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

The C-->G transition in the alpha 2-globin gene of a normal alpha alpha-chromosome is responsible for the Hb G-Philadelphia variant in Sardinians

Sequencing of alpha-globin genes of 18 Sardinian heterozygotes for the Hb G-Philadelphia [alpha 68(E17)Asn-->Lys] variant, with four active alpha genes and circulating level of the variant of about 27%, showed the AAC-->AAG change at codon 68 of the alpha 2-globin gene (alpha(G)alpha/alpha alpha). Two heterozygotes with level of about 37% were the carriers of the same mutation on the same alpha 2 gene, and of the alpha 2 alpha 1 hybrid gene, because of the 3.7-kb deletion, in trans (alpha(G)alpha/-alpha(3.7)). In Black people, the same C-->G mutation occurs on the hybrid gene (-alpha(G)3.7), whereas in Caucasians the Lys for Asn change is because of the C-->A transversion occurring on the alpha 2 gene of a normal alpha alpha arrangement. The identification of the C-->G mutation on the normal alpha alpha chromosome points to an undescribed genotype for this rather common variant, which is probably because of the high rate of recombination between the duplicated alpha-globin genes.

Characterization and locus assignment of two alpha-globin variants present in the Maltese population: Hb St. Luke's [alpha95(G2)Pro-->Arg] and Hb Setif [alpha94(G1)Asp-->Tyr]

Two types of alpha-globin variants were found in 0.2% of a large number of newborn from Malta. The two hemoglobins were identified from tryptic maps on a Vydac C18 column and by alpha-globin gene sequencing as Hb St. Luke's (isoelectric point = 7.18+/-0.017) and Hb Setif (isoelectric point = 7.26+/-0.010). Hb St. Luke's [alpha95(G2)Pro-->Arg] was found to result from a C-->G mutation at the second position of codon 95 on an alpha1-globin gene, and Hb Setif [alpha94(G1) Asp-->Tyr] resulted from a G-->T mutation at the first position of codon 94 on an alpha2-globin gene. Quantification of Hb St. Luke's (11.1+/-1.12%) and Hb Setif (14.7+/-2.22%) in peripheral blood hemolysates indicated that, in the absence of either an alpha- or a beta-thalassemia allele, the protein products of the alpha1- and alpha2-globin genes were nearly equal in quantity.

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.

Translation inhibition by an mRNA coding region secondary structure is determined by its proximity to the AUG initiation codon

In the present study we investigate the impact of highly stable coding region secondary structures on mRNA translation efficiency. By introducing antisense segments into the 3'non-translated region of human alpha-globin mRNA we are able to synthesize a series of transcripts in which site-specific secondary structures are introduced without altering the primary structure of the 5' non-translated region, the coding region, or the encoded protein product. Coding region duplexes in close proximity to the AUG initiation codon are found to inhibit translation severely to a degree equal to that of a duplex that extends into the 5' non-translated region. In contrast, mRNAs containing duplexes positioned further 3' in the coding region translate at levels that are significantly higher although are still below those of native alpha-globin mRNA. The primary determinant of translation inhibition by coding region duplexes appears to be the proximity of the duplex to the AUG initiation codon and reflects a parallel inhibition of monosome formation. These data demonstrate that extensive coding region secondary structures suppress translation to a minimal or to a substantial degree depending on their distance from the initiation codon.

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.

Inactivation of human alpha-globin gene expression by a de novo deletion located upstream of the alpha-globin gene cluster

Synthesis of normal human hemoglobin A, alpha 2 beta 2, is based upon balanced expression of genes in the alpha-globin gene cluster on chromosome 16 and the beta-globin gene cluster on chromosome 11. Full levels of erythroid-specific activation of the beta-globin cluster depend on sequences located at a considerable distance 5' to the beta-globin gene, referred to as the locus-activating or dominant control region. The existence of an analogous element(s) upstream of the alpha-globin cluster has been suggested from observations on naturally occurring deletions and experimental studies. We have identified an individual with alpha-thalassemia in whom structurally normal alpha-globin genes have been inactivated in cis by a discrete de novo 35-kilobase deletion located approximately 30 kilobases 5' from the alpha-globin gene cluster. We conclude that this deletion inactivates expression of the alpha-globin genes by removing one or more of the previously identified upstream regulatory sequences that are critical to expression of the alpha-globin genes.

Locus assignment of human alpha globin mutations by selective amplification and direct sequencing

We describe a simple approach for molecular characterization and locus assignment of structural mutants by direct sequencing of enzymatically amplified DNA selective to alpha 1 and alpha 2 globin gene regions. Nucleotide substitution of two structural variants (Stanleyville II alpha 2(78Lys) and J Mexico alpha 2(54Glu) were determined and their encoding loci were specified. The amplified segment encompasses sequences upstream of the CAAT box to downstream of the Poly(A) addition signal. Hence all of the alpha globin structural variants and most of the nondeletion alpha thalassaemic mutants should be characterizable by this approach.

Locus assignment of human alpha-globin structural mutants by selective enzymatic amplification of alpha 1 and alpha 2-globin cDNAs

We have used the powerful methodology of DNA enzymatic amplification in order to assign human alpha-globin structural mutants to one of the two highly homologous alpha-globin genes. Selectively amplified alpha 1 and alpha 2-globin cDNAs were dot-blotted and further hybridized to synthetic oligonucleotides encompassing either the normal or the mutated sequences. The generated signals corresponded specifically to one of the two alpha-globin genes. Using this approach the alpha-globin structural mutants J-Buda and G-Pest were found to be encoded by the alpha 2 and the alpha 1-globin genes, respectively. Furthermore, the exact nucleotide changes were determined. We propose this technique to serve as a simple and definitive method for assigning alpha-globin structural mutants.

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.

Mouse placental 57-kDa calcium-binding protein: I. Cloning of cDNA and characterization of developmental expression

A cDNA clone to the mouse placental 57-kDa calcium-binding protein (MCaBP) [29] was isolated by immunoscreening a mouse placenta cDNA library constructed in the expression phage vector, lambda gt 11. The MCaBP cDNA was 0.7 kb in size, with restriction sites for StuI and Bg/II, and its identity to the MCaBP was confirmed by mRNA hybrid selection. RNA blot hybridization revealed a predominant, 3.9-kb transcript of the MCaBP in day-18 mouse placenta. The expression of the MCaBP during development was analyzed with respect to the levels of protein activity, translatable MCaBP mRNA, and total MCaBP transcript.

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.

Different hematologic phenotypes are associated with the leftward (-alpha 4.2) and rightward (-alpha 3.7) alpha+-thalassemia deletions

We have compared the phenotypes of the two common deletion forms of alpha+-thalassemia by analysis of umbilical cord blood samples from Melanesia. Homozygotes for the leftward, 4.2-kilobase, deletion (-alpha 4.2) had significantly higher levels of Hb Bart's at birth than homozygotes for the rightward, 3.7-kilobase, deletion (-alpha 3.7). Compound heterozygotes for each deletion had intermediate values. Although deletion forms of alpha 0 thalassemia were not found in this survey, nondeletion alpha-thalassemia was present at low frequency. Since the predominant rightward deletion in this population, -alpha 3.7III, entirely removes the alpha 1-gene and the 4.2-kilobase deletion deletes the alpha 2-gene, these data indicate that the alpha 2-globin gene has a higher output than the alpha 1-gene, on single alpha-gene chromosomes.

Alkaline phosphatase conjugated protein A as a sensitive reagent to immunoscreen an expression cDNA plasmid library: isolation of cDNA to the calcium-binding protein of the chick embryonic chorioallantoic membrane

A highly efficient immunoscreening procedure has been developed to isolate cDNA clones to the calcium-binding protein (CaBP) of the chick embryonic chorioallantoic membrane (CAM). A library of total CAM cDNA was constructed using the expression plasmid vector, pUC 19. Bacterial clones containing plasmids with CaBP cDNA inserts were detected immunohistochemically based on their expression of hybrid CaBP protein sequences. For immunodetection, nitrocellulose bacterial colony replicas were treated with specific antibodies to the CaBP followed by incubation with Staphylococcus aureus Protein A conjugated with alkaline phosphatase (AP) which served as a secondary immunoreagent. Positive clones were then histochemically identified based on AP enzyme activity. The identity of the immunopositive clones was further verified by in vitro translation of mRNA selected by hybridization to the cloned cDNA. The AP-based immunoscreening procedure yields stable reaction products with relatively low background, and should find general application for isolating specific cDNA clones from expression cDNA libraries.

Translational profiles of alpha 1-, alpha 2-, and beta-globin messenger ribonucleic acids in human reticulocytes

In human reticulocytes, the critical balancing of alpha- and beta-globin synthesis may be controlled in part by differential translation of the three major adult globin messenger RNAs (mRNAs), alpha 1, alpha 2, and beta. In this study, we determined, as a parameter of translational efficiency, the relative ribosome loading of these three mRNAs. Using oligonucleotide probes specific for the alpha 1- and alpha 2-globin mRNAs, we find that these two mRNAs have identical translational profiles. Their distribution contrasts with that of beta-globin mRNA, which is present on heavier polyribosomes and is less prevalent in pre-80S messenger ribonucleoprotein fractions. The relative distribution of alpha- vs. beta-globin mRNA is consistent with more efficient beta-globin translation. In contrast, the parallel distributions of alpha 1- and alpha 2-globin mRNAs suggests they are translated with equal efficiencies. Considering the relative concentrations of the two alpha-globin mRNAs in normal reticulocytes, this result predicts a dominant role for the alpha 2-globin locus in human alpha-globin expression.

Isolation and characterization of a cDNA encoding a human liver/bone/kidney-type alkaline phosphatase

Alkaline phosphatases (ALPs) [orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] isolated from human liver, bone, and kidney (L/B/K) exhibit very similar biochemical and immunologic properties that differentiate them from other human ALPs, such as those characteristically found in placenta and intestine. Despite their similarities, the L/B/K ALPs produced in different tissues show slight physical differences. To examine structural and evolutionary relationships between the various ALPs, a cDNA corresponding to L/B/K ALP mRNA has been isolated. A lambda 11 cDNA expression library was constructed using poly(A) RNA from the osteosarcoma cell line Saos-2 and screened with anti-liver ALP antiserum. The 2553-base-pair cDNA contains an open reading frame that encodes a 524 amino acid polypeptide with a predicted molecular mass of 57.2 kDa. This ALP precursor protein contains a presumed signal peptide of 17 amino acids followed by 37 amino acids that are identical to the amino-terminal sequence determined from purified liver ALP. In addition, amino acid sequences of several CNBr peptides obtained from liver ALP are found within the cDNA-encoded protein. The deduced L/B/K ALP precursor polypeptide shows 52% homology to human placental ALP and 25% homology to Escherichia coli ALP precursor polypeptides. Sixty percent nucleotide homology exists between the human L/B/K and placental cDNAs over the protein coding regions. The 5' and 3' untranslated regions of the L/B/K ALP cDNA, 176 and 805 base pairs, respectively, show no homology to the corresponding regions of placental ALP cDNA.

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.