Ž . Molecular Brain Research 52 1997 284–289 Research report Reduced steady-state levels of mitochondrial RNA and increased mitochondrial DNA amount in human brain with aging Antoni Barrientos a,b , Jordi Casademont a , Francesc Cardellach a , Xavier Estivill b , Alvaro Urbano-Marquez a , Virginia Nunes b, ) a Muscle Research Unit, Department of Internal Medicine, Hospital Clinic, UniÕersity of Barcelona, Villarroel 170, Barcelona 08036, Spain b ( ) Department of Molecular Genetics, Cancer Research Institute IRO , AutoÕıa de Castelldefels, Km 2.7, 08907 Hospitalet del Llobregat, Barcelona, Spain ´ Accepted 19 August 1997 Abstract The contribution of the mitochondrial genetic system in the degenerative processes of senescence remains unclear. This study deals with age-related changes in brain mtDNA expression in humans. Brain tissue from the frontal lobe cortex was obtained from autopsy of 13 humans aged between 21 and 84 years. No structural changes were detected in mtDNA, increased mtDNA content and reduced Ž . steady-state level of mitochondrial transcripts and transcription ratio mtRNArmtDNA were associated with aging. These findings suggest that the increase of the mtDNA levels could be considered as an inefficient compensatory mechanism to maintain the normal levels of mtRNA transcripts. This unbalanced mitochondrial condition could play a role in the process of senescence in human brain. q 1997 Elsevier Science B.V. Keywords: Aging; Mitochondrial DNA; Mitochondrial transcription; Brain 1. Introduction Aging is characterized by a generalized physiological decline. It has been proposed that mitochondrial dysfunc- w x tion may have a role in this process 1–3 , especially in highly oxidative phosphorylation-dependent tissues such as brain where the toxic free radicals derived from the respi- ratory chain activity may damage the mitochondrial DNA Ž . wx mtDNA 4 . In the last few years, it has been hypothe- sized that the accumulation of somatic mutations in the mtDNA secondary to this toxic effect might contribute to w x the neurological impairment associated with aging 5,6 . However, since only a low proportion of mutated mtDNA molecules has been consistently found in aged individuals, other changes not related to the structure of mtDNA may play a role in aging. For example, studies in brain from rat have shown that concentrations of mitochondrial RNA Abbreviations: mtDNA: mitochondrial DNA; mtRNA: mitochondrial RNA; nDNA: nuclear DNA; nRNA: nuclear RNA; PCR: polymerase chain reaction ) Ž .Ž . Corresponding author. Fax: q34 3 -2632251; e-mail: vnunes@iro.es Ž . mtRNA decline with age, which has been related to a wx reduced mitochondrial transcription rate 7 rather than to a wx drop in mtDNA levels 8 . To our knowledge such studies have not been performed in humans, where brain is one of the most clinically relevant targets in senescence. Herein, we report the effect of aging on the content of mtDNA and mtRNA in the frontal cortex of 13 human brains. 2. Material and methods Brain tissue from the cortical region of frontal lobe was obtained from autopsy of individuals without any clinical history suggestive of mitochondrial encephalomyopathy. Brain tissue was obtained within 12 h of death and was stored at y808C until use. Frontal cortex consisted of cortex in front of the precentral gyrus and above the sylvian fissure. The families of all the individuals gave informed consent and the protocol was approved by the ethics committee of the hospital. Total DNA was extracted from brain following standard wx procedures 9 . To detect rearranged mtDNA molecules, Southern blot analyses were performed with undigested 0169-328Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.