Going all over the internet is this bitter-sweet video:
Below is some of what a quick sweep of reading teaches me about myotonia congenita. (This is hardly a definitive survey of this disorder! Share what you know, I’m learning too.)
Myotonia congenita, or congenital myotonia is a skeletal muscle-locking disorder. In affected individuals, when the muscles contract, they do not immediately relax again: for a short period the affected muscles stiffen. If the individuals are standing up, they’ll fall over, as the kittens in the video do.
Startle reflexes can cause sudden muscle contraction, hence startles can induce falling in these individuals.
Congenital myotonia, is a genetic condition most commonly found in goats, where it is informally called ‘fainting goat syndrome’, but is also known to occur in dogs and humans.
(Some reports suggest is is most common in northern Finnish people. Many recessive genetic conditions are found to be particularly common in an area where isolation or a population bottleneck has caused the traits of the founder population to be more common in that group of people than elsewhere.)
The literature refers to both recessive (Becker-type) and dominant (Thomsen-type) autosomal congenital myotonia in humans. As you’ll know congenital forms are those present from before birth. Autosomal means the gene that when altered causes the disease is not on the sex chromosomes, but on the autosomes, which are the same in both sexes (which, in turn, affects how they are inherited).
Our autosomes have two copies of each gene, one from each parent. Recessive genetic disorders are those where both parents need to carry a defective copy of the relevant gene and both copies of the gene in the offspring are defective before the trait (in this case myotonia) is seen. In dominant disorders, the defective copy of the gene dominates over the normal copy so that only one defective gene is needed in offspring before myotonia is seen.
The cause of congenital myotonia are mutations in the CLCN1 gene that codes for a chloride voltage-gated channel that is active in skeletal muscle.*
How do these defective channel proteins cause congenital myotonia?
Cells are enclosed in membranes, so are organelles within the cells.
Membranes, including those in muscle cells, traffic ions between the outside of the cell or organelle and the inside, through the membrane.
Ions carry a small charge. Moving ions across a membrane can alter the balance of charge across the membrane. The balance of charges across membranes are used in muscle contraction.
Within these membranes are ion channel proteins. These large proteins form cylindrical tubes or pores through the membrane (see to left).
Some types of ion channels are gated, latching the ions though the channel.
Voltage-gated channels, as they are known, are controlled by the charge balance across the membrane.**
So, for example, in response to high positive charge inside the cell, they may allow negatively-charged chloride ions to enter the cell to restore the balance of charge across the membrane.
Muscles with defective CLCN1 genes are slow to import negatively-charged chloride ions, so the muscles contract for longer than in normal individuals.
Sadly, both of the kittens have now died. A memorial video has been posted – to save you the trip, I’ve hosted it below:
In one scene, just over a minute into the video, the tabby (Charlie) seems to have ‘fallen’ over whilst eating. Having recovered enough he has starting eating while still flat on his side.
* There are other variants of (non-congenital) myotonia such as potassium-aggravated myotonia, which involves a defective potassium channel, or hyperkalemic periodic paralysis.
** Other types of gating exist, such as ligand-induced gating.
I have listed below a selection of open-access references for those interested in further reading.
(Rats, I posted this before getting these in: I’ll update this later… promise. In the meantime some DOIs for those that know what to do!)
Other articles on Code for life: