DREAM complex and aging: a brake on DNA repair

DREAM complex and aging are connected through one of the cell’s most fundamental maintenance systems: DNA repair. The more active this protein complex is, the more strongly it represses a broad set of DNA-repair genes. A study in Nature Aging now links this molecular brake not only to mutation accumulation, but also to lifespan across mammals and features of Alzheimer’s disease.

DNA is damaged continuously. Ultraviolet radiation, chemicals and ionizing radiation can cause lesions, but many arise simply as by-products of normal cellular activity. Cells therefore possess several repair systems that detect damaged DNA, remove incorrect segments or restore broken strands.

These systems, however, are not equally active in every cell. One particularly striking difference exists between germ cells and the somatic cells that make up most of the body. Protecting germline DNA is essential because it will be transmitted to the next generation. Some repair pathways are more restricted in somatic tissues.

One mechanism behind this difference is DREAM, a protein complex that regulates the transcription of many genes. Earlier work showed that DREAM binds near numerous DNA-repair genes and represses their expression. In simple terms, cells possess the repair machinery, but DREAM limits how much of that machinery is produced.

From individual cells to entire species

The new study asked whether this regulation has measurable consequences over a lifetime.

The researchers first analyzed 110,824 individual cells from 21 tissues of 18 mice of different ages. Rather than measuring the DREAM complex directly, they inferred its activity from the expression pattern of genes normally repressed by DREAM. They also estimated the burden of somatic mutations accumulated by individual cells.

Across many tissues, the same pattern emerged: cells with higher DREAM-associated activity tended to carry more mutations. Importantly, the association was also detectable when cells of the same age and tissue were compared, making it less likely that the pattern simply reflected differences between organs or the age of the animal.

The researchers then expanded the comparison across mammalian evolution. They examined 803 samples from the liver, kidney and brain of 92 mammalian species. Species with lower DREAM-associated activity tended to have longer maximum lifespans.

That does not mean DREAM determines how long an animal lives. Cross-species comparisons are correlational, and longevity is influenced by many physiological and evolutionary variables. But the result is consistent with another established observation: many long-lived species accumulate somatic mutations more slowly.

What happens when the brake is removed

A particularly important test came from genetically modified mice in which DREAM function was disrupted. Their brains contained 4.2% fewer single-base substitutions and 19.6% fewer small insertions and deletions than those of control mice.

This moves the evidence beyond an association between DREAM activity and mutation burden. Direct genetic disruption of the pathway was accompanied by reduced mutation accumulation.

It still does not mean that “switching off DREAM slows aging.” The study did not demonstrate that these mice lived longer or aged more slowly. DREAM regulates the cell cycle, cellular quiescence and other biological programs in addition to DNA repair. Altering such a central regulatory system could have consequences that cannot be predicted simply from mutation counts.

The authors also examined data from 90 people, including 80 individuals with Alzheimer’s disease or neurodegenerative pathology and 10 cognitively normal individuals. The analysis covered more than 800,000 cells from 11 tissues. Lower DREAM-associated activity was associated with later disease onset and a lower probability of severe neuropathology.

Here, the distinction between association and causation is essential. These human data do not show that high DREAM activity causes Alzheimer’s disease, nor that inhibiting the complex could prevent it.

Why would cells have such a brake?

At first, it may seem odd that evolution has retained a mechanism capable of limiting repair of the genome. But DREAM has another major role: it helps cells enter and maintain quiescence and controls genes involved in cell-cycle progression. Keeping every DNA-repair pathway maximally active in every cell at all times could carry energetic costs or interfere with other cellular programs.

DREAM therefore cannot simply be regarded as a harmful complex that should be switched off.

The importance of the study lies elsewhere. It connects several previously separate observations into a coherent picture: DREAM represses DNA-repair genes; its activity tracks somatic mutation burden within tissues; across species it is associated with lifespan; and experimentally disrupting DREAM reduces mutation accumulation in the mouse brain.

That makes DREAM an intriguing target for research into the biology of aging. But there remains a considerable distance between identifying such a molecular mechanism and developing a safe way to alter human aging.