Synthesis of haemoglobin Wayne in erythroid cells

A 3'UTR mutation affects beta-globin expression without altering the stability of its fully processed mRNA

Determinants of mRNA stability are frequently positioned in the 3'UTR where they are not subject to disruption by actively translating ribosomes. Two related individuals with beta thalassaemia who carry a beta-globin gene containing a 13 nt deletion in its 3'UTR have recently been described. Its position within the 3'UTR, as well as its relative distance from other known functionally important elements, suggested that the deletion might overlay previously unrecognized determinants of beta-globin mRNA stability. We studied the impact of the Delta13 mutation on beta-globin gene expression in vitro and in vivo. The adverse effect of the Delta13 mutation on beta-globin expression was confirmed in studies utilizing reticulocytes from a betaDelta13 heterozygote, which indicated a sixfold reduction in the relative level of the mutant mRNA. Additional in vitro analysis indicated that the deletion did not affect the capacity of the betaDelta13 mRNA to assemble an mRNA-stabilizing mRNP 'beta-complex'. Unexpectedly, functional tests in both primary erythroid cells and in a transgenic mouse model demonstrated that the betaDelta13 mRNA was fully stable, suggesting that the Delta13 mutation affects accumulation of the fully processed mRNA at an earlier step. Consistent with this, there was a relative excess of unprocessed betaDelta13 mRNA in erythroid progenitors from a betaDelta13 heterozygote. Taken together, these results define a new thalassaemic determinant, which acts to decrease beta-globin mRNA levels by inhibiting the efficiency of nuclear processing events, and suggest a previously unanticipated complexity to the role of the 3'UTR elements in the regulation of beta-globin gene expression.

Structural and functional analysis of an mRNP complex that mediates the high stability of human beta-globin mRNA

Human globins are encoded by mRNAs exhibiting high stabilities in transcriptionally silenced erythrocyte progenitors. Unlike alpha-globin mRNA, whose stability is enhanced by assembly of a specific messenger RNP (mRNP) alpha complex on its 3' untranslated region (UTR), neither the structure(s) nor the mechanism(s) that effects the high-level stability of human beta-globin mRNA has been identified. The present work describes an mRNP complex assembling on the 3' UTR of the beta-globin mRNA that exhibits many of the properties of the stability-enhancing alpha complex. The beta-globin mRNP complex is shown to contain one or more factors homologous to alphaCP, a 39-kDa RNA-binding protein that is integral to alpha-complex assembly. Sequence analysis implicates a specific 14-nucleotide pyrimidine-rich track within its 3' UTR as the site of beta-globin mRNP assembly. The importance of this track to mRNA stability is subsequently verified in vivo using mice expressing human beta-globin transgenes that contain informative mutations in this region. In combination, the in vitro and in vivo analyses indicate that the high stabilities of the alpha- and beta-globin mRNAs are maintained through related mRNP complexes that may share a common regulatory pathway.

Erythroid cell-specific determinants of alpha-globin mRNA stability

Although globin mRNAs are considered prototypes of highly stable messages, the mechanisms responsible for their longevity remain largely undefined. As an initial step in identifying potential cis-acting elements or structures which contribute to their stability, we analyzed the defect in expression of a naturally occurring alpha 2-globin mutant, alpha Constant Spring (CS). The CS mutation is a single-base change in the translation termination codon (UAA-->CAA) that allows the ribosome to read through into the 3' nontranslated region (NTR). The presence of CS mRNA in transcriptionally active erythroid precursors and its absence (relative to normal alpha-globin mRNA) in the more differentiated transcriptionally silent erythrocytes suggest that this mutation disrupts some feature of the alpha-globin mRNA required for its stability. Using a transient transfection system, we demonstrate that in murine erythroleukemia cells the CS mRNA is unstable compared with the normal alpha 2-globin mRNA. The analyses of several other naturally occurring and site-directed mutant alpha-globin genes in murine erythroleukemia cells indicate that entry of a translating ribosome into the 3' NTR targets the message for accelerated degradation in erythroid cells. In contrast, both the CS and alpha 2-globin mRNAs are stable in several nonerythroid cell lines. These results suggest that translational readthrough disrupts a determinant associated with the alpha 2-globin 3' NTR which is required for mRNA stability in erythroid cells.

Erythroid cell-specific determinants of alpha-globin mRNA stability

Although globin mRNAs are considered prototypes of highly stable messages, the mechanisms responsible for their longevity remain largely undefined. As an initial step in identifying potential cis-acting elements or structures which contribute to their stability, we analyzed the defect in expression of a naturally occurring alpha 2-globin mutant, alpha Constant Spring (CS). The CS mutation is a single-base change in the translation termination codon (UAA-->CAA) that allows the ribosome to read through into the 3' nontranslated region (NTR). The presence of CS mRNA in transcriptionally active erythroid precursors and its absence (relative to normal alpha-globin mRNA) in the more differentiated transcriptionally silent erythrocytes suggest that this mutation disrupts some feature of the alpha-globin mRNA required for its stability. Using a transient transfection system, we demonstrate that in murine erythroleukemia cells the CS mRNA is unstable compared with the normal alpha 2-globin mRNA. The analyses of several other naturally occurring and site-directed mutant alpha-globin genes in murine erythroleukemia cells indicate that entry of a translating ribosome into the 3' NTR targets the message for accelerated degradation in erythroid cells. In contrast, both the CS and alpha 2-globin mRNAs are stable in several nonerythroid cell lines. These results suggest that translational readthrough disrupts a determinant associated with the alpha 2-globin 3' NTR which is required for mRNA stability in erythroid cells.

Heterogeneity in the globin chain composition of a human minor fetal hemoglobin component

The first hemoglobin found to contain an acetyl blocking group was the minor human fetal hemoglobin, Hb FI, present as 10-15% of the total fetal hemoglobin in umbilical cord blood red cells. Acetylation occurs at the amino-terminal glycine of the gamma-globin chain. Assays for the acetyl group by two different methods gave values less than the 2 per tetramer expected for a fully acetylated hemoglobin. We have purified acetylated fetal hemoglobin FIc to homogeneity. The globin chain composition of Hb FIc has been examined by both globin chain separation on CM-cellulose and by tryptic peptide mapping by HPLC. The identities of the gamma globin chains and of the gamma T-1 peptides were confirmed by amino acid analysis. Globin chain separation profiles showed the presence of 22.3 +/- 7.0% of gamma 0 globin (of the total gamma globin) in Hb FIc. Accordingly, the tryptic peptide maps of Hb FIc tetramers also showed the presence of a similar amount of gamma 0T-1 peptide. The gamma 0T-1 peptide was not present in the maps of isolated gamma Ic globin. It is evident that column purified Hb FIc contains a certain percentage of non-acetylated gamma-globin chains, thus indicating a hybrid globin chain composition for this minor fetal hemoglobin component.

'GPI Roma', a new glucose phosphate isomerase deficient variant: in vivo occurrence of postsynthetic modifications of the mutant enzyme

In a 5-year-old Italian girl with severe congenital hemolytic anemia, red cell GPI deficiency was proven, and found to be due to a new variant, 'GPI Roma.' The parents are first cousins and have been proven to be heterozygous for this variant. GPI Roma was slightly unstable to heat and exhibited a slightly increased Michaelis constant for fructose-6-phosphate. A single predominant fast-migrating GPI form existed in the patient's white blood cells, while the electrophoretic pattern in the red cells was composed, in addition to this 'fast band,' of a major band migrating as normal GPI and of an additional slow band. It is shown that this phenomenon may be ascribed to postsynthetic events modifying the charge of the mutant enzyme.