What it is
The affected gene carries the instructions for a protein that works like a pump in the walls of the blood vessels of the brain and other organs: it pushes certain substances back out of the brain and helps the body get rid of them. When the gene is changed, the pump does not work, and some drugs can build up in the brain to levels a dog without the change would not reach. That is when signs appear, such as drooling, unsteadiness or trembling.
The change is inherited. A dog that receives it from both parents is more sensitive, but a dog with one changed copy of the gene can also be more sensitive than usual.
In daily life the mutation makes no difference: the dog runs, eats and plays like any other. It matters only when your dog needs certain drugs, so tell every vet who sees your dog about it beforehand, and keep the genetic test result with the rest of its papers.
Signs you may notice
The mutation itself causes no signs, so you cannot recognise it from the dog's looks or behaviour. The only way to find out is a genetic test, and its result is worth sharing with every vet who treats the dog.
When to see a vet
If your dog becomes unsteady, drools, trembles or seems confused after being given any drug, it needs a vet immediately. Tell your vet about the mutation, or the chance of it, before any drug is given.
This is general information, not veterinary advice. For your own dog's health, tests and treatment, see a vet.