The ultimate corrective therapy for severe Hb abnormalities would be gene therapy. This might involve inserting normal Hb genes into the patient’s haemopoietic cells in vitro and then transplanting these cells back into the patient after ablative treatment had been given to remove the abnormal bone marrow. However, numerous problems remain to be overcome before gene therapy for Hb defects becomes a practical option.
METABOLIC DISORDERS OF THE RED CEll
Red cell metabolism
The mature red cell has no nucleus, mitochondria or ribosomes and is therefore unable to synthesize proteins. Red cells have only limited enzyme systems but they are of major importance in maintaining the viability and function of the cells. In particular, energy is required in the form of ATP for the maintenance of the flexibility of the membrane and the biconcave shape of the cells to allow passage through small vessels, and for regulation of the sodium and potassium pumps to ensure osmotic equilibrium. In addition, it is essential that Hb is maintained in the reduced state.
The enzyme systems esponsible for producing energy and reducing power are:
GLYCOLYTIC (EMBDEN-MEYERHOF) PATHWAY, in which glucose is metabolized to pyruvate and lactic acid with production of ATP
HEXOSE MONOPHOSPHATE (PENTOSEPHOSPHATE)
PATHWAY, which provides reducing power for the red cell in the form of NADPH About 90% of glucose is metabolized by the former and 10% by the latter. The importance of the hexose monophosphate shunt is that it maintains glutathione (GSH) in a reduced state. Glutathione is important in combating oxidative stress to the red cell, and failure of this mechanism may result in:
1 Rigidity due to cross-linking of spectrin, which decreases membrane flexibility and causes ‘leakiness’ of the red cell membrane 2 Oxidation of the Hb molecule, producing methaemoglobin and precipitation of globin chains as Heinz bodies localized on the inside of the membrane; these bodies are removed from circulating red cells by the spleen 2,3-DPG is formed from a side-arm of the glycolytic pathway. It binds to the central part of the Hb tetramer, fixing it in the low affinity state. A decreased affinity with a shift in the oxygen dissociation curve to the right enables more oxygen to be delivered to the tissues.
Glucose-6-phosphate dehydrogenase (G6PD) deficiency The enzyme G6PD holds a vital position in the hexose monophosphate shunt. G6PD deficiency is a common condition that presents with a haemolytic anaemia and affects millions of people throughout the world, particularly in Africa, around the Mediterranean, the Middle
East and South East Asia.
The gene for G6PD is sex-linked, being carried on the X chromosome. The deficiency therefore affects males. It is carried by females, who show half the normal levels of the enzyme and who have some protection against
There are over 400 structural types of G6PD. The commonest with normal activity are type B, which is present in almost all Caucasians and about 70% of Blacks, and type A, which is present in about 20% of Blacks. There are many variants with reduced activity but only two are common. In the African, or A type, the degree of deficiency is mild (enzyme activity about 10% of normal). Haemolysis is self-limiting as the young red cells newly produced by the bone marrow have nearly normal enzyme activity. However, in the Mediterranean type, both young and old red cells have very low enzyme activity. After an oxidant shock the Hb level may fall precipitously; death may follow unless the condition is recognized and the patient is transfused urgently.
• Acute drug-induced haemolysis
• Favism (ingestion of fava beans)
• Chronic haemolytic anaemia
• Neonatal jaundice
• Infections and acute illnesses will also precipitate haemolysis in patients with G6PD deficiency
The clinical features are due to rapid intravascular haephosphate DPG, diphosphoglycerate; F, fructose; G, glucose; GSH, reduced glutathione; (SSG, oxidized glutathione; P, phosphate; PG, phosphoglycerate. molysis with symptoms of anaemia, jaundice and haemoglobinuria.
BLOOD COUNT is normal between attacks.
DURING AN ATTACK the blood film may show irregularly contracted cells, bite cells (cells with an indentation of the membrane), blister cells (cells in which the Hb appears to have become partially detached from the cell membrane), Heinz bodies (best seen on films stained with methyl violet) and reticulocytosis.
HAEMOLYSIS is evident.
G6PD DEFICIENCY can be detected using several screening tests, such as demonstration of the decreased ability of G6PD-deficient cells to reduce dyes. The level of the enzyme may also be directly assayed.
• Any offending drugs should be stopped
• Underlying infection should be treated
• Blood transfusion may be life-saving
• Splenectomy is not usually helpful
Pyruvate kinase deficiency
This is the most common defect of red cell metabolism after G6PD deficiency, affecting thousands rather than millions of people. The site of the defect is shown in Fig. 6.21. There is reduced production of ATP causing rigid red cells. Homozygotes have haemolytic anaemia and splenomegaly. It is inherited as an autosomal recessive.
ANAEMIA of variable severity is present (Hb 5- 10 g dr ‘). The oxygen dissociation curve is shifted to the right as a result of the rise in intracellular 2,3-DPG (Fig. 6.21), and this reduces the severity of symptoms due to anaemia.
BLOOD FILM shows distorted (,prickle’) cells and a reticulocytosis.
PYRUVATE KINASE activity is low (affected homozygotes have levels of 5-20%).
Blood transfusions may be necessary during infections and pregnancy. Splenectomy may improve the clinical condition and is usually advised for patients requiring frequent transfusions.
In addition to G6PD and pyruvate kinase deficiencies, there are a number of rare enzyme deficiencies that need specialist investigation.