alpha-Thalassemia caused by an unstable alpha-globin mutant

In a previous study, molecular cloning of the alpha-globin genes from a patient with nondeletion Hb-H disease (genotype--/alpha alpha) showed that a single nucleotide mutation (CTG to CCG) in one of the genes resulted in a leucine to proline substitution. This paper describes the approach we used to detect the abnormal alpha-globin chain. The chain was identified using a cell-free translation system. It turned over rapidly both in vitro and in vivo in the patient's reticulocytes. The unusual feature of this unstable alpha-globin is that the alpha-globin deficiency causes alpha-thalassemia. Simple heterozygotes for this lesion (alpha Pro alpha/alpha alpha) resemble alpha-thalassemia carriers and do not exhibit the hemolytic anemia usually associated with unstable hemoglobins.

Different rates of mRNA translation balance the expression of the two human alpha-globin loci

The relative translational efficiency of the two human alpha-globin mRNAs, alpha 1 and alpha 2, was measured in vitro in a rabbit reticulocyte lysate system. To differentiate the translational products of these two mRNAs which normally encode an identical alpha-globin protein product, we used reticulocyte mRNA from a recently described Chinese subject. In this subject, an electrophoretically distinct alpha-globin mutant is encoded at the alpha 2 locus and both the alpha 1- and alpha 2-globin genes are deleted from the homologous chromosome (--/alpha 125Pro alpha). As in normal controls, the concentration of alpha 2-globin mRNA exceeded alpha 1 by approximately 3-fold. However, alpha 1- and alpha 2-globin proteins were synthesized by this reticulocyte mRNA at equal rates. This data suggests that the equal expression of the two alpha-globin genes observed in human erythrocytes results from a balance between the 3-fold excess of alpha 2-globin mRNA and a 3-fold higher translational efficiency of alpha 1-globin mRNA. The disparate translational efficiencies of the two alpha-globin mRNAs may be determined by the divergent structure of their 3' noncoding regions.

Hematologic improvement following splenectomy for hemoglobin-H disease

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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.

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.

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.

Dual pathways for ribonucleic acid turnover in WI-38 but not in I-cell human diploid fibroblasts

The turnover rates of 3H-labeled 18S ribosomal ribonucleic acid (RNA), 28S ribosomal RNA, transfer RNA, and total cytoplasmic RNA were very similar in growing WI-38 diploid fibroblasts. The rate of turnover was at least twofold greater when cell growth stopped due to cell confluence, 3H irradiation, or treatment with 20 mM NaN3 or 2 mM NaF. In contrast, the rate of total 3H-protein turnover was the same in growing and nongrowing cells. Both RNA and protein turnovers were accelerated at least twofold in WI-38 cells deprived of serum, and this increase in turnover was inhibited by NH4Cl. These results are consistent with two pathways for RNA turnover, one of them being nonlysosomal and the other being lysosome mediated (NH4Cl sensitive), as has been suggested for protein turnover. Also consistent with the notion of two pathways for RNA turnover were findings with I-cells, which are deficient for many lysosomal enzymes, and in which all RNA turnover was nonlysosomal (NH4Cl resistant).

Cloning and complete nucleotide sequence of human 5'-alpha-globin gene

We have cloned one of the two human alpha-globin genes and report its complete nucleotide sequence. The gene is 832 base pairs (bp) long from the 5'-cap site to the 3'-polyadenylylation site. The amino acid coding sequences are separated into three segments (exons) by two short (117 and 140 bp) intervening sequences. Highly conserved regions are identified in the 5'-flanking region, intron-exon junctions, and 3' noncoding regions that may have functional significance.

Bidirectional Effects of Interaction between Parental Thymocytes and F1 Spleen Cells in the Production of Graft vs Host Splenomegaly

The addition of 1.5 × 107 F1 spleen cells to inocula of parental thymocytes in sublethally irradiated F1 hosts influenced the resultant splenomegaly. The influence of the F1 cells was bidirectional; under certain conditions they increased the splenomegaly while under others they decreased it. Analysis of these conditions revealed that the effect the F1 cells had depended on the activity of the parental thymocytes. Active thymocytes produced small spleens when inoculated alone; the smaller the spleen the greater the synergy with F1 cells. On the other hand less active thymocytes occasionally produced some splenomegaly on their own. When they did, added F1 cells reduced the spleen size (acted “antergistically”). The mechanism(s) by which these bidirectional effects of F1 cells may be expressed are discussed.

The response of T cells to histocompatibility-2 antigens. Dose-response kinetics

The DNA synthetic response of a wide variety of parental thymocyte doses was studied in the lymphoid tissues of lethally irradiated F(1) mice. The response curves of the thymocytes were strikingly similar in shape to those of T cells responding to antigens, such as sheep red cells, which are more labile than the histocompatibility antigens of the F(1) host. The response was characterized by a dose-dependent latent period of 1-3 days, followed by a sharp increase in activity and a significant subsequent shutoff. Larger thymocyte doses tended to shorten the latent period. A comparison of the responses of different cell doses to one another indicated that the response usually simulated a suppressed first-order reaction. However, occasional multiple order reactions were observed. We have interpreted these data to indicate that both positive and negative interactions occur between thymocytes, without the mediation of B cell products such as conventional antibodies.

Suppressor T Cells

The antigen-induced DNA synthetic response of a number of different thymocyte populations was studied in the spleens of lethally irradiated recipients by incorporation of 125I 5-iodo-2-deoxyuridine. The response of certain cell combinations was not only less than the sum of the two responding alone but less than one of the individual cell populations in the combination. The following combinations had this effect: 1) “Educated” and normal thymocytes < “educated” alone in the response to sheep red cells (SRBC); 2) parental and F1 thymocytes < parental alone in F1 hosts; 3) cortisone resistant (in a low dose) and normal thymocytes < normal alone in the response to SRBC. Thus, thymocytes are capable of suppressing the antigen-induced response of other thymocytes without the mediation of B cells or their product (i.e., antibody).