New mutations in the ataxia telangiectasia gene

We report the detection of four new mutations in the ataxia telangiectasia gene (ATM). Reverse-transcribed RNA extracted from cultured cells was analysed for mutations by polymerase chain reaction amplifications and restriction endonuclease fingerprinting. Three deletions and a base substitution are described. The deletions reported here would result in severe disruptions of the ATM gene product by leading either to a protein truncation (a 4-bp deletion) or the loss of stretches of 53 and 58 amino acids (a 159-bp deletion and a 174-bp deletion, respectively); whereas the base substitution would lead to an amino acid change from a highly conserved glycine to an arginine residue.

The Gly/Glu polymorphism of the neurotrophin 3 gene: allele frequencies in a Caucasian population and relevance for psychiatric disorders

Using a modified method for the determination of the genotype of the neurotrophin 3 Gly/Glu polymorphism by PCR, we investigated allele frequencies in patients suffering from different psychiatric diseases as well as in healthy controls. There was a much lower frequency of the allele Glu among Caucasians (0.006) than previously reported for a Japanese population (0.203). No homozygotes and only three heterozygotes for the rare allele could be detected: the latter belonged to the patient group and suffered from endogenous depression (two patients) and hebephrenia (one patient). Due to its very low frequency it seems unlikely that the allele Glu plays a decisive role in the aetiopathogenesis of schizophrenic psychoses or other psychiatric diseases. However, it cannot be excluded that it represents a risk or vulnerability factor.

A null mutation allele in the CNTF gene and schizophrenic psychoses

The maldevelopmental theory postulates disturbances in neural development as crucial factors in the aetiopathogenesis of schizophrenia. Neurotrophic factors, including ciliary neurotrophic factor (CNTF), play a central role in the regulation of such development. A mutation has been described for the CNTF gene, whereby subjects homozygous for the mutation lack CNTF. The polymerase chain reaction was used to amplify the CNTF gene region containing this mutation in whole blood genomic DNA. The mutation was detected by analysis of restriction fragment length polymorphisms. Patients suffering from schizophrenic psychosis (ICD-10 criteria) (51 from Würzburg, 83 from Barcelona), and healthy controls (62 from Würzburg, 50 from Barcelona) were investigated. In the Würzburg group, the frequency of subjects homozygous or heterozygous for the mutation was significantly higher among schizophrenic patients than in controls. However, no difference could be detected in the Spanish sample; the possible reasons for the different allele distribution in the two patient groups is discussed. It is concluded that the CNTF null mutation may be relevant to the aetiopathogenesis of schizophrenia in some patients, but further work is required to identify specifically the patient group for which it is important.

Association between a null mutation in the human ciliary neurotrophic factor (CNTF) gene and increased incidence of psychiatric diseases?

We report a possible association between a null mutation in the human ciliary neurotrophic factor (CNTF) gene and psychiatric diseases. Prior findings that the mutant allele frequency is not significantly elevated in patients suffering from neurological diseases are confirmed. The frequency of the mutant allele was higher among psychiatric patients (0.192, n = 297) than among healthy controls and neurological patients (0.142, n = 267). This difference (one-tailed 2 x 2 chi-square test, P < 0.05) might be evidence that disturbances in the neurotrophic factor system could play a crucial role in the etiopathogenesis of psychiatric disorders, mainly psychoses.

Age-related human mtDNA deletions: a heterogeneous set of deletions arising at a single pair of directly repeated sequences

Deletions in mtDNA accumulate during the human aging process, arising from either intramolecular illegitimate recombination or strand slippage during replication, which results in subgenomic mtDNA molecules. We identify here two classes of mtDNA deletions--class A deletions, which are homogeneous at the breakpoints, with all subgenomic molecules therefore being identical in size, and class B deletions, which arise from a less stringent process that gives rise to heterogeneity at the breakpoints, with the subgenomic molecules being of slightly different sizes. A novel approach is described that offers a global overview of the populations of different deletions in individual tissues. It is based on PCR cycle-sequencing reactions that are carried out directly on mtDNA segments, amplified by PCR from total cellular DNA. The results show a clear size homogeneity of the subgenomic mtDNA molecules representative of class A, which carry a commonly detected 4,977-bp deletion occurring at a pair of 13-bp directly repeated sequences. In this case, precisely one copy of the repeat is retained in the subgenomic molecules. We then describe a class B situation comprising a family of at least nine closely related 8.04-kb deletions involving the same pair of 5-bp direct repeats. In this situation, the breakpoints differ at the base-pair level (8,037-8,048-bp deletions); the subgenomic molecules retain > 1 copy, 1 copy, or < 1 copy of the 5-bp repeat. In different tissues from either the same individual or among different individuals, there is a widely variable occurrence of particular deletions in the subgenomic mtDNA population within this class B set. Class B deletions offer a new approach for studying the accumulation of mtDNA deletions, thereby providing insight into the independent somatic origin of mutated mtDNA molecules, both in aging and in mitochondrial diseases. We also report a convenient method for ascertaining whether a given PCR product results from the amplification of a subgenomic mtDNA template, on the basis of the selective degradation of full-length mtDNA molecules prior to PCR.

Mitochondrial DNA mutation associated with aging and degenerative disease

Previous theories of aging based on somatic mutation neglected mtDNA, which has a high propensity for mutational error. Knowledge of yeast mtDNA mutations and their functional effects, and of human mtDNA mutations identified in the mitochondrial cytopathies, provides for a concept of aging based on the cumulative effect of mutations affecting human mtDNA. An essential feature of this concept is heteroplasmy, representing mixtures of normal and mutant mtDNA at the cellular and mitochondrial level, resulting in a "tissue mosaic" of focal bioenergetic deficits. Direct evidence for the concept is provided by (i) focal loss of staining for mitochondrially encoded enzymes, such as cytochrome c oxidase, in tissues of aged individuals (humans and rats) and (ii) an age-related increase in deletional mutations in mtDNA demonstrable by application of the polymerase chain reaction to DNA templates from individuals of different ages.

Mitochondrial DNA mutation and the ageing process: bioenergy and pharmacological intervention

A comprehensive hypothesis concerning the contribution of mitochondrial DNA (mtDNA) mutations to the human ageing process is reviewed and the implications for cellular bioenergy loss and pharmacological therapy are considered. The central idea is that random mutations in the population of mtDNA molecules of each cell occur throughout life, and that this is a major contributor to the gradual loss of cellular bioenergy capacity within tissues and organs, associated with general senescence and diseases of ageing. An elaboration of four major aspects of the general proposition, together with relevant supporting data, is presented. (1) An extensive array of deletions in mtDNA of many tissues of humans and other mammals has been observed to occur in an age-related manner. (2) The preservation and selection of fully functional mtDNA molecules in the female germ line cells is proposed to occur via a human mtDNA cycle, in which selective amplification of a limited number of mtDNA templates occurs during oocyte development. This proposal explains the endowment of normal neonates with a mtDNA complement minimally contaminated by damaged mtDNA molecules. The phenomena of maternal inheritance and rapid fixation of sequence variants of mtDNA in mammals, as well as selection of cells based on mitochondrial function, are taken into account. (3) Tissue bioenergy mosaics result from accumulated mtDNA damage during ageing, representing different rates of cellular bioenergy loss within individual cells of a tissue. The random segregation of mtDNA during cell division will also further contribute to the tissue energy mosaic. Cells unable to meet their particular bioenergy demand will become non-functional, leading to cell death; the bioenergy threshold is different for the various cell types in the tissues of the body. (4) In order to bioenergetically resuscitate cells and tissues suffering from impaired mitochondrial functions as a result of the ageing process, we propose that redox compounds may be used therapeutically in the pharmacological configurations of a by-pass strategy or as a redox sink therapy. The role of these compounds is to maintain at least part of the mitochondrial respiratory chain function (by-pass) as well as to maintain adequate levels of cellular NAD+ (redox sink) for ATP synthesis, predominantly by the cytosolic glycolytic pathway, with some contribution from mitochondrial oxidative phosphorylation.

Multiple mitochondrial DNA deletions in an elderly human individual

We have used the polymerase chain reaction (PCR) to study deletions in the mitochondrial DNA (mtDNA) of an elderly human individual. An extended set of PCR primers has been utilised to identify 10 mitochondrial DNA deletions in a 69-year-old female subject with no known mitochondrial disease. The particular deletions visualised as PCR products depended on the primer pairs used, such that the more distantly separated PCR primers enabled visualisation of larger deletions. Some deletions were common to the heart, brain and skeletal muscle, whereas others were apparently specific to individual tissues. DNA sequencing analysis of PCR products showed that short direct repeat sequences (5 to 13 bp) flanked all deletion breakpoints; in most cases one copy of the repeat was deleted. It is proposed that the accumulation of such multiple deletions is a general phenomenon during the ageing process.

Characterization of two novel human retropseudogenes related to the calmodulin-encoding gene, CaMII

Two human genomic clones (lambda hg22 and lambda hg29), containing two novel calmodulin (CaM) retropseudogenes, were isolated and characterized. The two pseudogenes show high similarity with the human CaMII cDNA, hCE1 [SenGupta et al., J. Biol. Chem. 262 (1987) 16663-16670] and the CaMII-type retropseudogene, hCE2 (CaMII-psi 1) [SenGupta et al., Nucleic Acids Res. 17 (1989) 2868]. One of them, in clone lambda hg22 (CaMII-psi 2), shows all the characteristics of a processed pseudogene. In clone lambda hg29 (CaMII-psi 3), however, an Alu repetitive sequence was detected immediately upstream from the ancestral 5'-untranslated region. Downstream from the truncated 3'-untranslated region, three additional copies of Alu repetitive sequences flanking about 750 nucleotides of unknown origin were found. Such a processed retropseudogene flanked by multiple Alu repeats may be a target for further recombination events. The three retropseudogenes CaMII-psi 1, CaMII-psi 2 and CaMII-psi 3 are estimated to be about 49, 21 and 25 million years old, respectively.

Mitochondrial gene mutation: the ageing process and degenerative diseases

Polymerase chain reaction (PCR) amplification was carried out on total DNA from a range of autopsy tissues from deceased human subjects with no known mitochondrial disease, aged from birth (80 minutes) to 87 years. We report the finding of an age-related 5 kb deletion in the mitochondrial genomes of these subjects. The deletion occurs between nucleotide positions 8470 and 13459 of the mitochondrial genome, and is flanked by a 13 bp direct repeat. All tissues from adult subjects showed the presence of mitochondrial DNA molecules with the deletion after a 30 cycle PCR amplification; by contrast the deletion was not similarly detected in any of the infant tissues analysed. However, the occurrence of the deletion was detected in the infant tissues after 60 PCR cycles of MtDNA amplification. It is concluded that such deletions are not necessarily associated with particular mitochondrial diseases but occur naturally, and with increasing frequency with age. A consequence of the accumulation of this deletion could be a progressive decrease with age of bioenergetic capacity which in turn could influence the rate of ageing and predispose to age-associated degenerative diseases.