Focal caspase activation underlies the endplate myopathy in slow-channel syndrome.

Slow-channel syndrome (SCS) is a progressive neuromuscular disorder caused by abnormal gating of mutant acetylcholine receptors (AChRs) in the neuromuscular junction (NMJ). The pathological hallmark is selective degeneration of the NMJ termed endplate myopathy. Endplate myopathy consists of a combination of ultrastructural abnormalities, including degenerating subsynaptic nuclei, mitochondria, and postsynaptic folds, caused by localized cation overload through mutant AChRs. Because some of these changes resemble those seen in programmed cell death, we evaluated SCS muscle for evidence of focal activation of apoptotic pathways. Using antisera specific for the activated forms of caspases, the family of cysteine proteases that underlies apoptosis, we demonstrated that active forms of initiator and effector caspases are selectively localized at the NMJ in SCS. In comparison with an electron microscopic assessment of the abnormalities seen in endplate myopathy, we found that activated caspases were present at between 15 and 57% of endplates, similar to the proportion of endplates with degenerating mitochondria or vacuoles. This greatly exceeds the number of NMJs exhibiting nuclear degeneration. These findings provide the first evidence supporting the view that caspase activation in human disease can play a prominent role in localized cellular degenerative processes without causing nuclear or cell death.

Effect of inherited abnormalities of calcium regulation on human neuromuscular transmission.

Synaptotagmins are abundant synaptic proteins that represent the best candidate for the calcium sensor at the nerve terminal. The pore-forming, voltage-sensing transmembrane alpha-1 subunit of the P/Q voltage-gated calcium channel (or Ca(v)2.1) encoded by the CACNA1A gene is another major component of the process of action potential-evoked exocytosis at the adult mammalian neuromuscular junction. Defects of these proteins, in nonhuman species, result in severe disruption of rapid synaptic transmission. This paper investigates the molecular bases of inherited presynaptic deficits of neuromuscular transmission in humans. Patients with congenital presynaptic failure, including two patients with episodic ataxia type 2 (EA-2) due to CACNA1A mutations, were studied with muscle biopsy, microelectrode studies, electron microscopy, DNA amplification, and sequencing. All patients, including EA-2 patients, showed selective failure of the action potential-dependent release without reduction of the spontaneous release of neurotransmitter. In addition, patients with EA-2 showed partial blockade of neuromuscular transmission with the N-type blocker omega-conotoxin not seen in controls. The EM showed a varied degree of increased complexity of postsynaptic folds. Mutational analysis in candidate genes, including human synaptotagmin II, syntaxin 1A, synaptobrevin I, SNAP 25, CACNA1A, CACNB2, and Rab3A, was unrevealing. Although no mutations in candidate genes were found in patients with inborn presynaptic failure, functional and structural similarities between this group and patients with EA-2 due to CACNA1A mutations suggest a common pathogenic mechanism.

Two novel mutations in the COLQ gene cause endplate acetylcholinesterase deficiency.

Congenital myasthenic syndromes with endplate acetylcholinesterase deficiency are very rare autosomal recessive diseases, characterized by onset of the disease in childhood, general weakness increased by exertion, ophthalmoplegia and refractoriness to anticholinesterase drugs. To date, all reported cases are due to mutations within the gene encoding ColQ, a specific collagen that anchors acetylcholinesterase in the basal lamina at the neuromuscular junction. We identified two new cases of congenital myasthenic syndromes with endplate acetylcholinesterase deficiency. The two patients showed different phenotypes. The first patient had mild symptoms in childhood, which worsened at 46 years with severe respiratory insufficiency. The second patient had severe symptoms from birth but improved during adolescence. In both cases, the absence of acetylcholinesterase was demonstrated by morphological and biochemical analyses, and heteroallelic mutations in the COLQ gene were found. Both patients presented a novel splicing mutation (IVS1-1G-->A) affecting the exon encoding the proline-rich attachment domain (PRAD), which interacts with acetylcholinesterase. This splicing mutation was associated with two different mutations, both of which cause truncation of the collagen domain (a known 788insC mutation belonging to one patient and a novel R236X to the other) and may impair its trimeric organization. The close similarity of the mutations of these two patients with different phenotypes suggests that other factors may modify the severity of this disease.