Dinucleotide deletions in neuronal transcripts: a novel type of mutation in non-familial Alzheimer's disease and Down syndrome patients

Frameshifted beta-amyloid precursor protein (APP+1) is a secretory protein, and the level of APP+1 in cerebrospinal fluid is linked to Alzheimer pathology

Molecular misreading of the beta-amyloid precursor protein (APP) gene generates mRNA with dinucleotide deletions in GAGAG motifs. The resulting truncated and partly frameshifted APP protein (APP+1) accumulates in the dystrophic neurites and the neurofibrillary tangles in the cortex and hippocampus of Alzheimer patients. In contrast, we show here that neuronal cells transfected with APP+1 proficiently secreted APP+1. Because various secretory APP isoforms are present in cerebrospinal fluid (CSF), this study aimed to determine whether APP+1 is also a secretory protein that can be detected in CSF. Post-mortem CSF was obtained at autopsy from 50 non-demented controls and 122 Alzheimer patients; all subjects were staged for neuropathology (Braak score). Unexpectedly, we found that the APP+1 level in the CSF of non-demented controls was much higher (1.75 ng/ml) than in the CSF of Alzheimer patients (0.51 ng/ml) (p < 0.001), and the level of APP+1 in CSF was inversely correlated with the severity of the neuropathology. Moreover the earliest neuropathological changes are already reflected in a significant decrease of the APP+1 level in CSF. These data show that APP+1 is normally secreted by neurons, preventing intra-neuronal accumulation of APP+1 in brains of non-demented controls without neurofibrillary pathology.

Neuronal expression of GFAP in patients with Alzheimer pathology and identification of novel GFAP splice forms

Glial fibrillary acidic protein (GFAP) is considered to be a highly specific marker for glia. Here, we report on the expression of GFAP in neurons in the human hippocampus. Intriguingly, this neuronal GFAP is coded by out-of-frame splice variants and its expression is associated with Alzheimer pathology. We identified three novel GFAP splice forms: Delta 135 nt, Delta exon 6 and Delta 164 nt. Neuronal GFAP is mainly observed in the pyramidal neurons of the hippocampus of Alzheimer and Down syndrome patients and aged controls, but not in neurons of patients suffering from hippocampal sclerosis. Apparently, the hippocampal neurons in patients with Alzheimer's disease pathology are capable of expressing glia-specific genes.

Coexpression and accumulation of ubiquitin +1 and ZZ proteins in livers of children with alpha(1)-antitrypsin deficiency

The ZZ variant of alpha(1)-antitrypsin deficiency (AATD) is well known to cause liver damage and cirrhosis in some affected children. Ubiquitin abnormality was recently shown to be significant in AATD in childhood cirrhosis. Molecular misreading (MM), defined as faulty transcription of genomic information from DNA into mRNA, as well as its translation into mutant proteins, has been documented in many pathologic processes where aggregation of abnormal proteins occurs. The misread protein, ubiquitin-B(+1) (UBB(+1)), was recently identified in the hallmarks of various neurological disorders. The objective of this study was to determine whether MM of ubiquitin occurs in AATD. Twelve explanted liver specimens from AATD-affected children with cirrhosis were retrieved from archival sources, along with 10 control liver specimens obtained from autopsies of age-matched children with no clinical, gross anatomic, or histologic evidence of liver disease. Double immunofluorescence studies using rabbit polyclonal antibodies against UBB(+1) and AAT were performed on consecutively sectioned tissue. UBB(+1) immunoreactivity was colocalized with AAT in all cirrhotic AATD livers. The control livers were consistently negative. Ubiquitin MM is prominent in AATD-affected cirrhotic livers. This indicates that for children with AATD and cirrhosis, ubiquitin MM is a necessary cofactor to the aggregation of mutant ZZ isoform of AATD.

Translational misreading: a tRNA modification counteracts a +2 ribosomal frameshift

Errors during gene expression from DNA to proteins via transcription and translation may be deleterious for the functional maintenance of cells. In this paper, extensive genetic studies of the misreading of a GA repeat introduced into the lacZ gene of Escherichia coli indicate that in this bacteria, errors occur predominantly by a +2 translational frameshift, which is controlled by a tRNA modification involving the MnmE and GidA proteins. This ribosomal frameshift results from the coincidence of three events: (1) decreased codon-anticodon affinity at the P-site, which is caused by tRNA hypomodification in mnmE(-) and gidA(-) strains; (2) a repetitive mRNA sequence predisposing to slippage; and (3) increased translational pausing attributable to the presence of a rare codon at the A-site. Based on genetic analysis, we propose that GidA and MnmE act in the same pathway of tRNA modification, the absence of which is responsible for the +2 translational frameshift. The difference in the impact of the mutant gene on cell growth, however, indicates that GidA has at least one other function.

Biological ageing research in the Netherlands

After an introduction on the development of biological ageing research in the Netherlands during the past decades, 606 papers on aging published by Dutch institutes in the period 1991-2000, collected from PubMed, were analysed for their relevance to research into biological ageing. For the period 1996-2000, the total number of research papers on biological ageing amounted to 142, which accounts for 23% of all publications on ageing in that period. The number of publications per year did not change. On the basis of these papers and additional information provided by research groups a comprehensive overview of biological ageing research in the Netherlands is presented, together with an extensive literature list. Ageing of the central nervous system (CNS), of the endocrinological system and of the cardiovascular system are the topics most studied. It is concluded that general biological ageing research has not increased in the Netherlands over the last ten years, and that the infrastructure for basic biological ageing research in the Netherlands is weak.

Novel frameshift mutations near short simple repeats

In patients with Alzheimer's disease or Down's syndrome, the cerebellar cortex exhibits protein deposits in neurofibrillary tangles and neuritic plaques. Recently, the deposits have been shown to contain protein fragments of ubiquitin-B and amyloid precursor protein (APP) with an aberrant carboxyl terminus resulting from frameshift mutations (dinucleotide deletions; DeltaGU or DeltaGA) in or adjacent to GAGAG motifs in their mRNAs, a process referred to as molecular misreading. We have now used a bacterial expression system with the green fluorescent protein as a reporter to screen gene transcripts from aged controls, Alzheimer's disease, and Down's syndrome for molecular misreading. Novel frameshift mutations at a number of locations in the transcripts of the ubiquitin-B and APP genes were discovered (DeltaGA, DeltaG, DeltaGU, DeltaGG, DeltaCA, DeltaAU, DeltaA, DeltaAA, DeltaC, DeltaU, and insertion of an A). Interestingly, most mutations were in close proximity of short simple repeats (GAGAG, GGUGGU, GAGACACACA, UCAUCAUCA, CAAACAAA, and GAAGAAGAA), demonstrating that the GAGAG motif does not constitute the only hot spot for transcriptional errors. Unlike the previously detected aberrant APP fragments, some of the novel ones have the potential to generate the neurotoxic peptide beta-amyloid. We conclude that during aging molecular misreading is a widespread phenomenon.

In vivo ribozyme targeting of betaAPP+ mRNAs

In Alzheimer's disease (AD) and Down's syndrome (DS) patients, posttranscriptional alterations of sequences encoded by exon 9 and exon 10 of the beta-amyloid precursor protein (betaAPP) mRNA result in mutant proteins (betaAPP+) that colocalize with neurofibrillary tangles and senile plaques. These aberrant messages may contribute to the development of sporadic or late-onset Alzheimer's disease; thus, eliminating them or attenuating their expression could significantly benefit AD patients. In the present work, self-cleaving hammerhead ribozymes targeted to betaAPP exon 9 (Rz9) and betaAPP+ mutant exon 10 (Rz10) were examined for their ability to distinguish between betaAPP and betaAPP+ mRNA. In transiently transfected A-204 cells, quantitative confocal fluorescence microscopy showed that Rz9 preferentially lowered endogenous betaAPP. In contrast, in transient cotransfection experiments with betaAPP+ mRNAs containing a wild-type exon 9 and mutant exon 10 (betaAPP-9/betaAPP-10+1), or a mutant exon 9 and wild-type exon 10 (betaAPP-9+1/betaAPP-10) we found that Rz9 and Rz10 preferentially reduced betaAPP+ -mutant exon 10 mRNA in a concentration and a ribozyme-dependent manner.

Molecular misreading. A new type of transcript mutation in gerontology

Molecular misreading is a novel process that causes mutations in neuronal transcripts. It is defined as the inaccurate conversion of genomic information from DNA into nonsense transcripts and the subsequent translation into mutant proteins. As a result of dinucleotide deletions (delta GA, delta GU, delta CU) in and around GAGAG motifs in mRNA the reading frame shifts to the +1 frame, and subsequently the so-called +1 proteins are synthetized. +1 Proteins have a wild-type NH2 terminus and from the site of the dinucleotide deletion onwards an aberrant, nonfunctional COOH terminus. Molecular misreading was found in the rat vasopressin gene associated with diabetes insipidus and in the human genes linked to Alzheimer's disease (AD), that is, beta-amyloid precursor protein (beta APP) and ubiquitin-B (UBB). Moreover, beta APP+1 and UBB+1 proteins accumulate in the neuropathological hallmarks of AD. Inasmuch as these +1 proteins were also found in elderly, nondemented control patients, but not in younger ones (< 72 years), molecular misreading may act as a factor that becomes manifest in aged people. A hotspot for dinucleotide deletions is GAGAG motifs. Because statistically an average of 2.1 GAGAG motifs per gene can be expected, other genes expressed in other tissues may undergo molecular misreading as well. Indeed, we recently detected +1 proteins in proliferating cells present in tissues such as the liver, epididymis, parotid gland, and neuroblastoma cell lines. Therefore, molecular misreading can be regarded as a general biological source of transcript errors that may be involved in cellular derangements in numerous age-related pathologic conditions apart from Alzheimer's disease.

Molecular misreading of genes in Down syndrome as a model for the Alzheimer type of neurodegeneration

The occurrence of +1 frameshifted proteins, such as amyloid precursor protein (APP+1) and ubiquitin-B (UBB+1) in Down syndrome (DS) has been linked to the onset of Alzheimer's disease (AD). In DS and AD patients, but also in elderly non-demented persons, these co-called +1 proteins accumulate in the neuropathological hallmarks (neurofibrillary tangles, dystrophic neurites of the neuritic plaques and neuropil threads) and may have deleterious effects on neuronal function. Frameshifts are caused by dinucleotide deletions in GAGAG motifs in messenger RNA and are now thought to be the result of unfaithful transcription of normal DNA by a novel process termed "molecular misreading". In the present review some of the critical events in molecular misreading are discussed, the emphasis being on DS.