DNA Is Not the Only Mode of Biological Inheritance

A little over a decade ago, a clutch of scientific studies was published that seemed to show that survivors of atrocities or disasters such as the Holocaust and the Dutch famine of 1944-45 had passed on the biological scars of those traumatic experiences to their children.

The studies caused a sensation, earning their own BBC Horizon documentary and the cover of Time – and no wonder. The mind-blowing implications were that DNA wasn’t the only mode of biological inheritance, and that traits acquired by a person in their lifetime could be heritable. Since we receive our full complement of genes at conception and it remains essentially unchanged until our death, this information was thought to be transmitted via chemical tags on genes called “epigenetic marks” that dial those genes’ output up or down. The phenomenon, known as transgenerational epigenetic inheritance, caught the public imagination, in part because it seemed to release us from the tyranny of DNA. Genetic determinism was dead.

A model of DNA methylation – the process that modulates genes. The influence of environment or lifestyle on this process is being studied

A decade on, the case for transgenerational epigenetic inheritance in humans has crumbled. Scientists know that it happens in plants, and – weakly – in some mammals. They can’t rule it out in people, because it’s difficult to rule anything out in science, but there is no convincing evidence for it to date and no known physiological mechanism by which it could work. One well documented finding alone seems to present a towering obstacle to it: except in very rare genetic disorders, all epigenetic marks are erased from the genetic material of a human egg and sperm soon after their nuclei fuse during fertilisation. “The [epigenetic] patterns are established anew in each generation,” says geneticist Bernhard Horsthemke of the University of Duisburg-Essen in Germany.

Different people define epigenetics differently, which is another reason why the field is misunderstood. Some define it as modifications to chromatin, the package that contains DNA inside the nuclei of human cells, while others include modifications to RNA. DNA is modified by the addition of chemical groups. Methylation, when a methyl group is added, is the form of DNA modification that has been studied  most, but DNA can also be tagged with hydroxymethyl groups, and proteins in the chromatin complex can be modified too.

Researchers can generate genome-wide maps of DNA methylation and use these to track biological ageing, which as everyone knows is not the same as chronological ageing. The first such “epigenetic clocks” were established for blood, and showed strong associations with other measures of blood ageing such as blood pressure and lipid levels. But the epigenetic signature of ageing is different in different tissues, so these couldn’t tell you much about, say, brain or liver. The past five years have seen the description of many more tissue-specific epigenetic clocks.

Mill’s group is working on a brain clock, for example, that he hopes will correlate with other indicators of ageing in the cortex. He has already identified what he believes to be an epigenetic signature of neurodegenerative disease. “We’re able to show robust differences in DNA methylation between individuals with and without dementia, that are very strongly related to the amount of pathology they have in their brains,” Mill says. It’s not yet possible to say whether those differences are a cause or consequence of the pathology, but they provide information about the mechanisms and genes that are disrupted in the disease process, that could guide the development of novel diagnostic tests and treatments. If a signal could be found in the blood, say, that correlated with the brain signal they’ve detected, it could form the basis of a predictive blood test for dementia.

Source: https://www.theguardian.com/

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