-gene deletion have a 1 in 4 chance of having a baby with the lethal hydrops fetalis condition.

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2. Common Hemoglobin Variants in Thailand

1. HbE (Hemoglobin E)
   • Caused by a mutation in the beta-globin gene that results in an abnormal hemoglobin (E).
   • On electrophoresis, people with HbE trait often show about 25–40% HbE and the rest mostly HbA (with normal to slightly increased HbA2).
   • People with HbEE disease (homozygous E) show a very high percentage (often 85–95%) of HbE and some HbF.
   • HbE/β-thalassemia can range from mild to severe depending on whether the beta-thal mutation is β^0 or β^+.
2. Hb Bart’s
   • Seen in newborns with alpha-thalassemia.
   • It’s made of gamma chains (γ4) due to an absence or severe reduction of alpha chains (common in (--/--) or (--/-α) at birth).
   • High levels of Hb Bart’s indicate severe alpha chain deficiency (e.g. hydrops fetalis).
3. Hb Constant Spring (Hb CS)
   • A mutation in the alpha-globin stop codon that produces an abnormally long alpha chain.
   • Common in Southeast Asians, especially Thai.
   • Often written as α^CS; can coexist with alpha-thal deletions, causing conditions like HbH with Constant Spring (HbH-CS), which is often more severe than typical HbH disease.

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3. Laboratory Interpretation: Thai Focus

3.1 Hemoglobin Electrophoresis Patterns

1. Normal Adult
   • HbA > 95%
   • HbA2: 1.5–3.5%
   • HbF: <2%
   • No abnormal variants (e.g., E, CS, etc.)
2. Beta-Thalassemia Trait (Minor)
   • HbA2 > 3.5% (key hallmark)
   • Mildly ↑ HbF (often <5%)
   • Slightly ↓ or normal HbA
   • Usually mild or no anemia (mild microcytosis: low MCV, MCH)
3. Beta-Thalassemia Major
   • Little to no HbA
   • HbF predominant (often >90% in untreated major)
   • HbA2 variable (can be normal or ↑)
   • Severe clinical symptoms, requires regular transfusions
4. HbE Trait
   • HbE around 25–40%
   • HbA around 60–75%
   • Normal or slightly ↑ HbA2
5. HbEE Disease (Homozygous E)
   • HbE up to 85–95%
   • Some HbF (5–15%), may vary
   • Little or no HbA
6. HbE/β-Thalassemia
   • Typically, HbE + HbF + (possible small fraction of HbA if β^+).
   • If β^0, you may see almost no HbA, mainly HbE and HbF.
7. Alpha-Thalassemia Trait
   • Typically normal Hb Electrophoresis (no clear changes in HbA, A2, or F).
   • If HbH disease (3-gene deletion), you’ll see a band for HbH (β4) on special electrophoresis or on brilliant cresyl blue stain.
   • Hb Bart’s band can be seen in neonates with severe alpha-thal.
8. Hb Constant Spring
   • Often detected on special electrophoresis (or HPLC) because it migrates close to HbA or slightly different, but can be overlooked.
   • Might be reported as “Hb Constant Spring” fraction.

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4. Calculating Risks for Children

When both parents have abnormal hemoglobin patterns, we combine their genotypes to find possible outcomes. Let’s look at a few common Thai scenarios:

4.1 Beta-Thalassemia Trait ×\times× Beta-Thalassemia Trait

• Each parent: β / β^0 (or β / β^+).
• Child outcomes (classic Mendelian):
  1. 25% normal (β / β)
  2. 50% carriers (β / β^0 or β / β^+)
  3. 25% thalassemia major (β^0 / β^0 or β^+ / β^+)

4.2 Alpha-Thalassemia-1 Trait (Cis deletion --/αα) ×\times× Alpha-Thalassemia-1 Trait (Cis deletion --/αα)

• Child outcomes:
  1. 25% normal (αα/αα\alpha \alpha / \alpha \alphaαα/αα)
  2. 50% alpha-thalassemia-1 trait (−−/αα--/\alpha \alpha−−/αα)
  3. 25% Hb Bart’s hydrops fetalis (−−/−−--/--−−/−−) → lethal

4.3 HbE Trait ×\times× Beta-Thalassemia Trait

• Mother: (β / β^E) – “HbE trait”
• Father: (β / β^0 or β / β^+) – “β-thal trait”
• Possible child genotypes:
  • (β / β) – normal
  • (β / β^E) – HbE trait
  • (β^0 / β^E) or (β^+ / β^E) – HbE/β-thal (can be mild to severe depending on β^0 vs β^+)
  • Exact percentages can vary because it depends on whether the father has β^0 or β^+. Typically:
    • If father is β^0, 25% chance normal, 25% chance E trait, 50% chance E/β^0-thal.
    • If father is β^+, the ratio changes similarly but the clinical severity of E/β^+ can be milder than E/β^0.

4.4 Alpha-Thalassemia Trait ×\times× Beta-Thalassemia Trait

• Typically, the child can inherit both alpha-thal trait and beta-thal trait.
• They do not cause a “new” combined disease as severe as alpha^0/beta^0, but the child may have microcytosis from both sides. Usually each gene set is inherited independently, so you calculate the risk for alpha-thal trait separately from the risk for beta-thal trait, then combine them.

4.5 Hb Constant Spring Inheritance

• If one parent carries α^CS (e.g., α^CS/α) and the other parent is normal, about 50% of children will get the α^CS gene.
• If combined with alpha-thal deletions (like --/α^CS α), children can develop HbH Constant Spring disease, which is more severe than typical HbH.

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5. Practical Calculation Example

Let’s say:

• Mother’s Lab: Beta-thalassemia trait (HbA2 = 4.5%) + Alpha-thal-2 trait (-α/αα).
• Father’s Lab: HbE trait (HbE 30%, HbA 70%) + normal alpha genes (αα/αα).

We can break it down:

1. For Beta Genes:
   • Mother: β / β^0 (or β^+)
   • Father: β / β^E Child can receive from mother either β (normal) or β^0 (thal). Child can receive from father either β (normal) or β^E.
   • Possible β-genotypes in child:
     • β / β (normal)
     • β / β^E (HbE trait)
     • β^0 / β (β-thal trait)
     • β^0 / β^E (HbE/β-thal disease)
2. For Alpha Genes:
   • Mother: (-α/αα) – alpha-thal-2 trait
   • Father: (αα/αα) – normal alpha Child can inherit from mother either (-α) or (αα). From father always (αα).
   • Possible α-genotypes in child:
     • (αα / αα) = normal alpha (50%)
     • (-α / αα) = alpha-thal-2 trait (50%)

So for each pregnancy, you combine these possibilities:

• Beta side has 4 possible genotypes.
• Alpha side has 2 possible genotypes.

Hence, in total, 4 × 2 = 8 possible genotype combinations, each with 12.5% probability (if we assume β^0, not β^+). Some examples:

1. (β/β) + (αα/αα) = Completely normal
2. (β/β) + (-α/αα) = Alpha-thal-2 trait
3. (β/β^E) + (αα/αα) = HbE trait
4. (β/β^E) + (-α/αα) = HbE trait + Alpha-thal-2 trait
5. (β^0/β) + (αα/αα) = Beta-thal trait
6. (β^0/β) + (-α/αα) = Beta-thal trait + Alpha-thal-2 trait
7. (β^0/β^E) + (αα/αα) = HbE/β-thal disease
8. (β^0/β^E) + (-α/αα) = HbE/β-thal disease + Alpha-thal-2 trait

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6. Summary Tips for Thai Clinical Settings

1. Always Check CBC (MCV, MCH). A very low MCV (<80 fL) with normal iron status often suggests thalassemia trait, especially in Thailand.
2. Electrophoresis is critical for diagnosing β-thal trait (high HbA2) and detecting HbE, Hb Constant Spring (special HPLC sometimes needed), or other variants.
3. DNA Analysis is essential for diagnosing alpha-thalassemia accurately, especially when the electrophoresis is normal but clinical suspicion (microcytosis) is high.
4. Couple Screening: In Thailand, screening both partners for thalassemia is highly recommended before pregnancy. If both carry a severe form (like β^0 or -- alpha), the risk of a thalassemia major or hydrops fetalis child is 25%.
5. Genetic Counseling: Once the mother and father’s genotypes are known, use Mendelian ratios (Punnett squares) to estimate the chances for each child.
6. Hb Bart’s usually appears in newborns with severe alpha gene deletions. High levels after birth indicate a strong possibility of (--/--).
7. Hb Constant Spring can exacerbate alpha-thal disease (like HbH).

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Final Takeaway

• Identify the father and mother’s exact genotype (both alpha and beta status if possible).
• Use simple Mendelian genetics to calculate the probability of each child’s outcome.
• Interpret hemoglobin electrophoresis results in the context of Thai-prevalent mutations (HbE, Constant Spring, alpha-thal-1 cis deletions, etc.).
• Confirm with DNA analysis when alpha-thal is suspected but electrophoresis appears normal.