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Prepublished online as a Blood First Edition Paper on September 12, 2002; DOI 10.1182/blood-2002-07-1975.

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Blood, 1 February 2003, Vol. 101, No. 3, pp. 791-800

REVIEW ARTICLE

Hemoglobin H disease: not necessarily a benign disorder

David H. K. Chui, Suthat Fucharoen, and Vivian Chan

From the Department of Pathology and Molecular Medicine, McMaster University Faculty of Health Sciences, Hamilton, ON, Canada; Thalassemia Research Center, Institute of Science and Technology for Research and Development, Mahidol University, Salaya Campus, Puttamonthon, Nakornpathom, Thailand; and University Department of Medicine, University of Hong Kong and Queen Mary Hospital, Hong Kong, China.


    Introduction
Top
Introduction
alpha -Thalassemia mutations
Genotypes of Hb H...
Pathophysiology
Clinical and laboratory...
Alpha-thalassemia/mental...
Acquired Hb H disease...
Diagnosis
Treatment
Genetic counseling
Summary
References

Introduction

The hemoglobin molecule is a tetramer consisting of 2 pairs of globin chains, each of which contains a heme group. During fetal development, the major hemoglobin is Hb F (alpha 2gamma 2). In a normal adult, the major hemoglobin is Hb A (alpha 2beta 2). The alpha -globin gene cluster is located on chromosome 16pter-p13.3 and is made up of one embryonic zeta -globin and 2 alpha -globin genes in tandem (in cis): 5'-zeta 2-alpha 2-alpha 1-3'.1 Because each person has 2 chromosomes 16, most people have 4 alpha -globin genes. The beta -globin gene cluster, 5'-epsilon -Ggamma -Agamma -delta -beta -3' is located on chromosome 11p15.5.

alpha -Thalassemia is caused by mutations or deletions affecting either one or more alpha -globin genes, leading to decreased or absent alpha -globin chain production from the affected gene(s).2 The deletion or inactivation of only one alpha -globin gene usually results in insignificant hematologic findings. When 2 alpha -globin genes are deleted or inactivated, either both on the same chromosome 16 (in cis) or one on each of the 2 chromosomes 16 (in trans), the affected person is well but has borderline anemia, as well as microcytic and hypochromic red blood cells.3

When 3 alpha -globin genes become inactive because of deletions with or without concomitant nondeletional mutations, the affected individual would have only one functional alpha -globin gene. These people usually have moderate anemia and marked microcytosis and hypochromia. In affected adults, there is an excess of beta -globin chains within their erythrocytes that will form beta 4 tetramer, also known as Hb H. This hereditary disorder is known as Hb H disease.3

The most severe form of alpha -thalassemia is that of fetuses lacking all alpha -globin genes. Some succumb early in gestation. Most develop hydrops fetalis syndrome and die in utero during the second or the third trimester of pregnancy, or shortly after birth.3-5


    alpha -Thalassemia mutations
Top
Introduction
alpha -Thalassemia mutations
Genotypes of Hb H...
Pathophysiology
Clinical and laboratory...
Alpha-thalassemia/mental...
Acquired Hb H disease...
Diagnosis
Treatment
Genetic counseling
Summary
References

Deletional mutations

Deletions removing one alpha -globin gene, such as the rightward (-alpha 3.7) or leftward (-alpha 4.2) single alpha -globin gene deletions, are results of misalignment crossovers during meiosis (Figure 1). These deletions are very common and are found in 30% of African Americans and up to 60% to 80% of people living in parts of Saudi Arabia, India, Thailand, Papua New Guinea, and Melanesia.6,7 These single alpha -globin gene deletions and other point mutations involving a single alpha -globin gene (see "Nondeletional mutations") are known as the alpha +-thalassemia mutations.2


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Figure 1. Common deletions in the zeta -alpha -globin gene cluster. The 3 red boxes represent the 3 active globin genes, the one embryonic zeta 2-globin gene, and the alpha 2- and alpha 1-globin genes. They span approximately 26 Kb in length from zeta 2 to alpha 1 globin genes. The blue lines represent the common alpha -thalassemia deletions. Figure adapted with permission from Cambridge University Press2 and Blood.5

There are more than 20 known natural deletions that remove both alpha -globin genes on the same chromosome 16 (in cis) or the complete zeta -alpha -globin gene cluster, and they are known as the alpha 0-thalassemia mutations.2 Some deletions might measure 100 to 300 kb or more in length. In addition, there are rare deletions that silence alpha -globin gene expression by removing the HS-40 regulatory sequences upstream of the zeta -alpha -globin gene cluster.2

The (--SEA) type of alpha 0-thalassemia deletion of approximately 19.3 kb in length removing both alpha -globin genes in cis but sparing the embryonic zeta -globin gene is common in Southeast Asia. It is found in 14% of people living in Northern Thailand, for example.5 This mutation is the most common cause for Hb H disease (Table 1) and hydrops fetalis syndrome in that part of the world. In addition, the (--FIL), (--MED), and -(alpha )20.5 deletions are relatively common in the Philippines and in the Mediterranean region, respectively (Figure 1).

                              
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Table 1. alpha -Globin genotypes in 319 patients with Hb H disease from California, Hong Kong, and Ontario31,33,41

Nondeletional mutations

In contrast to beta -thalassemia, nondeletional alpha +-thalassemia mutations are relatively uncommon. There is an inherent difficulty in deciphering these mutations, because nucleotide sequencing of the alpha -globin genes with their high GC content is not an easy task. In recent years, increasing numbers of nondeletional alpha +-thalassemia mutations have been described. More than 30 of these mutations are tabulated in the human globin gene mutation database on the World Wide Web (http://globin.cse.psu.edu). The alpha 2-globin gene normally accounts for 2 to 3 times more alpha -globin mRNA and alpha -globin chain production than the alpha 1-globin gene.8 Therefore, alpha -thalassemia point mutations of the alpha 2-globin gene generally cause more severe anemia than the same mutations involving the alpha 1-globin gene.

Most of these mutations affect the transcriptional or translational processes of alpha -globin chain production. Others, notably Hb Constant Spring (alpha 2 codon 142 TAA>CAA or Terright-arrowGln) lead to markedly decreased and abnormally elongated alpha -globin chains.2 Recently, it was reported that as many as 15% of patients previously thought to be carriers of Hb Constant Spring based on hemoglobin analysis were in fact carriers of Hb Pakse (alpha 2 codon 142 TAA>TAT or Terright-arrowTyr).9,10,141 Other mutations produce highly unstable alpha -globin chains or hemoglobins, such as Hb Agrinio (alpha 2 codon 29 CTG>CCG or Leuright-arrowPro), Hb Suan-Dok (alpha 2 codon 109 CTG>CGG or Leuright-arrowArg), Hb Quong Sze (alpha 2 codon 125 CTG>CCG or Leuright-arrowPro), and Hb Pak Num Po (alpha 1 codon 131/132 +T), and result in an alpha -thalassemia phenotype.11-14


    Genotypes of Hb H disease
Top
Introduction
alpha -Thalassemia mutations
Genotypes of Hb H...
Pathophysiology
Clinical and laboratory...
Alpha-thalassemia/mental...
Acquired Hb H disease...
Diagnosis
Treatment
Genetic counseling
Summary
References

Deletional and nondeletional Hb H disease

Hb H disease is commonly caused by a deletion removing both alpha -globin genes on one chromosome 16, plus a deletion removing only a single alpha -globin gene on the other chromosome 16 such as the (-alpha 3.7) or (-alpha 4.2) deletions. These are known as "deletional Hb H disease."11,15-33

In a smaller proportion of patients, Hb H disease is caused by a deletion removing both alpha -globin genes on one chromosome 16, plus an alpha +-thalassemia point mutation, or small insertion/deletion, involving either the alpha 2- or alpha 1-globin gene on the other chromosome 16. These are collectively labeled as the "nondeletional Hb H disease." Patients with nondeletional Hb H disease usually are more anemic, more symptomatic, more prone to have significant hepatosplenomegaly, and more likely to require transfusions.11,18,20,24,25,31,32

Rarely, homozygosity or compound heterozygosity of nondeletional alpha +-thalassemia mutation(s) involving alpha 2-globin gene on each of the 2 chromosomes 16, can lead to a phenotype similar to Hb H disease. They include initiation codon mutation (ATG>ACG), Hb Sallanches (codon 104 TGC>TAC or Cysright-arrowTyr), or polyadenylation signal mutation (AATAAA>AATAAG).25,29,34-37 Even in this small subset of patients, their phenotypes can vary from relatively mild to very severe, requiring regular transfusions.

Population genetics

Hb H disease is found in many parts of the world, including Southeast Asian, Middle Eastern, and Mediterranean populations. It is particularly prevalent in Southeast Asia and in southern China, because of high carrier frequencies of the (--SEA), and to a lesser extent, the (--FIL) types of alpha -thalassemia deletions there.5,38,39 In Thailand with a population of 62 million people, it is estimated that 7000 infants with Hb H disease are born annually, and that there are 420 000 patients with Hb H disease in that country.40

The genotypes of 319 patients with Hb H disease from California, Hong Kong, and Ontario were reported during the past 2 years.31,33,41 These genotypes are summarized in Table 1 and serve to illustrate the heterogeneity of Hb H disease genotypes, especially in many multicultural communities in North America and elsewhere. Of those patients, 266 patients or 83% (95% confidence interval [CI], 79%-87%) have deletional Hb H disease. The most common genotype is (--SEA/-alpha 3.7) found in 175 patients (55%), followed by (--SEA/-alpha 4.2) 37 patients (12%), and (--FIL/ -alpha 3.7) 36 patients (11%).

Fifty-three patients or 17% (95% CI, 13%-21%) have nondeletional Hb H disease. The most prevalent genotype among this subgroup is (--SEA/alpha Constant Spring alpha ) found in 31 patients (10%). In Thailand, nondeletional Hb H disease with alpha Constant Spring is even more common, reportedly found in 40% to 50% of patients with Hb H disease.18

Among the 638 chromosomes examined in these 319 patients with Hb H disease, (--SEA) is found in 263 chromosomes (41%), (-alpha 3.7) in 224 chromosomes (35%), (-alpha 4.2) in 42 chromosomes (7%), (--FIL) in 38 chromosomes (6%), and (alpha Constant Spring alpha ) in 32 chromosomes (5%). The remaining 14 other mutations are found in 39 chromosomes (6%).

In the Mediterranean region, the most common deletion removing both alpha -globin genes in cis is the (--MED) deletion. Among 78 Cypriot patients with Hb H disease, 79% had the (--MED) deletion and 17% had the -(alpha )20.5 deletion.28


    Pathophysiology
Top
Introduction
alpha -Thalassemia mutations
Genotypes of Hb H...
Pathophysiology
Clinical and laboratory...
Alpha-thalassemia/mental...
Acquired Hb H disease...
Diagnosis
Treatment
Genetic counseling
Summary
References

In Hb H disease, there is a deficiency of alpha -globin mRNA and alpha -globin chains. These are reflected in the alpha /beta globin mRNA ratio being below 0.5, and the alpha /beta globin chain synthetic ratios ranging from 0.2 to 0.7.3,11,15,18 During fetal development, the excess gamma -globin chains form gamma 4 tetramers (Hb Bart). In adults, the excess beta -globin chains form beta 4 tetramers (Hb H). These homotetramers have high oxygen affinity, lack heme-heme interaction, and deliver only insignificant amounts of oxygen to tissues.

Hb H (beta 4) is relatively unstable and can be oxidized to form intracellular precipitates. If present within developing erythroblasts, these precipitates are thought to cause intramedullary early erythroid cell death and ineffective erythropoiesis. More often, these precipitates are formed in circulating erythrocytes with time and become attached to cell membrane. They cause local oxidative damage, membrane dysfunction, and shortened red cell survival.42 Hb H disease erythrocytes are rigid, and their membrane is more stable than normal.43 The membrane associated with Hb Constant Spring is even more rigid.44 These properties retard the passage of red blood cells through microvasculature and can promote erythrophagocytosis.45

There is evidence that pyrexia can enhance formation of intraerythrocytic Hb H inclusion bodies that might account for the acute hemolytic crisis and precipitous drop in hemoglobin associated with infections in some patients.11,46-48 Loss of normal membrane phospholipid asymmetry, particularly the exposure of phosphatidylserine on thalassemic cell surface, likely mediates intramedullary erythroblast apoptosis and engulfment of abnormal or aging circulating erythrocytes by macrophages.49-52 Erythrocyte membrane-associated immunoglobulin G (IgG) and complement components may also have similar roles in promoting erythrophagocytosis.45

It is generally accepted that hemolysis is the major cause of anemia in Hb H disease, although ineffective erythropoiesis also plays a pathogenetic role in this syndrome.15,51,53-56 In contrast, ineffective erythropoiesis is the dominant cause for the anemia in beta -thalassemia intermedia and major.

Nondeletional Hb H disease is generally more severe.11,18,20,24,25,31,32 Given that alpha 2-globin gene produces 2.5 times more alpha -globin mRNA than the alpha 1-globin gene, the combined alpha -globin mRNA production from both alpha -globin genes on a normal chromosome 16 could be assigned 3.5 "arbitrary units" for this discussion.8,45 In an alpha -globin gene cluster harboring a point mutation inactivating alpha 2-globin gene, the alpha -globin mRNA production by the remaining and normal alpha 1-globin gene is merely 1.0 "unit." However, with the single alpha -globin gene deletion of the (-alpha 3.7) type as a result of a misalignment crossover event during meiosis, the alpha -globin mRNA production by the remaining single hybrid alpha -globin gene is 2.0 "units."45 This consideration provides at least partial explanation for the observation that patients with nondeletional Hb H disease such as (--SEA/alpha Constant Springalpha ) are more severely affected than those with deletional Hb H disease such as (--SEA/-alpha 3.7).31,45

In some cases, the nondeletional alpha -globin gene mutation leads to highly unstable alpha -globin chains or variant hemoglobin, such as Hb Quong Sze (alpha 2 codon 125 CTG>CCG or Leuright-arrowPro).13 In addition to the more severe alpha -globin chain deficiency, the intracellular aggregates of hyperunstable variant alpha -globin chains or hemoglobins might also cause additional membrane damage and dysfunction. In one report, patients with nondeletional Hb H disease were found to have higher malonyldialdehyde levels, which is a secondary product of lipid peroxidation, and lower levels of vitamin E.57


    Clinical and laboratory findings
Top
Introduction
alpha -Thalassemia mutations
Genotypes of Hb H...
Pathophysiology
Clinical and laboratory...
Alpha-thalassemia/mental...
Acquired Hb H disease...
Diagnosis
Treatment
Genetic counseling
Summary
References

Hb H disease is generally thought to be a mild disorder. However, there is a marked phenotypic variability, ranging from asymptomatic, to need for periodic transfusions, to severe anemia with hemolysis and hepatosplenomegaly, and even to fatal hydrops fetalis syndrome in utero. Patients with identical alpha -globin genotypes can have different phenotypes,30 suggesting that there are other yet undefined genetic and/or environmental factors that can affect phenotypic expression of Hb H disease.58,59

Anemia

The hemoglobin level, mean erythrocyte corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH) of patients with Hb H disease are tabulated in Table 2. There are significant variations in these values between individuals. For example, in one report, hemoglobin levels ranged between 70 and 129 g/L among 10 male patients and between 70 and 114 g/L among 41 female patients, all with deletional Hb H disease.33 MCV ranged from 51 to 73 fL. There are elevated reticulocyte counts (5%-10%), polychromasia, and moderate anisopoikilocytosis.60

                              
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Table 2. Hemoglobin, MCV and MCH in patients with Hb H disease

The MCV in nondeletional Hb H disease is higher than in deletional Hb H disease (Table 2). Anemia is more pronounced in nondeletional Hb H disease and so is reticulocytosis. This finding may partially account for the higher MCV. alpha -Thalassemic erythrocytes are hyperhydrated, especially those inherited with Hb Constant Spring.43 In 15 patients with nondeletional Hb H disease/Hb Constant Spring, their Hb was 82 ± 19 g/L, and MCV was 76 ± 7 fL. In 5 other patients with nondeletional Hb H disease/Hb Quong Sze, their Hb was 86 ± 10 g/L, and MCV was 71 ± 7 fL. (V.C., unpublished observations, August 2002). It is postulated, although unproven, that the early closing of the K-Cl cotransporter in alpha -thalassemic erythrocytes, in contrast to beta -thalassemic erythrocytes, prevents loss of K-Cl and water and leads to erythrocyte hyperhydration and higher MCV.45

There are occasions, such as increased hemolysis secondary to infections, fever, ingestion of oxidative compounds or drugs, aregenerative anemia as a result of Parvovirus B19 and other infections, hypersplenism, and pregnancy, when severe anemia may ensue and transfusions become necessary. In Thailand, 29% of 221 patients with deletional Hb H disease and 50% of 136 patients with nondeletional Hb H disease had been transfused.18 In Hong Kong, 46% of 114 patients with Hb H disease had been transfused, but only very few patients required regular transfusions.31

The microcytic and hypochromic anemia found in Hb H disease might lead to the erroneous diagnosis of iron deficiency anemia. On occasions, inappropriate iron supplement treatment or invasive investigations to rule out possible gastrointestinal bleeding are prescribed.

Iron overload

Iron overload as manifested by markedly elevated serum ferritin levels is present in 70% to 75% of adult patients with Hb H disease.61-63 Raised serum ferritin levels are correlated with increasing age but are unrelated to previous history of transfusions, iron supplement, or herbal medicine treatment.22,31 It is likely that iron absorption is increased in Hb H disease, secondary to enhanced erythropoiesis as a result of hemolysis and anemia.64 Excessive alcohol consumption is an additional risk factor.62

In Hong Kong, 60 patients underwent computed tomographic (CT) scan of liver, and 51 of them (85%) had a signal-intensity ratio of less than 1, indicative of iron overload.31,65 In others, liver biopsy revealed increased liver iron content and hepatic fibrosis and/or cirrhosis, without serologic evidence of either hepatitis B or C.22,31 In addition, 25 asymptomatic adult patients had cardiac echocardiography done, and all showed abnormal left ventricular diastolic function.31 There was a trend of increasing cardiac abnormality with increasing serum ferritin levels. Three patients had heart failure as a result of cardiac iron overload. Another patient developed hemosiderosis and diabetes mellitus.66 These findings underscore the importance of monitoring adult patients with Hb H disease for iron overload and to initiate iron chelation therapy when indicated.

There are reports from the Mediterranean region that iron overload in Hb H disease is uncommon.11,30,67 The apparent discrepancy might be due to differences in other genetic or environmental factors. In Sardinia, some adult patients had serum ferritin levels at or close to 1000 µg/L.25 Long-term follow-up on those apparently unaffected patients should be informative.

Hepatosplenomegaly

Hepatomegaly was present in 70% of 502 patients in Thailand, 60% of 153 patients in Sardinia, and 14% of 88 patients in Taiwan.15,22,25 Jaundice was found in 25% of patients in Thailand, 20% to 30% in Sardinia, and 43% in Taiwan.18,22,25 Splenomegaly is also prevalent in Hb H disease, found in 79% of patients in Thailand, 60% in Sardinia, and 47% in Taiwan.15,22,25 In Hong Kong, a third of 27 patients with nondeletional Hb H disease had undergone splenectomy. In contrast, only 5% of 87 patients with deletional Hb H disease were splenectomized.31

Cholelithiasis

Seventy-seven adult patients in Hong Kong had abdominal ultrasonography done. Twenty-six patients (34%) were found to have cholelithiasis, and 5 of them underwent cholecystectomy.31 Nineteen percent of 88 patients in Taiwan and 15% of 81 patients in Thailand had cholelithiasis.22,68 A promoter polymorphism in uridine diphosphate (UDP)-glucuronosyl-transferase, as in Gilbert disease, is a risk factor for cholelithiasis in hereditary spherocytosis, beta -thalassemia, and sickle cell disease.69-73 Whether this correlation also holds true for Hb H disease awaits confirmation.

Growth and development in children

Thirteen percent of children with Hb H disease in Hong Kong had a growth rate below the third percentile.31 In another report, 2 infants with Hb H disease were followed from birth to 6 months of age.74 At birth, the total hemoglobin was approximately 155 g/L, but only 110 to 120 g/L represented functional hemoglobins (Hb F and Hb A). The other 30 to 40 g/L were Hb Bart (gamma 4) and some Hb H (beta 4), which have markedly impaired ability to deliver oxygen to tissues. At 1 to 2 months of age, the functional hemoglobin fractions fell to 70 g/L. By 3 months, these fractions had returned to 85 to 95 g/L. These anecdotal reports underscore the importance of a systematic investigation of the natural history of Hb H disease during early infancy and childhood.37

Pregnancy

There is usually an increasing severity of anemia during pregnancy, possibly related to expansion of blood volume. Hb level may sometimes fall to 60 g/L or even less, necessitating the need for blood transfusion to maintain the health of the mother and the developing fetus.15,25,75,76 However, other women go through pregnancies without the need for transfusions.77

In 34 pregnancies among 29 Thai women, Hb fell to 71 ± 20 g/L during the second and third trimesters.75 Preeclampsia occurred in 18% of the cases, and 9% developed congestive heart failure. There was one case each of miscarriage and perinatal death and 3 cases of premature births. Galanello et al25 reported that there were 7 miscarriages (12%) among 58 pregnancies in 24 Italian women with Hb H disease. It is vitally important to carry out prospective and in-depth studies of pregnancies in women with Hb H disease to define the risks and criteria for treatment.

Another important issue is related to testing the woman's partner and genetic counseling. This issue is discussed in more detail in "Genetic counseling."

Hb H hydrops fetalis syndrome

This devastating syndrome represents the most severe form of Hb H disease and was first described in detail in 1985.78 It is the subject of a recent review.79 The alpha -globin genotypes of the affected fetuses consist of deletion of both alpha -globin genes in cis or the complete zeta -alpha -globin gene cluster on one chromosome 16 and a nondeletional mutation involving alpha 2-globin gene on the other chromosome 16.79-81 These fetuses suffer from severe anemia and hypoxia in utero, with its sequelae such as edema, pericardial effusion, congenital anomalies, and even death.

The pathophysiology of this serious inherited disorder is not well understood. One hypothesis is that the nondeletional alpha -globin gene mutations, such as alpha 2 codon 35 TCC>CCC or Serright-arrowPro, codon 59 GGC>GAC or Glyright-arrowAsp, or codon 66 CTG>CCG or Leuright-arrowPro, result in highly unstable hemoglobin and severe anemia, akin to the dominant beta -thalassemia syndrome.79-82

With the mutation alpha 2 codon 30 Delta GAG, hydrops fetalis occurs only with the coinheritance of a deletion removing the complete zeta -alpha -globin gene cluster.80 Other individuals who have inherited the same Delta GAG mutation together with the Southeast Asian type of deletion removing both alpha -globin genes in cis, but sparing the embryonic zeta -globin gene, survive to adulthood, albeit with moderate to severe anemia (Hb, 72-105 g/L).31,80,83

More recently, another case of this severe syndrome was reported in a Turkish newborn, who died despite active resuscitation, soon after birth at the 30th week of gestation. This infant's alpha -globin genotype was (--MED/alpha AATAAA>AATAAGalpha ).84,142

Hb H disease in combination with other hemoglobinopathies

Patients with both Hb H disease and heterozygosity for beta -globin variants such as Hb S, Hb C, or Hb E have hemoglobin levels similar to most patients with Hb H disease.85-89 The proportions of the variant hemoglobins are low (20%-25%), because of the lower affinity of the alpha -globin chains for the variant beta -globin chains as compared with the normal beta -globin chains.90 There are exceptions, such as beta J-Iran (codon 77 CAC>GAC or Hisright-arrowAsp) that is a negatively charged variant beta -globin chain.91 With Hb H disease, the proportion of this variant Hb increases to 65%.91 These patients with variant beta -globin chains as well as those with beta -thalassemia trait have low Hb H levels and scanty Hb H inclusion bodies.

Hb E is very common in Southeast Asia. In Thailand, interactions of Hb H disease with Hb E and/or beta -thalassemia give rise to many different complex genotypes. They are broadly known as AE-Bart disease (Hb H disease and heterozygosity for Hb E), and EF-Bart disease (Hb H disease and homozygosity for Hb E or Hb E/beta -thalassemia).88,89 They present with thalassemia intermedia phenotype with moderate to severe anemia and with decreased or no Hb H and inclusion bodies detected.

Patients who have Hb H disease together with heterozygosity for beta New York (codon 113 GTG>GAG or Valright-arrowGlu) have severe anemia (Hb between 34 and 68 g/L) and hepatosplenomegaly.92 The variant beta New York has higher affinity for the alpha -globin chains to form Hb New York that is an unstable hemoglobin. This worsens the deficiency of alpha -globin chains already present in Hb H disease and causes even more severe anemia. Coinheritance of Hb H disease and several other uncommon variant beta -globin chains has been described, such as Hb J Bangkok (codon 56 GGC>GAC or Glyright-arrowAsp), Hb Pyrgos (codon 83 GGC>GAC or Glyright-arrowAsp), and Hb Hope (codon 136 GGT>GAT or Glyright-arrowAsp).93-95

Coinheritance of Hb H disease and beta -thalassemia heterozygosity leads to anemia usually more severe than beta -thalassemia heterozygote alone, but less than common Hb H disease.96-98 However, there are exceptions and some patients require transfusions.28,98 They have marked microcytosis and hypochromia, but low or absent Hb H and Hb H inclusion bodies. Hb A2 levels are elevated, as in most other beta -thalassemia heterozygotes.96-98

Other findings

The following findings associated with Hb H disease have been reported: dysmorphic facial features, skeletal changes, retinopathy, extramedullary hematopoietic tumors, risk of thromboembolic disorders, and leg ulcers.3,99-102

Erythrocyte glucose-6-phosphate dehydrogenase (G6PD) deficiency is prevalent in the same populations in whom thalassemias are prevalent. Therefore, coinheritance of both Hb H disease and G6PD deficiency is to be expected.47,103 Recently in Hong Kong, a newborn was found to have Hb H disease (--SEA/alpha Constant Springalpha ) and G6PD deficiency (0.1 U/g Hb in cord blood). At birth, the infant was anemic with Hb of 125 g/L and had hepatosplenomegaly. At 1 year of age, his Hb fell to 42 g/L soon after an episode of infection and fever (E. S. K. Ma, unpublished observations, August 2002).


    Alpha-thalassemia/mental retardation (ATR) syndromes
Top
Introduction
alpha -Thalassemia mutations
Genotypes of Hb H...
Pathophysiology
Clinical and laboratory...
Alpha-thalassemia/mental...
Acquired Hb H disease...
Diagnosis
Treatment
Genetic counseling
Summary
References

ATR-16 syndrome

The affected children have chromosomal rearrangements that delete as much as 1 to 2 Mb of chromosome 16 short-arm (16p) telomere, including the zeta -alpha -globin gene complex, resulting in monosomy for the 16p telomere, and alpha -thalassemia phenotype.104 In some, the chromosomal abnormalities can be visualized by cytogenetic analysis or fluorescent in situ hybridization (FISH). In most cases, one parent carried a balanced translocation that the child inherited in an unbalanced fashion. In others, the abnormality arose as de novo events. If the affected child also inherited a common single alpha -globin gene deletion from the other parent, the child would have Hb H disease.105

It is thought that mental retardation and other congenital anomalies observed in this syndrome are caused by the deletion of dosage-sensitive genes on one 16p telomere and other concomitant chromosomal abnormalities. Children with deletions of up to 350 kb in length from one 16p telomere, including the complete zeta -alpha -globin gene complex, have alpha -thalassemia trait but are developmentally normal.106 It is critical for understanding the genetic basis of mental retardation to identify these genes on 16p telomere that can affect severe cognitive and developmental abnormalities when only one copy is deleted.105,107

ATR-X syndrome

This syndrome is an X-linked disorder, caused by mutations of the ATRX gene located on chromosome Xq13.3.108 It is more common than the ATR-16 syndrome. The affected males usually have very severe intellectual and physical handicaps and many other congenital anomalies. They often have characteristic facial features. Genital abnormalities, such as ambiguous genitalia, and skeletal deformities are present in 90% of these patients.109 They present with an alpha -thalassemia phenotype but with considerable variations with Hb H inclusion bodies found in none to up to 32% of circulating erythrocytes.105

The functions of ATRX protein in vivo are not yet understood and are under active investigation.110 It contains motifs that have been implicated in the regulation of transcription mediated by chromatin modification, protein-protein interaction, adenosine triphosphatase (ATPase), and helicase activities and that affect genomic methylation pattern.111,112 The highly variable phenotypes in patients suggest that ATRX interacts with other genetic factors to bring about normal expression of many genes of developmental importance, including the alpha - but not the beta -globin genes.105


    Acquired Hb H disease in myeloproliferative disorders
Top
Introduction
alpha -Thalassemia mutations
Genotypes of Hb H...
Pathophysiology
Clinical and laboratory...
Alpha-thalassemia/mental...
Acquired Hb H disease...
Diagnosis
Treatment
Genetic counseling
Summary
References

This syndrome has been described mostly in elderly men who have primary marrow disorders such as erythroleukemia, myelodysplasia, refractory anemia with excessive blasts, acute leukemia, and so forth. Its molecular pathology has not been defined, although it is conceivable that the down-regulation of alpha -globin gene expression in these patients is the result of somatic mutation(s) in transcription factor(s).

This and the relatively rare ATR syndromes provide important insight into the biology of alpha -globin gene regulation and expression, as well as other developmental pathways. They are the subjects of 2 recent reviews3,105 and are beyond the scope of the present manuscript.


    Diagnosis
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Introduction
alpha -Thalassemia mutations
Genotypes of Hb H...
Pathophysiology
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Alpha-thalassemia/mental...
Acquired Hb H disease...
Diagnosis
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Summary
References

Hb H disease is characterized by moderate hypochromic and microcytic anemia (see "Anemia"). Formation of Hb H (beta 4) precipitates within red cells (Hb H inclusion bodies) can be induced by incubation with mild oxidants such as Brilliant Cresyl Blue.113 These inclusion bodies are numerous and can easily be seen by light microscopy in most circulating erythrocytes. In addition, Hb H ranging between 5% and 30% can be detected by hemoglobin electrophoresis, isoelectric focusing (IEF), or high-performance liquid chromatography (HPLC).60 Hb H levels are usually higher in nondeletional Hb H disease than in deletional Hb H disease. It should be noted that some commercial electrophoretic methods are incapable of detecting Hb H.114 Hb A2 level is usually down in the 1.0% to 2.0% range.60

Patients who have Hb H disease and concomitant heterozygous beta -hemoglobinopathies, such as Hb S, Hb C, Hb E, or beta -thalassemia, have low or absent Hb H (beta 4) and many fewer Hb H inclusion bodies.85-89,96-98 These hematologic findings may confound the diagnosis of the underlying Hb H disease.

Correct DNA-based genotyping is necessary for genetic counseling and prenatal diagnosis. Southern blotting is often used for diagnosing deletional mutations. More recently, Gap-polymerase chain reaction (PCR) methods have been developed for many of the common deletions.115-117 In large deletions that remove the complete zeta -alpha -globin gene cluster, the diagnosis is more difficult and often depends on family studies and Southern blotting analysis using probes distal to the deletional breakpoints.118,119

For point mutations, or small deletion/insertion mutations, the diagnosis is often based on direct nucleotide sequencing of the PCR-amplified product of either alpha 2- or alpha 1-globin gene.79-81,83,84 Reverse dot-blot by hybridization with allele specific oligonucleotide probes and multiplex ARMS (PCR-based amplification refractory mutation system) assays have been developed for rapid diagnosis of several alpha +-thalassemia point mutations.120,121

Hb Bart (gamma 4) is a fast-moving hemoglobin that can be determined by electrophoresis, IEF, or HPLC. Newborns who have inherited Hb H disease have 20% to 40% of Hb Bart at birth.60 Universal newborn screening for Hb H disease was implemented in 1996 in California, based on an elevated level of a fast-moving peak on HPLC of hemolysates eluted from newborn blood spots impregnated on filter papers.41 Of 101 newborns found to have 25% or more of this peak, 89 were confirmed by DNA diagnostics to have Hb H disease. Table 3 shows that in California, there are many more newborns with Hb H disease than with other commonly screened for inherited metabolic disorders such as phenylketonuria and galactosemia.

                              
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Table 3. Number of newborns diagnosed annually through the universal newborn screening program in California41

The (--SEA) type of alpha 0-thalassemia deletion is very common in Southeast Asia. An enzyme-linked immunosorbent assay (ELISA) to detect embryonic zeta -globin chains in peripheral blood erythrocytes was reported to be a simple, rapid, and reliable screening test to identify adult carriers of the (--SEA) alpha 0-thalassemia deletion.122-125 This test should help to identify couples at risk of conceiving fetuses with Hb H disease and also homozygous alpha 0-thalassemia hydrops fetalis syndrome.


    Treatment
Top
Introduction
alpha -Thalassemia mutations
Genotypes of Hb H...