BEIJING—Southwest China's hill country has long been stalked by a killer whose victims sometimes drop dead in midsentence. Two years ago, researchers unmasked the likely villain as a mushroom, new to science, which no one had realized is poisonous. Now they have isolated its toxins and propose an astonishing modus operandi.
Over the past few decades, health authorities in China have blamed more than 400 deaths on Yunnan sudden unexplained death. The syndrome exhibits a curious pattern. It strikes almost exclusively during the summer rainy season in highland villages in a narrow altitude band in Yunnan and possibly neighboring provinces. In 2010, an investigation spearheaded by epidemiologist Zeng Guang of the Chinese Center for Disease Control and Prevention (CDC) in Beijing identified the culprit as a small white mushroom (Science, 9 July 2010, p.132), since named Trogia venenata.
In the latest work, LIU Jikai, a medicinal chemist at Kunming Institute of Botany, and Chinese CDC colleagues set out to pin-point the poisons. After months of pains-taking effort, LIU says, “we isolated almost every unusual compound” in the mushroom. Three were toxic: two novel unsaturated amino acids and γ-guanidinobutyric acid, a compound normally found in the brain that induces seizures in lab animals. Unlike virtually all known natural amino acids, which have left-handed chirality, the Trogia pair is right-handed. Unsaturated amino acids are known toxins in other poisonous mushrooms, says Kimiko Hashimoto, an organic chemist at Kyoto Pharmaceutical University in Japan who with colleagues isolated a vicious muscle-melting toxin from Russula subnigricans, a mushroom found in Asia and North America. Puzzlingly, however, T. venenata's toxins are not particularly potent. “The toxicities of the newly isolated amino acids are rather weak,” Hashimoto says.
Also contrary to expectations, the Trogia toxins had only a mild effect on the heart. Mice fed an extract from the mushroom died. But the enzyme creatine kinase, a marker for heart attacks and other manifestations of muscle damage, was only slightly elevated in mouse blood—certainly not high enough to explain the toxins' lethality. Drilling deeper into the toxicology data, the team discovered that their mice had extremely low blood sugar. The “profound hypoglycemia” triggered by the mushroom extract “may explain the fatal outcome in humans and experimental animals,” Liu and colleagues report in the 5 March issue of Angewandte Chemie.
Such a mechanism for a toxin is not unprecedented. Ackee, a West African fruit grown widely in the Caribbean, contains hypoglycin, an aptly named toxin that blocks cells from using fatty acids as an energy source. As a result, cells rapidly use up glucose, starving them of energy. After ackee ripens, most hypoglycin in the fruit is converted to benign compounds. But eating the unripe fruit or its seeds can be fatal. Hypoglycin causes Jamaican vomiting sickness, in which victims can suffer seizures, coma, and in rare instances, death.
Similarities in the structures of the mushroom's amino acids and hypoglycin, LIU says, suggest that they “belong to the same class of chemical compounds.” Trogia and ackee poisonings are not mirror images of each other. In Yunnan, few people had vomited, says U.S. CDC epidemiologist Robert Fontaine, who is on assignment as a senior adviser to the Chinese CDC. However, he notes, some victims would suffer seizures and lapse into comas before dying. And researchers in Kunming isolated one of Trogia's exotic lefty amino acids from the blood of a previously healthy 27-year-old man who had died suddenly in August 2009, in a village in Yunnan that had experienced repeated sudden-death clusters.
What's happening at the molecular level may be the key to unlocking the riddle of Yunnan sudden unexplained death. Hypoglycin blocks beta oxidation, or the production of ATP from fatty acids, and glucose regeneration, causing hypoglycemia. If Trogia toxins were simply suppressing blood sugar, victims might pass out—the brain uses only glucose as an energy source—but they would stand a good chance of recovery. The heart, on the other hand, needs a lot of energy and relies on beta oxidation. “A depressed capacity of heart muscle to use fats as energy” may explain T. venenata's lethality, Fontaine says.
LIU suspects that Trogia's toxins, milquetoast on their own, may have powerful synergistic effects, perhaps abetted by γ-guanidinobutyric acid. The team plans more toxicology tests.
Not everyone buys that explanation. SHI Wuxiang, an epidemiologist at Dali University in Yunnan who has studied the syndrome, says that relatives have insisted that some victims never ate the mushroom, while in other cases several family members consumed it but only one or two fell ill. He believes the cause may be minerals in the soil washed into the water supply by summer downpours.
Fontaine says that if hypoglycemia is indeed the mechanism, then Trogia toxins should only be fatal if blood sugar falls below a threshold. Eating an insufficient quantity to reach that threshold may have no effect, which would explain why some family members do not succumb. More damning, there hasn't been a single reported case of Yunnan sudden unexplained death since Chinese CDC mounted a campaign in 2009 to warn villagers about T. venenata, Fontaine says. “It just stopped,” he says. “If something pops up again, then we'll say, ‘Whoops, maybe we were wrong and go back and investigate.’” But as a public health threat, he says, the case is closed.
(Source: http://www.sciencemag.org/content/335/6074/1293.full)
