A single gene may provide important clues to the causes of some types of deafness, research suggests.

When the FGF20 gene was “knocked out” of mice, the animals appeared perfectly healthy but had absolutely no ability to hear.

In humans, the gene has already been associated with inherited deafness in otherwise healthy families.

FGF20 codes for one of a family of proteins called fibroblast growth factors. The molecules play key roles in embryonic development, tissue maintenance and wound healing.

Disabling the gene caused a loss of outer hair cells, a special sensory cell in the inner ear responsible for amplifying sound.

Mice lacking FGF20 were missing about two-thirds of their outer hair cells. However, the number of inner hair cells which transmit amplified sound signals to the brain, appeared normal. “This is the first evidence that inner and outer hair cells develop independently of one another,” said lead researcher Sung-Ho Huh, from Washington University School of Medicine in St Louis, US. “This is important because most age-related and noise-induced hearing loss is due to the loss of outer hair cells.”

The findings were reported in the online journal Public Library of Science Biology.

The FGF20 signalling pathway must be activated by day 14 of a mouse embryo’s development to produce a normal inner ear, the scientists learned.

If it is switched on at day 15, the inner ear fails to develop properly.

This critical “window” may explain why mammals are incapable of regenerating outer hair cells, unlike birds and other vertebrates.

Rachel Baker, research programme manager at the charity Action on Hearing Loss, said: “Understanding how cells in the inner ear grow is crucial in developing new therapies for protecting and restoring hearing.

This research could be particularly relevant for sufferers of age-related or noise-induced hearing loss, which affect a growing number of people in the UK.

“We are pleased to be funding a new project being run by this research team, which is further investigating the role of FGF20 in hearing. In the future, this could lead to studies to test whether reactivation of FGF20 can regenerate damaged parts of the inner ear.”