A beta zero-thalassemic beta-globin RNA that is labile in bone marrow cells is relatively stable in HeLa cells

Synthesis and activity of a novel inhibitor of nonsense-mediated mRNA decay

A new route to a tetracyclic lactam was developed and the product, called VG1, was found to inhibit nonsense-mediated mRNA decay at μM concentrations.

During efforts to prepare the known compound NMDI1, a new tetracyclic compound, called VG1, was prepared in six steps. This compound was found to have good activity as an inhibitor of nonsense-mediated mRNA decay.

When a ribosome encounters a premature termination codon

In mammalian cells, aberrant transcripts harboring a premature termination codon (PTC) can be generated by abnormal or inefficient biogenesis of mRNAs or by somatic mutation. Truncated polypeptides synthesized from these aberrant transcripts could be toxic to normal cellular functions. However, mammalian cells have evolved sophisticated mechanisms for monitoring the quality of mRNAs. The faulty transcripts harboring PTC are subject to nonsense-mediated mRNA decay (NMD), nonsense-mediated translational repression (NMTR), nonsense-associated alternative splicing (NAS), or nonsense-mediated transcriptional gene silencing (NMTGS). In this review, we briefly outline the molecular characteristics of each pathway and suggest mRNA quality control mechanisms as a means to regulate normal gene expression. [BMB Reports 2013; 46(1): 9-16]

Unspliced precursors of NMD-sensitive β-globin transcripts exhibit decreased steady-state levels in erythroid cells

Nonsense-mediated mRNA decay (NMD) is a quality control mechanism that detects and rapidly degrades mRNAs carrying premature translation-termination codons (PTCs). Mammalian NMD depends on both splicing and translation, and requires recognition of the premature stop codon by the cytoplasmic ribosomes. Surprisingly, some published data have suggested that nonsense codons may also affect the nuclear metabolism of the nonsense-mutated transcripts. To determine if nonsense codons could influence nuclear events, we have directly assessed the steady-state levels of the unspliced transcripts of wild-type and PTC-containing human β-globin genes stably transfected in mouse erythroleukemia (MEL) cells, after erythroid differentiation induction, or in HeLa cells. Our analyses by ribonuclease protection assays and reverse transcription-coupled quantitative PCR show that β-globin pre-mRNAs carrying NMD-competent PTCs, but not those containing a NMD-resistant PTC, exhibit a significant decrease in their steady-state levels relatively to the wild-type or to a missense-mutated β-globin pre-mRNA. On the contrary, in HeLa cells, human β-globin pre-mRNAs carrying NMD-competent PTCs accumulate at normal levels. Functional analyses of these pre-mRNAs in MEL cells demonstrate that their low steady-state levels do not reflect significantly lower pre-mRNA stabilities when compared to the normal control. Furthermore, our results also provide evidence that the relative splicing efficiencies of intron 1 and 2 are unaffected. This set of data highlights potential nuclear pathways that might be promoter- and/or cell line-specific, which recognize the NMD-sensitive transcripts as abnormal. These specialized nuclear pathway(s) may be superimposed on the general NMD mechanism.

Nonsense-mediated decay of human HEXA mRNA

Nonsense-mediated mRNA decay (NMD), the loss of mRNAs carrying premature stop codons, is a process by which cells recognize and degrade nonsense mRNAs to prevent possibly toxic effects of truncated peptides. Most mammalian nonsense mRNAs are degraded while associated with the nucleus, but a few are degraded in the cytoplasm; at either site, there is a requirement for translation and for an intron downstream of the early stop codon. We have examined the NMD of a mutant HEXA message in lymphoblasts derived from a Tay-Sachs disease patient homozygous for the common frameshift mutation 1278ins4. The mutant mRNA was nearly undetectable in these cells and increased to approximately 40% of normal in the presence of the translation inhibitor cycloheximide. The stabilized transcript was found in the cytoplasm in association with polysomes. Within 5 h of cycloheximide removal, the polysome-associated nonsense message was completely degraded, while the normal message was stable. The increased lability of the polysome-associated mutant HEXA mRNA shows that NMD of this endogenous mRNA occurred in the cytoplasm. Transfection of Chinese hamster ovary cells showed that expression of an intronless HEXA minigene harboring the frameshift mutation or a closely located nonsense codon resulted in half the normal mRNA level. Inclusion of multiple downstream introns decreased the abundance further, to about 20% of normal. Thus, in contrast to other systems, introns are not absolutely required for NMD of HEXA mRNA, although they enhance the low-HEXA-mRNA phenotype.

Effects of nonsense mutations on nuclear and cytoplasmic adenine phosphoribosyltransferase RNA

We have analyzed Chinese hamster ovary (CHO) cell mutants bearing nonsense codons in four of the five exons of the adenine phosphoribosyltransferase (aprt) gene and have found a pattern of mRNA reduction similar to that seen in systems studied previously: a decrease in steady-state mRNA levels of 5- to 10-fold for mutations in exons 1, 2, and 4 but little effect for mutations in the 3'-most exon (exon 5). Nuclear aprt mRNA levels showed a similar decrease. Nonsense-containing aprt mRNA decayed at the same rate as wild-type mRNA in these cell lines after inhibition of transcription with actinomycin D. Nonsense-containing aprt mRNA is associated with polysomes, ruling out a model in which stable residual mRNA escapes degradation by avoiding translation initiation. A tetracycline-responsive form of the aprt gene was used to compare the stability of nonsense-containing and wild-type aprt mRNAs without globally inhibiting transcription. In contrast to measurements made in the presence of actinomycin D, after inhibition of aprt transcription with tetracycline, a nonsense-mediated destabilization of aprt mRNA was indeed demonstrable. The increased rate of decay of cytoplasmic aprt mRNA seen here could account for the nonsense-mediated reduction in steady-state levels of aprt mRNA. However, the low levels of nonsense-bearing aprt mRNA in the nucleus suggest a sensibility of mRNA to translation or translatability before it exits that compartment. Quantitation of the steady-state levels of transcripts containing introns revealed no accumulation of partially spliced aprt RNA and hence no indication of nonsense-mediated aberrancies in splicing. Our results are consistent with a model in which translation facilitates the export of mRNA through a nuclear pore. However, the mechanism of this intriguing nucleocytoplasmic communication remains to be determined.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

When cells stop making sense: effects of nonsense codons on RNA metabolism in vertebrate cells

It appears that no organism is immune to the effects of nonsense codons on mRNA abundance. The study of how nonsense codons alter RNA metabolism is still at an early stage, and our current understanding derives more from incidental vignettes than from experimental undertakings that address molecular mechanisms. Challenges for the future include identifying the gene products and RNA sequences that function in nonsense mediated RNA loss, resolving the cause and consequences of there apparently being more than one cellular site and mechanism for nonsense-mediated RNA loss, and understanding how these sites and mechanisms are related to both constitutive and specialized pathways of pre-mRNA processing and mRNA decay.

Nuclear degradation of nonsense mutated beta-globin mRNA: a post-transcriptional mechanism to protect heterozygotes from severe clinical manifestations of beta-thalassemia?

Nonsense mutations of the beta-globin gene are a common cause of beta-thalassemia. It is a hallmark of these mutations not only to cause a lack of protein synthesis but also a reduction of mRNA expression. Both the pathophysiologic significance and the underlying mechanisms for this surprising phenomenon have so far remained enigmatic. We report that the reduction of the fully spliced mutant beta-globin mRNA already manifests itself within the nucleus. In contrast, the levels of mutant pre-mRNA are normal. The promoter and the 5'-untranslated region (5'-UTR) of the herpes simplex virus type 1 thymidine kinase (HSV1 Tk) gene can independently circumvent this recognition/response mechanism in cis and restore nonsense mutated beta-globin mRNA expression to normal levels. These two genetic elements can thus exert a dominant influence on the post-transcriptional control of nonsense mutated beta-globin gene expression. While wild-type mRNA levels are restored by fusion of the HSV1 Tk 5'-UTR to the nonsense mutated beta-globin reading frame, translation of a wildtype reading frame in such a hybrid is precluded. In contrast, the HSV1 Tk promoter appears to efficiently deliver the mRNA to the translational apparatus. The 5'-UTR and the promoter sequences therefore control the nuclear fate of nonsense mutated beta-globin mRNA by separable pathways. The nuclear mRNA degradation mechanisms examined here may prevent the synthesis of C-terminally truncated beta-globin chain fragments and may protect heterozygotes from clinically relevant symptoms of beta-thalassemia.

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.

Molecular basis of nanomelia, a heritable chondrodystrophy of chicken

Nanomelia is a recessively inherited connective tissue disorder of chicken affecting cartilage development. Other investigators have demonstrated that it involves low aggrecan production and diminished aggrecan mRNA levels. Based on genetic linkage studies showing a high likelihood that the mutation responsible for the nanomelic phenotype lay within the aggrecan gene, a series of experiments was performed to define the molecular basis of the trait. Aggrecan mRNA was present in the nucleus of the nanomelic chondrocyte but greatly reduced in the cytoplasmic compartment, a finding suggestive of a premature stop codon within the aggrecan transcript. Since no defect in mRNA splicing could be demonstrated by ribonucleasease protection studies, direct DNA sequencing was initiated by polymerase chain reaction of the mRNA and of genomic DNA. A stop codon was demonstrated at codon 1513, which is located in the eighth repeat of the chondroitin sulfate 2 domain of the large tenth exon. The mutation creates a unique BasBI restriction site which readily distinguishes the mutant and wild-type alleles.

Defective splicing of mRNA from one COL1A1 allele of type I collagen in nondeforming (type I) osteogenesis imperfecta

Osteogenesis imperfecta (OI) type I is the mildest form of heritable bone fragility resulting from mutations within the COL1A1 gene. We studied fibroblasts established from a child with OI type I and demonstrated underproduction of alpha 1 (I) collagen chains and alpha 1 (I) mRNA. Indirect RNase protection suggested two species of alpha 1 (I) mRNA, one of which was not collinear with fully spliced alpha 1 (I) mRNA. The noncollinear population was confined to the nuclear compartment of the cell, and contained the entire sequence of intron 26 and a G-->A transition in the first position of the intron donor site. The G-->A transition was also identified in the genomic DNA. The retained intron contained an in-frame stop codon and introduced an out-of-frame insertion within the collagen mRNA producing stop codons downstream of the insertion. These changes probably account for the failure of the mutant RNA to appear in the cytoplasm. Unlike other splice site mutations within collagen mRNA that resulted in exon skipping and a truncated but inframe RNA transcript, this mutation did not result in production of a defective collagen pro alpha 1 (I) chain. Instead, the mild nature of the disease in this case reflects failure to process the defective mRNA and thus the absence of a protein product from the mutant allele.

Two mutations in the beta-globin polyadenylylation signal reveal extended transcripts and new RNA polyadenylylation sites

Two mutations in the beta-globin poly(A) signal were identified in Israeli patients with beta +-thalassemia by sequence analysis following PCR. One is a point mutation (AATAAA----AATAAG) and the other is a 5-base-pair deletion (AATAAA----A----). The mutant genes were used to investigate the function of the poly(A) signal in vivo and to evaluate the mechanism whereby these mutations lead to a thalassemic phenotype. Analysis of RNA derived from peripheral blood demonstrated the presence of elongated RNA species in patients carrying either mutation. Other aspects of RNA processing (initiation, splicing) were unimpaired. RNA obtained from the patients carrying the point mutation contained four discrete, extended RNA species, 1500-2900 nucleotides long, which were found to be polyadenylated. Some normal cleavage-polyadenylylation was also observed. The 5-base-pair deletion completely abolished cleavage at the normal site. This deletion mutation resulted in a phenotype of beta +-thalassemia, thus providing evidence that the extended mRNAs are translatable in vivo. Furthermore, additional transcripts, greater than 5 kilobases, presumably mRNA precursors, were found in all RNA samples, including those of nonthalassemic controls. The extended transcripts of the poly(A) mutants, together with the high molecular weight precursors, suggest that the human beta-globin gene transcription unit is significantly longer than previously recognized.

Nonsense codons in human beta-globin mRNA result in the production of mRNA degradation products

Human beta zero-thalassemic beta-globin genes harboring either a frameshift or a nonsense mutation that results in the premature termination of beta-globin mRNA translation have been previously introduced into the germ line of mice (S.-K. Lim, J.J. Mullins, C.-M. Chen, K. Gross, and L.E. Maquat, EMBO J. 8:2613-2619, 1989). Each transgene produces properly processed albeit abnormally unstable mRNA as well as several smaller RNAs in erythroid cells. These smaller RNAs are detected only in the cytoplasm and, relative to mRNA, are longer-lived and are missing sequences from either exon I or exons I and II. In this communication, we show by using genetics and S1 nuclease transcript mapping that the premature termination of beta-globin mRNA translation is mechanistically required for the abnormal RNA metabolism. We also provide evidence that generation of the smaller RNAs is a cytoplasmic process: the 5' ends of intron 1-containing pre-mRNAs were normal, the rates of removal of introns 1 and 2 were normal, and studies inhibiting RNA synthesis with actinomycin D demonstrated a precursor-product relationship between full-length mRNA and the smaller RNAs. In vivo, about 50% of the full-length species that undergo decay are degraded to the smaller RNAs and the rest are degraded to undetectable products. Exposure of erythroid cells that expressed a normal human beta-globin transgene to either cycloheximide or puromycin did not result in the generation of the smaller RNAs. Therefore, a drug-induced reduction in cellular protein synthesis does not reproduce this aspect of cytoplasmic mRNA metabolism. These data suggest that the premature termination of beta-globin mRNA translation in either exon I or exon II results in the cytoplasmic generation of discrete mRNA degradation products that are missing sequences from exon I or exons I and II. Since these degradation products appear to be the same for all nonsense codons tested, there is no correlation between the position of translation termination and the sites of nucleolytic cleavage.

Nonsense codons in human beta-globin mRNA result in the production of mRNA degradation products

Human beta zero-thalassemic beta-globin genes harboring either a frameshift or a nonsense mutation that results in the premature termination of beta-globin mRNA translation have been previously introduced into the germ line of mice (S.-K. Lim, J.J. Mullins, C.-M. Chen, K. Gross, and L.E. Maquat, EMBO J. 8:2613-2619, 1989). Each transgene produces properly processed albeit abnormally unstable mRNA as well as several smaller RNAs in erythroid cells. These smaller RNAs are detected only in the cytoplasm and, relative to mRNA, are longer-lived and are missing sequences from either exon I or exons I and II. In this communication, we show by using genetics and S1 nuclease transcript mapping that the premature termination of beta-globin mRNA translation is mechanistically required for the abnormal RNA metabolism. We also provide evidence that generation of the smaller RNAs is a cytoplasmic process: the 5' ends of intron 1-containing pre-mRNAs were normal, the rates of removal of introns 1 and 2 were normal, and studies inhibiting RNA synthesis with actinomycin D demonstrated a precursor-product relationship between full-length mRNA and the smaller RNAs. In vivo, about 50% of the full-length species that undergo decay are degraded to the smaller RNAs and the rest are degraded to undetectable products. Exposure of erythroid cells that expressed a normal human beta-globin transgene to either cycloheximide or puromycin did not result in the generation of the smaller RNAs. Therefore, a drug-induced reduction in cellular protein synthesis does not reproduce this aspect of cytoplasmic mRNA metabolism. These data suggest that the premature termination of beta-globin mRNA translation in either exon I or exon II results in the cytoplasmic generation of discrete mRNA degradation products that are missing sequences from exon I or exons I and II. Since these degradation products appear to be the same for all nonsense codons tested, there is no correlation between the position of translation termination and the sites of nucleolytic cleavage.

Translation to near the distal end of the penultimate exon is required for normal levels of spliced triosephosphate isomerase mRNA

The translation of human triosephosphate isomerase (TPI) mRNA normally terminates at codon 249 within exon 7, the final exon. Frameshift and nonsense mutations within the TPI gene that cause translation to terminate prematurely at or upstream of codon 189, within exon 6, result in a decreased level of TPI mRNA (I.O. Daar and L.E. Maquat, Mol. Cell. Biol. 8:802-813, 1988). For all mutations in this group, the decrease is to the same extent, i.e., to approximately 20% of the normal level. We show here that a second group of nonsense mutations that cause translation to terminate prematurely at or downstream of codon 208, in exon 6, did not affect TPI mRNA abundance. Deletion analysis demonstrated that the abundance of translationally active TPI mRNA is a function of both the distance and the polarity of the nonsense codon relative to the final intron in TPI pre-mRNA. Our results indicate that if translating ribosomes are unable to progress to at least a certain position within the penultimate exon relative to the final intron, then the level of the corresponding mRNA will be abnormally low. Studies inhibiting RNA synthesis with dactinomycin demonstrated that a block in translation does not affect the half-life of mature TPI mRNA. The simplest interpretation of our data is that the translation of TPI mRNA in the cytoplasm facilitates the splicing of TPI pre-mRNA or the transport of TPI mRNA across the nuclear envelope or both.

Translation to near the distal end of the penultimate exon is required for normal levels of spliced triosephosphate isomerase mRNA

The translation of human triosephosphate isomerase (TPI) mRNA normally terminates at codon 249 within exon 7, the final exon. Frameshift and nonsense mutations within the TPI gene that cause translation to terminate prematurely at or upstream of codon 189, within exon 6, result in a decreased level of TPI mRNA (I.O. Daar and L.E. Maquat, Mol. Cell. Biol. 8:802-813, 1988). For all mutations in this group, the decrease is to the same extent, i.e., to approximately 20% of the normal level. We show here that a second group of nonsense mutations that cause translation to terminate prematurely at or downstream of codon 208, in exon 6, did not affect TPI mRNA abundance. Deletion analysis demonstrated that the abundance of translationally active TPI mRNA is a function of both the distance and the polarity of the nonsense codon relative to the final intron in TPI pre-mRNA. Our results indicate that if translating ribosomes are unable to progress to at least a certain position within the penultimate exon relative to the final intron, then the level of the corresponding mRNA will be abnormally low. Studies inhibiting RNA synthesis with dactinomycin demonstrated that a block in translation does not affect the half-life of mature TPI mRNA. The simplest interpretation of our data is that the translation of TPI mRNA in the cytoplasm facilitates the splicing of TPI pre-mRNA or the transport of TPI mRNA across the nuclear envelope or both.

Ribonucleoprotein and protein factors bind to an H-DNA-forming c-myc DNA element: possible regulators of the c-myc gene

We have located a positive, cis-acting DNA sequence element within the 5' flanking DNA of the c-myc gene (-125 base pairs). This DNA sequence element has a large purine-pyrimidine strand asymmetry and can assume the H-DNA conformation. A factor with the properties of a ribonucleoprotein (RNP) interacts with this DNA region. The interaction of the c-myc DNA sequence element and the RNP involves an RNase H-sensitive mechanism and, therefore, may involve an RNA.DNA hybrid. In addition, a protein factor(s) binds to this DNA sequence element. DNA footprinting and mutant oligonucleotide binding/competition assays implicate a punctate, poly(G.C) recognition/binding sequence for the RNP factor, whereas the major protein factor requires two ACCCT sequence motifs for maximal binding. These results suggest that RNP and protein factors act as positive transcriptional regulators of the c-myc gene, perhaps by altering DNA topology.

The mRNA transcripts from a mutant beta-globin gene derived from splicing at preferential cryptic sites

We have analyzed mRNA transcripts from beta-globin genes carrying a homozygous point mutation at the 5' splicing site of the first intron, using a method allowing in vivo analysis of mRNA transcripts. As expected, this mutation decreases normal splicing of mRNA when cryptic splicing are utilized. We have observed that, in reticulocytes, most mature mRNA transcribed from beta-globin genes derives from specific sites of abnormal splicing. Our results differ from those previously obtained using mutant beta-globin genes introduced in cultured cells and indicate a preferential processing of the abnormal globin mRNA species in red cell precursors.

Translation affects immunoglobulin mRNA stability

When termination codons were introduced into exons of the gene for Ig mu chain, steady-state levels of mu mRNA were reduced, both at the pre-B cell stage and at the plasma cell stage. A termination codon in the variable region gene segment and a termination codon in the second exon of the constant region gene segment had effects of similar magnitude. When the termination codon was deleted, the original level of mRNA was restored. The rate of mu gene transcription was the same whether or not a termination codon was present. Therefore, the termination codons must reduce the amount of the mRNA by reducing its stability. Since the introduced termination codons prematurely terminate translation and, in so doing, change the ribosome load on the mRNA, we conclude that mu mRNA stability is conferred in part by ribosomal protection from enzymatic degradation. We propose that the differences in mu mRNA stability during B lymphocyte differentiation are due to different amounts of ribosomes available for translation.

New amber mutation in a beta-thalassemic gene with nonmeasurable levels of mutant messenger RNA in vivo

We have identified a beta-thalassemia gene that carries a novel nonsense mutation in a Chinese patient. This mutation, a G to T substitution at the first position of codon 43, changes the glutamic acid coding triplet (GAG) to a terminator codon (TAG). Based on oligonucleotide hybridization studies of 78 Chinese and Southeast Asian beta-thalassemia chromosomes, we estimate that this mutation accounts for a small minority of the beta-thalassemia mutations in that population. Study of the expression of this cloned gene in a transient expression system demonstrated a 65% decrease in levels of normally spliced mutant beta-globin mRNA. However, the study of reticulocyte RNA isolated from an individual heterozygous for this mutation demonstrated a total absence of this mutant mRNA in vivo. The basis for this big discrepancy between the level of accumulated mRNA in vivo and in vitro is probably the result of differences in the stabilities of the mutant mRNA in erythroid cells.

Post-transcriptional regulation and DNA undermethylation of intracisternal A particle genes in embryonal carcinoma cell lines

Northern blot analysis and in vitro nuclear transcription assays were performed in order to clarify conflicting reports on the expression of intracisternal A particle (IAP) genes in embryonal carcinoma (EC) cell lines. Results demonstrate that post-transcriptional mechanisms control the final steady-state levels of IAP RNA in EC cells. IAP genes were further found to be undermethylated in IAP-expressing EC cell lines.

Beta-thalassemia: analysis of mRNA precursors of a mutant human globin gene with defective splicing using peripheral blood nucleated red blood cells

Studies on the effects of thalassemic mutations on gene function in vivo have clinical as well as scientific implications. Usually these studies have been performed on nucleated red blood cell (RBC) precursors normally present in bone marrow. Many patients with beta-thalassemia are splenectomized and may have high levels of nucleated RBC, orthochromatic normoblasts, in their peripheral blood (1-5% of total RBC). The possibility of exploiting these cells instead of bone marrow as a source for nuclear and cytoplasmic RNA for expression studies was investigated. A simple procedure was developed for enrichment for normoblasts in blood samples withdrawn from patients prior to transfusion. Globin transcripts were analyzed in RNA purified from 12 patients. Unspliced precursor beta-mRNA molecules were observed in a patient with beta o-thalassemia, homozygous for a mutation at the 5' IVS2 splice site of the beta-globin gene. Detailed analyses showed that his mature beta-mRNA was larger than normal, and that a cryptic 5' splice site, approximately 50 nucleotides downstream from the normal one, was utilized. We conclude that peripheral blood can be used as a reliable source of RNA for the analysis of the effects of beta-thalassemia mutations on gene expression and the relationship to the clinical condition. Moreover, this procedure facilitates the comparison of in vivo gene expression with the results obtained from DNA transfection experiments with cloned beta-thalassemia genes.