Duplication of structural genes for hemoglobin alpha and beta chains in man

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.

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.

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.

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.

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.

Evolutionary conservation of linkage groups: additional evidence from murid and cricetid rodents

In Mus musculus, family Muridae, the glucosephosphate isomerase (Gpi-l), pink-eyed dilution (p), albinism (c), and beta-type globin (Hbb) loci are known to be linked in the order Gpi-l-p-c-Hbb. In Rattus norvegicus, another murid rodent, the p, c, and Hbb loci are known to be linked in the same order and with similar recombination frequencies. In Peromyscus maniculatus, family Cricetidae, it was previously known that p and c are linked and by analogy to Mus musculus that linkage group should be bounded by Gpi-l near p and by a beta-globin locus near c. Linkage has now been established between Gpi-l and the Hbe globin locus in Peromyscus. However, the observed recombination frequency in Peromyscus (16.3%) is significantly lower than in Mus, suggesting that perhaps a chromosomal inversion has occurred during the evolutionary divergence of the two rodent families. Linkage relationships were also tested between the Hbc1, Hbd1, and Hbe1 globin variants. Hbc1 (presumably an alpha-type globin) segregated independently from Hbd1 and Hbe1 (presumably beta-type globins). No recombination was observed between Hbd1 and Hbe1. Those two globin genes may be alleles at a single locus, although circumstantial evidence suggests that they represent tightly linked duplicate loci.

Thalassemia and pregnancy: results of an antenatal screening program

A thalassemia screening program was implemented at our institution using the finding of a mean corpuscular volume less than 80 fl as the index of abnormality. Further evaluation using hemoglobin (Hb) electrophoresis and serum iron studies was carried out according to the scheme detailed below. A diagnosis of thalassemia was made in 33 women (42 pregnancies). Eight patients had alpha-thalassemia trait, 23 beta-thalassemia trait, and two Hb H disease. Thalassemia trait did not have any adverse effect on pregnancy outcome. In two couples the fetuses were at risk for homozygous disease and in one couple the fetus was at risk for sickle cell beta-thalassemia. The screening program described is an effective and inexpensive means of detecting thalassemia in an antenatal population and is applicable to most every clinic or office setting.

Demonstration of two alpha-globin genes per human haploid genome for normals and Hb J Mexico

Complementary DNA (cDNA) was prepared with viral RNA-dependent DNA polymerase using human globin messenger RNA (mRNA) as template. By selective hydridization to globin mRNA from beta-thalassaemics a probe which was greater than 85% complementary to alpha-globin mRNA was purified. This was hybridized in cDNA excess to human genomic DNA, and the rate and extent of hybridization confirmed that there are two genes for alpha-globin per haploid genome. Cellular DNA was also prepared from peripheral blood from cases expressing the alpha-globin chain mutant Hb J Mexico to varying extents. This DNA was identical in hybridization behaviour to normal DNA demonstrating that the imbalanced mutant chain synthesis seen physiologically is not due to a gene deletion.

Multiple structural genes for the alpha chain of canine (Canis familiaris) hemoglobin

There are two forms of the alpha chain of canine hemoglobin differing only at residue 130. One form (taualpha) contains threonine at this position, the other (Aalpha) contains alanine. Studies of two Labrador dog families strongly support the existance of multiple alpha-chain structural genes as the basis of the alpha-chain heterogeneity. There must be at least one gene locus for Talpha and one for Aalpha; the exact number of loci has not been determined. Two other dog breeds, the Basenji and the Beagle, also have both Talpha and Aalpha chains.

Studies of the proporation and synthesis of haemoblogin C Philadelphia in red cells of heterozygotes, a homozygote, and a heterozygote for both haemoglobin G and alpha thalassaemia

The proportion of Hb G Philadelphia (alpha68-Asn leads to Lys) in heterozygotes has been found to have a well-defined bimodal distribution around means of 33% and 46% Hb G. microcytosis and hypochromia are consistently associated with the latter group, who also have a decreased ratio of alpha/beta-chain synthesis in the peripheral blood, but these characters are not linked to the Hb-Galpha gene, because a parent with microcytosis and 46% Hb Galpha may have offspring with 33% Hb G without significant microcytosis. In one family a subject with Hb G and Hb G2 but no Hb A or Hb A2 is presumably a homozygote for alphaG. This subject has microcytosis and a decreased ratio of alpha/beta chain synthesis. In another family a subject with Hbs H, G and G2 but without Hbs A or A2 is heterozygous for both Hb G and alpha thalassaemia I. These findings are compatible with the hypothesis that the alphaG mutation occurs on a chromosome with only a single alpha-chain locus and that the expression in heterozygotes as 46% or 33% Hb G is determined by the homologous chromosome in trans having either one or two normal alphaA genes respectively. The significance of this polymorphism for chromosomes carrying alpha-chain genes is discussed.

Trimodality in the proportion of hemoglobin G Philadelphia in heterozygotes: evidence for heterogeneity in the number of human alpha chain loci

The extent of variability in the number of human hemoglobin (Hb) alpha chain loci has not yet been conclusively determined. There is evidence that in some populations individuals may possess two alpha-chain loci, while in other populations only one locus is present. Electrophoresis of peripheral blood from 53 heterozygotes for Hb G Philadelphia (alpha 68 Asn leads to Lys) revealed that the proportion of Hb G is trimodally distributed, with modes at approximately 20, 30, and 40% Hb G. Familial, hematologic, and statistical studies suggest that hte proportion of Hb G is not random but is genetically controlled and inversely correlated with mean cell volume. Two alternative genetic models are proposed to explain these findings: one assums alpha-thalassemia, while the other postulates variability in the number of alpha-chain loci in the American Black population. Biosynthetic studies of blood from 15 subjects revealed balanced synthesis of alpha and beta globin chains in heterozygotes from all three classes, strongly supporting variable gene dosage rather than alpha-thalassemia as the mechanism underlying the observed trimodality in the proportion of Hb G. Incompatibilities between out results and current concepts of alpha-thalassemia are discussed in the context of differences between Black compared with Oriental and Italian forms of Hb H disease.

Is haemoglobin G alpha Philadelphia linked to alpha-thalassaemia?

The question, "Is Hb G alpha Philadelphia linked to alpha-thalassaemia?" was first posed because the abnormal haemoglobin is found in heterozygotes at a concentration greater than 25%, the proportion predicted from a 4 alpha-chain gene model. Globin chain biosynthesis was studied in a West Indian family in which one parent had beta + thalassaemia and the other was heterozygous for the G alpha Philadelphia chain gene. The former had a globin chain production ratio alpha/beta well above 1, while the latter had a ratio significantly less than 1. One child of the marriage had inherited the beta + thalassaemia from one parent and the G alpha Philadelphia chain gene from the other and showed the typical picture of alpha/beta-thalassaemia (alpha/beta ratio slightly above normal). It is explained in the discussion that the evidence favours a close linkage of 2 alpha-chain genes.

Homozygous cases for hemoglobin J Mexico (alpha54 (E3)Gln replaced by Glu) evidence for a duplicated alpha gene with unequal expression

Hemoglobin J Mexico has been found in five generations of a large Algerian family. Nine subjects have 55% Hb J although their parents, siblings and offspring may have 31%, the usual quantity found in heterozygotes. Those with 55% Hb J are considered to be homozygous for a chromosome carrying both a normal alpha chain locus and a locus for alphaJ. The proportion of the abnormal hemoglobin in all the subjects is in favor of an unequal expression of both loci, the amount of protein synthesis directed by the alpha J gene being greater than that directed by the alpha A. In two heterozygotes a slightly higher proportion of the Hb J (38%) suggests the presence of a single normal alpha chain locus in trans. An associated alpha-thalassemia was excluded by biosynthetic studies in this family.