Thalassaemias Lecture Notes

Overview

• Thalassaemias are hereditary blood disorders resulting from mutations in the genes responsible for hemoglobin production. They follow an autosomal recessive inheritance pattern, meaning that both parents must carry a copy of the mutated gene for a child to be affected.

• These disorders are characterized by a decreased or absent synthesis of either the alpha (α) or beta (β) polypeptide chains of hemoglobin, leading to ineffective erythropoiesis and resulting in a range of symptoms from mild to severe anemia.

Geographic Distribution

• Historically, thalassaemia genes have been prevalent in tropical and subtropical regions, where malaria was also endemic. This geographic overlap was particularly evident in the 1940s, prior to the eradication of malaria in southern Europe.

• The prevalence of thalassaemias in areas such as the Mediterranean, Middle East, and Southeast Asia is thought to be a result of natural selection, as carriers of the thalassaemia trait exhibit some protection against malaria.

Types of Thalassaemia

• Thalassaemias are classified into two main types: α-Thalassaemia and β-Thalassaemia, each with distinct genetic mechanisms and clinical features.

α-Thalassaemia

• α-Thalassaemia usually arises from gene deletions, which can vary in number and severity. The normal human genetic configuration consists of 4 copies of the alpha gene (two from each parent located on chromosome 16).

Clinical Forms and Gene Deletions

1. 4 Gene deletions:

   • Incompatible with life, leading to death in utero (Hydrops Fetalis).

   • Associated with the formation of Barts haemoglobin (Hb g_4), which is unable to carry oxygen effectively. This condition is predominantly found in Southeast Asia and parts of the Mediterranean region.

2. 3-Alpha gene deletions:

   • Result in a condition known as Hemoglobin H disease, manifesting as moderately severe anemia.

   • Patients typically have elevated levels of Hemoglobin H (b_4) and may develop splenomegaly and hepatosplenomegaly as their bodies attempt to compensate for the ineffective erythropoiesis.

Gene Arrangements

• The α-like gene family is located on chromosome 16, while the ß-like gene family resides on chromosome 11. Misalignment of α-globin genes during meiosis can result in gene deletions, such as a notable -α 4.2 kb deletion, which contributes to α-Thalassaemia.

Hb Constant Springs

• This condition arises from a point mutation in the stop codon of the alpha chain, which results in an extended protein that includes an extra 31 amino acids at the C-terminal end. This leads to low production rates of the alpha chain, contributing to the clinical manifestations of the disorder.

β-Thalassaemia

Classification

Genetic Lesions

• Compared to α-Thalassaemias, β-Thalassaemia is characterized by genetic mutations that are more localized to substitutions or deletions.

1. Gene Deletion:

   • For instance, a common mutation of Indian origin involves the removal of 600 base pairs from the 3’ end of the β-globin gene, which severely disrupts hemoglobin production.

2. Transcriptional Mutations:

   • Single base substitutions can occur in essential promoter regions, such as CAAT and TATA boxes, resulting in modest reductions in α-globin chain production and leading to a milder phenotype known as β+ thalassaemia.

3. Splicing:

   • A notable point mutation observed in IVS1 G-> A creates an alternative splice site, which disrupts normal RNA splicing. This can lead to either complete absence of β-globin or reduced production.

4. Non-Functional RNA:

   • β o Thalassaemia is frequently seen in Sardinia. It occurs due to substitution of cytosine with thymine at codon 39, introducing a premature stop codon in mRNA, which leads to an absence of functional β-globin production.

Haemoglobin Development

• The development of hemoglobin involves various genes located on chromosomes, specifically:

  • Embryonic Hb Genes located at 16p13.3 include genes such as $ε$, $Ψε1$, $μ$, $Ψα$, $α2$, $α1$, and $θ$.

  • Fetal hemoglobin genes are active during fetal development, and after birth, adult Hb genes located at 11p15.5 take over, including $ε$, $Gγ$, $Aγ$, $Ψβ$, $δ$, and $β$.

• Hemoglobin production transitions from yolk sak-derived embryonic Hb (like Gower and Portland) to fetal Hb (HbF: $α2γ2$) and eventually to adult Hb (HbA: $α2β2$) produced in the liver and bone marrow.

Diagnosis

• Diagnosis of thalassaemia is multifaceted. Common symptoms include: Hemolytic anemia due to ineffective erythropoiesis, characterized by the presence of hypochromic and microcytic red blood cells (MCV < 70 fl).

• Iron studies reveal normal ferritin levels, which helps in differentiating thalassaemia from iron deficiency anemia.

• Comprehensive diagnostic techniques include:

  • HPLC or Electrophoresis: To identify abnormal hemoglobin variants such as Hb Bart's or Hb H, with normal HbA2 levels being raised in β-Thalassaemia minor.

  • GAP-PCR: Useful in detecting large deletions by exploiting the amplification of sequences that become closely spaced as a result of gene deletion.

  • MLPA: Multiplex ligation-dependent probe amplification is a refined technique capable of identifying major deletions in the α- and β-globin genes.

Prevention

• Genetic counseling plays a crucial role in managing the risk of thalassaemia in at-risk populations. This involves testing potential parents for carrier status to assess the risk of offspring inheriting severe forms of thalassaemia, such as Hydrops Fetalis or Hemoglobin H disease.

• Antenatal screening programs can assist in early diagnosis and management of affected pregnancies.

Treatment

• Asymptomatic carriers typically require no treatment; however, iron supplementation is indicated if iron deficiency is also present.

• For individuals with Thalassaemia Intermedia, treatment may involve blood transfusions to manage symptoms related to growth impairment and skeletal deformities.

• Individuals with Thalassaemia Major require regular hypertransfusion therapy to maintain hemoglobin levels above 95g/L, alongside iron chelation therapy to prevent the complications of iron overload.

• A multidisciplinary approach involving specialized nursing, cardiologists, and genetic counselors is critical for effective management of the condition.

Potential Cure

• A potential cure for thalassaemia is hematopoietic stem cell transplantation, which is highly dependent on HLA matching. An identical twin represents the ideal match, but unrelated matched donors can serve as a viable alternative. Efforts to use virus-specific T-cells aim to prevent cellular rejection during the transplantation process.

Summary

• Thalassaemias represent a significant public health challenge characterized by disorders of globin synthesis due to various genetic anomalies. α-Thalassaemia primarily involves gene deletions, while β-Thalassaemia is marked by more complex genetic lesions and mutations.

• While treatment options are available, ongoing research is aimed at developing more effective therapies and potential cures, with encouraging progress being made.