Much research has linked mitochondrial dysfunction with aging and age-related syndromes such as Alzheimer’s disease (AD), but it remains unclear whether harm to these cellular powerhouses precedes or results from disease processes. Two new studies offer support for the former. From analysis of a series of mouse mutants, researchers in Sweden and Germany report in the August 21 Nature that mitochondrial DNA mutations present at birth can speed aging in animals with normal nuclear genomes. The finding raises the possibility that maternally inherited mitochondrial DNA variants could predispose to age-related disorders as well. Earlier, a research team in Spain reported in the June 22 Annals of Neurology that reduced mitochondrial DNA content in human spinal fluid serves as a marker for preclinical AD. The CSF changes showed up at least a decade before AD symptoms. While the studies differ, they both suggest mitochondrial changes as a driving force, rather than a consequence, of the aging process and AD pathogenesis.
Researchers led by Nils-Göran Larsson at the Max Planck Institute for Biology of Ageing, Cologne, Germany, and Lars Olson of the Karolinska Institute, Stockholm, Sweden, previously engineered mice with a proofreading-deficient mitochondrial polymerase, PolgA. They found that the animals racked up mitochondrial DNA (mtDNA) mutations faster than wild-type mice and aged prematurely (ARF related news story). In the current study, co-first authors Jaime Ross of the Karolinska and James Stewart of the Max Planck took the analysis one step further, asking whether premature aging could result from mtDNA mutations already present at birth.
PolgA mutant mice transmit mtDNA mutations through their germline (Ameur et al., 2011), making them suitable for studying the role of inherited mtDNA variants. Ross and colleagues crossed mice to generate lines with normal nuclear genomes (wt-N) but with or without inherited mtDNA mutations, as well as PolgA mutant mice with or without inherited mtDNA mutations.
Among wt-N mice, Larsson said, “if they inherited mtDNA mutations from their mother, they aged a bit more quickly.” PolgA mutant mice with inherited mtDNA mutations aged even faster, and displayed signs of premature aging, such as heart enlargement and reduced respiratory chain function. Furthermore, when Stewart and colleagues performed magnetic resonance imaging (MRI) on the brains of the mice to look for neuronal degeneration, they were surprised to find malformations in about a third of the PolgA mutant mice that inherited mtDNA mutations. The abnormalities included elongated hippocampi, striatal disturbances, and white matter changes, but no two mice looked the same. Brain malformations did not show up in mice with normal PolgA or without inherited mtDNA mutations. This suggests that “in order to get these malformations, (the mice) must inherit mtDNA mutations from their mother and have ongoing mutagenesis,” Larsson told Alzforum.
In the biomarker study, researchers led by Ramon Trullas of the Institute of Biomedical Research, Barcelona, Spain, took a different approach to ask whether mitochondrial changes precede the known biochemical signatures of AD. Based on their recent findings linking synaptic breakdown with mitochondrial transport (Clayton et al., 2012), first author Petar Podlesniy and colleagues wondered if release of mtDNA from damaged synapses into cerebrospinal fluid (CSF) could serve as a marker of neurodegeneration in AD. First, they verified they could use PCR to quantify free-floating mtDNA in human CSF—something not previously reported, they claim. The researchers measured mtDNA using real-time quantitative DNA amplification.
They then measured mtDNA in cell-free CSF samples from people with probable AD, asymptomatic at-risk individuals with low CSF Aβ1-42, age-matched healthy elderly, and, as an additional control, people with frontotemporal lobar degeneration (FTLD). Mitochondrial DNA content was substantially lower in probable AD and at-risk asymptomatic patients relative to the two control groups. The researchers were puzzled, but the findings held up in separate cohorts of sporadic AD cases and young presymptomatic patients with dominant presenilin 1 mutations. That CSF mtDNA levels were already low in presymptomatic patients suggested to the authors that mitochondrial dysfunction precedes symptom onset.
What might cause CSF mtDNA levels to drop in preclinical AD? At this point, Trullas has little insight into mechanisms. In light of a recent study reporting a 50 percent decrease in mtDNA copy number in postmortem AD brain (Coskun et al., 2010), he said their “best explanation is that this decrease in CSF mtDNA is an index of low mtDNA copy number in brain neurons.” Consistent with this idea, Podlesniy and colleagues measured the ratio of mtDNA to nuclear DNA in cultured cortical neurons from APP/PS1 transgenic mice and found they had 28 percent fewer mtDNA copies per cell compared to wild-type neurons.
“The two papers deal with different issues but ultimately circle back and tell a bigger story. They are both consistent with the mitochondrial cascade hypothesis,” said Russell Swerdlow of the University of Kansas Medical Center, Kansas City. This hypothesis holds that changes in mitochondrial function initiate the cascade that leads to AD, as opposed to beta-amyloid changes being upstream (see Swerdlow et al., 2010).
Bruce Yankner of Harvard Medical School, Boston, Massachusetts, said the mtDNA mutant mouse analyses were “elegantly done” and beg the question of whether inheritance of mtDNA mutations speeds aging in people as well. He suggested analyzing fibroblasts or lymphocytes from presymptomatic patients or people with AD risk factors (e.g. low CSF Aβ, apolipoprotein E4 allele) to see if they have more mtDNA mutations than usual.
In the biomarker study, Yanker said he would have liked to see analogous measurements in the APP/PS1 mice to determine if CSF mtDNA falls, and when, during the course of pathogenesis. If the human CSF findings can be replicated by other research groups, he thinks the assay is “worth pursuing because [a drop in CSF mtDNA] might be a useful very early biomarker, and may tell us something about the role of mitochondria in the pathogenesis of AD.” Flint Beal of Weill Cornell Medical College, New York, agreed. Beal added that the work opens up further questions—for example, how specific the reduction in CSF mtDNA is to AD, and how far in advance of symptom onset it occurs.—Esther Landhuis
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