TOKYO A Japanese researcher at the center of discredited research that was initially hailed as a potential breakthrough for stem-cell treatment, killed himself after months of stress and exhaustion, officials said on Tuesday.
Yoshiki Sasai, co-author of the high-profile research that had seemed to offer hope for replacing damaged cells or even growing new human organs, was found early on Tuesday at the Riken institute where he worked in Kobe, western Japan, police and the institute said.
"It is confirmed as a suicide," said a police spokesman. "It was a hanging."
Sasai, 52, had been hospitalized in March for stress and become less receptive to media inquiries during the controversy over the team's research, said Riken spokesman Satoru Kagaya.
The scientist "had seemed completely exhausted" in their last phone conversation around May or June, Kagaya told a televised news conference.
As deputy director of Riken's Center for Developmental Biology, Sasai supervised the work of lead author Haruko Obokata, which took the world of molecular biology by storm when it was published in the British journal Nature in January.
It was retracted after months of controversy that made front-page news in Japan and tarnished the country's reputation for scientific research.
"It is very unfortunate that this happened," said the government's top spokesman, Chief Cabinet Secretary Yoshihide Suga. "Mr Sasai contributed greatly in the field of developmental biology and was an internationally renowned researcher."
Riken president Ryoji Noyori expressed "deep regret over the loss of an irreplaceable scientist."
In what looked like game-changing discovery, Obokata, Sasai and the other authors described simple ways to reprogram mature animal cells back to an embryonic-like state, allowing them to generate many different types of cells.
But questions soon arose about the research, as other scientists could not replicate the startling claims. Riken said its investigation found Obokata had plagiarized and fabricated parts of the papers, raising doubts about the credibility of Japanese science.
After defending her work for months against Riken's claims, Obokata agreed in June to retract the papers, which Nature did in early July.
Despite the retractions of the research papers, Sasai never wavered in his belief that Stimulus-Triggered Acquisition of Pluripotency, or STAP, cells could exist, Japanese media said.
Obokata was "very shocked" at Sasai's suicide and was being assisted by two Riken staffers, Kagaya said.
Sasai left five suicide notes, including two addressed to senior Riken officials, he said. He would not disclose the contents or to whom the other letters were addressed.
Sasai started receiving counseling in April and recently had trouble communicating due to side-effects of medical treatments he was undergoing, media reported.
(Reporting by Megumi Lim; Additional reporting by Kaori Kaneko; Editing by William Mallard and Robert Birsel)