Haemoglobinopathy Diagnosis

Barbara J. Bain

• Éditeur: Wiley-Blackwell
• Année: 2020
• ISBN:
  9781119579991
• Édition: 3
• Sujet: Médecine
• Sous-sujet: Hématologie

An updated, essential guide for the laboratory diagnosis of haemoglobin disorders

This revised and updated third edition of Haemoglobinopathy Diagnosis offers a comprehensive review of the practical information needed for an understanding of the laboratory diagnosis of haemoglobin disorders. Written in a concise and approachable format, the book includes an overview of clinical and laboratory features of these disorders. The author focuses on the selection, performance, and interpretation of the tests that are offered by the majority of diagnostic laboratories. The book also explains when more specialist tests are required and explores what specialist referral centres will accomplish. The information on diagnosis is set in a clinical context.

The third edition is written by a leading haematologist with a reputation for educational excellence. Designed as a practical resource, the book is filled with illustrative examples and helpful questions that can aide in the retention of the material presented. Additionally, the author includes information on the most recent advances in the field. This important text:

• Contains a practical, highly illustrated, approach to the laboratory diagnosis of haemoglobin disorders

• Includes "test-yourself" questions and provides an indispensable tool for learning and teaching

• Presents new material on antenatal screening/prenatal diagnostic services

• Offers myriad self-assessment case studies that are ideal for the trainee

Written for trainees and residents in haematology, practicing haematologists, and laboratory scientists, Haemoglobinopathy Diagnosis is an essential reference and learning tool that provides a clear basis for understanding the diagnosis of haemoglobin disorders.

1
Haemoglobin and the genetics of haemoglobin synthesis

Haemoglobins and their structure and function

The haemoglobin molecule contained within red blood cells is essential for human life, being the means by which oxygen is transported to the tissues. Other functions include the transport of carbon dioxide (CO₂) and a buffering action (reduction of the changes in pH that would otherwise be expected when an acid or an alkali enters or is generated in a red cell). A normal haemoglobin molecule has a molecular weight of 64–64.5 kDa and is composed of two dissimilar pairs of polypeptide chains, each of which encloses an iron‐containing porphyrin designated haem (Fig. 1.1). Haem is essential for oxygen transport while globin serves to protect haem from oxidation, renders the molecule soluble and permits variation in oxygen affinity. The structure of the haemoglobin molecule produces an internal environment of hydrophobic radicals, which protects the iron of haem from water and thus from oxidation. External radicals are hydrophilic and thus render the haemoglobin molecule soluble. Both haem and globin are subject to modification. The iron of haemoglobin is normally in the ferrous form (Fe²⁺). Haem is able to combine reversibly with oxygen so that haemoglobin can function as an oxygen‐transporting protein. Oxidation of iron to the ferric form (Fe³⁺) is a less readily reversible reaction, converting haem to haematin and haemoglobin to methaemoglobin, a form of haemoglobin that cannot transport oxygen. Auto‐oxidation of haemoglobin to methaemoglobin is a normal process. About 3% of haemoglobin undergoes this process each day with about 1% (0.4–1% in one study) of haemoglobin being methaemoglobin [1, 2]. Methaemoglobin is converted back to haemoglobin mainly by the action of the enzyme NADH‐cytochrome b5‐methaemoglobin reductase.

The haemoglobin molecule can also combine with CO₂ and is responsible for about 10% of CO₂ transport from the tissues to the lungs; transport is by reversible carbamation of the N‐terminal groups of the α chains of haemoglobin. Because carbamated haemoglobin has a lower oxygen affinity than the non‐carbamated form, binding of the CO₂ produced by the metabolic processes in tissues facilitates oxygen delivery to tissues. In addition, non‐oxygenated haemoglobin can carry more CO₂ than oxygenated haemoglobin so that unloading of oxygen to the tissues facilities the uptake and transport of CO₂. Because of its buffering action (mopping up of protons, H⁺), haemoglobin also contributes to keeping CO₂ in the soluble bicarbonate form and thus transportable. The reaction CO₂ + H₂O → HCO₃⁻ + H⁺ is facilitated.

Haemoglobin also has a role in nitric oxide (NO) transport and metabolism, being both a scavenger of NO and an active transporter. NO is produced in endothelial cells and neutrophils by the action of nitric acid synthase [2–5]. It has a very high affinity for oxyhaemoglobin so that blood levels are a balance between production and removal by binding to oxyhaemoglobin. NO is a potent vasodilator, but this effect is limited by its binding to haemoglobin. The iron atom of a haem group of oxyhaemoglobin (preferentially the haem enclosed in the haem pocket of an α chain), binds NO. A haemoglobin molecule with NO bound to two haem groups strikingly favours the deoxy conformation so oxygen is readily released. NO–haemoglobin is subsequently converted to methaemoglobin with release of NO and production of nitrate ions, which are excreted. Since deoxyhaemoglobin has a much lower affinity for NO, hypoxic conditions could leave more NO free and lead to vasodilation, which is of potential physiological benefit. In addition, deoxyhaemoglobin can convert nitrite to NO, again favouring vasodilation.

Fig. 1.1 Diagrammatic representation of the quaternary structure of haemoglobin and the tertiary structure of a haemoglobin monomer (a β globin chain containing a haem group). Upper case letters indicate homologous α helixes.

Nitric oxide also causes S‐nitrosylation of a conserved cysteine residue (Cys93, E15) of the β globin chain of oxyhaemoglobin to form S‐nitrosohaemoglobin. This occurs in the lungs. In this circumstance, the bioactivity of NO may be retained with NO being delivered to low molecular weight thiol‐containing molecules to reach target cells such as the smooth muscle of blood vessels. Oxygenation of haemoglobin favours S‐nitrosylation. Conversely, deoxygenation favours release of NO. This may be an important physiological process with NO ...