Date: 25th June 2021
Congenital myasthenia (CM) is a group of devastating inherited neuromuscular diseases, caused by genetic changes in at least 32 genes which are important for the formation, function, and maintenance of neuromuscular synapses. They all include muscle weakness that gets worse with physical activity, and in the main manifest in infancy or early childhood. Mutations in Dok7, an adaptor protein, are a major cause of CM and are responsible for 10–20% of all cases. Patients with the disease face lifelong muscle weakness and the few treatments that are available only partially ablate the clinical symptoms. Now, researchers have described a mouse model for this common form of CM, unearthing a new therapeutic target to which they designed synthetic antibodies. Targeted antibody therapy rescued mice with the Dok7 mutation, restoring neuromuscular synapse formation, reversing movement deficits and early lethality.
The formation and maintenance of neuromuscular synapses requires the assembly of highly specialised presynaptic and postsynaptic membranes, which involves the coordinated action of several key molecules. One such molecule is muscle-specific kinase (MUSK), a receptor tyrosine kinase that acts as a master regulator of synaptic differentiation, activation of MUSK is dependent on the protein DOK7.
The most common disease-causing mutation (DOK71124_1127 dup) produces a truncated form of the protein, leading to the loss of two tyrosine residues that are phosphorylated and recruit other proteins. Previous theories have postulated these are important for anchoring acetylcholine and have focused around this loss of attachment sites in preventing assembly of the cellular machinery for building the synapses.
Now, researchers at NYU Grossman School of Medicine and NYU Langone Health, US, led by Steven Burden and Shohei Koide, have unexpectedly discovered that CM deficits were due to a severe deficiency in phosphorylation and activation of muscle-specific kinase (MUSK) rather than a deficiency in DOK7 tyrosine phosphorylation in a mouse model for CM. Furthermore, early neonatal lethality and neuromuscular synapse formation could be rescued using a synthetic agonist antibody against MUSK in the Dok7CM mice.
To start the team generated a mouse model of the most common form of Dok7 CM (Dok71124_1127 dup; referred to as Dok7CM mice) and found that the mutations caused very early lethality, shortly after birth. This was likely due to a five-fold reduction in the number of synapses found in these mice furthermore, the remaining synapses were immature. Additional investigations showed, the truncated DOK7 protein was expressed at three-fold lower levels than the wild-type DOK7 protein which was still present in these mice. However, unexpectedly they found that a control cohort, where just the two tyrosine residues in the C-terminal were mutated did not cause early lethality and severe synapse abnormalities, going against the dogma that the loss of the two DOK7 tyrosine residues are the cause of CM.
As DOK7 functions as a dimer to dimerise MUSK, thereby stabilising MUSK tyrosine phosphorylation the team then asked whether MUSK tyrosine phosphorylation was altered in the Dok7CM mice. Indeed, this was the case and they found MUSK phosphorylation was reduced seven-fold.
But was this the cause of CM synapse defects? To address this, they reasoned that re-activation of MUSK may reverse these deficiencies and lethality. They therefore generated agonist antibodies targeting MUSK and treated the mice with them to stimulate MUSK phosphorylation.
Dok7CM mice injected with a control antibody lost weight and died rapidly within a week. However, those treated with the agonist antibody, called X17, saw a reversal in weight loss and early lethality. In fact, chronic dosing allowed most of the mice to grow to fertile adults and rescued synapse formation and maturation. Importantly, this was also accompanied by improvements in motor function.
Early lethality in human CM patients does not usually occur, so turning to a more clinically relevant therapy, the team determined whether the antibody treatment could reverse neuromuscular defects that develop in adulthood. Dok7CM mice were treated with X17 to overcome early lethality at either at P4, P24 and P44 or at P4 and P18 but then discontinued antibody treatment. Weight and mobility was maintained for 2-3 months after cessation of the therapy, but then started to decline. Upon re-treatment with X17, weight gain occurred in as little as two days, whilst motor performance was restored within a week.
The team here have discovered a new target for the treatment of CM, generating an agonist antibody to stimulate MUSK which rescued synapse formation and motor function, preventing early lethality in CM-model mice.
The study indicated that the therapeutic approach rescued synapse defects not only when they first form during development but also reversed synaptic dysfunction in adult animals. This is a crucial property for such a therapy to translate into the clinic as CM patients could then be treated as soon as they are diagnosed.
From a wider perspective this type of epistatic rescue could also provide a therapy for CM caused by mutations in other genes, as well as potentially for other neuromuscular disease or in fact other genetic disorders where the disease mechanism is understood and suitable targets have been identified.
Whilst, gene therapy is becoming a popular method for treating inherited diseases such as Charcot-Marie-Tooth disease, Huntington’s disease and Leber hereditary optic neuropathy to name just a few, this antibody therapeutic strategy avoids the complex requirements for gene therapy. It is able to circumvent treating the root cause of the disease, such a repairing mutated genes, which can be caused by more than one mutation in a single gene, or mutations in more than one gene. Furthermore such treatments often have to be personalised as individuals exhibit different combinations of defects. The beauty here, is that the antibody treatment could be used for a variety of patients, who exhibit mutations in any of the genes upstream of MUSK, offering a far broader reaching therapeutic.
For more information please see the press release at NYU Langone
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