What is wilson disease? What is the problem in the body?

Mutated ATP7B. The cells can't transport copper out of their cytoplasm as they should. This cases the complications of liver cell death and cirrhosis, brain damage, and so forth.
Genetics, copper. It is a genetic defect causing an over accumulation of copper in the body, especially the liver and the brain. Liver problems are often the first sign but neurologic symptoms are also common, especially later in the disease.