Negro alpha-thalassaemia is caused by deletion of a single alpha-globin gene

Copy number variation in human health, disease, and evolution

Copy number variation (CNV) is a source of genetic diversity in humans. Numerous CNVs are being identified with various genome analysis platforms, including array comparative genomic hybridization (aCGH), single nucleotide polymorphism (SNP) genotyping platforms, and next-generation sequencing. CNV formation occurs by both recombination-based and replication-based mechanisms and de novo locus-specific mutation rates appear much higher for CNVs than for SNPs. By various molecular mechanisms, including gene dosage, gene disruption, gene fusion, position effects, etc., CNVs can cause Mendelian or sporadic traits, or be associated with complex diseases. However, CNV can also represent benign polymorphic variants. CNVs, especially gene duplication and exon shuffling, can be a predominant mechanism driving gene and genome evolution.

Acquired haemoglobin H disease

Acquired haemoglobin H disease has been described in various premalignant haematological conditions and is most commonly associated with myelodysplastic and myeloproliferative syndromes. The condition is not restricted to any specific population group or geography. Affected individuals have no family or past history of alpha thalassaemia and these subjects usually suffer from severe uncompensated haemolysis. Extensive mapping and sequence analysis of the alpha globin gene cluster have demonstrated intact alpha globin genes, leading workers to conclude that an acquired in trans mechanism is responsible for the disorder. ATRX gene mutations on the X chromosome have been shown to be instrumental in the suppression of alpha globin gene expression. Despite recent advances in the understanding of its pathogenesis, the precise mechanism of acquired haemoglobin H disease remains a mystery.

Blood lactate concentrations during incremental exercise in subjects with sickle cell trait

The aim of the present study was to assess blood lactate concentrations ([LA], mmol x L(-1)) and oxygen uptake (VO2, L x min(-1), mL x kg(-1) x min(-1)) during incremental exercise in subjects with sickle cell trait (SCT) only, i.e., sedentary subjects with SCT without anemia and/or associated alpha thalassemia. Anemia was ruled out using hemoglobin (Hb) level, and alphathalassemia was ruled out using hemoglobin S (HbS) percentage and concomitant Hb level and mean corpuscular volume (MCV). Comparison was made with control subjects with normal Hb, matched for physical fitness, anthropometric data, and hematological parameters. All subjects underwent an incremental exercise test (IET) using an electromagnetic cycle ergometer. Ventilatory data, i.e., minute ventilation (VE, L x min(-1)), oxygen uptake (VO2, mL x min(-1), mL x Kg(-1) x min(-1)) carbon dioxide production (VO2, mL x min(-1)), ventilatory equivalent for O2(VE x VO2(-1))and for CO2 (VE x VO2(-1)), and respiratory exchange ratio (RER, VO2 x VO2(-1)), were collected every minute during IET and the recovery period using a breath-by-breath automated system. Heart rate (HR, beats x min(-1)) was measured every minute using an EKG. Blood sampling was done every minute during IET and the first 5 min of the recovery period, and then every 5 min until the 20th minute of recovery. [LA] were determined by an enzymatic method with a spectrophotometer. Comparisons of all mean cardioventilatory variables showed no significant differences in subjects with SCT versus controls during IET and recovery. In contrast, analysis of variance revealed significantly lower time courses of [LA] during IET (P < 0.05) and recovery (P < 0.05), whereas time courses of VO2 were similar (P > 0.05). We conclude that the lower [LA] exhibited by subjects with SCT during incremental exercise and the subsequent recovery was not associated with concomitant oxygen uptake impairment.

Alpha-thalassaemia

alpha-Thalassaemias are genetic defects extremely frequent in some populations and are characterized by the decrease or complete suppression of alpha-globin polypeptide chains. The gene cluster, which codes for and controls the production of these polypeptides, maps near the telomere of the short arm of chromosome 16, within a G + C rich and early-replicating DNA region. The genes expressed during the embryonic (zeta) or fetal and adult stage (alpha 2 and alpha 1) can be modified by point mutations which affect either the processing-translation of mRNA or make the polypeptide chains extremely unstable. Much more frequent are the deletions of variable size (from approximately 3 to more than 100 kb) which remove one or both alpha genes in cis or even the whole gene cluster. Deletions of a single gene are the result of unequal pairing during meiosis, followed by reciprocal recombination. These unequal cross-overs, which produce also alpha gene triplications and quadruplications, are made possible by the high degree of homology of the two alpha genes and of their flanking sequences. Other deletions involving one or more genes are due to recombinations which have taken place within non-homologous regions (illegitimate recombinations) or in DNA segments whose homology is limited to very short sequences. Particularly interesting are the deletions which eliminate large DNA areas 5' of zeta or of both alpha genes. These deletions do not include the structural genes but, nevertheless, suppress completely their expression. Larger deletions involving the tip of the short arm of chromosome 16 by truncation, interstitial deletions or translocations result in the contiguous gene syndrome ATR-16. In this complex syndrome alpha-thalassaemia is accompanied by mental retardation and variable dismorphic features. The study of mutations of the 5' upstream flanking region has led to the discovery of a DNA sequence, localized 40 kb upstream of the zeta-globin gene, which controls the expression of the alpha genes (alpha major regulatory element or HS-40). In the acquired variant of haemoglobin H (HbH) disease found in rare individuals with myelodysplastic disorders and in the X-linked mental retardation associated with alpha-thalassaemia, a profound reduction or absence of alpha gene expression has been observed, which is not accompanied by structural alterations of the coding or controlling regions of the alpha gene complex. Most probably the acquired alpha-thalassaemia is due to the lack of soluble activators (or presence of repressors) which act in trans and affect the expression of the homologous clusters and are coded by genes not (closely) linked to the alpha genes. The ATR-X syndrome results from mutations of the XH2 gene, located on the X chromosome (Xq13.3) and coding for a transacting factor which regulates gene expression. The interaction of the different alpha-thalassaemia determinants results in three phenotypes: the alpha-thalassaemic trait, clinically silent and presenting only limited alterations of haematological parameters, HbH disease, characterized by the development of a haemolytic anaemia of variable degree, and the (lethal) Hb Bart's hydrops fetalis syndrome. The diagnosis of alpha-thalassaemia due to deletions is implemented by the electrophoretic analysis of genomic DNA digested with restriction enzymes and hybridized with specific molecular probes. Recently polymerase chain reaction (PCR) based strategies have replaced the Southern blotting methodology. The straightforward identification of point mutations is carried out by the specific amplification of the alpha 2 or alpha 1 gene by PCR followed by the localization and identification of the mutation with a variety of screening systems (denaturing gradient gel electrophoresis (DGGE), single strand conformation polymorphisms (SSCP)) and direct sequencing.

Rapid analysis of -alpha 3.7 thalassaemia and alpha alpha alpha anti 3.7 triplication by enzymatic amplification analysis

In this report we describe a PCR-based method for the diagnosis of the most common form of alpha thalassaemia, the -alpha 3.7 deletion which occurs throughout all tropical and subtropical regions of the world. The same procedure also identifies the reciprocal recombinant chromosome (alpha alpha alpha anti 3.7). Restriction mapping of the PCR products has enabled us to distinguish between the type I (-alpha 3.7 I), type II (-alpha 3.7 II) and type III (-alpha 3.7 III) deletions. This strategy will be very useful in screening programmes of alpha thalassaemia occurring on its own or in association with beta thalassaemia and sickle cell disease.

Human alpha-globin gene expression is silenced by terminal truncation of chromosome 16p beginning immediately 3' of the zeta-globin gene

The high level expression of the human alpha-globin genes in erythroid tissue appears to require a set of DNaseI hypersensitive sites located upstream of the human alpha-globin gene cluster. These sequences, termed the locus control region (LCR), include two erythroid specific and a number of less restricted DNaseI hypersensitive sites. In this report we describe an individual with alpha-thalassemia associated with a truncation of the short arm of chromosome 16 that removes the LCR region and inactivates the adjacent intact alpha-globin genes. This genetic study supports the critical role of the LCR in the transcriptional activation of the human alpha-globin gene cluster and substantiates the importance of LCR deletions in the etiology of alpha-thalassemia.

Effects of alpha-thalassemia and sickle polymerization tendency on the urine-concentrating defect of individuals with sickle cell trait

A defect in urine concentrating ability occurs in individuals with sickle cell trait (HbAS). This may result from intracellular polymerization of sickle hemoglobin (HbS) in erythrocytes, leading to microvascular occlusion, in the vasa recta of the renal medulla. To test the hypothesis that the severity of the concentrating defect is related to the percentage of sickle hemoglobin present in erythrocytes, urinary concentrating ability was examined after overnight water deprivation, and intranasal desmopressin acetate (dDAVP) in 27 individuals with HbAS. The HbAS individuals were separated into those who had a normal alpha-globin genotype (alpha alpha/alpha alpha), and those who were either heterozygous (-alpha/alpha alpha) or homozygous (-alpha/-alpha) for gene-deletion alpha-thalassemia, because alpha-thalassemia modulates the HbS concentration in HbAS. The urinary concentrating ability was less in the alpha alpha/alpha alpha genotype than in the -alpha/alpha alpha or -alpha/-alpha genotypes (P less than 0.05). After dDAVP, the urine osmolality was greater in patients with the -alpha/-alpha genotype than with the -alpha/alpha alpha genotype (882 +/- 37 vs. 672 +/- 38 mOsm/kg H2O) (P less than 0.05); patients with the -alpha/alpha alpha genotype had greater concentrating ability than individuals with a normal alpha-globin gene arrangement. There was an inverse linear correlation between urinary osmolality after dDAVP and the percentage HbS in all patients studied (r = -0.654; P less than 0.05). A linear correlation also existed for urine concentrating ability and the calculated polymerization tendencies for an oxygen saturation of 0.4 and O (r = -0.62 and 0.69, respectively). We conclude that the severity of hyposthenuria in HbAS is heterogeneous. It is determined by the amount of HbS polymer, that in turn is dependent upon the percentage HbS, which is itself related to the alpha-globin genotype.

A newly-characterized alpha-thalassaemia-1 deletion removes the entire alpha-like globin gene cluster in an Italian family

We describe a new deletional form of alpha thalassaemia which encompasses the entire alpha-like globin gene cluster in a 15-year-old boy of Southern Italian descent. The deletion removes approximately 31 kb, the 5'-end point is located approximately 4 kb upstream of the xi gene, while the 3'-end point maps between the alpha 1- and theta 1-globin genes. The interaction of this deletion with the common-alpha 3.7 form gives origin to a classical form of haemoglobin (Hb) H disease in the propositus of this study. Deletional forms of xi alpha-thalassaemia are uncommon in the Mediterranean basin; as for other unusual xi alpha-thalassaemia forms, heterozygotes for this mutation may escape detection in population surveys based on zeta and alpha probes.

Thalassemia: molecular pathology and management

Recent advances in molecular biology have allowed us to develop an almost complete picture of the molecular pathology of the thalassemia syndromes. The different classes of mutations that are responsible for the thalassemia syndromes will be discussed along with the special insights they have provided into the controls of eukaryotic gene expression. While management of these disorders has not kept pace with our understanding of their cause, there have been notable advances in treatment. Perhaps even more exciting is what the future holds, as the continued march of molecular biology is melded with novel approaches to the definitive treatment of thalassemias.

Hemoglobin J Guantanamo [alpha 2 beta 2 128 (H6) Ala----Asp] in association with hemoglobin C and alpha-thalassemia in a family from Benin

Hemoglobin J (HbJ), Guantanamo, which had been described but once in the literature, was found in a family originating from Benin; this second case was found to be in association with hemoglobin C (HbC) and alpha-thalassemia. High-performance liquid chromatography (HPLC) procedures and microsequencing were used for characterization of the aminoacid substitution. The main hematological disorder, in relation with the instability of Hb J Guantanamo, seems to be a worsening of the rheological properties of the red blood cells (RBC), as demonstrated by ektacytometric studies. Oxygen-binding properties of the RBC were almost normal, but a slight decrease in cooperativity and lowered Bohr and 2,3-diphosphoglycerate (DPG) effects were observed for pure stripped Hb J Guantanamo. The expression of the electrophoretic charge difference was partly masked, as is often observed when the structural abnormality is situated in or near a contact area.

Dysfunctional alpha-globin genes in hemoglobin H disease in blacks: variation in restriction fragment size permits the detection of the -alpha/-alpha T genotype

Hemoglobin H (HbH) disease is most often due to deletion of three of the four alpha-globin genes (genotype --/--alpha). In black subjects although the -alpha/chromosome is common, the --/haplotype is very rare and few examples of HbH disease have been detected. We have studied three black siblings with HbH by restriction endonuclease mapping of the alpha-like gene complex (5'-zeta-psi zeta-psi alpha 2-psi alpha 1-alpha 2-alpha 1-3') using zeta- and alpha- specific probes. The presence of size differences in the previously described hypervariable region between the zeta and psi zeta genes results in a restriction fragment length polymorphism which permitted the detection of single alpha genes on both number 16 chromosomes in these subjects. Quantitative DNA hybridization by a slot-blot technique confirmed that their genomes contained two alpha-globin genes. The results establish that in these black subjects HbH disease is associated with dysfunctional alpha-globin genes (genotype: -alpha/-alpha T).

Molecular basis for nondeletion alpha-thalassemia in American blacks. Alpha 2(116GAG----UAG)

An American black woman was found to have the phenotype of moderately severe alpha-thalassemia normally associated with the loss of two to three alpha-globin genes despite an alpha-globin gene map that demonstrated the loss of only a single alpha-globin gene (-alpha/alpha alpha). Several individuals in her kindred with normal alpha-globin gene mapping studies (alpha alpha/alpha alpha) had mild alpha-thalassemia hematologic values consistent with the loss of one to two alpha-globin genes. These data suggested the presence of a nondeletion alpha-thalassemia defect in this family which segregates with the intact alpha alpha gene cluster. An abnormally migrating and highly unstable alpha-globin gene product was demonstrated by in vitro translation of the reticulocyte mRNA from the proposita and this mutant alpha-globin protein was mapped to the alpha 2-globin gene by hybrid-selected translation. The abnormal alpha 2-globin gene was cloned and sequenced. A single base mutation that results in a premature termination codon was identified at codon 116 (GAG----UAG). The defined alpha-globin genotype of the proposita (-alpha/alpha 116UAG alpha) and the positioning of this nonsense mutation at the alpha 2-globin gene locus are fully consistent with the observed alpha-thalassemia phenotype.

'Neutropenia' in black west Indians

A prospective case control study routine haematological parameters was conducted in 294 healthy Black and White age/sex-matched subjects. The most important finding relevant to clinical practice was a reduction of total white cell count in Blacks due mainly to reduced neutrophil numbers. Twenty-one percent of sickle negative Blacks had white cell counts below the lowest value seen in Whites. The haemoglobin concentration, erythrocyte mean cell volume and monocyte count were also significantly lower amongst Blacks though lymphocyte counts were higher. The racial differences in haemoglobin and white count were not accounted for by differences in smoking and drinking habits. They were also found when Blacks with sickle cell trait were compared to age/sex-matched Whites and in others taking the oral contraceptive pill. Awareness of racial group should aid interpretation of routine tests and avoid unnecessary investigation of normal 'neutropenic' Blacks.

Hemoglobin abnormalities. An evaluation on new-born infants and their mothers in a maternity unit close to Brazzaville (P.R. Congo)

In order to evaluate the polymorphism of hemoglobin in a population of Equatorial Africa, we undertook a prospective study of 146 births at a rural maternity hospital close to Brazzaville (P.R. Congo). This showed among the mothers 31 (22%) carriers of the sickle cell trait (AS), six with delta mutation, and two with beta-thalassemia trait. Among the children, 27 (18.5%) had sickle cell trait and one had sickle cell homozygosity. The frequency of the HbF Sardinia trait was 7.5%. This and other studies suggested a dilution gradient from Europe to Africa. Hemoglobin Bart's could be visually detected in 23.3% of the new-born babies. We attempted to distinguish between those infants with a high level of Hb Bart's (Bart's ++ group: 13.7%) and a group with a detectable Hb Bart's level that in our experimental conditions is between 1 and 2% (Bart's + group: 9.6%). Mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) were 88.6 +/- 5.7 fl and 29.0 +/- 2.1 pg in the Bart's ++ group; 94.5 +/- 10.9 fl and 30.6 +/- 4.1 pg in the Bart's + group; whereas they were 101.0 +/- 8.7 fl and 33.9 +/- 2.5 pg in the control group. Since iron deficiencies are very rare in new-borns and selecting according to published data on black people as homozygous alpha-thalassemia of the type I (-alpha/-alpha), individuals of the Bart's ++ group whose MCV was below 95 fl and MCV below 30 pg, the gene frequency is estimated to be 34% and that of heterozygotes (-alpha/alpha alpha) 45%. These high frequencies were confirmed in AS mothers: 45% showed a significant decrease of the S fraction.

Influence of HbS levels upon the hematological and clinical characteristics of sickle cell trait

The variable concentration of HbS in individuals with sickle cell trait led us to study the relationship between HbS level and selected vascular events in 355 hospitalized black men with sickle cell trait. There were significant differences in hemoglobin concentration and mean corpuscular volume found in four groups divided by their HbS level, the lowest proportion of HbS (less than 30%) being associated with the lowest hemoglobin concentration (12.6 g/dl) and MCV (77 fl). The percent HbS did not influence the incidence of pulmonary embolism, thrombophlebitis, myocardial infarction, cerebrovascular accident, or idiopathic hematuria. Our results suggest that HbS level does not influence vascular disease, and while certain hematological alterations occur, they are very unlikely to have any clinical significance. Regardless of the proportion of HbS, sickle cell trait in black men is benign.

The effect of Hb F and alpha-thalassemia on the red cell indices in sickle cell anemia

This study examines the effect of different levels of fetal hemoglobin (Hb F) and the presence or absence of genes for alpha-thalassemia on the red cell indices and degree of anemia among 102 patients with homozygous sickle cell disease (S/S) between the ages of 15 and 62 years. Patients were divided into those with an average Hb F of less than 10 gm/L ("low" Hb F group) and those with greater than 10 gm/L ("high" Hb F group). alpha-Thalassemia was assessed by restriction enzyme analysis of DNA by the Southern blotting technique. Homozygosity for the beta(s) gene was confirmed by restriction enzyme analysis of DNA using the enzyme Mst II. There were 51 patients with four alpha-globin genes, 28 of whom had "high" and 23 "low" Hb F levels. Fifty-one patients had alpha-thalassemia, 38 of whom were heterozygous and 13 homozygous for the 3.7 kb alpha-thalassemia deletion. Nine had "high" and 31 had "low" Hb F. Irrespective of alpha-globin genotype, patients in the high Hb F group had a higher mean Hb, Hct, MCV, and MCH than those in the low HB F group. In patients without alpha-thalassemia Hb F was positively correlated with MCV and MCH (p less than 0.001), patients with high Hb F levels having macrocytosis confirmed by microhematocrit studies. Patients with alpha-thalassemia had a lower MCHC than patients with four alpha-globin genes and this was not significantly affected by the level of Hb F. The combination of alpha-thalassemia and high levels of Hb F appears to result in a distinctive S/S phenotype that is similar to the type of S/S disease described in Southern India.

The interaction of coexistent alpha-thalassemia and sickle cell anemia: a model for the clinical and cellular results of diminished polymerization?

Review of the studies on the interaction of alpha-thalassemia and sickle cell anemia reveals a consistently beneficial effect on the severity of anemia and the rheologic and cellular properties of sickle cells. The predominantly vaso-occlusive clinical aspects of sickle cell anemia studied are not uniformly benefited by the presence of alpha-thalassemia. These variable results may be related to the detrimental effects of higher hematocrit on whole blood viscosity. The effects of alpha-thalassemia on sickle cell anemia are probably related to a fundamental effect on MCHC and polymerization, although coexistent effects on other properties of sickle cells and the possibility that the MCHC differences may be secondary to sickling effects cannot be excluded. In total, alpha-thalassemia is an imperfect model for the effects of polymerization on the clinical consequences of sickle cell disease. An important inference from the published literature is that the severity of anemia may be a more important determinant of the ultimate clinical consequence of sickle cell anemia, life expectancy, than has previously been appreciated. Although the effect of alpha-thalassemia is an imperfect model for modified polymerization, studies of the phenotypic result of the interaction of these two genotypes will provide further insight into the causes of clinical diversity in sickle cell disease.

Interaction between HBS-beta-o-thalassemia and alpha-thalassemia

We have defined the clinical and laboratory characteristics of a group of patients with HbS-beta o-thalassemia plus alpha-thalassemia, by analysis of erythrocyte indices, hemoglobin A2 and F levels, globin biosynthesis studies and alpha-globin gene mapping. Patients with HbS-beta o + alpha-thalassemia closely resembled individuals with HbS-beta o-thalassemia except for balanced globin synthesis ratios and a lower HbF level. The frequency of painful crises, leg ulceration, aseptic necrosis of bone and acute chest syndrome was similar in HbS-beta o + alpha-thalassemia patients and controls with sickle cell anemia (HbSS), HbSS-alpha-thalassemia and HbS-beta o-thalassemia. These findings are consistent with previous work which failed to show a reduction in the vaso-occlusive severity of sickle cell disease by the coexistence of alpha-thalassemia.

The effects of alpha-thalassaemia in HbSC disease

In HbSC disease, as in sickle cell anaemia, there is a spectrum of clinical severity. A reduced mean corpuscular volume and haemoglobin concentration, traits typical of thalassaemia, might retard sickling. We therefore ascertained the prevalence of alpha-thalassaemia in 53 adults with HbSC disease and related alpha-globin gene deletion to the haematologic and clinical findings. Alpha-globin genotype was identified by restriction endonuclease gene mapping. Indirect ophthalmoscopy and fluorescein angiography were used to document the presence of proliferative retinopathy. Bone necrosis and infarction were determined roentgenographically or by radionuclide scanning. Either heterozygous or homozygous alpha-thalassaemia-2 was present in 26% of patients. There was no relationship between alpha-globin genotype and haematocrit, pain crises, bone lesions, proliferation retinopathy or clinical severity score.

Instability of beta E-messenger RNA during erythroid cell maturation in hemoglobin E homozygotes

Hemoglobin E interacts with beta-thalassemia to produce a disorder of variable severity that is the most common form of symptomatic thalassemia in Southeast Asia. The beta E-globin gene acts as a mild thalassemia gene; there are low levels of beta E-messenger RNA (mRNA) in reticulocytes, and preliminary evidence had suggested that this might be due to instability of the beta E-mRNA. Analysis of beta E-mRNA levels in the nuclei and cytoplasm of bone marrow erythroblasts compared with reticulocytes has shown higher levels of beta E-mRNA in the former, providing direct evidence that this is the case.

Thalassemia: recent insights into molecular mechanisms

Recent advances in defining the molecular basis for the thalassemia syndromes are discussed. We now realize that the causes of the thalassemia phenotype are diverse and include gene deletions, nuclear RNA processing defects, nonsense mutations, fusion genes, termination codon mutants, and unstable globin chains.

Interaction of chromosomes bearing 1, 2 or 3 alpha-globin genes in an American black family with alpha-thalassemia

Utilizing restriction endonuclease mapping and molecular hybridization we have determined the number and arrangement of the alpha-globin genes in members of an American Black family in which alpha-thalassemia is present. In addition to chromosomes bearing 0, 1 or 2 alpha-genes, an unusual chromosome bearing 3 alpha-globin genes was detected in 3 family members. In 2 family members the 3 alpha-globin gene chromosome was present opposite a chromosome containing a single alpha-globin gene; these cases represent the first reports of the alpha alpha alpha/-alpha genotype. The presence of the stigmata of "mild" alpha-thalassemia trait in one of these subjects indicates that the 3 alpha-gene chromosome probably does not direct the synthesis of significantly more alpha-globin chains than does the 2 alpha-gene chromosome.

Molecular characteristics of a non-deletion alpha-thalassaemia of the Po River Delta

The form of alpha-thalassaemia of the Po river delta presents haematological and globin biosynthetic characteristics similar to alpha-thalassaemia-1 but never gives rise to HB H disease nor to hydrops foetalis. In alpha-thalassaemic subjects originally from this region globin mRNA translation and alpha-globin gene arrangement have been investigated. The data obtained indicate that alpha-globin synthesis and reticulocyte alpha-globin mRNA are reduced by one fourth; in addition, since the defect in alpha-globin synthesis does not change with cell ageing, a possible instability of alpha-globin mRNA is excluded. Restriction enzyme analysis of the DNA shows a normal alpha-globin gene organization. This form of alpha-thalassaemia is therefore of the non-deletion type; its molecular lesion is either at the level of alpha-globin mRNA transcription or processing. The fact that this, as well as other forms of non-deletion alpha-thalassaemia, have a phenotypic expression similar to alpha-thalassaemia 1 is discussed.

Human liver aldehyde dehydrogenase in Chinese and Asiatic Indians: gene deletion and its possible implications in alcohol metabolism

Two separate human liver aldehyde dehydrogenases exist which show differences in substrate specificity, cation inhibition or activation, and molecular weight. In this paper we report a common absence of enzyme 2 in Chinese which may be taken to indicate a gene deletion coding for this enzyme. The possible implication of thie gene deletion among Chinese is discussed.

The genetics and molecular basis of alpha thalassaemia in association with Hb S in Jamaican Negroes

We have studied seven Jamaican Negro families in whom the genes for alpha thalassaemia and the sickle cell mutation (betas) were independently segregated. Using a combination of techniques we identified two alpha thalassaemia phenotypes which resemble the severe (alpha thalassaemia 1) and mild (alpha thalassaemia 2) determinants previously described in Orientals. This study has enabled us to clearly correlate the phenotype of alpha thalassaemia with the genotype in this population. Furthermore, since in each family alpha thalassaemia was present in association with the gene for the sickle cell mutation we have determined the proportion of Hb S in the peripheral blood of individuals with the alpha alpha/alpha alpha, -alpha/alpha alpha and -alpha/-alpha genotype who are also heterozygous for the betas mutation. Genetic analysis in these families shows that in each case subjects with the alpha thalassaemia 1 phenotype are homozygous for the alpha thalassaemia 2 defect (-alpha/-alpha). We have found no instances of the genotype --/alpha alpha in this population which may explain the rarity of the severe alpha thalassaemia syndromes in Jamaica. Restriction mapping data in the alpha thalassaemia 2 homozygotes from this population shows that the (-alpha/) haplotype results from a deletion of one of the linked pair of alpha globin genes and that this has probably arisen by an unequal crossover between non-homologous alpha genes.

Observations on the epidemiology of sickle cell disease

The four common genotypes of sickle cell disease in Jamaica are homozygous sickle cell (SS) disease, sickle cell-haemoglobin C (SC) disease, sickle cell-B+ thalassemia, and sickle cell-BO thalassaemia with respective incidence at birth of 3.2, 2.0. 0.34, and 0.16 per 1000 live births. Haematological indices, clinical features, and over-all prognosis vary between these genotypes and also between patients within individual genotypes. Although symptomatic selection has tended to emphasize more severely affected patients, this wide variation of clinical and haematological severity is especially apparent in SS disease. Factors contributing to this variability in SS disease include the persistence of foetal haemoglobin, the association with alpha thalassemia, and the interaction with environmental factors of which socioeconomic status is the most obvious. Further elucidation of factors determining the severity of SS disease will increase understanding of the pathogenetic mechanisms in the disease and may also identify new possibilities for therapeutic intervention.

Defective synthesis of HbE is due to reduced levels of beta E mRNA

Haemoglobin E (alpha 2 beta 2(26)Glu leads to Lys) is one of the commonest haemoglobin variants. There are an estimated 30 million carriers of the beta E gene in South-East Asia, where they comprise more than 50% of the population in some areas; however, the reasons for this high frequency have never been adequately explained. Homozygotes for HbE may be midly anaemic, but they do not have any clinical disability. However, individuals heterozygous for both beta E and beta thalassaemia (HbE/beta thalassaemia) have a severe clinical disorder which in some cases may approach that seen in homozygous beta thalassaemia and which is by far the commonest form of symptomatic thalassaemia in the Indian subcontinent and South-East Asia. Haemoglobin E is the only common structural variant which interacts with beta thalassaemia to produce a severe disorder and the underlying mechanism of the interaction is not known. We have studied several homozygotes and heterozygotes for HbE and show here that the beta E chain is inefficiently synthesized and produces the phenotype of a mild form of beta thalassaemia; hence, when inherited together with beta thalassaemia it causes a marked beta-chain deficit. Furthermore, the mechanism for the defective production of beta E chains seems to be a reduction of beta E mRNA, a most unexpected finding in a disorder caused by a single amino acid substitution and presumably by a single nucleotide change in the DNA of the beta globin gene.

Proportion of hemoglobin G Philadelphia (alpha 268 Asn leads to Lys beta 2) in heterozygotes is determined by alpha-globin gene deletions

In humans the alpha-globin genes are duplicated and closely linked. Whereas individuals heterozygous for most alpha-chain mutations possess approximately 25% abnormal hemoglobin, heterozygotes for the alpha-chain variant Hb G Philadelphia synthesize either 33% or 50% Hb G. Both variable gene dosage and interaction with alpha-thalassemia have been proposed to explain this observation. To differentiate between these models, we have performed restriction endonuclease mapping and hematological studies on individuals with Hb G from four families. In every case the alpha G locus was carried on an EcoRI or EcoRI + BamHI fragment approximately 4 kilobases shorter than that bearing the two linked alpha A loci of hematologically normal individuals. Bgl II digestion revealed that the alpha G gene is the only alpha locus on the affected chromosome. Erythrocyte indices and alpha/beta synthesis ratios indicated that the alpha G chromosome confers alpha-thalassemia. In addition to the alpha G gene, subjects who synthesized 33% Hb G possessed two alpha A genes on the homologous chromosome and exhibited the mild form of alpha-thalassemia trait ("silent carrier"). Subjects who synthesized 50% Hb G possessed a single alpha A gene trans to the alpha G locus and displayed the more pronounced form of alpha-thalassemia trait. One subject, who synthesized 100% alpha G chains and had Hb G-Hb H disease, was found to have a single nonfunctional alpha gene trans to the alpha G gene. Thus the proportion of Hb G synthesized by heterozygotes is determined by interaction with alpha-globin gene deletions cis and trans to the alpha G locus.

alpha-Thalassaemia in Sardinian infants

A haemoglobin survey carried out in southern Sardinian newborn infants showed an overall incidence of 12.9% with haemoglobin Bart's of more than 1%. The distribution was trimodal: low (1 to 2%), intermediate (2 to 10%), and high (about 25%). A considerable overlap was seen between the first two groups. Both the 1 to 2% and 2 to 10% groups had thalassaemia-like red cell indices at birth. Newborn infants ascertained as having alpha-thalassaemia at follow-up did not necessarily have unbalanced alpha/non-alpha chain synthesis at birth. At follow-up examination two subjects in the 25% group had developed haemoglobin H disease, and the 2 to 10% group had thalassaemia-like red cell indices and unbalanced globin chain synthesis ratios indicative of heterozygous alpha-thalassaemia. The 1 to 2% group either had normal or slightly reduced alpha-chain synthesis ratios, indicative of the silent alpha-thalassaemia carrier state. Two subjects with 2.0% and 2.5% haemoglobin Bart's at birth had heterozygous beta-thalassaemia at follow-up. Therefore, they were double heterozygotes for alpha- and beta-thalassaemia with alpha/beta-globin chain synthesis ratios of 0.81 and 0.86. Genotype assessment in a few families showed that infants with haemoglobin Bart's levels of more than 2% may have one of the genotypes --alpha/ --alpha or -- --/alpha alpha.

Organization of alpha-globin genes and mRNA translation in subjects carrying haemoglobin Hasharon (alpha 47 Asp replaced by His) from the Ferrara Region (Northern Italy)

In subjects carrying the haemoglobin Hasharon mutation (alpha 47 replaced by His), originally from the delta of the Po river (Northern Italy), the concentration of the alpha-globin variant has been evaluated and found to be approximately 32%, a value definitely higher than that reported for the same mutant haemoglobin in other regions. Restriction enzyme analysis has been carried out on the DNA from these subjects; the data obtained indicate the presence of three alpha-globin genes per diploid cell. Family studies further show that the two normal genes are located on one chromosome and the Hasharon gene on the other. The origin of the single alpha-gene in the Hasharon-carrying subjects of the Ferrara region is discussed in connection with their haematological and biosynthetic data.

Negro alpha-thalassaemia: genetic studies in homozygous sickle cell disease

Interaction with the alpha-thalassaemia phenotypes lowers the proportion of Hb S in the sickle cell trait and influences the mean cell volume and proportional Hb A2 in homozygous sickle cell (SS) disease. By assigning somewhat arbitrary values to the alpha-thalassaemia 1 and alpha-thalassaemia 2 phenotypes in these conditions, it has been possible to investigate the patterns of inheritance of alpha-thalassaemia in black populations. The results strongly support the hypothesis that the alpha-thalassaemia 1 phenotype represents homozygosity for alpha-thalassaemia 2.

Clinical implications of recent advances in hemoglobin disorders

The greater availability of sophisticated diagnostic procedures has led to the discovery of more than 350 abnormal human hemoglobins. Whereas most are clinically silent, in a sizeable number of variants, function anomalies and disease states. Their more clinically relevant aspects are discussed.

Hb F synthesis in sickle cell anaemia: a comparison of Saudi Arab cases with those of African origin

Fetal haemoglobin (Hb F) synthesis has been studied in 22 cases of sickle cell anemia (SS) from Saudi Arabia and compared with an equal number of cases of African origin. Among the Saudi Arabs gamma chain synthesis ranged from 4.0% to 19.9% of the total non-alpha chain synthesis (mean 8.1%) while the corresponding range for the Negro cases was < 0.3% to 4.6% (mean 1.7%). In both groups the peripheral blood Hb F level was on average 3--4 times higher than the proportion synthesized, indicating that the selective survival of Hb F containing cells (F cells) was an important factor in determining the final Hb F levels. Among the Saudi Arab cases there was a significant negative correlation between the degree of F cell enrichment and either the Hb F level of the percentage gamma chain synthesis. No such correlation was observed among the Negro cases. A high proportion of the cases in both groups were carriers of alpha thalassaemia in addition to SS, but no effect of alpha thalassaemia on Hb F production was observed.

Hematological and clinical features of sickle cell diseases: a review

The differentiation, hematologic features and clinical manifestations of patients with the various sickling disorders are reviewed. The deficiencies in our current knowledge about the spectrum of the clinical course of patients with these conditions is discussed. The interaction of alpha thalassemia with sickle cell anemia and its possible effect upon the severity of the disease is summarized. The apparent milder disease in certain groups of patients with sickle cell anemia in whom there is an associated elevation of hemoglobin F is contrasted with the controversy surrounding the effects of hemoglobin F levels in the patients of African origin.