Doing a little detective work, I think I was able to find some more information about the genetic test for JLPP in BRT dogs. First, there is the description of the disease on the University of Missouri website:
JLPP Juvenile Laryngeal Paralysis & Polyneuropathy (JLPP) in Black Russian Terriers
Recently, Dr. Nicolas Granger reported a syndrome in Black Russian Terrier pups in Europe characterized by difficulties breathing and weakness shortly after weaning age. We have now seen affected dogs in the USA. The purpose of this article is to provide information about the condition so that breeders and veterinarians can be alert to any future cases and help us find the gene responsible.
What is Juvenile Laryngeal Paralysis & Polyneuropathy? The brain controls muscles via signals that travel through nerves. A disease that affects the nerves is called a polyneuropathy: poly- (many), neuro- (nerves), -pathy (a disease). Due to a quirk in the way an embryo develops, one of the longest nerves in the body supplies the muscles of the voice box (larynx). The vocal folds vibrate as air moves over them allowing a dog to bark. When the dog breathes in, muscles in the larynx pull the vocal folds aside so that air can move easily into their lungs. These nerves also help to close the larynx when the dog swallows so they do not choke on their food.
If nerves are unable to convey messages properly, the muscles become weak or paralyzed. The longest nerves are often affected first; hence laryngeal paralysis is the first symptom. The vocal folds cannot be pulled out of the way as the dog breaths in. They vibrate noisily and can obstruct the flow of air into the lungs particularly when the dog is exercised or hot. The dog may also choke on their food or water or regurgitate, which can result in pneumonia.
The next longest nerves in the body go to the back legs, thus they are affected next. The dogs have difficulty getting up and wobble as they walk. Eventually the front legs will also be affected. The symptoms do not occur until after weaning age, and thus the disease is called juvenile laryngeal paralysis/polyneuropathy or JLPP for short.
I was able to find a paper that came out just a month ago that describes these very symptoms in Leonbergers and Saint Bernards, and this paper also cites the Nicolas Granger paper:
PLOS Genetics: An ARHGEF10 Deletion Is Highly Associated with a Juvenile-Onset Inherited Polyneuropathy in Leonberger and Saint Bernard Dogs
I'm not sure whether this article is behind a paywall, but if it is, let me know, and I will quote parts of it. The paper describes JLPP in Leonbergers in a very similar way as that described in BRT's by the University of Missouri people. They describe the mutation and how they test for it in Leonbergers and Saint Bernards. A good question for Dr. O'Brien would be, is the BRT mutation the same mutation as that described in Leonbergers in this paper?