21 August 2009. As is true with getting into college or receiving a job offer, it’s not just pedigree but also life experiences that may determine whether a person will develop Alzheimer disease. So suggests an analysis of identical twins—one who died of AD, one without AD—reported this month in the publicly accessible journal PLoS ONE. Researchers led by Paul Coleman, Sun Health Research Institute, Sun City, Arizona, examined postmortem brain tissue and found that cortical neurons from the AD twin had reduced DNA methylation, a biochemical process that can disrupt genes’ accessibility for transcription by attaching methyl groups to individual nucleotides.
In an earlier study (Mastroeni et al., 2008), first author Diego Mastroeni and colleagues found lower levels of DNA methylation, as well as reduced expression of DNA methyltransferase and other methylation regulators, in affected brain areas of sporadic AD patients. “This led to the question of whether these epigenetic effects we saw in AD were related to the [people’s] genes or to their life experience,” said Coleman, who is also a professor emeritus at the University of Rochester Medical Center, New York. In the sporadic AD study, genetic backgrounds were all over the map—which is why the scientists leaped at the opportunity to analyze epigenetic markers in identical twins discordant for AD. “This was a situation in which the genetic background would be quite similar, if not identical, and anything we saw could be attributed to life experience,” Coleman said. Other research has shown that identical twins who are genetically prone to AD can differ markedly in their age of onset and degree of pathology (Brickell et al., 2007).
The twins in the current study were white males who attended the same schools and worked as chemical engineers. One encountered extensive pesticides in his work and died at age 76 after a 16-year battle with Alzheimer disease. The other worked in a different environment and was cognitively normal when he died of prostate cancer at age 79. Pathologically, their brains could not have looked more different. At the time of his death, the twin with AD had an anterior temporal neocortex riddled with amyloid plaques and neurofibrillary tangles, the two key pathological hallmarks of AD. In his non-demented brother, however, “we had to hunt through the brain sections in order to find even one neurofibrillary tangle,” Coleman told ARF. The cognitively intact man also had comparatively higher expression of 5-methylcytosine, a marker of methylated cytosine-guanine (CpG) sites on DNA, in neurons, reactive astrocytes, and microglia of brain areas typically vulnerable to AD.
Apart from disease status, DNA methylation appears to vary with age and environmental factors. In a recent analysis of 217 non-pathological human tissues, published this month in PLoS Genetics (Christensen et al., 2009), researchers report that genes in CpG islands become increasingly methylated as people get older, whereas genes outside of these methylation hotspots lose methylation with age. Methylation status also correlated with environmental exposures such as tobacco smoking in that analysis, led by Karl Kelsey at Brown University. In an earlier study, Manel Estreller and colleagues at the Spanish National Cancer Center, Madrid, analyzed identical twins and found that DNA methylation status was very similar when the siblings were young but diverged more and more as they got older (Fraga et al., 2005). Those papers “make the case for environmental and aging effects on methylation,” Coleman said of the Estreller and Kelsey studies. “Our research shows that the concept of life events affecting DNA methylation may apply to development of the AD phenotype. It also stresses the potential importance of epigenetic phenomena in molecular mechanisms of AD.”
The new data may have ramifications for interpreting studies of AD genetics. “One study will find that, yes, this gene is a risk factor for AD, and others say, no, it’s not, and the statistics have some uncertainty in them,” Coleman said. “We raise the question of whether the probabilistic nature of the relationship between some genes and AD may be due to the fact that the genetic effects can be modulated by life experience.”
A recent study in Iceland may offer a case in point. Researchers at the University of Iceland and at deCODE Genetics, Reykjavik, reported a drastically shortened lifespan over the last 20 years in people with a hereditary amyloid angiopathy, and attribute this to diet changes that may have exacerbated the effects of a genetic mutation tied to the disease (Palsdottir et al., 2008 and ARF related news story). Studies in AD mouse models that overexpress mutant amyloid precursor protein (TgCRND8 and 129Sv) offer another example of a diet-gene interaction. When put on a diet deficient in folate, B1, and B6, the AD mice had reduced brain methylation activity in conjunction with amyloid-β overproduction and cognitive impairment (Fuso et al., 2008).
A link between epigenetics and AD also came up in a recent investigation led by Axel Schumacher at the Klinikum Rechts der Isar, Munich, Germany. However, unlike the current study, which reveals global demethylation in affected brain areas of the AD twin, Schumacher’s showed that most DNA methylation changes in AD brains are subtle and restricted to specific genes, including several involved in amyloid-β processing (PSEN1, ApoE) and methylation homeostasis (MTHFR, DNMT1) (Wang et al., 2008 and ARF related news story). In an e-mail to ARF, Schumacher noted that analyzing late-stage disease tissue makes it hard to determine whether the observed epigenetic phenotypes are the cause or the result of the disease. In the new study, “the global demethylation in the affected brain areas may indicate that specific components of the epigenetic machinery, such as DNA maintenance methylation, were inactivated, which in turn could indicate that the observed epigenetic patterns result from the course of the disease,” he wrote (see full comment below).
Coleman hopes to address this possibility in a genomewide study to identify specific genes affected by DNA methylation in AD, he told ARF. Future work in this area may benefit from a new approach that uses flow cytometry and state-of-the-art sequencing techniques to quantify the number of methylated molecules in a sample. Its developers show the method is sensitive enough to detect one methylated molecule in about approximately 5,000 unmethylated molecules in DNA from plasma or fecal samples. In a report published online 16 August in Nature Biotechnology (Li et al., 2009), researchers led by Sanford Markowitz, Case Western Reserve University, Cleveland, Ohio, and Bert Vogelstein at Johns Hopkins University School of Medicine, Baltimore, Maryland, have used the technology to detect early-stage colorectal cancer.—Esther Landhuis.
Mastroeni D, McKee A, Grover A, Rogers J, Coleman PD. Epigenetic Differences in Cortical Neurons from a Pair of Monozygotic Twins Discordant for Alzheimer’s Disease. Aug 2009. PLoS ONE 4(8). Abstract