Hemoglobinopathies Flashcards
What are hemoglobinopathies
• Globin chain production (e.g. thalassaemia)
• Structure of the globin chain (e.g. sickle cell disease)
Combined defects of globin chain production and structure, e.g. sickle cell B-thalassaemia.
What are thalassemias
The thalassaemias affect people throughout the world (Fig. 8.21). Normally, there is balanced (1:1) production of o and B chains. The defective synthesis of globin chains in thalassaemia leads to ‘imbalanced’ globin chain production, leading to precipitation of globin chains within the red cell precursors and resulting in ineffective erythropoiesis. Precipitation of globin chains in mature red cells leads to haemolysis.
B-Thalassaemia
In homozygous B-thalassaemia, either no normal B chains are produced (B) or B-chain production is very reduced (B*. There is an excess of a chains, which precipitate in erythroblasts and red cells causing ineffective erythropoiesis and haemolysis. The excess & chains combine with whatever B, & and y chains are produced, resulting in increased quantities of HbA2 and HbF and, at best, small amounts of HbA. In heterozygous B-thalassaemia there is usually symptomless microcytosis with or without mild anaemia. Table 8.10 shows the findings in the homozygote and heterozygote for the common types of B-thalassaemia.
Molecular genetics
The molecular errors accounting for over 200 genetic defects leading to B-thalassaemia have been characterized. Unlike in o-thalassaemia, the defects are mainly point mutations rather than gene deletions. The mutations result in defects in transcription, RNA splicing and modification, translation via frame shifts and nonsense codons producing highly unstable B-globin, which cannot be utilized.
What are the divisions of beta thalassemia
Thalassaemia minor (or trait), the symptomless heterozygous carrier state
• Thalassaemia intermedia, a moderate anaemia, not requiring regular transfusions (with a number of different genotypes)
• Thalassemia major (generally homozygous
B-thalassaemia), severe anemia requiring regular transfusions.
What is thalassemia minor
This common carrier state (heterozygous B-thalassaemia) is asymptomatic. Anaemia is mild or absent. The red cells are hypochromic and microcytic with a low MCV and MCH, and it may be confused with iron deficiency. However, the two are easily distinguished, as in thalassemia trait the serum ferritin and the iron stores are normal (Table 8.2). The RDW is usually normal (see p. 373). Hb electrophoresis usually shows a raised HbA2 and often a raised HbF (Fig. 8.22). Iron should not be given to these patients unless they also have proven coincidental iron deficiency.
What is thalassemia intermedia
Thalassemia intermedia includes patients who are symptomatic with moderate anaemia (Hb 70-100 g/L), who do not require regular transfusions.
Thalassaemia intermedia may be due to a combination of homozygous mild B+- and a-thalassaemia, where there is reduced o-chain precipitation and less ineffective erythropoi-esis and haemolysis. The inheritance of hereditary persistence of HbF with homozygous B-thalassaemia also results in a milder clinical picture than unmodified B-thalassaemia major because the excess a chains are partially removed by the increased production of y chains.
Patients may have splenomegaly and bone deformities.
Recurrent leg ulcers, gallstones and infections are also seen.
It should be noted that these patients may be iron overloaded despite a lack of regular blood transfusions. This is caused by excessive iron absorption which results from the underlying dyserythropoiesis
What is thalassemia major (Cooley’s anaemia)
Most children affected by homozygous B-thalassaemia present during the first year of life with:
• Failure to thrive and recurrent bacterial infections
• Severe anaemia from 3 to 6 months when the switch from y- to B-chain production should normally occur
• Extramedullary haemopoiesis that soon leads to hepatosplenomegaly and bone expansion, giving rise to the classical thalassaemic facies (Fig. 8.23a).
Skull X-rays in these children show the characteristic ‘hair on end’ appearance of bony trabeculation as a result of expansion of the bone marrow into cortical bone (Fig. 8.23b).
The expansion of the bone marrow is also shown in an X-ray of the hand (Fig. 8.23c).
The classic features of untreated thalassemia major are generally only observed in patients from countries without good blood transfusion support.
How is beta-thalassemia managed
The aims of treatment are to suppress ineffective erythropoi-esis, prevent bony deformities and allow normal activity and development.
• Long-term folic acid supplements are required.
• Regular transfusions should be given to keep the Hb above 100 g/L. Blood transfusions may be required every 4-6 weeks.
• If transfusion requirements increase, splenectomy may help, although this is usually delayed until after the age of 6 years because of the risk of infection. Prophylaxis against infection is required for patients undergoing splenectomy (see p. 406).
• Iron everload caused by repeated transfusions (transfusion haemosiderosis) may lead to damage to the endocrine glands, liver, pancreas and the myocardium by the time patients reach adolescence. Magnetic resonance imaging (myocardial T2- relaxation time) is useful for monitoring iron overload in thalassemia; both the heart and the liver can be monitored. The standard iron-chelating agent remains desferrioxamine, although it has to be administered parenterally. Desferrioxamine is given as an overnight subcutaneous infusion on 5-7 nights each week. Ascorbic acid 200 mg daily is given, as it increases the urinary excretion of iron in response to desferrioxamine. Often young children have a very high standard of chelation as it is organized by their parents. However, when the children become adults and take on this role themselves they often rebel and chelation with desferrioxamine may become problematic. Deferiprone, an oral iron chelator, has been available for some years, and results on a new once-daily oral iron chelator, deferasirox, indicate that it is safe, similar in effectiveness to desterrioxamine and is being increasingly used.
• Intensive treatment with desferrioxamine has been reported to reverse damage to the heart in patients with severe iron overload, but excessive doses of desferrioxamine may cause cataracts, retinal damage and nerve deafness. Infection with Yersinia enterocolitica occurs in iron-loaded patients treated with desterrioxamine. Iron overload should be periodically assessed by measuring the serum ferritin and by assessment of hepatic iron stores by MRI.
• Bone marrow transplantation has been used in young patients with HLA-matched siblings. It has been successful in patients in good clinical condition with a
3-year mortality of <5%, but there is a high mortality (>50% in patients in poor condition with iron overload and liver dysfunction.
• Prenatal diagnosis and gene therapy are discussed on page 43.
• Patients’ partners should be tested. If both partners have B-thalassaemia trait, there is a one in four chance of such pregnancy resulting in a child having B-thalassaemia major. Therefore, couples in this situation must be offered prenatal diagnosis
What is the molecular genetics of alpha-thalassemia
In contrast to $-thalassaemia, a-thalassaemia is often caused by gene deletions, although mutations of the a-globin genes may also occur. The gene for a-globin chains is duplicated on both chromosomes 16, i.e. a normal person has a total of four a-globin genes. Deletion of one o-chain gene (at) or both a-chain genes (a0) on each chromosome 16 may occur (Table 8.11). The former is the most common of these abnormalities.
• Four-gene deletion (deletion of both genes on both chromosomes); there is no a-chain synthesis and only Hb Barts (4) is present. Hb Barts cannot carry oxygen and is incompatible with life (Table 8.9 and Table 8.11).
Infants are either stillborn at 28-40 weeks or die very shortly after birth. They are pale, oedematous and have enormous livers and spleens - a condition called hydrops fetalis.
• Three-gene deletion; HbH disease, which is common in parts of Asia, has four B chains with low levels of HbA and Hb Barts. HAz is normal or reduced. HbH does not transport oxygen and precipitates in erythroblasts and erythrocytes. There is moderate anaemia (Hb
70-100 g/L) and splenomegaly (thalassaemia intermedia). The patients are not usually transfusion-dependent.
• Two-gene deletion (-thalassaemia trait); there is microcytosis with or without mild anaemia. HbH bodies may be seen on staining a blood film with brilliant cresyl blue.
• One-gene deletion; the blood picture is usually normal.
Globin chain synthesis studies for the detection of a reduced ratio of a to 3 chains may be necessary for the definitive diagnosis of a-thalassaemia trait.
Less commonly, a-thalassaemia may result from genetic defects other than deletions, for example mutations in the stop codon producing an a chain with many extra amino acids (Hb Constant Spring). It has a more severe clinical course than HbH with severe anaemia often precipitated by infection.
What are sickle syndromes
Sickle cell haemoglobin (HbS) results from a single-base mutation of adenine to thymine, which produces a substitution of valine for glutamic acid at the sixth codon of the B-globin chain (a2ß26lu-sval). In the homozygous state (sickle cell anaemia), both genes are abnormal (HbSS), whereas in the heterozygous state (sickle cell trait, HbAS) only one chromosome carries the gene. As the synthesis of HbF is normal, the disease usually does not manifest itself until the HbF decreases to adult levels at about 6 months of age.
The sickle gene is commonest in Africans (up to 25% gene frequency in some populations) but is also found in India, the Middle East and Southern Europe.
What is the pathogenesis of sickle syndromes
Deoxygenated HbS molecules are insoluble and polymerize.
The flexibility of the cells is decreased and they become rigid and take up their characteristic sickle appearance (Fig. 8.24). This process is initially reversible but, with repeated sickling, the cells eventually lose their membrane flexibility and become irreversibly sickled. This is due to dehydration, partly caused by potassium leaving the red cells via calcium activated potassium channels called the Gados channel. These irreversibly sickled cells are dehydrated and dense and will not return to normal when oxygenated. Sick-ling can produce:
impaired passage of cells through the microcirculation, leading to obstruction of small vessels and tissue infarction.
Sickling is precipitated by infection, dehydration, cold, acidosis or hypoxia. In many cases, the cause is unknown, but adhesion proteins on activated endothelial cells (VCAM-
1) may play a causal role, particularly in vaso-occlusion when rigid cells are trapped, facilitating polymerization. HbS releases its oxygen to the tissues more easily than does normal Hb, and patients therefore feel well despite being anaemic (except of course during crises or complications).
Depending on the type of haemoglobin chain combina-tions, three clinical syndromes occur:
• Homozygous HbSS have the most severe disease
Combined heterozygosity (HbSC) for HbS and C (see below) who suffer intermediate symptoms
• Heterozygous HbAS (sickle cell trait) have no symptoms (see p. 395).
What are some clinical features of sickle cell anaemia
Vaso-occlusive crises
An early presentation may be acute pain in the hands and feet (dactylitis) owing to vaso-occlusion of the small vessels.
Severe pain in other bones, e.g. femur, humerus, vertebrae, ribs, pelvis, occurs in older children/adults. These attacks vary greatly in frequency from patient to patient and sometimes in the same patient from year to year; however, as a generalization, a patient with moderately severe sickle cell anaemia may have around three hospital admissions a year from painful vaso-occlusive crises. Fever often accompanies the pain.
Pulmonary hypertension
Pulmonary hypertension is a known consequence of sickle cell anaemia, occurring in 30-40% of patients, and is associated with an increased mortality. The ‘hyperhaemolytic para-digm’ (HHP) proposes that haemolysis in sickle cell disease leads to increased cell-free plasma Hb, which consumes NO, leading to a state of NO deficiency, endothelial dysfunction and a high prevalence of pulmonary hypertension. The basis of the HHP has recently been questioned.
Acute chest syndrome
This occurs in up to 30%, and pulmonary hypertension and chronic lung disease are the commonest causes of death in adults with sickle cell disease. The acute chest syndrome is caused by infection, fat embolism from necrotic bone marrow or pulmonary infarction due to sequestration of sickle cells.
It comprises shortness of breath, chest pain, hypoxia and new chest X-ray changes due to consolidation. The presentation may be gradual or very rapid, leading to death in a few hours. Management is with pain relief, high-flow supplemental oxygen and antibiotics (cefotaxime and clarithromycin), which should be started immediately. Exchange transfusion will reduce the amount of HbS to <20% if there is no improve-ment. Ventilation (CAP) may be necessary. Infections can be due to Chlamydia and mycoplasma, as well as Streptococcus pneumoniae.
Anaemia
Chronic haemolysis produces a stable haemoglobin level, usually in the 60-80 g/L range, but an acute fall in the hae-moglobin level can occur owing to:
• Splenic sequestration
• Bone marrow aplasia
• Further haemolysis due to drugs, acute infection or associated G6PD deficiency.
Splenic sequestration
Vaso-occlusion produces an acute painful enlargement of the spleen. There is splenic pooling of red cells and hypo-volaemia, leading in some to circulatory collapse and death.
The condition occurs in childhood before multiple infarctions have occurred. The latter eventually leads to a fibrotic non-functioning spleen. Liver sequestration can also occur.
Bone marrow aplasia
This most commonly occurs following infection with erythro-virus Big, which invades proliferating erythroid progenitors.
There is a rapid fall in haemoglobin with no reticulocytes in the peripheral blood, because of the failure of erythropoiesis in the marrow.
What are some long-term problems of sickle cell anaemia
Growth and development. Young children are short but regain their height by adulthood. However, they remain below the normal weight. There is often delayed sexual maturation, which may require hormone therapy.
Bones are a common site for vaso-occlusive episodes, leading to chronic infarcts. Avascular necrosis of hips, shoul-ders, compression of vertebrae and shortening of bones in the hands and feet occur. These episodes are the common cause for the painful crisis. Osteomyelitis is commoner in sickle cell disease and is caused by Staphylococcus aureus, Staph. pneumoniae and salmonella (see p. 534). Occasion-ally, hip joint replacement may be required.
Infections are common in tissues susceptible to vasooc-clusion, e.g. bones, lungs, kidneys.
Leg ulcers occur spontaneously (vaso-occlusive ep-sodes) or following trauma and are usually over the medial or lateral malleoli. They often become infected and are quite resistant to treatment, sometimes blood transfusion may facilitate ulcer healing.
Cardiac problems occur, with cardiomegaly, arrhythmias and iron overload cardiomyopathy. Myocardial infarctions occur due to thrombotic episodes which are not secondary to atheroma.
Neurological complications occur in 25% of patients, with transient ischaemic attacks, fits, cerebral infarction, cerebral haemorrhage and coma. Strokes occur in about 11% of patients under 20 years of age. The most common finding is obstruction of a distal intracranial internal carotid artery or a proximal middle cerebral artery. About 10% of children without neurological signs or symptoms have abnormal blood-flow velocity indicative of clinically significant arterial stenosis; such patients have very high risk of stroke. It has now been demonstrated that if children with stenotic cranial artery lesions, as demonstrated on transcranial Doppler ultrasonography, are maintained on a regular programme of transfusion that is designed to suppress erythropoiesis so that no more than 30% of the circulating red cells are their own, about 90% of strokes in such children could be prevented.
Cholelithiasis. Pigment stones occur as a result of chronic haemolysis.
Liver. Chronic hepatomegaly and liver dysfunction are caused by trapping of sickle cells.
Renal. Chronic tubulointerstitial nephritis occurs (see p. 596).
Priapism. An unwanted painful erection occurs from vaso-occlusion and can be recurrent. This may result in impotence.
Treatment is with an a-adrenergic blocking drug, analgesia and hydration.
Eye. Background retinopathy, proliferative retinopathy, vitreous haemorrhages and retinal detachments all occur.
Regular yearly eye checks are required.
Pregnancy. Impaired placental blood flow causes spontaneous abortion, intrauterine growth retardation, preeclampsia and fetal death. Painful episodes, infections and severe anaemia occur in the mother.
What are some investigations to make when sickle cell anaemia is suspected
Blood count. The level of Hb is in the range 60-80 g/L with a high reticulocyte count (10-20%).
• Blood films can show features of hyposplenism (see
Fig. 8.29) and sickling (Fig. 8.24).
• Sickle solubility test. A mixture of HbS in a reducing solution such as sodium dithionite gives a turbid appearance because of precipitation of HbS, whereas normal Hb gives a clear solution. A number of commercial kits such as Sickledex are available for rapid screening for the presence of HbS, e.g. before surgery in appropriate ethnic groups and in the A&E department.
• Hb electrophoresis (Fig. 8.22) is always needed to confirm the diagnosis. There is no HbA, 80-95% HbSS and 2-20% HbF.
• The parents of the affected child will show features of sickle cell trait.
How is sickle cell anaemia managed
Precipitating factors (see above) should be avoided or treated quickly. The complications requiring inpatient management are shown in Table 8.12.
Acute painful attacks require supportive therapy with intravenous fluids, and adequate analgesia. Oxygen and antibiotics are only given if specifically indicated. Crises can be extremely painful and require strong, usually narcotic, analgesia. Morphine is the drug of choice. Milder pain can sometimes be relieved by codeine, paracetamol and NSAIDs (Box 8.1).
Prophylaxis is with penicillin 500 mg daily and vaccination with polyvalent pneumococcal and Haemophilus influenza type b vaccine (see p. 406). Folic acid is given to all patients with haemolysis.
Anaemia
Blood transfusions should only be given for clear indications.
Patients with steady state anaemia, those having minor surgery or having painful episodes without complications should not be transfused. Transfusions should be given for heart failure, TIAs, strokes, acute chest syndrome, acute splenic sequestration and aplastic crises. Before elective operations and during pregnancy, repeated transfusions may be used to reduce the proportion of circulating HbS to <20% to prevent sickling. Exchange transfusions may be necessary in patients with severe or recurrent crises, or before emergency surgery. Transfusion and splenectomy may be lifesaving for young children with splenic sequestration. A full blood crossmatching compatibility screen should always be performed.
Hydroxycarbamide (hydroxyurea) is the first drug which has been widely used as therapy for sickle cell anaemia. It acts, at least in part, by increasing HbF concentrations.
Hydroxycarbamide has been shown in trials to reduce the episodes of pain, the acute chest syndrome, and the need for blood transtusions.
Inhaled nitric oxide is a new approach to the treatment of painful crises in sickle cell anemia based on the hyper-haemolytic paradigm discussed briefly above. However, it is yet to become an established therapy based on randomized controlled trials.
Stem cell transplantation has been used to treat sickle cell anaemia although in fewer numbers than for thalassae-mia. Children and adolescents younger than 16 years of age who have severe complications (strokes, recurrent chest syndrome or refractory pain) and have an HLA-matched donor are the best candidates for transplantation.
Counselling
A multidisciplinary team should be involved, with regular clinic appointments to build up relationships. Adolescents require careful counselling over psychosocial issues, drug and birth control.
What is the prognosis for sickle cell anaemia
Some patients with HbSS die in the first few years of life from either infection or episodes of sequestration. However, there is marked individual variation in the severity of the disease and some patients have a relatively normal lifespan with few complications.
