A woman who underwent an experimental treatment to try to cure her paralysis, in which stem cells were collected from her nasal cavity and implanted in her spine, suffered a tumour-like growth eight years later, the New Scientist reports.
The American, whose name has not been made public, underwent the procedure at a hospital in Portugal. It was hoped that the cells would support and guide the growth of neurons, repairing the damage to her spine which had paralysed her.
Instead, eight years later she was complaining of worsening pain at the implant site. Last year, at the age of 28, she was operated on again, with surgeons removing a three-centimetre long growth.
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The 'tumour' was composed mainly of nasal tissue, with bits of bone and nerves which had not connected with her spinal nerves.
The growth was also secreting a "thick copious mucus-like material". It was the pressure from this mucus which is thought to have been causing pain.
The magazine speculates that the nasal stem cells, which came from the patient's own nose, had retained their ability to produce mucus because they had been implanted directly, rather than being grown in a lab to eliminate undesired cells.
Jean Peduzzi-Nelson, a researcher from Wayne State University in Detroit who advised the Portuguese team, says they tried the treatment on 140 people in all. She said many patients had had a "remarkable recovery".
She added: "I am saddened to learn of this adverse event. However, the incidence of this problem is less than one per cent."
A stem cell researcher from Harvard Medical School, George Daley, told the New Scientist the news was "sobering". He said: "It speaks directly to how primitive our state of knowledge is about how cells integrate and divide and expand."
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