Deletion of alpha-globin genes in haemoglobin-H disease demonstrates multiple alpha-globin structural loci

Clonal hematopoiesis and acquired genetic abnormalities of the red cell: An historical review

Several syndromes affecting the red cell that mimic those induced by germline mutations may result from a somatic mutation that accompanies a myeloid malignancy. These syndromes are most notable in cases of myelodysplastic syndrome, but they are not limited to any one category of myeloid neoplasm. Their occurrence in males exceed the male predominance that is evident in myeloid neoplasms. The syndromes include disorders of globin chain synthesis (α- and β-thalassemia), heme synthesis (erythropoietic porphyria and erythropoietic uroporphyria), red cell membrane structure (elliptocytosis and spherocytosis), red cell enzyme activity (pyruvate kinase deficiency, glucose-6-phosphate dehydrogenase deficiency) and lowered expression of red cell ABO blood group antigens. This historical review describes the path to uncovering these acquired syndromes and their causal somatic mutations, where known. These syndromes often go unrecognized because of the dominant concern of the primary neoplasm. They may add to the healthcare needs of the patient.

Genome structural variation in human evolution

Structural variation (SV) is a large difference (typically >100 bp) in the genomic structure of two genomes and includes both copy number variation and variation that does not change copy number of a genomic region, such as an inversion. Improved reference genomes, combined with widespread genome sequencing using short-read sequencing technology, and increasingly using long-read sequencing, have reignited interest in SV. Recent large-scale studies and functional focused analyses have highlighted the role of SV in human evolution. In this review, we highlight human-specific SVs involved in changes in the brain, population-specific SVs that affect response to the environment, including adaptation to diet and infectious diseases, and summarise the contribution of archaic hominin admixture to present-day human SV.

Molecular Diagnostics and Genetic Counseling in Primary Congenital Glaucoma

Primary congenital glaucoma (PCG) is a childhood irreversible blinding disorder with onset at birth or in the first year of life. It is characterized by the classical traid of symptoms viz. epiphora (excessive tearing), photophobia (hypersensitivity to light) and blepharospasm (inflammation of eyelids). The only anatomical defect seen in PCG is trabecular meshwork dysgenesis. PCG shows autosomal recessive mode of inheritance with considerable number of sporadic cases. The etiology of this disease has not been fully understood but some genes like CYP1B1, MYOC, FOXC1, LTBP2 have been implicated. Various chromosomal aberrations and mutations in mitochondrial genome have also been reported. Molecular biology has developed novel techniques in order to do genetic and biochemical characterization of many genetic disorders including PCG. Techniques like polymerase chain reaction, single strand conformational polymorphism and sequencing are already in use for diagnosis of PCG and other techniques like protein truncation testing and functional genomics are beginning to find their way into molecular workout of this disorder. In the light of its genetic etiology, it is important to develop methods for genetic counseling for the patients and their families so as to bring down its incidence. In this review, we ought to develop a genetic insight into PCG with possible use of molecular biology and functional genomics in understanding the disease etiology, pathogenesis, pathology and mechanism of inheritance. We will also discuss the possibilities and use of genetic counseling in this disease.

How to cite this article: Faiq M, Mohanty K, Dada R, Dada T. Molecular Diagnostics and Genetic Counseling in Primary Congenital Glaucoma. J Current Glau Prac 2013;7(1):25-35.

Genomic views of distant-acting enhancers

In contrast to changes in protein-coding sequences, the significance of noncoding DNA variation in human disease has been minimally explored. A recent torrent of genome-wide association studies suggests that noncoding variation represents a significant risk factor for common disorders, but the mechanisms by which they contribute to disease remain largely obscure. Distant-acting transcriptional enhancers - a major category of functional noncoding DNA - are likely involved in many developmental and disease-relevant processes. Genome-wide approaches for their discovery and functional characterization are now available and provide a growing knowledgebase for the systematic exploration of their role in human biology and disease susceptibility.

Hereditary pancreatitis caused by a double gain-of-function trypsinogen mutation

Hereditary pancreatitis, an autosomal dominant disease with approximately 80% penetrance, can be caused by both 'gain-of-function' missense and copy number mutations in the cationic trypsinogen gene (PRSS1). Here we demonstrate a heterozygous hybrid PRSS2 (encoding anionic trypsinogen)/PRSS1 gene in a French white family with hereditary pancreatitis, by means of quantitative fluorescent multiplex PCR and RT-PCR analyses. The hybrid gene, in which exons 1 and 2 are derived from PRSS2 and exons 3-5 from PRSS1, apparently resulted from a non-allelic homologous recombination (NAHR) event between the chromosome 7 homologs or sister chromatids during meiosis. Interestingly, this hybrid gene causes the disease through a combination of its inherent 'double gain-of-function' effect, acting simultaneously as a 'quantitative' copy number mutation and a 'qualitative' missense mutation (i.e. the known disease-causing p.N29I mutation). Our finding reveals a previously unknown mechanism causing human inherited disease, enriches the lexicon of human genetic variation and goes beyond the known interaction between copy number variations (CNVs) and single nucleotide substitutions in health and disease. Our finding should also stimulate more interest in analyzing both types of genetic variation whenever one tries to determine the contribution of a specific locus to a given disease phenotype.

Copy number variants and genetic traits: closer to the resolution of phenotypic to genotypic variability

A considerable and unanticipated plasticity of the human genome, manifested as inter-individual copy number variation, has been discovered. These structural changes constitute a major source of inter-individual genetic variation that could explain variable penetrance of inherited (Mendelian and polygenic) diseases and variation in the phenotypic expression of aneuploidies and sporadic traits, and might represent a major factor in the aetiology of complex, multifactorial traits. For these reasons, an effort should be made to discover all common and rare copy number variants (CNVs) in the human population. This will also enable systematic exploration of both SNPs and CNVs in association studies to identify the genomic contributors to the common disorders and complex traits.

Heterogeneity in alpha-thalassemia interactions in Malays, Chinese and Indians in Malaysia

AIM

Interactions between different determinants of alpha-thalassemia raises considerable problems, particularly during pregnancies where antenatal diagnosis is necessary. This study aims to determine the different types of deletional alpha-thalassemia and Hemoglobin Constant Spring (HbCS), and their frequency in Malays, Chinese and Indians in Malaysia.

METHODS

DNA from 650 pregnant women from the Antenatal Clinic of the University of Malaya Medical Center in Kuala Lumpur, Malaysia who showed mean cell volume < or =89 fL and/or mean cell hemoglobin < or =28 pg were analyzed for the double alpha-globin gene South-East Asian deletion (--SEA), the -alpha3.7 and -alpha4.2 single alpha-globin gene deletions and HbCS.

RESULTS

One hundred and three (15.8%) of the pregnant women were confirmed as alpha-thalassemia carriers: 25 (3.8%) were alpha-thalassemia-1 carriers with the --SEA/alphaalpha genotype, 64 (9.8%) were heterozygous for the -alpha3.7 rightward deletion (-alpha3.7/alphaalpha), four (0.6%) were heterozygous for the -alpha4.2 leftward deletion (-alpha4.2/alphaalpha), nine (1.4%) were heterozygous for HbCS (alphaCSalpha/alphaalpha) and one (0.2%) was compound heterozygous with the -alpha3.7/alphaCSalpha genotype. The double alpha-globin gene --SEA deletion was significantly higher in the Chinese (15%) compared to the Malays (2.5%) and not detected in the Indians studied. The -alpha3.7 deletion was distributed equally in the three races. HbCS and -alpha4.2 was observed only in the Malays.

CONCLUSION

The data obtained gives a better understanding of the interactions of the different alpha-thalassemia determinants in the different ethnic groups, thus enabling more rapid and specific confirmation of alpha-thalassemia in affected pregnancies where antenatal diagnosis is necessary.

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.

Studies on the in vitro and in vivo expression of a dysfunctional alpha-globin gene

In a black family with members having alpha-thalassemia and hemoglobin H (HbH) disease, a deletion of an AG dinucleotide at the 3' end of exon 1 near the junction with intron 1 was shown previously to produce a dysfunctional alpha-thalassemia gene with a reading frame-shift and a nonsense codon (Safaya S, Rieder RF: J Biol Chem 263:4328-4332, 1988). We have found that the same mutation is responsible for alpha-thalassemia and HbH disease in a second unrelated black family (Bellevue R, Dosik H, Rieder RF: Br J Haematol 41:193-202, 1979). Despite the loss of two nucleotides from the consensus sequence at the 5' splice donor site of intron 1, studies employing an in vitro plasmid-based expression system indicated that the mutant alpha-globin mRNA was spliced normally and expressed in amounts equal to normal alpha-globin mRNA in COS-7 cells. The correct processing of the mRNA in these studies is probably due to the presence of a tandem repeat of the affected AG dinucleotide. However, in reticulocytes from subjects bearing the mutant gene, we were unable to detect any of the abnormal mRNA. These findings suggest that there is accelerated post-transcriptional loss of mRNA bearing a premature terminator codon.

The different types of alpha-thalassemia-2: genetic aspects

The alpha-thal-2 haplotype is the most common cause of alpha-thal and is found in a great many of the world's populations. It is most commonly due to the deletion of a single alpha-globin structural gene, and the deletion of a single alpha locus using gene mapping methods provides objective diagnostic evidence for this haplotype. Seven varieties of nondeletion syndromes have also been described, but these seem to be responsible for a small minority of alpha-thal in all populations studied. Detailed structural analyses of the deletional types have revealed that they were caused by a wide variety of DNA recombinant events, the racial difference of which remain poorly understood.

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).

Combination of three alpha-globin gene loci deletions and hemoglobin New York results in a severe hemoglobin H syndrome

A chinese patient with hemoglobin (Hb) H and Hb New York is described. Unlike classical Hb H disease, the additional beta New York chain defect resulted in a more marked alpha/beta-globin chain imbalance in this patient with only one alpha-gene, accounting for her severe anemia (3.4-6.8 g/dl) during the steady state.

Molecular basis of hereditary elliptocytosis due to protein 4.1 deficiency

Hereditary elliptocytosis is a heterogeneous group of red-cell disorders characterized morphologically by oval-shaped erythrocytes. It is an infrequent but well-defined cause of hemolytic anemia. To investigate the molecular abnormality in this disorder, we examined the arrangement of the gene encoding protein 4.1, a membrane skeletal protein of human erythrocytes, in an Algerian family with hereditary elliptocytosis and severe hemolytic anemia due to complete deficiency of protein 4.1. Southern blot analysis of genomic DNA from hematologically normal and abnormal members of this family showed that the mutant gene had a DNA rearrangement upstream from the initiation codon for translation. The mRNA from the mutant locus was aberrantly spliced, and that abnormality was the most likely explanation for the deficiency of protein 4.1 in the family. We assigned the gene for erythrocyte protein 4.1 to the short arm (p) of chromosome 1, within a region from band 32 to the terminus (1p32----1pter). Other cases of hereditary elliptocytosis of unknown cause have been mapped to the same region of chromosome 1 by linkage to the Rh locus. The chromosomal location of the gene that codes for protein 4.1 suggests that hereditary elliptocytosis in one class of patients with the disorder may be caused by a mutation of this gene.

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.

A leftward deletional alpha+ thalassemia found in East Sicily in conjunction with heterozygous beta-thalassemia

Two types of alpha+ thalassemia (-alpha l) have been described, respectively termed leftward and rightward, which correspond to nonhomologous crossing-over in different homology zones X and Z within the alpha-globin gene cluster. Up to now the leftward type has been described only in Asiatic populations, whereas the rightward type is universally distributed. We report here a first case of leftward deletion observed in a Sicilian male. This raises the question of an identical or not crossing-over event.

Diagnosls of α thalassemia in the newborn. cord blood survey utilizing gene mapping

Application of the technique of gene mapping has made possible accurate assessment of the alpha thalassemia defect in cord blood samples obtained in a Sydney teaching hospital. Results showed a 5.2% incidence of the alpha+ thalassemia haplotype in the population being tested. Various hematological parameters such as hemoglobin, mean corpuscular hemoglobin, hemoglobin EPG pattern at pH 8.9 and the percentage of hemoglobin Bart's determined by elution at pH 6.7 were measured to assess which would be the most useful in diagnosis of alpha thalassemia at this time of life. The most consistent test proved to be hemoglobin EPG pattern at pH 8.9, and reasons for this are discussed.

Structural and evolutionary analysis of the two chimpanzee alpha-globin mRNAs

Two distinct alpha-globin mRNAs were detected in chimpanzee reticulocyte mRNA using a primer extension assay. DNA copies of these two mRNAs were cloned in the bacterial plasmid pBR322, and their sequence was determined. The two alpha-globin mRNAs have obvious structural homology to the two human alpha-globin mRNAs, alpha 1 and alpha 2. Comparison of the two chimpanzee alpha-globin mRNAs to each other and to their corresponding human counterparts revealed evidence of a recent gene conversion in the human alpha-globin complex and a marked heterogeneity in the rate of structural divergence within the alpha-globin gene.

A mouse model for beta-thalassemia

A mutation that produces an absolute deficiency of normal beta-major globin polypeptides has been recovered from a DBA/2J male mouse. Most mice homozygous for the deficiency survived to adulthood and reproduced but were smaller at birth than their littermates and demonstrated a hypochromic, microcytic anemia with severe anisocytosis, poikilocytosis, and reticulocytosis and the presence of inclusion bodies in a high proportion of circulating erythrocytes. Mice heterozygous for the deficiency demonstrated a mild reticulocytosis but were not clinically anemic. Analysis of globin chain synthesis in vitro by 3H-leucine incorporation revealed that beta-globin synthesis was nearly normal (95%) in heterozygotes and about 75% of normal in deficiency homozygotes. Molecular characterization of the mutation by restriction analysis revealed a deletion of about 3.3 kb of DNA, including regulatory sequences and all coding blocks for beta-major globin. Based on genetic and hematological criteria, mice homozygous for the mutant allele, designated Hbbth-1, represent the first animal model of beta-thalassemia (Cooley's anemia), a severe genetic disease of humans.

Duplication of the hemoglobin alpha-chain gene in sheep: characterization of a new alpha-chain variant present in animals possessing the alpha Leu and the II alpha His chains

The structural characterization of a third type of alpha chain, detected in the hemoglobin of sheep also possessing the alpha Leu and the II alpha His chains, is reported. The new alpha-chain variant is an allele of the common alpha Leu chain controlled by the 1 alpha locus and differs from it in the replacement of the serine residue at position 8 with an alanine (I alpha 8 Ser leads to Ala). The alanine variant was observed in 38 of 206 sheep, whose hemolysates were analyzed by isoelectric focusing, and was found exclusively among the 57 animals also possessing the II alpha His chain.

Homozygous deletional alpha + thalassaemia associated with unequal expression of the two remaining alpha 1 genes (alpha 1A and alpha 1Q)

A Cambodian family presenting several haemoglobinopathies, Hb E, Hb Q and alpha + thalassaemia, has been investigated. DNA analysis showed that the thalassaemia syndrome corresponds to a leftward type (4.2 kb) deletional form of alpha + thalassaemia. Genotypes found in the family are: propositus -alpha A/-alpha Q, beta A/beta E., mother and older sister alpha A alpha A/-alpha Q, beta A/beta E., father alpha A alpha A/-alpha A, beta A/beta A. The propositus consistently presents an alpha Q/alpha A chain ratio of 60/40 although both chains are products of alpha 1 loci. The relatively higher expression of the alpha Q chain is not observed in the mother and therefore makes it unlikely to reflect anything other than differential expression of the maternal -alpha Q/ and paternal -alpha A/ haplotypes. This observation raises the possibility that both haplotypes are not strictly identical and that the region of the cross-over event is important for alpha gene expression.

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.

Homozygous alpha thalassemia/Hb G Philadelphia

Microcytic red cells from a 70 year old Negro man with mild anemia contained only hemoglobin G-Philadelphia. Red cells from all of his children had low-normal MCV's, and contained 32-34 percent of the abnormal hemoglobin. Oxygen affinity of his blood and stability of his hemolysate were normal, suggesting that his mild anemia was not caused by the the abnormal hemoglobin. Restriction endonuclease analyses of DNA from the proband and his offspring showed that the alpha G-Philadelphia globin gene exists in only one copy per chromosome. The new gene was probably created by an unequal cross-over which deleted an alpha globin coding sequence (derived from one or both alpha globin genes), as well as some or all of the DNA sequence between those genes.

Influence of two alpha-globin gene deletions on homozygous beta 0-thalassemia

The favorable influence of alpha-thalassemia due to a deletion of two alpha-globin genes (alpha-thal-l) on homozygous beta 0-thalassemia in an adult Chinese woman is described. Her clinical and hematologic condition was milder than usually seen in homozygous beta 0-thalassemia. With the help of repeated blood transfusions immediately before delivery she gave birth to a physically healthy baby who had the alpha-thal-l trait due to two alpha-globin gene deletions with Hb Bart's in the cord blood.

Duplication/deletion polymorphism 5' - to the human beta globin gene

DNA sequence analysis of the human beta globin locus has identified an array of simple tandem repeated sequences upstream from the beta globin structural gene. Comparison of several cloned human beta globin alleles demonstrated a high frequency of sequence heteromorphism at this site apparently due to duplication or deletion of single units of the repeat array. At least two such duplication/deletion events are necessary to account for the observed variation. No other sequence variation was observed, suggesting that duplication/deletion events within the tandem repeat array may be at least 13 to 14 times more frequent than nucleotide substitutions in the surrounding DNA.

beta zero thalassemia in Sardinia is caused by a nonsense mutation

We report the characterization of a molecular lesion of beta thalassemia in Sardinia. Beta thalassemia in this area is predominantly the beta zero type with low levels of beta-globin mRNA. Translation assay of this messenger RNA in a cell-free system showed beta-globin chain synthesis only with the addition of an amber (UAG) suppressor transfer RNA. Double-stranded complementary DNA prepared from reticulocyte mRNA from a Sardinian patient was cloned in a bacterial plasmid and a beta-globin complementary DNA containing clone was isolated and sequenced. At the position corresponding to amino acid number 39, a single nucleotide mutation converted a glutamine codon (CAG) to an amber termination codon (UAG). We previously reported an amber nonsense mutation at amino acid 17 as a cause of Chinese beta zero thalassemia. Thus, beta zero thalassemia in Sardinia represents the second example of a nonsense mutation, and we predict that other beta zero thalassemias with mutations at various points along the beta-globin chain will be found to form a discrete subgroup of beta zero thalassemia. These experiments further illustrate the heterogeneity of lesions that lead to defective globin chain synthesis in beta thalassemia.

Two types of triplicated alpha-globin loci in humans

DNA from healthy Malaysian newborns was studied on gene maps after digestion with different restriction endonucleases. Of 65 newborns, two were found to be carriers of two different variants of triplicated alpha-globin loci. In variant no. 1, found in an Malay, the three alpha-globin genes are in an elongated DNA fragment on digestion with Eco RI and Bam HI. The third alpha-globin gene was found in a additional 3.7-kb fragment on digestion with Hpa I, Bgl II and Hind III. In variant no. 2, a new type of triplicated alpha-globin loci, found in a Chinese, the three alpha-globin genes reside in an elongated DNA fragment longer than that of variant no. 1 on digestion with Eco RI and Bam HI. The third alpha-globin gene was found in an additional 4.2-kb fragment on digestion with Hpa I and Hind III. Digestion of this variant DNA with Bg1 II produced an abnormal 16.7-kb fragment in addition to the normal 7.0-kb Bgl-II fragment. The locations of the restriction sites in the two types of triplicated alpha-globin loci are compatible with a mechanism of unequal crossing over following two different modes of misalignment.

Differentiation of the mRNA transcripts originating from the alpha 1- and alpha 2-globin loci in normals and alpha-thalassemics

The alpha-globin polypeptide is encoded by two adjacent genes, alpha 1 and alpha 2. In the normal diploid state (alpha alpha/alpha alpha) all four alpha-globin genes are expressed. Loss or dysfunction of one or more of these genes leads to deficient alpha-globin production and results in alpha-thalassemia. We present a technique to differentially assess the steady-state levels of the alpha 1- and alpha-2-globin messenger RNA (mRNA) transcripts and thus delineate the relative level of expression of the two alpha-globin loci in a variety of alpha-thalassemia states. Only alpha 1 mRNA was produced in the alpha-thalassemia-2 haplotype (-alpha) (one of the two alpha-globin genes deleted from chromosome 16). This confirms previous gene mapping data which demonstrate deletion of the alpha 2 gene. The triple alpha-globin gene haplotype (alpha alpha alpha) is the reciprocal of the alpha-thalassemia-2 haplotype and thus contains an extra alpha 2-globin gene. RNA from this haplotype contained a greater than normal level of alpha 2-relative to alpha 1-globin mRNA. This data implies that the extra alpha 2 gene in the triple alpha-globin haplotype is functional. We detected a relative instability of the alpha 2-globin mRNA encoding the alpha-globin structural mutant Constant Spring. This instability may contribute to the low level of expression of the alpha-Constant Spring protein. In a Chinese patient with nondeletion hemoglobin-H disease (- -/alpha alpha T) (both alpha-globin genes are present but not fully functional) a normal ratio was maintained between the levels of alpha 1- and alpha 2-globin mRNA, implying that mRNA production from both alpha-globin genes is suppressed in a balanced manner. These observations extended previous findings concerning the structural rearrangements in the deletion types of alpha-thalassemia and the pathophysiology of two nondeletion variants.

Alpha-thalassemia and the production of different alpha chain variants in heterozygotes

The production of five alpha chain variants (Hb G-Georgia, Hb St. Luke's, Hb Lloyd, Hb Montgomery, and Hb G-Philadelphia) in heterozygotes was evaluated through hematological observations, hemoglobin quantification, and biosynthetic studies. All heterozygotes for Hb St. Luke's and Hb Lloyd and most heterozygotes with Hb G-Georgia and Hb Montgomery had normal hematology and average sigma alpha/beta values of about 1.1. They were assigned a normal genotype (alpha alpha G/alpha alpha), although the proportions of Hb St. Luke's and Hb G-Georgia were low (10 to 13%) and those of Hb Lloyd and Hb Montgomery twice as high (20%). Data from short-term incubations confirmed this genotype for some of these heterozygotes. Isolated Hb St. Luke's and Hb G-Georgia gave low alpha G/beta values (0.2 and 0.3) indicating that these Hb variants were defective at the level of Hb assembly. Isolated Hb Montgomery and Hb G-Philadelphia, however, gave higher alpha G/beta values of 0.6 and 0.8, respectively. A second type of variability existed among Hb G-Georgia (20 vs. 13%), Hb Montgomery (28 vs. 20%), and Hb G-Philadelphia (47 vs. 34%) heterozygotes, in whom the levels of Hb G differed. The occurrence of higher levels of these three alpha chain heterozygosities was associated with hematological or biosynthetic evidence of a mild or moderate alpha chain deficiency due to an alpha-thalassemia-2 heterozygosity (alpha alpha G/alpha O alpha or alpha O alpha G/alpha alpha) or a homozygosity (alpha O alpha G/alpha O alpha), respectively.

Homology and concerted evolution at the alpha 1 and alpha 2 loci of human alpha-globin

The identical structure of two racially distinct alpha 1-globin alleles and the high degree of homology between the alpha 1- and alpha 2-globin loci indicate that mechanisms exist for suppression of allelic polymorphisms and for exchange of genetic information within the alpha-globin gene complex.

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.

Hemoglobin Shuangfeng (alpha 27 (B8) Glu substituting for Lys): a new unstable hemoglobin variant

An unstable hemoglobin variant was found in a 24-year-old Chinese of Han origin. It had an electrophoretic mobility slightly faster than that of Hb A2, but could not be separated from it completely by cellulose acetate electrophoresis (pH 8.6). Chromatography on DE-52 column showed that the variant amounted to approximately 13% of the total hemoglobin. Structural analysis showed that the hemoglobin had the structure alpha 27 (B8) Glu substituting for Lys. It was named Hemoglobin Shuangfeng. The alpha 27 Glu residue is an invariable residue located near the alpha 1 beta 1 contact.

Analysis of globin gene structure in patients with beta thalassemia by restriction endonuclease mapping

Twenty-six DNA samples from individuals either heterozygous or homozygous for beta thalassemia were analyzed by restriction endonuclease digestion, agarose gel electrophoresis, and Southern blot analysis to define DNA fragments containing portions or all of the beta globin gene. A total of twenty-seven genes affected by a beta thalassemia mutation and twenty-seven genes affected by a beta thalassemia mutation and twenty-two normal beta globin genes were examined in Italian, Greek, or Asian individuals. With all four restriction endonucleases used, the fragments generated from DNA of thalassemic individuals were identical to those found in DNA from normal. Thus, gross rearrangement or deletion within the genomic region containing the beta globin gene is not characteristic of mutations which cause a thalassemia. A third patient homozygous for pancellular hereditary persistence of fetal hemoglobin was shown to have complete deletion of the delta and beta globin genes.

A new genetic basis for hemoglobin-H disease

We studied 11 families with alpha-thalassemia from the Qatif population of eastern Saudi Arabia to determine the genetic and molecular basis of hemoglobin-H disease, which is being encountered in this area with increasing frequency. The results show that there are two common alpha-thalassemia haplotypes, a deletion (-alpha/) determinant and a nondeletion (alpha alpha T/) determinant, which interact to produce a series of overlapping phenotypes. The most severe, hemoglobin-H disease, results from the homozygous state for the nondeletion determinant--a pattern of inheritance not previously recognized for this condition. Its molecular and genetic properties are thus different from those that produce the condition in Oriental or Mediterranean populations.

Hematological evaluation of patients with various combinations of alpha-thalassemia

Six patients and their parents from five different families with Hb H have been evaluated clinically and hematologically. Previous studies using restriction endonuclease mapping technique indicated that alpha-thalassemia determinants in these cases are heterogeneous. Only one of the five cases have the usual genotype for Hb H, which is characterized by an alpha-DNA-specific fragment of 20 kb long by Eco RI digestion. Three cases from two different families have Hb H disease with alpha-specific DNA fragments of 22.5 kg/2.6 kg long; and the other two have alpha-specific DNA fragments of 20 kb/2.6 kg long, in Eco RI digestion of the cellular DNA. The hematological examination of the parents suggests that the alpha-thalassemia condition associated with the Eco RI fragment of alpha-specific cellular DNA of approximately 22.5 kb long produces an alpha-thal-2-like clinical condition, while the other alpha-thalassemia determinant associated with a fragment 2.6 kg long results in an alpha-thal-1-like clinical condition. The clinical and hematological findings of the cases with 22.5 kb/2.6 kb fragment patterns were more severe than the case with the 20 kb/2.6 kb combination. This study suggests that variation in the clinical and hematological findings among patients with Hb H disease may well reflect a heterogeneity of the genotype combination.

The genetic basis of Hb Q-H disease

A Chinese family has been studied in which two siblings have haemoglobin Q-H disease. Using a combination of haematological and haemoglobin analysis, globin chain synthesis, analysis of alpha/beta globin messenger RNA ratios and restriction endonuclease mapping, it has been shown that each of these siblings has received one chromosome on which both alpha chain genes have been deleted and another on which there is only a single alpha chain locus which carries the alpha Q mutation. Their genotype is thus --/-alpha Q. Despite the fact that the haemoglobin Q mutation in this family is carried on a chromosome with a single alpha chain locus, heterozygous carriers for the variant have only 25% or less haemoglobin Q. Our observations indicate that the molecular basis for haemoglobin Q-alpha thalassaemia is similar to that for the common form of haemoglobin H disease in Orientals. Furthermore, they provide clear evidence that the level of an alpha chain variant in heterozygous carriers is not a reliable reflection of the number of alpha globin genes.

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.

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.

The molecular basis of disorders of human hemoglobin synthesis

The structure and organization of the human globin genes at the nucleotide level has been established by restriction endonuclease digestion of cellular DNA, and by the isolation and purification of these genes in phage vectors. With this approach it has been possible to define alterations at the DNA level resulting in a group of inherited diseases of man known as the thalassemia syndromes, and related disorders. Combined with other known genetic and biochemical data, these studies provide a framework for understanding the pathogenesis of these disorders at the molecular level.

The human alpha-globin gene. The protein products of the duplicated genes are identical

In order to determine whether any heterogeneity exists in the human alpha-globin chain, i.e. whether the products of the duplicated genes are identical, we have determined the total sequence of 14 alpha-globin chains: seven of these were abnormal, while six were normal chains from the same individuals, with one additional sample which consisted of the alpha chains from a normal control. In the individuals heterozygous for an alpha-chain abnormality, the product of a single alpha-gene could be isolated from that of the three others using the differing physicochemical properties of the mutant haemoglobins. In the special situation of a double heterozygosity for an alpha-chain abnormality, the products of the two mutated genes were separated from each other and from the mixture of the products of the two normal genes. They were then investigated independently, this approach increased the precision of our work. During the course of the present investigation, sequence determinations were mainly performed on large fragments of the chains, which were purified exclusively by gel chromatography. In this way mixtures of products of several genes could be studied, thereby overcoming the risk of losing peptides differing slightly in sequence. Such loss may often occur when using ion-exchange procedures to purify small peptides. Our results show the absence of any heterogeneity at the level of the gene products of the duplicated alpha loci. Thus the human alpha-globin chain has to be considered as homogeneous.

Linkage of alpha G-Philadelphia to alpha-thalassemia in African-Americans

We have studied the inheritance of the alpha-chain hemoglobin variant Hb G-Philadelphia (alpha 2(68 Asn leads to Lys)Beta 2) in two African-American families. Expression of the alpha-globin loci was monitored by the percentage of Hb G in these individuals. The variant represented approximately 33% of the total adult hemoglobin in some and 50% in others. alpha-Globin gene fragments were analyzed by using restricton endonucleases that cleave outside (EcoRI), within (HindIII), and between (Bgl II) the normal duplicated alpha-globin loci (alpha alpha/alpha alpha). Individuals having 33% variant lack one functioning alpha gene (alpha G/alpha alpha); those with 50% variant lack two genes, one missing on each chromosome (alpha G/alpha). Inheritance of alpha G was therefore linked to that of a chromosome with only one functional alpha-globin gene locus. This locus is probably the result of a nonhomologous crossover. Our results also suggest equal expression of the alpha-globin loci in humans because the percentages of the variant could be explained solely on the basis of the total number of alpha genes present. The percentages of Hb G as well as other hematologic data all were consistent with the number of alpha-globin genes identified by restriction endonuclease mapping. Gene mapping yields a more precise determination of the number of alpha-globin genes than does study of globin synthesis.

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.

alpha Thalassemia and the expression of hemoglobin G-Philadelphia

The findings presented for these two families help to explain the inheritance of alpha thalassemia, alpha-chain variants, and the relative expression of alpha genes. An 18.0-kb EcoRI fragment contains only one functional alpha gene, whereas a 20.5-kb fragment contains two. Individuals homozygous for the 18.0-kb EcoRI fragment also lack the 4.1-kb HindIII fragment that normally connects the centers of the duplicated alpha genes. These findings are consistent with a deletion involving the 5' alpha-gene locus. Presence of alpha G-Philadelphia in both families was found in association with the 18.0-kb EcoRI fragment; this short fragment was also found in an individual with alpha A. Inheritance of alpha G-Philadelphia at one alpha-gene locus was therefore also linked to the inheritance of alpha thalassemia due to a deletion involving the second alpha gene. The high percentage (46% to 48%) of alpha G found in some family members was due to alpha-thalassemia trait, or deletion of two alpha genes (-alpha G/-alpha); others with levels of variant of 32% to 34% were shown to have three functional alpha genes (-alpha G/alpha alpha). The genetic expression of the four normal alpha genes therefore appears to be equal and furthermore implies the existence of separate independently functioning transcriptional units for each of these genes in humans. It would be interesting to analyze the alpha genes in Afro-Americans reported to have alpha G-Philadelphia in the 20% to 25% range to determine whether the inheritance of alpha G can be linked to a normal alpha gene.

Determination of alpha thalassaemia phenotypes by messenger RNA analysis

The possibility of using alpha/beta globin messenger RNA (mRNA) ratios to distinguish between the carrier states for different forms of alpha thalassaemia has been explored. Alpha/beta globin mRNA ratios were determined in the red cells of a series of normal individuals and in members of four Cypriot families, one Thai and one Chinese family in which at least one person has haemoglobin H disease. It was found that there was a clear distinction in the ratios between normals, alpha thalassaemia 1 carriers, alpha thalassaemia 2 carriers, and those with haemoglobin H disease. This method should be a valuable addition to haematological analysis, haemoglobin synthesis and restriction mapping of DNA for the further elucidation of the genetics of alpha thalassaemia.