Overview
This lecture covers haemoglobinopathies, using thalassemia as a worked example of Mendelian disease pathophysiology. It moves through normal haemoglobin/globin gene structure, the molecular basis and gene-dosage effects of α- and β-thalassemia, genetic heterogeneity, the resulting clinical spectrum (major/intermedia/minor), population and ethnic distribution (including Māori α-thalassemia trait), diagnostic red cell indices, and finally treatment (transfusion, chelation, bone marrow transplant, hydroxyurea, CRISPR/Casgevy) and public health prevention strategies (carrier screening, premarital screening programmes).
Haemoglobin structure and normal globin genetics
- Haemoglobin has four components: two globin protein chains (2α + 2β) each carrying a haem group with a central Fe2+; iron + protoporphyrin = haem, haem + globin peptides = haemoglobin.
- Chromosome 16 carries the α-globin genes: two identical copies of α (α1, α2) per chromosome, so 4 functional α gene copies normally (αα/αα).
- Chromosome 11 carries β, δ and γ globin genes (only one copy of β per chromosome).
- Normal combinations: α2β2 = HbA (adult, majority); α2δ2 = HbA2 (<3.5%); α2γ2 = HbF (fetal, <1% in adults).
- Haemoglobin types across development: embryo ζ2ε2 → fetus α2γ2 (HbF) → adult α2β2 (HbA).
- After birth there is a switch from γ- to β-globin expression; γ is repressed by the transcription factor BCL11A. Before birth γ chain synthesis is high (~100%) and β low; after birth γ synthesis falls sharply while β rises sharply, α chain synthesis stays constant (~100%) throughout. Onset of symptoms in transfusion-dependent thalassemia (TDT) and sickle cell disease (SCD) coincides with this postnatal switch.
Haemoglobinopathies: overview and classification
- Among the most common Mendelian disorders: about 5% of the world’s people are carriers; ~0.5 million people affected per year, mainly by HbS (sickle cell disease).
- Abnormal/structural variants (usually affecting the beta chain): e.g. HbS, HbC, HbE.
- Thalassemia (α or β): ~70,000 cases/year worldwide. Three severity categories:
- Major: no globin product (α0 or β0). Causes anaemia, splenomegaly, abnormal bone development, and (via transfusion) iron overload. Life expectancy <5 years untreated, 30+ years treated.
- Intermedia: reduced but variable amount of globin produced; intermediate disease severity.
- Minor/trait: carrier state (α+ or β+), clinically unaffected or mild; detectable microcytic anaemia but no clinical disease.
- Ways mutations can produce different effects (revision, non-silent globin mutations):
- Decreased/loss of function, e.g. β-globin loss → β-thalassemia (autosomal recessive).
- Abnormal/gain of function, e.g. Hb Hammersmith → Hb precipitation → cell lysis (autosomal dominant).
- Novel properties, e.g. HbS → sickle cell disease (usually autosomal recessive).
- Ectopic gene expression, e.g. hereditary persistence of fetal haemoglobin.
Genetic heterogeneity
A single clinical picture (thalassemia) can arise via several distinct genetic mechanisms:
- Allelic heterogeneity: more than one allele at a single locus produces the disease, e.g. different deletions cause α-thalassemia; different mutations cause β-thalassemia.
- Locus heterogeneity: more than one gene (locus) can produce the same clinical phenotype, e.g. thalassemia can result from mutations in either the α-globin or the β-globin gene.
- Modifier genes/environment: other gene variants or environmental factors alter phenotype, e.g. the range of presentations in thalassemia intermedia depends partly on iron status.
Alpha-thalassemia: gene dosage and clinical states
- Normally 4 functional α gene copies (αα/αα, two per chromosome 16).
- Clinical severity is graded by number of functional α genes remaining (Table 11-3, Genetics in Medicine):
| Functional α genes | Genotype | α-globin production | Clinical state |
|---|---|---|---|
| 4 | αα/αα | 100% | Normal |
| 3 | αα/α− | 75% | Silent carrier |
| 2 | α−/α− or αα/− − | 50% | α-Thalassemia trait (mild anaemia, microcytosis) |
| 1 | α−/− − | 25% | HbH (β4) disease (moderately severe haemolytic anaemia) |
| 0 | − −/− − | 0% | Hydrops fetalis / homozygous α-thalassemia (Hb Bart’s, γ4) |
- Mechanistically, deletion of both α genes on chromosome 16 leaves β and γ chains without α partners to combine with: excess β chains form β4 tetramers (HbH), and excess γ chains form γ4 tetramers (Hb Barts). Chromosome 11 (β/δ/γ) is unaffected in α-thalassemia.
- Māori population: α-thalassemia trait found in an estimated ~3% of Māori. As there are normally four α gene copies, this mild anaemia is likely a carrier state (α,−/α,− or α,−/α,α) and will not by itself produce a “Major” (0 or 1 functional gene) phenotype in the next generation. A family study (NZ Med J 1989, Parker et al., PMID 2471123) found 10 of 12 Māori family members had the 3.7 kb deletion form of α-thalassemia (2 homozygous −α/−α, 8 heterozygous −α/αα); anaemia itself was not significant, but degree of hypochromia and microcytosis correlated with α-globin gene status, supporting α-thalassemia as a significant contributor to chronic mild anaemia in Māori.
Beta-thalassemia: mechanism and inheritance
- Deletion of both α genes on chromosome 16 causes α-thalassemia (above); by contrast β-thalassemia arises from mutations (not typically deletions) in the β-globin gene on chromosome 11 — allelic heterogeneity, with many different mutations described (see regional variation below).
- Mechanistically: with reduced/absent β chain production, HbA is decreased, and relatively more δ and γ globin expression occurs as compensation, producing HbA2 >3.5% and HbF >1% (these elevated HbA2/HbF levels are diagnostic clues).
- Inheritance: autosomal recessive. A cross of two β-thalassemia minor (heterozygous carrier) parents yields, per offspring: 25% normal (β/β), 50% β-thalassemia minor (carrier), 25% β-thalassemia major (β0/β0).
- The β-globin gene structure (from its DNA sequence) includes 3 exons and 2 introns; mutations may affect the coding sequence, promoter regions (CAAT and TATA boxes in the 5′ flanking region), or RNA splicing (GT/AG dinucleotides at intron-exon junctions) or the AATAAA polyadenylation signal — i.e. mutation may affect coding, promoters, or splicing. [slide does not elaborate further on specific mutation types beyond this classification]
Regional and ethnic distribution
- Carrier (minor/trait) rates are high in some ethnic groups: Asian, African, Mediterranean populations, e.g. Sardinia 12%, Cyprus 14% (β trait/carriers).
- A geographic distribution map shows thalassemia, HbS, HbC, ovalocytosis, HbE and Pk deficiency concentrated across Africa, the Mediterranean, the Middle East, and parts of Asia.
- Proposed evolutionary benefit of carrier status: decreased risk of malaria in children (heterozygote advantage), consistent with the geographic overlap between thalassemia carrier frequency and historical malaria endemicity.
- Within the Mediterranean region, the relative proportion of different β-thalassemia mutations (e.g. Cd 8, Cd 39 (C→T), IVSI-1 (G→A), IVSI-6 (C→T), IVSI-110 (G→A), IVSII-1 (G→A), IVSII-745 (C→G), FSC6, and others) varies by location, illustrating allelic heterogeneity by region — e.g. Cd 39 (C→T) is prominent in more central/western Mediterranean areas, while IVSI-110 (G→A) is prominent elsewhere.
The transcript flags that the Asian-region mutation-proportion map (analogous to the Mediterranean one) was cut off in the source slide and its content could not be described.
Diagnosis: red cell indices
Red blood cell indices distinguish normal, β-thalassemia major, and β-thalassemia minor (carrier):
| Index | Normal (M/F) | β-Thal Major | β-Thal Minor |
|---|---|---|---|
| MCV (fl) | M 89.1±5.01 / F 87.6±5.5 | 50–70 | <79 |
| MCH (pg) | M 30.9±1.9 / F 30.2±2.1 | 12–20 | <27 |
| Hb (g/dL) | M 15.9±1.0 / F 14.0±0.9 | <7 | M 11.5–15.3 / F 9.1–14 |
- Thalassemia carriers/minor/trait have a detectable microcytic anaemia on blood smear (hypochromic, microcytic cells, sometimes target cells) but no clinically significant disease.
- Iron stores in thalassemia are usually normal or raised (distinguishing it from iron-deficiency microcytic anaemia).
Complications of thalassemia major
Untreated/undertreated thalassemia major produces a wide range of complications, reflecting chronic anaemia, iron overload, and marrow expansion: transfusion-transmitted infections, bone expansion (“hair-on-end” skull appearance), hypopituitarism, excess melanin skin pigmentation (“bronze diabetes”), hypothyroidism, hyperparathyroidism, pulmonary hypertension and embolism, cardiomyopathy, venous thrombosis, haemosiderosis and cirrhosis of the liver, extramedullary haematopoiesis, splenomegaly, diabetes mellitus, arthropathy, delayed puberty/secondary sexual characteristics, testicular or ovarian failure, osteoporosis, and short stature.
Treatment
- Supportive: blood transfusion (with leukoreduction and viral testing) — the mainstay, but causes iron overload, which requires iron chelation therapy (deferoxamine, deferiprone, deferasirox). Endocrinopathies are managed with hormone replacement; osteoporosis with vitamin D (and osteoclast-targeted therapy).
- Curative: bone marrow (haematopoietic stem-cell) transplant (bone marrow, cord blood, or unrelated donor) — a cure but rarely used (~25/year in the UK).
- Intermedia (mild cases): hydroxyurea, which increases expression of γ (fetal) globin.
- Investigational/experimental: new drugs promoting RBC formation (in trials); erythropoietin; other fetal haemoglobin modifiers (e.g. butyrate); antioxidants.
- Gene therapy / future therapy: CRISPR-Cas gene editing to raise fetal (γ) globin expression — Casgevy, approved for treating both thalassemia and sickle cell disease (US approval 12/2023; NHS/UK 2025, treating an estimated 50/year in the UK). Mechanism: Cas9 with guide RNA targets the erythroid enhancer region of the BCL11A gene (the transcription factor that normally represses γ-globin/HbF), disabling that repression. In trial data, fetal haemoglobin rose from ~11% (control) to ~29% (edited).
- Genetic disorders in general can be treated at different levels of gene expression (DNA → RNA → protein → clinical phenotype → family), each with a corresponding strategy (Figure 13-1, Genetics in Medicine):
- Mutant gene: modification of the somatic genotype (i. transplantation, e.g. bone marrow transplant in β-thalassemia; ii. gene therapy, e.g. γc cytokine receptor gene transfer in X-linked SCID); or pharmacological modulation of gene expression (e.g. decitabine to increase HbF in sickle cell disease, investigational).
- Mutant mRNA: RNA interference to degrade mutant mRNA (e.g. RNAi for transthyretin amyloidosis).
- Mutant protein: protein replacement (e.g. glucocerebrosidase in Gaucher disease, factor VIII in haemophilia A); or enhancement of residual function (e.g. pyridoxine in classic homocystinuria).
- Metabolic/biochemical dysfunction: disease-specific compensation, dietary (e.g. low-phenylalanine diet in PKU) or pharmacologic (e.g. sodium benzoate in urea cycle defects).
- Clinical phenotype: medical intervention (e.g. transfusion in thalassemia) or surgical intervention (e.g. correction of congenital heart disease).
- The family: genetic counselling (e.g. after a child born with trisomy 21), carrier screening (e.g. Tay-Sachs disease), presymptomatic diagnosis (e.g. Huntington disease).
- For thalassemia specifically, the interventions highlighted are: modification of the somatic genotype (CRISPR-Cas editing, 2023), pharmacological modulation of gene expression, medical intervention (transfusion), and genetic counselling.
Public health prevention
- Population screening for carriers is possible (screening test: microcytic anaemia) and has reduced incidence; alternatively, screening can be limited to affected families or intending parents.
- Alternative approach: prenatal/genetic screening of the fetus, with termination as an option if affected.
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Premarital Screening and Genetic Counselling (PMSGC) programmes are described as a genetics public-health success story: in Mediterranean countries with mandatory PMSGC (Cyprus 1973, Italy 1975, Greece 1975), the percentage of expected thalassemia major births fell from near 100% in 1972 toward low levels by the 1990s, with Cyprus declining earliest/fastest, followed by Greece, then Italy; the UK (which does not have the same mandatory programme) plateaued around 50–55%.
- Other countries later implemented mandatory PMSGC: Turkey (1995), Iran (1997), Palestinian territories (2000), Jordan (2004), Saudi Arabia (2004), Bahrain (2005), Iraqi Kurdistan (2008), United Arab Emirates (2011).
- Ethical and social issues in population screening: affects large numbers of people, particularly in developing countries, with potentially 70,000 affected births at stake; raises questions of autonomy and informed consent since results affect marriage and reproductive decisions; and screening programmes differ in effectiveness across different cultures and countries.
Self-test
- Define the difference between thalassemia major, intermedia, and minor/trait in terms of globin product amount and clinical severity.
- Distinguish allelic heterogeneity from locus heterogeneity, using thalassemia as the example for each.
- Describe the molecular mechanism by which loss of both α-globin genes leads to HbH disease and Hb Barts formation.
- Using the α-thalassemia gene-dosage table, predict the clinical state and approximate α-globin production for someone with only 1 functional α gene.
- Explain why β-thalassemia typically shows elevated HbA2 and HbF, in mechanistic terms.
- A patient of Māori ethnicity has mild, otherwise unexplained microcytic anaemia. Explain what genetic state is most likely responsible, and why this would not necessarily predict a “major” thalassemia phenotype in their children.
- Compare the MCV, MCH and Hb values that distinguish a β-thalassemia carrier from someone with β-thalassemia major.
- Two people who are both β-thalassemia minor (carriers) have a child. What are the possible genotypes/phenotypes of their offspring, and in what proportions?
- List at least five systemic complications of poorly managed thalassemia major, and explain the underlying process(es) driving them.
- Describe the mechanism by which Casgevy (CRISPR gene editing) treats transfusion-dependent thalassemia.
- Distinguish loss-of-function, gain-of-function, and novel-property globin mutations, giving the disease example and inheritance pattern for each.
- Explain how premarital/carrier screening programmes reduced thalassemia major births over time, and identify one ethical issue this raises.
- Integrative: trace how a mutation in the β-globin gene (at the DNA level) can lead through to the population-level pattern of declining thalassemia major births seen in Mediterranean screening data, referencing the different levels of intervention (gene, mRNA, protein, phenotype, family) at which treatment or prevention can occur.
Answers
Reveal answers
- Major: no globin product (α0 or β0) — most severe, causing anaemia, splenomegaly, abnormal bone development, and (via transfusion) iron overload, with life expectancy <5 years untreated (30+ years treated). Intermedia: reduced but variable amount of globin produced — intermediate severity disease. Minor/trait: carrier state (α+ or β+) with detectable microcytic anaemia but no clinical disease.
- Allelic heterogeneity is more than one allele at the same locus causing disease, e.g. different deletions cause α-thalassemia, or different mutations cause β-thalassemia. Locus heterogeneity is more than one gene (locus) producing the same clinical phenotype, e.g. thalassemia can result from mutations in either the α-globin gene or the β-globin gene.
- With both α genes deleted, there are no α chains to pair with β or γ chains. Excess β chains associate with each other to form β4 tetramers (HbH); excess γ chains associate to form γ4 tetramers (Hb Barts).
- 1 functional α gene = 25% α-globin production, genotype α−/− −, corresponding to HbH (β4) disease — moderately severe haemolytic anaemia.
- With reduced/absent β-globin production, less HbA (α2β2) is made, so there is relatively more δ and γ globin expression as compensation, producing HbA2 (α2δ2) >3.5% and HbF (α2γ2) >1%.
- Most likely an α-thalassemia trait/carrier state (α,−/α,− or α,−/α,α) — found in an estimated ~3% of Māori. Because there are normally four α gene copies, a carrier with 2 remaining functional genes will not by itself produce a “Major” (0 or 1 functional gene) phenotype in offspring.
- β-thalassemia carrier (minor): MCV <79 fl, MCH <27 pg, Hb 11.5–15.3 g/dL (males) or 9.1–14 g/dL (females). β-thalassemia major: MCV 50–70 fl, MCH 12–20 pg, Hb <7 g/dL — markedly lower across all indices.
- 25% normal (β/β), 50% β-thalassemia minor/carrier, 25% β-thalassemia major (β0/β0) — autosomal recessive inheritance pattern.
- Any five of: transfusion-transmitted infections, “hair-on-end” bone expansion, hypopituitarism, bronze skin pigmentation (“bronze diabetes”), hypothyroidism, hyperparathyroidism, pulmonary hypertension/embolism, cardiomyopathy, venous thrombosis, haemosiderosis/liver cirrhosis, extramedullary haematopoiesis, splenomegaly, diabetes mellitus, arthropathy, delayed puberty, gonadal failure, osteoporosis, short stature. These largely arise from chronic haemolytic/ineffective erythropoiesis (marrow expansion, extramedullary haematopoiesis, splenomegaly) and from iron overload due to repeated transfusion (endocrine, cardiac, hepatic damage).
- Cas9 with a guide RNA targets the erythroid enhancer region of the BCL11A gene, disabling the transcription factor that normally represses γ-globin (fetal haemoglobin) expression after birth. This raises fetal haemoglobin (shown rising from ~11% to ~29% after editing), which compensates for the deficient adult β-globin.
- Loss of function, e.g. β-globin loss causing β-thalassemia — autosomal recessive. Gain of function, e.g. Hb Hammersmith causing Hb precipitation and cell lysis — autosomal dominant. Novel properties, e.g. HbS causing sickle cell disease — usually autosomal recessive.
- Population/carrier screening (detecting microcytic anaemia) identifies carriers before or during pregnancy, allowing informed reproductive choices (or prenatal diagnosis with termination as an option), which reduces the number of affected (major) births over time, as seen in the declining Mediterranean data after mandatory PMSGC programmes began (Cyprus 1973, Italy/Greece 1975). Ethical issue: screening results affect marriage and reproductive decisions, raising autonomy and informed consent concerns, especially given the large numbers of people and pregnancies potentially affected in developing countries.
- A β-globin gene mutation (DNA level) disrupts coding, promoter or splicing function, reducing/abolishing functional β-globin mRNA and protein (RNA/protein levels), which reduces HbA and shifts compensation toward HbA2/HbF; clinically this produces the anaemia and complications of thalassemia (phenotype level), which can be addressed by medical intervention (transfusion) or curative gene/cell-level intervention (bone marrow transplant, CRISPR editing of BCL11A to raise HbF). At the family/population level, carrier screening and genetic counselling identify carrier couples before conception, and premarital screening programmes translate this into a measurable decline in thalassemia major births over time, as seen in the Mediterranean screening data.