A genetic characteristic usually associated with youthfulness and good health increases the risk of brain cancer, research has shown.

Longer telomeres, the protective caps on the ends of chromosomes, are generally thought to be a sign of slow biological ageing.

But scientists found two common gene variants linked to long telomeres that significantly raise the risk of deadly brain cancers known as gliomas.

The variants in the TERT and TERC genes are respectively carried by 51 per cent and 72 per cent of the population.

US lead researcher Kyloe Walsh, from the University of California at San Francisco, said: “Though longer telomeres might be good for you as a whole person, reducing many health risks and slowing ageing, they might also cause some cells to live longer than they’re supposed to, which is one of the hallmarks of cancer.”

Both the genes in question are known to regulate the action of telomerase, the enzyme that maintains telomere length.

Like the plastic ends on shoe laces, telomeres prevent DNA from fraying and becoming jumbled. Each time a cell divides, they shorten, until a point is reached when the cell ceases to function.

Telomere shortening occurs at different rates in different people, leading to variations in biological age.

The new study, published in the journal Nature Genetics, involved a huge investigation of telomere length in almost 40,000 people.

While longer telomeres may heighten the chances of brain cancer, the research also linked shorter telomeres with a significantly increased risk of heart disease.

TERT variants have been implicated in lung, prostate, testicular and brain cancers, and TERC variants in leukaemia, bowel cancer and multiple myeloma.

In some cases, the variants promoted longer telomeres and in other cases shorter telomeres.

This suggested that “both longer and shorter telomeres may be pathogenic, depending on the disease under consideration,” said the authors.