The Mystery of Stiff Person Syndrome

Neuroskeptic iconNeuroskeptic
By Neuroskeptic
Feb 14, 2011 6:30 AMNov 5, 2019 12:17 AM


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"Stiff Person Syndrome" (SPS) is a rare neurological disease with a silly name but serious symptoms.

Not in fact a disorder caused by an overdose of Viagra, the defining feature of SPS is uncontrollable muscle rigidity, which comes and goes in bouts, but generally gets worse over time. However, other symptoms are seen including depression, anxiety, and other neurological features such as cerebellar ataxia.

What causes SPS? Well, it's been known for over 20 years that most SPS patients have antibodies against the enzyme GAD65, which is required for the production of GABA, the main inhibitory neurotransmitter in the brain. The body shouldn't be producing antibodies against its own proteins, but unfortunately this does happen quite often, for various reasons, and the result is autoimmune diseases.

So this all seems to make sense. We know that GABA causes muscle relaxation by reducing the brain's input to the muscles. This is why GABA drugs like Valium are muscle-relaxants, and it's part of the reason why drunk people tend to stagger around.

This also explains the anxiety symptoms, because Valium and beer make you less anxious, while drugs that block GABA cause panic attacks. Anti-GAD65 antibodies block GAD, so less GABA gets made. So SPS is autoimmunity against GAD65. Mystery solved?

Not quite. Anti-GAD65 antibodies are also seen in most people with Type I diabetes, but the vast majority of diabetics luckily don't suffer SPS. Mystery remains.

Two studies just out investigated exactly what the antibodies produced by SPS patients do. Geis et al purified the antibodies from a 53 year old woman with SPS and serious anxiety, and injected them into the brains of some rats.

The rats became very anxious. Here's what the cowardly critters did in a standard rodent anxiety test: they avoided the open spaces, which are naturally scary to rodents, who prefer dark, enclosed places:

This was associated with reduced GABA production.

Meanwhile Manto et al found that anti-GAD65 antibodies from another patient with SPS caused very different effects in rat brains compared to the antibodies derived from a patient with autoimmune cerebellar ataxia, but no SPS symptoms. They also found that two kinds of off-the-shelf anti-GAD65 antibodies commonly used in research had different effects as well.

Taken together this all suggests that SPS is caused by anti-GAD65 antibodies, but they have to be a particular type. Different antibodies cause different symptoms even though they all bind to GAD65.

Presumably this is because it's a big protein, and antibodies could bind to any part of it. Only ones that block the "business end" - the part which actually catalyzes the formation of GABA - will cause problems. A bit like how if you get shot in the heart, that's the end of you, but get shot in the foot and it probably won't be.

Geis, C., et al. (2011). Human Stiff-Person Syndrome IgG Induces Anxious Behavior in Rats PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0016775

Manto MU, Hampe CS, Rogemond V, & Honnorat J (2011). Respective implications of glutamate decarboxylase antibodies in stiff person syndrome and cerebellar ataxia. Orphanet journal of rare diseases, 6 (1) PMID: 21294897

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