Yes, Addison's disease is considered to be a immune mediated hereditary condition. Some breeds are more predisposed to the condition then the others. The bull mastiff is not a breed considered prone to the condition but it has occurred in all breeds.
Here is a link you may find interesting. It doesn't discuss your breed of dog in particular but it does contain a lot of information about genetics and Addison's:
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1481556
Thank you
Theresa
A Voice for Your Pet
15 years experience in small animal medicine