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Canadian researchers have discovered why a common chemotherapy drug causes hearing loss in some childhood cancer patients, paving the way for a simple saliva or blood test that can predict who is most likely to develop the problem.

After analyzing more than 1,800 genetic variations in 220 key genes, British Columbia scientists have found two genetic variations that for children who have them, means they will suffer serious hearing loss after taking cisplatin.

The study, published in the journal Nature Genetics, also has implications for adults, some of whom suffer hearing loss after taking cisplatin treatment for cancers of the ovaries, liver, stomach and bladder. Each year, an estimated 300 children receive the generic drug for cancers of the brain, bone, neuroblastomas and germ cell tumours.

“This is personalized genomics,” Michael Hayden, director of the Centre for Molecular Medicine and Therapeutics at the Child & Family Research Institute in Vancouver, said in a telephone interview. “…The other challenge is coming up with a test that's very cheap that can be used at the point of care.”

Doctors have long known patients metabolize drugs differently and the one-size-fits-all approach to medication simply doesn't work. One recent example is the breast cancer drug tamoxifen: A test can predict whether it will do little or nothing for some patients due to their genetic makeup.

In this study, the drug cisplatin works – it is lifesaving – but can also leave an enormous burden – deafness or hearing loss, which contributes to learning difficulties.

Susan Kerr first encountered this when her son, Jacob, then 5, was receiving cancer treatment for neuroblastoma, which presented itself with a bulky mass in his abdomen. As they sat on the deck of Ronald McDonald house in Vancouver, he could not hear the birds tweeting on a spring day.

“To tell me he has to go through all this and now he can't hear the birds,” Ms. Kerr says of Jacob, now 10, and who suffers high frequency hearing loss. “That was the one that hit me through the heart.”

Jacob can hear his parents whispering a room away but he cannot hear a thermometer beep. Speaking loudly doesn't help. At a concert, he needs earplugs to protect his hearing.

A Grade 5 pupil who loves to play soccer, hockey and basketball, Jacob, who lives in Victoria, says tests reveal his hearing has changed, but “most of the time I don't notice the difference.”

But the difference was measurable for researchers who examined the genes of 54 patients from BC Children's Hospital. They found 33 children, or 61 per cent, suffered serious hearing loss requiring hearing aids or cochlear implants after cisplatin therapy.

Specifically, those with variations in the thiopurine methyltransferase [TMPT] gene and the catechol-O-methyltransferase gene [COMT] had a 98 per cent chance of becoming deaf after having the drug infused, a finding replicated on 112 patients from children's hospitals across Canada.

Researchers can predict whether children with those genetic variations will suffer serious hearing loss, but not the other half of children on cisplatin. Though they also suffer hearing loss, no one yet knows why, though it could be the type of tumour or other treatments used with cisplatin.

Bruce Carleton, co-principal investigator and a senior clinician scientist at the Child and Family Research Institute, said the finding is crucial as it can help tailor treatment for pediatric cancer patients, by helping determine who is at risk of hearing loss and providing them a different therapy.

“I am really excited about this because for 20 years, I have been trying to figure out this plague in medicine; to dose to toxicity, and then stop. It's primitive but it's the best thing we can do,” Dr. Carleton said. “…The idea that we could stop therapy before permanent disability occurs is now a possibility.”

That is certainly what Dana Tent hopes. When her son, Stefan, was diagnosed with a brain tumour at 15 months of age, he was given cisplatin and suffered high-frequency hearing loss as a result.

Now five years old and in kindergarten, Stefan keeps asking his mother when he can stop wearing hearing aids.

“We basically have this conversation once a week,” Ms. Tent said in a telephone interview from Coquitlam, B.C. “I really hope that the kids in the future, they don't have to deal with it.”