Telomeres are to chromosomes what plastic caps are to the ends of shoelaces – they stop them unravelling as they age. Without telomeres, chromosomes would gradually lose their genetic information as cells replicate. For this reason – backed by some research – scientists believe longer telomeres are better for health. But now a new genomic study suggests longer telomeres may also increase the risk of developing deadly brain cancers known as gliomas.

Led by the University of California San Francisco (UCSF), the study focuses on two common variants of telomere-related genes known as TERT and TERC that lead to longer telomeres. TERT is carried by 51% of people and TERC is carried by 72%.

However, it is rather unusual for common variants to increase risk of disease, so the researchers suggest perhaps a genetic balancing act is going on between benefit and risk. On the benefit side, carrying longer telomere variants generally promotes overall health, while on the risk side, carrying them can also raise risk of high-grade gliomas, but for most carriers, the benefit trumps the risk, as gliomas, although nearly always fatal, are relatively rare.

“There are clearly high barriers to developing gliomas, perhaps because the brain has special protection,” explains senior author, Margaret Wrensch, professor of Neurological Surgery and Epidemiology and Biostatistics at UC San Francisco, who with colleagues reports the findings in the journal Nature Genetics.

To illustrate the point, she notes that the comment “I’ve never been sick in my life,” is not uncommon from people diagnosed with glioma.

One explanation for this genetic balancing act between benefit and risk could be that in general, slowing cellular aging is good for overall health. But perhaps this also means some cells will live longer than they are supposed to, which is a hallmark of cancer.

In a large genomic dataset that the researchers used in their analysis, covering 40,000 people, they found shorter telomeres were linked to higher risk of cardiovascular disease – a good example of where longer telomeres are generally linked to better health.