They all result from defects in the ‘ignition system’ of voluntary muscle. This is explained here. You will also find an A to Z of myasthenia here.

 

When an electrical impulse arrives from the brain, it causes release of packages of ACh – the ‘ignition keys’. These cross the short gap and latch into the special ‘ignition locks’ – the ACh receptors (AChR) which they then trigger. The spare ACh is broken down by AChE and the resulting fragments are re- cycled (by an enzyme, ChAT) to produce new ACh in the nerve terminals.

The AChRs are completely different in the voluntary and ‘automatic’ muscles (so we've given them V and U shapes). However, the nerve endings (and ACh) are similar.

Their Calcium channels – which are attacked by the auto-antibodies in LEMS patients – are located right at the point of ACh release on the nerve endings.

Only the (V-shaped ‘nicotinic’) AChRs in voluntary muscle are affected in the autoimmune and congenital myasthenias: only the (U-shaped ‘muscarinic’) AChRs in automatic muscle are blocked by Propantheline.

 

Research Landmarks

1935

Brenda Reid, a patient at the Myasthenia centre at The New End Hospital in Hampstead London approached Dr Lange, a consultant surgeon, to help her set up a support group for myasthenics. He suggested starting out as a committee within the muscular dystrophy (MD) Group.

The plot thickened greatly with early work on how nerve → muscle triggering normally works. Again in 1935, acetylcholine (ACh) was finally identified as the crucial ignition key (chemical transmitter). Later on, various researchers interested in nerve → nerve and nerve → muscle signalling realised that some snake toxins must affect the AChRs. For example, among the complicated mix of toxins in their venoms, cobras and other snakes have ‘alpha (α) neurotoxins’ which target the AChR almost irreversibly. By blocking it from binding ACh, they cause paralysis.

1963

Drs Chang and Lee (in Taiwan) found that, in their local Banded Krait snake (Bungarus multicinctus), this toxin (called ‘α-BuTx’) was so potent and selective that it could be tagged with colours or radioactivity and used to label the AChRs in microscope sections and so measure their numbers.

This key progress enabled Dr Doug Fambrough and his colleagues in 1973 (in the USA) to show that MG patients had fewer ACh receptors than normal people. This applies to both the autoimmune and most inherited myasthenias. Their AChR loss entirely accounts for their defective nerve → muscle ignition. Humans have only modest reserves of AChRs, so losing just over half of them is enough to make us weak.

Around that time, other biochemists were trying to understand how receptors work. They realised that the electric organs of electric eels, skates and rays are like muscles working backwards, converting chemical into electrical energy. These organs are packed with AChRs and are a much richer source than muscle. These AChRs can be dissolved with detergents, and then purified using α-BuTx.

1973

Drs Jon Lindstrom and Jim Patrick (in California) wanted to make antibodies (to help in their research) and immunised some rabbits with this pure AChR. To their surprise, the rabbits soon became floppy (displaying symptoms of myasthenia). Moreover, when treated with pyridostigmine, they recovered dramatically.

That was a crucial breakthrough, partly because it proved that myasthenia could indeed be caused by ‘autoantibodies’ just as our late vice president, Professor Iain Simpson, had predicted in 1960. It also led to a very useful blood test for these antibodies, which is now a vital standard diagnostic tool. It uses radioactive α-BuTx to tag AChR (from human muscle tumour cells grown in the lab) and quickly detects these antibodies in about 85% of patients with generalised MG and in about 60% of those with MG restricted to eye muscles.

Early doubts about whether the anti-AChR antibodies in MG are destructive or protective were soon dispelled by showing that the antibody -containing fraction of MG blood, when injected into mice, transferred the electrical defects of MG and also led to loss of AChRs from their muscles.

In fact nature had already done that experiment: babies born to around one in eight mothers with myasthenia have a brief (two to four week) myasthenic weakness (known as neonatal MG) and fully recover afterwards. Like the other antibodies that protect against infections, those that attack the AChRs are also transferred across the placenta and in the milk, and cause a short term loss of the baby’s muscle receptors.

The antibodies destroy AChRs by activating the same biochemical processes (complement and phagocytosis) that normally kill bacteria. They also increase the natural rate of AChR breakdown and occasionally block it from binding ACh. In many MG patients, the antibodies prefer the slightly different (fetal) form of the AChR that is found in the unborn baby’s muscles. Very rarely, they can cause paralysis and even malformations in the baby during late pregnancy.

Plasma exchange provided further proof that antibodies were causing the trouble. The patient’s plasma (which contains the antibodies) is washed away from the bloodstream and is replaced with a substitute together with the patient’s own red cells.

It was first used in MG in 1976 when a patient with myasthenia agreed to be ‘guinea pig’ and was rewarded by getting strikingly better for some weeks.

Interestingly, about 15 per cent of patients with typical myasthenia are negative for anti-AChR antibodies (seronegative) in the standard test. They must have some antibodies because their MG clearly improves on plasma exchange. Prof Angela Vincent’s team in Oxford found that some of these seronegatives do have antibodies that recognise the AChR but only when it is clustered naturally on cell surfaces (Leite et al, 2008).

In 2000, they showed that others instead recognise ‘MuSK’. It is a nearby partner that helps to cluster the AChRs in the right place and then to keep them there. In their new test, ~5% of the 15% of seronegatives now test positive against MuSK in the UK, and ~10% in S.E. Europe. These and the AChR antibodies are not found in the same patient. ‘MuSK’ patients seem not to have pure ocular MG but often to have more ‘bulbar’ weakness (i.e. affecting the face, throat and breathing) with more muscle wasting that may respond less well to immuno-suppressive drugs.

Unlike many such tests, those for anti-AChR and anti-MuSK very rarely give false positive results. This is partly because such hard and fast diagnostic tests are so valuable. Prof Vincent is still working on the final hardcore (5%) who have neither anti-AChR nor anti-MuSK antibodies.

 

 

 

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