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

Haemoglobin GPhiladelphia and its interaction with haemoglobin S and alpha-thalassaemia in Nigerians

The diagnosis of Hb SS/GPhiladelphia disease was made in four young Nigerians from separate families. Their Hb electrophoretic patterns on cellulose acetate membrane at alkaline pH were similar to those obtained in sickle-cell haemoglobin C (HbSC) disease, but their clinical features and haematological data were consistent with the diagnosis of homozygous sickle-cell disease. Family studies also revealed that they had inherited an additional alpha-chain mutant haemoglobin. In one of the families, fingerprints of the globin peptides and amino acid analysis confirmed that the mutant haemoglobin was Hb GPhiladelphia (alpha 2 68 Asn----Lys beta 2 A). The results of the whole blood solubility test for sickle-haemoglobin provided firm support for the diagnosis of homozygous sickle-cell disease and distinguished clearly Hb SS/GPhiladelphia disease from Hb SC disease and Hb AS from Hb AGPhiladelphia heterozygotes. Restriction endonuclease mapping of the globin genes of the propositus and some relatives of one of the families revealed also that they were carriers of the alpha-thalassaemia-2 gene (deletion-type). The globin gene-analysis data indicate also that the alpha GPhiladelphia and alpha-thalassaemia genes are linked closely in Nigerians.

Molecular Interactions Between Hb α-G Philadelphia, HbC, and HbS: Phenotypic Implications for SC α-G Philadelphia Disease

We show here that αG-Phila.2βC2 has an increased rate of crystal nucleation compared to α2 βC2 (HbC). We conclude from this finding that position α68, the mutation site of αG-Phila.2 β2 (HbGPhiladelphia), is a contact site in the crystal of HbC. In addition, that HbS enhances HbC crystallization (additive to the effect of αG-Phila, as shown here) and that αG-Phila. inhibits polymerization of HbS are pathogenically relevant previously known facts. All of these findings help explain the phenotype of an individual simultaneously heterozygous for the βS, βC, and the αG-Phila. genes (SCα-G Philadelphia disease). This disease is characterized by a mild clinical course, abundant circulating intraerythrocytic crystals, and increased folded red cells. This phenotype seems to be the result of increased crystallization and decreased polymerization brought about by the opposite effects of the gene product of the αG-Phila. gene on the βC and βS gene products. Some of the intraerythrocytic crystals in this syndrome are unusually long and thin, resembling sugar canes, unlike those seen in SC disease. The mild clinical course associated with increased crystallization implies that, in SC disease, polymerization of HbS is pathogenically more important than the crystallization induced by βC chains. The SCα-G Philadelphia disease is an example of multiple hemoglobin chain interactions (epistatic effect among globin genes) creating a unique phenotype.

Molecular Interactions Between Hb α-G Philadelphia, HbC, and HbS: Phenotypic Implications for SC α-G Philadelphia Disease

We show here that αG-Phila.2βC2 has an increased rate of crystal nucleation compared to α2 βC2 (HbC). We conclude from this finding that position α68, the mutation site of αG-Phila.2 β2 (HbGPhiladelphia), is a contact site in the crystal of HbC. In addition, that HbS enhances HbC crystallization (additive to the effect of αG-Phila, as shown here) and that αG-Phila. inhibits polymerization of HbS are pathogenically relevant previously known facts. All of these findings help explain the phenotype of an individual simultaneously heterozygous for the βS, βC, and the αG-Phila. genes (SCα-G Philadelphia disease). This disease is characterized by a mild clinical course, abundant circulating intraerythrocytic crystals, and increased folded red cells. This phenotype seems to be the result of increased crystallization and decreased polymerization brought about by the opposite effects of the gene product of the αG-Phila. gene on the βC and βS gene products. Some of the intraerythrocytic crystals in this syndrome are unusually long and thin, resembling sugar canes, unlike those seen in SC disease. The mild clinical course associated with increased crystallization implies that, in SC disease, polymerization of HbS is pathogenically more important than the crystallization induced by βC chains. The SCα-G Philadelphia disease is an example of multiple hemoglobin chain interactions (epistatic effect among globin genes) creating a unique phenotype.

Case report: alpha G-Philadelphia, beta O-Arab, and beta C globins present in a single patient

The case of a 7-month-old Nigerian child who presented with anemia and microcytosis is described. Hemoglobin electrophoresis studies revealed a band with pronounced cathodic mobility. This represented a heterohybrid hemoglobin tetramer composed of an alpha-globin mutant, G-Philadelphia (alpha GPhil), and two variant beta-globin chains, beta C and beta O-Arab. The absolute amounts of alpha GPhil found in the propositus were less than expected for an alpha 2-globin gene product. It has not been established whether alpha G-Philadelphia interacting with beta O-Arab and beta C globin chains is the cause of the microcytosis.

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).

Haemoglobin gene frequencies in the Jamaican population: a study in 100,000 newborns

The gene frequencies of abnormal haemoglobins have been determined in a group of 100,000 Jamaican newborns screened over a period of 8 1/2 years. The population is predominantly of West African origin and the survey represents approximately one quarter of all island deliveries within the period of the study. The common beta globin chain abnormalities beta s and beta c occurred with gene frequencies of 0.055 and 0.019 respectively; beta thalassaemia was relatively rare. In contrast, alpha thalassaemia was quite common, occurring with a gene frequency of 0.183. In addition to these common abnormalities, the frequencies of 256 rare abnormal haemoglobins are described. This survey thus represents a complete and accurate documentation of the alpha and beta globin variants that occur in the Jamaican population.

The haematological puzzle of Hb J Cape Town is partly solved

Molecular studies have shown that the mutation giving rise to Hb J Cape Town (alpha 92 arg----gln) is situated on a chromosome from which the other alpha-globin gene has been deleted. The -alpha 3.7 deletion has resulted from crossing-over within segment I of the Z region of homology. There appears to be an unusually high proportion of the variant haemoglobin in heterozygotes which cannot be explained by gene dosage alone.

Locus assignment of alpha-globin structural mutations by hybrid-selected translation

The two human alpha-globin genes, alpha 1 and alpha 2 located 3.4 kilobases apart on chromosome 16, encode identical alpha-globin proteins. A mutation in either gene could result in a structural hemoglobinopathy. It has only recently become possible to assign an alpha-chain mutant to one of these two loci by using recombinant DNA technology. While definitive, this approach has necessitated the cloning and sequencing of the specific gene in question. We present an alternative approach which results in rapid and definitive assignment of an alpha-globin mutation to its encoding genetic locus. This approach uses the technique of hybrid-selected translation. Reticulocyte RNA from individuals with alpha-globin mutations can be fractionated into beta-, alpha 9 (total)-, alpha 1-, and alpha 2-globin mRNA by selective hybridization of each mRNA species to its respective complementary DNA (cDNA) immobilized on nitrocellulose paper. Each mRNA purified in this way can be translated in vitro, and the mRNA species (and hence gene locus) encoding the globin mutant can then be directly identified by gel analysis of the radiolabeled translation products. This procedure can be used to identify globin mutants as alpha or beta and to localize alpha-globin mutants to the alpha 1 or alpha 2 gene. We have used this technique to localize the two alpha-globin mutants, alpha 125Pro (Hb Quong Sze) and alpha 47HIS (Hb Hasharon), to the alpha 2 locus. This approach could potentially be expanded to serve as an alternative to peptide analysis for the initial characterization of all globin structural mutants.

Hemoglobin I mutation encoded at both alpha-globin loci on the same chromosome: concerted evolution in the human genome

Genetic analysis of an individual expressing an unexpectedly high level of hemoglobin I, an alpha-globin structural mutant, reveals that the mutation is present at both the alpha 1- and the alpha 2-globin gene loci. Kindred analysis confirms that the two affected genes are located in cis. The most likely explanation for this finding is that a recent conversion event occurred within the human alpha-globin gene cluster.

Hemoglobin Evanston (alpha 14 Trp----Arg). An unstable alpha-chain variant expressed as alpha-thalassemia

A new hematologic syndrome with phenotypic features of mild Hb H disease was identified in three children from two unrelated black American families. Erythrocytes from each of these children contained Hb H (beta 4) and Hb Barts (gamma 4), as well as a slowly migrating hemoglobin fraction that made up 7-10% of the total hemoglobin. The parents of the affected children all showed mild thalassemia-like changes, with one of the parents in each family also expressing the variant hemoglobin; in the latter individuals the mutant alpha-chains made up less than 2% of the total, and were present mainly or exclusively in combination with delta-chains in the form of a slowly migrating Hb A2. Purified Hb Evanston showed an increased oxygen affinity, but its Bohr effect, cooperativity, and 2,3-diphosphoglycerate effect were normal. The mutant hemoglobin appeared to have normal stability to heat and to isopropanol, and the stability of its alpha-chain in an extended time course synthesis study also appeared to be similar to that of alpha A. However, the results from short-term globin synthesis studies, and from mRNA translation in vitro, suggest that the two types of alpha-chains were synthesized at relatively equal rates, with a major fraction of the newly synthesized variant alpha-chains undergoing rapid catabolism. The hematologic data taken in combination with DNA hybridization and globin synthesis findings indicate that the proposita in each of these families has the genotype--, alpha A/--, alpha Ev. These observations suggest that two separate mechanisms are contributing to the alpha-thalassemia-like expression of Hb Evanston : the newly synthesized alpha EV-chains are unstable and are subject to early proteolytic destruction; and the mutant alpha-allele is linked to an alpha-globin gene deletion.

Alpha-globin gene deletions associated with alpha A and alpha G Philadelphia in an Algerian family that includes two Hb G homozygotes

An Algerian family with a high degree of consanguinity and including two homozygotes for Hb-G Philadelphia is presented. Whether homozygotes or heterozygotes, all subjects displayed microcytosis (with various degrees of poikilocytosis) and a moderately depressed alpha-globin chain synthesis. Hb H and Heinz bodies were absent. DNA mapping revealed the presence of a 3.7 kb deletion resulting from the rightward type of recombination event between alpha 2 and alpha 1 genes on both the alpha A/ and the alpha G/ chromosomes. Such data indicate that the -alpha A/ and -alpha G/ haplotypes are involved and suggest that the -alpha G/ haplotype, which is very rare in Algeria, has an African Black origin. In subjects with genotype (-alpha A/-alpha G) or (-alpha G/-alpha G), the output of the remaining alpha genes is sufficiently high to avoid the appearance of Hb H. This situation contrasts with that reported in an Algerian patient, who had a (-alpha A/-alpha A) genotype but who was producing Hb H (Whitelaw et al. 1980). The data collected from this family suggest that the -alpha A/ haplotypes are heterogeneous in Algerians.

Alternate organization of alpha G-Philadelphia globin genes among U.S. black and Italian Caucasian heterozygotes

Seven Hb G-Philadelphia (Hb G) heterozygotes from three Caucasian families from Northern Italy and Sardegna were found to have proportions of Hb G averaging 23%. This value is considerably lower than the 34% or 48% found in Blacks from the Southeastern U.S.A. in whom the alpha G gene is in linkage with alpha-thalassemia-2, i.e. the alpha o alpha G/alpha alpha or alpha o alpha G/alpha o alpha genotypes. Gene mapping identified tandem organization of the alpha G gene in cis with a normal alpha A gene, i.e. the alpha alpha G/alpha alpha genotype, among the Hb G heterozygotes from Italy. The data on the Italian heterozygotes are similar to those obtained by Bruzdzinski et al (14) on a Black family. These results indicate alternate organization of the alpha G genes probably across racial or ethnic boundaries. Comparison of the mean cellular globin amount of alpha G/alpha G gene/cell among Hb G heterozygotes with 4, 3, 2 or 1 alpha globin genes (i.e. alpha A + alpha G) revealed considerable reactivation of individual alpha genes in conditions of mild to severe alpha globin deficiencies.

The occurrence of the alpha G-Philadelphia-globin allele on a double-locus chromosome

Hb G-Philadelphia, an alpha-globin allele, is expressed as either 20%, 30%, or 40% of the total hemoglobin. Restriction analyses published thus far have shown that among persons with 30% and 40% hemoglobin (Hb) G the alpha G allele is seen only in a single-locus haplotype. We now report the identification of a second haplotype in which the alpha G allele is found in tandem with an alpha A allele. This haplotype has been found present in DNA from the members of one family in which Hb G is expressed as 20% of the total hemoglobin, determined by both cellulose acetate electrophoresis and high-performance liquid chromatography (HPLC). Synthesis was balanced in all individuals. The identification of a variant alpha-globin allele in two distinct haplotypes presents the possibility of independent mutation. However, an alternative explanation cannot be ruled out; namely, that the original allele may have become distributed among the two haplotypes by unequal crossing-over.

alpha Chain and gamma chain abnormal hemoglobins in newborn babies: structural and genetic aspects

Structural studies and quantitative analyses were conducted on the hemoglobin of 55 newborn babies. Seven alpha chain variants (G-Philadelphia, Montgomery, Inkster, I-Philadelphia, Matsue-Oki, Winnipeg, and O-Indonesia) were present in 26 heterozygous newborns (17 black, eight Caucasian, and one Indonesian). The relative amount of the alpha X containing abnormal Hb F of the Hb G-Philadelphia and Hb Winnipeg babies was less than observed in heterozygous adults, which may indicate a decreased rate of assembly of the alpha X-gamma dimer over that of the alpha X-beta dimer. Of the 29 newborns with gamma chain variants, 16 were Caucasian babies; of these 15 had a Hb A gamma F-Hull heterozygosity and one a Hb G gamma F-Marietta heterozygosity. Six black babies were heterozygous for Hb A gamma F-Texas-I and six for Hb G gamma F-Port Royal. One Japanese baby had a heterozygosity for A gamma F-Iwata and a second was heterozygous for A gamma TF-Yamaguchi. Quantitative analyses of the isolated normal Hb Fo as well as an evaluation of the relative amounts of the Hb Fx in the red cell lysates gave data useful for a speculation of the genetic condition in each of these babies. It was concluded that the babies with the Hbs F-Texas-I, F-Iwata, F-Hull, and F-Marietta were simple heterozygotes with either the G gamma x A gamma/G gamma x A gamma X or the G gamma x A gamma/G gamma X x A gamma genic arrangement. The babies with Hb F-Port Royal had a G gamma x G gamma X/G gamma x A gamma arrangement, which may result from a (to be determined) gene conversion. The newborn baby with Hb F-Yamaguchi has the G gamma x A gamma x/A gamma T-X.-. genic arrangement, suggesting the presence of three distinctly different gamma chain genes of which one, the A gamma T-X gene, produces an A gamma chain (with threonyl at position gamma 75 and an Asn at position gamma 80) at a level usually seen for G gamma rather than A gamma chains. These studies were greatly facilitated by the use of high pressure liquid chromatographic methods.

Mapping the alpha-globin genes in Hb J Mexico carriers

The organization of the alpha-globin genes was studied by restriction endonuclease mapping, in subjects carrying the alpha variant Hb J Mexico. A subject homozygous for Hb J synthesized both Hb J (about 55%) and Hb A and had two alpha loci per chromosome. His restriction site map was found to be identical to that obtained with a normal DNA, except for a mutant Bgl II site which was observed on the Hb J chromosome proximal to the 5' alpha-locus. We have also mapped the DNA of a compound heterozygote for Hb J and alpha-thalassemia, who synthesizes 38% Hb J and we have found a single alpha gene corresponding to a - alpha 3.7 haplotype on one chromosome and two alpha genes, respectively alpha J and alpha A, on the other.

Construction of human gene libraries from small amounts of peripheral blood: analysis of beta-like globin genes

We describe a rapid procedure for constructing cloned human genomic libraries from small amounts of peripheral blood. High molecular weight DNA is isolated from 5-20 ml peripheral blood, partially cleaved with Eco R1, and 8-22 kb fragments are cloned using bacteriophage Charon 4A and suitable E. coli host. Using the approach we have isolated and characterized several non-alpha globin clones from a Kurdish Jew with homozygous beta thalassemia. The ability to isolate suitable amounts of high molecular weight DNA from peripheral blood provides a relatively simple means of constructing human gene libraries representing a variety of hemoglobin disorders.

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.