BioEd Online

Younger brothers and sisters are usually considered pests for their whining and fighting. Now it seems they could also be a factor in whether older siblings grow a brain tumour.

The cause of brain tumours is a long-standing and impenetrable mystery, because they are very rare and it is difficult to collect together enough cases to find a common cause. But one idea is that viral infections are involved, as they are in causing cervical and other cancers.

To explore this, a team led by Andrea Altieri of the German Cancer Research Centre in Heidelberg, studied the number of brothers and sisters a person has as a surrogate measure for the number of infections they suffered. The idea, used by epidemiologists before, is that a greater number of snotty siblings generally exposes a child to more viruses.

The team found that people with four or more siblings (either younger or older) were twice as likely to have developed certain types of brain and nervous-system tumours than those with no siblings.

Sibling count is "one of the highest risk factors we know for nervous-system cancers," Altieri says.

All in the family

The team analysed more than 13,600 cases of nervous-system tumours in a Swedish cancer database, one of the largest family cancer registries in the world.

Children who had three or more younger siblings were up to 3.7 times more likely to develop certain tumours when they were children or young teenagers compared with those with no younger siblings. Having lots of older siblings did not produce the same risk.

The result suggests that infections passed from younger siblings to older ones, perhaps during late childhood, may somehow increase susceptibility to these cancers. Infections passed from older siblings to younger ones, on the other hand, may not.

Another explanation could be that first-born children face different risks than third- or fourth-born children because they are exposed to a different milieu of maternal hormones or other factors during pregnancy.

The study the largest yet to look at links between siblings and brain tumours is published this month in Neurology¹.

Altieri stresses that the evidence is too provisional to try and prevent cancers through frequent hand washing or other measures that fight infection.

Pass it on

The idea that children swapping infections might play a large role in some cancers has been posed before (see ' Common infections blamed for childhood leukaemia'). But studies into childhood infection and brain cancers have been inconsistent.

The real challenge, says Faith Davis who studies the epidemiology of tumours at the University of Illinois, Chicago, is to pin down whether one particular virus or bacterium specifically increases susceptibility to cancer. Alternatively, she says, it may be that infections generally alter the developing immune system so that it does not fight off cancerous cells normally.

There are established links between the state of the immune system and brain cancer, Davis says. People who suffer allergies or asthma, for example, seem to be at less risk of certain types of tumour. "What we need to do is tease out what's underlying it," she says.

There is a link between the immune system and cancer, Davis notes. People who suffer allergies or asthma, for example, seem to be at less risk of certain brain tumours. "What we need to do is tease out what's underlying it," she says.

Few risk factors have been definitively pinned down for brain tumours, although family history, certain rare inherited disorders, high doses of radiation and HIV are thought to increase risk.

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