mRNA Metabolism and hereditary disorders: a tale of surveillance and escape.

The exploration of the molecular origin of hereditary diseases focused on genes and proteins for many years. Recently, mRNA has gained increasing attention. Most human genes contain introns and a considerable proportion of transcripts are not only alternatively spliced but also regulated posttranscriptionally in manifold ways. mRNA processing as well as a complex network of interactions between the steps of gene expression and associated quality control mechanisms are guided by a multitude of regulative mRNA sequence elements. Therefore it is not surprising that mutations of such elements can cause or modify human disease. The purpose of this review is the illustration of principles of physiological and dysregulated mRNA metabolism as well as a survey of new analytical tools and therapeutic approaches.

Increased efficiency of mRNA 3' end formation: a new genetic mechanism contributing to hereditary thrombophilia.

The G-->A mutation at position 20210 of the prothrombin or coagulation factor II gene (F2) represents a common genetic risk factor for the occurrence of thromboembolic events. This mutation affects the 3'-terminal nucleotide of the 3' untranslated region (UTR) of the mRNA and causes elevated prothrombin plasma concentrations by an unknown mechanism. Here, we show that the mutation does not affect the amount of pre-mRNA, the site of 3' end cleavage or the length of the poly(A) tail of the mature mRNA. Rather, we demonstrate that the physiological F2 3' end cleavage signal is inefficient and that F2 20210 G-->A represents a gain-of-function mutation, causing increased cleavage site recognition, increased 3' end processing and increased mRNA accumulation and protein synthesis. Enhanced mRNA 3' end formation efficiency emerges as a novel principle causing a genetic disorder and explains the role of the F2 20210 G-->A mutation in the pathogenesis of thrombophilia. This work also illustrates the pathophysiologic importance of quantitatively minor aberrations of RNA metabolism.

Splicing and 3' end formation in the definition of nonsense-mediated decay-competent human beta-globin mRNPs.

Premature translation termination codons are common causes of genetic disorders. mRNAs with such mutations are degraded by a surveillance mechanism termed nonsense-mediated decay (NMD), which represents a phylogenetically widely conserved post-transcriptional mechanism for the quality control of gene expression. How NMD-competent mRNPs are formed and specified remains a central question. Here, we have used human beta-globin mRNA as a model system to address the role of splicing and polyadenylation for human NMD. We show that (i) splicing is an indispensable component of the human beta-globin NMD pathway, which cannot be compensated for by exonic beta-globin 'failsafe' sequences; (ii) the spatial requirements of human beta-globin NMD, as signified by the maximal distance of the nonsense mutation to the final exon-exon junction, are less constrained than in yeast; and (iii) non-polyadenylated mRNAs with a histone 3' end are NMD competent. Thus, the formation of NMD-competent mRNP particles critically depends on splicing but does not require the presence of a poly(A) tail.

Dominant beta-thalassaemia: a highly unstable haemoglobin is caused by a novel 6 bp deletion of the beta-globin gene.

Beta-thalassaemia is inherited as an autosomal recessive trait in most families. Particular interest has recently been focused on the molecular pathology of the rare forms with a dominant mode of inheritance. The index patient and her mother, who are described in this report, displayed typical clinical and haematological features of beta-thalassaemia intermedia with significant ineffective erythropoiesis and additional peripheral haemolysis. Molecular analysis demonstrated a heterozygous genotype for a novel 6 bp (TGGTCT) deletion of the beta-globin gene involving codons 33-35. This deletion results in the removal of two valine residues from the beta-globin chain at position 33/34 (B15/B16) and the substitution of the tyrosine residue at position 35 (C1) by an aspartic acid (beta 33-35 [B15-C1] Val-Val-Tyr-->0-0-Asp). According to the index patient's place of birth, this abnormal haemoglobin has been termed Hb Dresden. The stability of the variant and the normal beta-globin chains were similar during the incubation period of in vitro globin chain synthesis analysis. However, Hb Dresden is exquisitely unstable and cannot be detected in the peripheral blood by haemoglobin electrophoresis, high-performance liquid chromatography (HPLC) or isoelectric focusing. This instability can be explained by the vital structural role of the three affected amino acids that, in normal haemoglobin, establish a total of nine intermolecular bonds (five hydrophobic and four polar) at both the alpha1beta1 (alpha2beta2) and the alpha1beta2 (alpha2beta1) interface.

Binary specification of nonsense codons by splicing and cytoplasmic translation.

Premature translation termination codons resulting from nonsense or frameshift mutations are common causes of genetic disorders. Complications arising from the synthesis of C-terminally truncated polypeptides can be avoided by 'nonsense-mediated decay' of the mutant mRNAs. Premature termination codons in the beta-globin mRNA cause the common recessive form of beta-thalassemia when the affected mRNA is degraded, but the more severe dominant form when the mRNA escapes nonsense-mediated decay. We demonstrate that cells distinguish a premature termination codon within the beta-globin mRNA from the physiological translation termination codon by a two-step specification mechanism. According to the binary specification model proposed here, the positions of splice junctions are first tagged during splicing in the nucleus, defining a stop codon operationally as a premature termination codon by the presence of a 3' splicing tag. In the second step, cytoplasmic translation is required to validate the 3' splicing tag for decay of the mRNA. This model explains nonsense-mediated decay on the basis of conventional molecular mechanisms and allows us to propose a common principle for nonsense-mediated decay from yeast to man.

Vigilin is a cytoplasmic protein. A study on its expression in primary cells and in established cell lines of different species.

A fusion protein composed of about two vigilin domains and beta-galactosidase was used to raise polyclonal antibodies which were affinity-purified and employed for immunoblotting and immunohistochemistry. A protein of an apparent molecular mass of 155 kDa could be stained in extracts of a variety of cells from different species and organs. Immunohistological studies on single cells showed that vigilin is accumulated in the cytoplasm. During in vitro maintenance of primary cell cultures, as well as of a growth-factor-dependent cell line, vigilin expression decreases and ceases in senescent cells. In contrast, vigilin is constitutively expressed in all other transformed cell lines of various origin studies so far. Vigilin expression can be induced in peripheral blood lymphocytes by mitogen stimulation. These observations suggest an involvement of vigilin in processes of cell activation. Immunoblot experiments demonstrating the presence of vigilin in a broad range of eukaryotes, indicate a high degree of evolutionary conservation.