A melanoma gene mutation is far more common in South Islanders, raising new questions about excess sun exposure.
The disparity may be linked to sunburn, say the study's University of Otago researchers, whose findings are published in the journal Oncotarget.
New Zealand has the highest rate of melanoma in the world. Each year around 2200 new cases are diagnosed and more than 300 people die from the disease.
The researchers studied 20 mutated genes in samples from 529 patients with advanced melanoma.
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In line with overseas findings, the most common mutation was in a gene called BRAF, and one-third of the melanomas in the study showed changes in this gene. The BRAF mutation rates were similar in the north and south of New Zealand.
But the mutation rate for the NRAS gene was 38 per cent for South Islanders and 21 per cent for those from the North Island.
"The North Island rate is about the same as that found in other countries, so the South Island rate really sticks out like a sore thumb," said study co-leader Professor Mike Eccles.
The researchers suggest sunburn, or strong exposure to ultraviolet radiation, especially during the spring months in the South Island when vitamin D levels are lowest, could be a factor leading to high rates of NRAS mutations.
NRAS mutations were linked to a higher likelihood that the melanoma would be of the deadlier nodular type.
Eccles said preliminary data from an American study last year found that in a small number of patients, melanomas with NRAS mutations had higher response rates to new immunotherapy treatments such as Keytruda and Opdivo.
Neither of the new immunotherapies is state-funded in New Zealand and some patients are paying tens of thousands of dollars for treatment. Pharmac has proposed funding Opdivo from July.