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Testing times ahead

发布时间:2019-03-08 02:05:03来源:未知点击:

By Michael Day WITHIN two or three years, a simple blood test could tell you whether or not you are likely to develop the human form of mad cow disease. So claims John Collinge of Imperial College, London, who last week unveiled a diagnostic test based on biopsied tonsils. Collinge’s test looks for the prion protein thought to cause vCJD, the form of Creutzfeldt-Jakob disease linked to BSE-infected meat. When tissue containing this protein is digested with an enzyme and the resulting material is separated in a gel using an electric current, it produces a characteristic pattern. Just over two years ago, Collinge demonstrated his test using brain tissue (This Week, 2 November 1996, p 4). In a paper in last week’s issue of The Lancet (vol 353, p 183), he shows that the same pattern can be detected in the tonsils, spleen and lymph nodes of people suffering from vCJD. Four of the patients in Collinge’s study have died, and postmortem analysis of their brains confirmed his diagnosis. Another five are still alive. Prions are misshapen forms of a normal protein called PrP. In classical CJD, they can’t be detected in the tonsils, spleen and lymph nodes—all part of the body’s lymphatic system. But prions regularly show up in these tissues in some animal prion diseases, such as scrapie in sheep. In these cases, prions turn up in lymphatic tissues long before any symptoms of brain degeneration appear. That leads Collinge to believe that tonsil biopsies could provide advance warning for people who are incubating vCJD. “The likelihood is most people with new variant CJD will have rogue PrP in their lymph tissue now,” he says. Collinge thinks it probable that lymph tissue would test positive within months of infection. Already, plans are afoot in Britain for anonymous testing of tissues removed during surgery. The national CJD Surveillance Unit in Edinburgh is already carrying out one study, using an antibody test to detect prions in stored tonsils and appendixes. Collinge intends to try his test on more than 2000 fresh tonsils. Even if there are no positive results, neither study will be large enough to exclude the possibility of a large epidemic of vCJD. But just one positive result, if the sample is representative of the British population, could signal that tens of thousands of people are infected. PrP is carried on the surface of white blood cells, and Collinge believes it will be possible to detect vCJD prions in a blood sample. A test could be ready for use within three years, he says. There is no evidence that CJD can be passed on by blood transfusions. But a blood test would create huge pressure for routine screening of donated blood in Britain, the home of BSE, so that blood could be destroyed as a precaution if it tested positive. But screening would pose many problems, as HIV testing did in the 1980s, says John Barbara, microbiology consultant to the National Blood Authority. He thinks it would be difficult on ethical grounds to withhold positive results from people. But until there are drugs to treat vCJD,