Genetic Disorders of Haemoglobin: Sickle Cell Anaemia and Thalassemia

Each red cell contains approximately 640 million haemoglobin (Hb) molecules.
Haemoglobin consists of two pairs of polypeptide chains known as globin chains.
Each globin chain is folded around a pocket that contains a haem molecule.
Each haemoglobin can carry 4 molecules of oxygen.
Thus, each red blood cell transports approximately 2.56 billion molecules of oxygen (calculated as $4 imes 640 ext{ million}$ ).

Structure includes arrangements of globin proteins, specifically notably being haem-globin complexes.

Chemical structure characteristics:
- Hem includes several chemical groups including: H3C, H₂C configurations.
Essential components:
- Iron (Fe) is crucial for oxygen binding.

Haem Synthesis process involves:
  - Location: Mitochondria and Cytoplasm.
  - Starting Materials:
    - Succinyl-CoA + Glycine leads to the formation of Heme.
    - Production of Protoporphyrin IX and Protoporphyrinogen III.
Key steps:
- ALA (Amino Levulinic Acid) contributes to the synthesis pathway.
- Hydroxymethylbilane intermediary product forms before final heme creation.

Processes involved in breaking down haem include:
  - Enzymes:
    - Heme oxygenase that catalyzes the conversion of haem to biliverdin.
  - Biliverdin is subsequently converted to bilrubin through biliverdin reductase.
  - Iron from heme enables storage as Ferritin post-cathelation.

Globin Chains:
  - The α and ζ chains, totaling 141 amino acids, are located on chromosome 16.
  - The β, γ, δ and ε chains have 146 amino acids on chromosome 11.
  - 65% of Hb is synthesized during the erythroblast stage, while 35% occurs at the reticulocyte stage.
Profile of Globin Genes on Chromosomes:
- Chromosome 11 contains β globin genes, and Chromosome 16 contains α globin gene.

Sickle Cell Disease is caused by an inherited abnormality in haemoglobin structure, where HbS substitute valine for glutamic acid at position 6 of the β chain.
Clinical Features can affect various organs in the body, presenting a broad range of symptoms.
Nomenclature of SCD:
  - Sickle Cell Trait: Represented as AS, a heterozygous condition combining both normal and sickle hemoglobin.
  - Homozygous Disease: Represented as SS, where both genes inherit sickle hemoglobin.
  - Other variations include compound heterozygous conditions like HbSC and HbS/β-thalassemia.

Overview of variants includes:
  - HbC: Lysine substitutes glutamic acid.
  - HbD, HbO Arab, HbE: Other point mutations in β chains causing differing clinical severity.
  - HbS genotype nomenclature maps to corresponding clinical severities.

Irreversibly Sickled Cells (ISC) are critical pathologic markers.
Key effects of sickled RBCs:
- Increased mechanical fragility leads to hemolytic anemia.
- Viscosity elevated leads to vascular issues, blockage of microvasculature, and resultant tissue infarctions.

Reduced clinical disability associated with Sickle Cell Trait (HbAS), though vaso-occlusive crises can occasionally induce complications under stress conditions.
HbSC Disease presents variably; normal hemoglobin levels can lead to lower morbidity yet significant crises.
SCA (HbSS) clinical onset is generally after 6 months, with symptoms like anemia, jaundice, hand-foot syndrome, and autosplenectomy.

Overview of organ impacts includes:
  - CNS: Risk of stroke, cranial nerve issues.
  - Cardiac: Potential heart failure, cardiomegaly.
  - Hepatic: Risk of jaundice from hemolysis, gallstones.
  - Renal: Failure and inability to concentrate urine among other issues.

Thalassemia characterized by reduced or absent production of globin chains, impacting oxygen transport functionality.
Classification: Includes α, β, and other forms reflective of the specific globin type affected.
Inheritance Patterns: Predominantly Mendelian recessive traits influencing offspring from carrier parents to severe phenotypic expressions.

Involved in production of Hb Bart's (γ4 tetramers) and HbH (β4 tetramers) when there is an excess of β chains.
- Phenotypes range from silent carriers to major disease states.

Results in an excess of α chains due to insufficient β chain production, leading to severe anemia and hemolysis.
Clinical presentations vary based on genotype and timely transfusions, while also managing complications like iron overload due to prolonged transfusion therapy.

Diagnosis involves observing hematological parameters and responses to treatments. A regular blood transfusion schedule, managing iron overload, and preventative measures against infections are critical components of patient management.