As most of you know, the weakness in MG is caused by defects in the nerve -> muscle ignition system. Usually, there is an immune attack on the muscle surface near the nerve endings; it causes loss of the ignition locks (the acetylcholine receptors or AChRs for short). These AChRs crucially make sure that your muscles contract when you tell them to. Most patients with typical generalised MG weakness have antibodies that latch onto the AChRs and destroy them. These antibodies are routinely measured (with radioactive AChR) in many diagnostic labs. In Oxford, we test about 100 blood samples a week; the positive cases (about 1 in 10) can be given a firm diagnosis of MG.
We have known for a long time that about 12% of patients with typical generalised MG do not have antibodies against AChR, and are misleadingly called antibody-negative. In fact, they clearly get better with plasma exchange and steroid treatment, just like typical MG patients, so they must have some other harmful antibodies. These can be transferred from MG mums to their developing babies, occasionally causing weakness in them again as in typical MG.
In the MGA News (June 2001 Edition, P12), we said we had identified a new target for these antibodies, a different protein called MuSK. This MuSK is important because it helps the muscles cluster the AChRs opposite the nerve endings (see David Beesons diagram on P5). We have found antibodies against MuSK in about half of the previously antibody-negative patients ie about 6% of all MG patients. Interestingly, we find antibodies against AChR or MuSK in completely separate patients, so there must be at least two distinct types of MG. We are now trying to learn more about the antibodies against MuSK, and to see if there is anything different about the myasthenia or its ideal treatment in the patients who have them. So far, it seems that they often have particular trouble with swallowing, speaking and chewing, and never have a thymoma
After cloning the gene and mass-producing MuSK in cultured cells, we now purify it in small amounts. We make it slightly radioactive and then use it to measure the antibodies. Over the last year in Oxford, this test has given very clear results and thus a firm diagnosis for more patients. It took some time to get commercial companies involved, but RSR Ltd (Cardiff) will start producing a kit in the next month or two so that other labs in the UK or Europe can measure the antibodies. RSR will also sell this material to the Athena Diagnostics labs in the USA, to allow them to test patients on that side of the Atlantic.
Doctors who need this test should send 1 ml of serum in a screw-topped
polypropylene tube (double packed for the mail) to:
Prof Angela Vincent,
Weatherall Institute of Molecular Medicine,
John Radcliffe Hospital,
Im afraid we have to charge £30 per test to help cover the costs. Any profits go into our research not only for this form of MG, but also to help identify the target for antibodies in the mysterious remaining 6%.
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