New Delhi

A new study offers an explanation for why big dogs age faster -- possibly because of genes that "jump" from chromosome to chromosome and damage DNA -- even though, among mammals, an increased body size typically means a longer lifespan.

Mice are known to live for only a couple of years, while whales can live up to almost 200 years. However, within species, including dogs, the opposite is often true -- smaller individuals outlive younger ones.

The findings, published in the journal Science, reveal that ageing in bigger dogs is associated with a widespread loss of DNA methylation over time , a well-known chemical "signpost" of ageing and DNA regulation -- particularly in regions of the genome known as "jumping genes" or transposable elements.

Researchers, led by the US' Arizona State University, said a class of jumping genes called 'LINE1s' was found to be a key component of biological ageing differences.

LINE1s can copy and insert themselves throughout the genome, jumping from chromosome to chromosome and damaging DNA in the process, they said.

The team added that while LINEIs are usually kept in check by DNA methylation, their activity can increase when the regulatory signposts are lost -- a process linked to genomic instability, cancer and other age-related diseases.

The researchers analysed data from 864 dogs enrolled in the 'Dog Aging Project' and mapped genome-wide patterns of DNA methylation.

DNA methylation is a process that influences how much genes are turned on or off without changing the DNA sequence. It is part of the epigenome, which responds to environmental cues such as diet or stress.

"We demonstrate that shorter-lived dogs do not merely die earlier; they undergo (an) accelerated molecular aging specific to genomic locations and structures," the authors wrote.

"Larger dogs, males, and younger individuals showed molecular profiles consistent with accelerated epigenetic aging," they said.

"Dogs showed increased methylation in promoters of genes down-regulated with age in humans and decreased methylation in transposable elements, with canid-specific genes showing increased changes in large dogs," the team wrote.

Senior author Noah Snyder-Mackler, a professor at Arizona State University's school of life sciences, said: "Dogs provide an extraordinary model for understanding aging because they show dramatic variation in lifespan within a single species."

"Because they are companion animals, they can also be a powerful model with immediate relevance to human health. Living with us means that we share experiences and environments, while our love for our pets means we provide them food, exercise and lifelong healthcare -- meaning we can translate many findings in our shorter-lived pets to humans," Snyder-Mackler said.

The researchers also uncovered differences between female dogs, which have two X chromosomes (XX), and males, which have one X and one Y chromosome (XY). LINE1s on the X chromosome were found to be more methylated in males than in females.

The result suggests that females may experience a higher activity of LINE1s, potentially influencing sex-specific aging patterns.

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The study's findings point to potential targets of aging in dogs, with possible translational impacts in humans, the authors said.