JAG1

At least 226 mutations in the JAG1 gene have been identified in people with Alagille syndrome. Most of these mutations result in an abnormally short Jagged-1 protein that is missing the segment that normally spans the cell membrane (the transmembrane domain). Other mutations interfere with proper transport (trafficking) of the protein within the cell, preventing it from reaching the cell membrane. 

The loss of Jagged-1 protein at the cell membrane precludes its interaction with Notch proteins and prevents cell signaling. The lack of Notch signaling causes errors in development that result in missing or narrowed bile ducts in the liver, heart defects, distinctive facial features, and changes in other parts of the body. People with JAG1 gene mutations may have one or more of these problems. In particular, some affected individuals have a particular combination of heart defects known as tetralogy of Fallot without other signs or symptoms of Alagille syndrome.  The type and severity of problems associated with Alagille syndrome may differ even within the same family.

MedlinePlus Genetics provides information about Critical congenital heart disease

Increased activity (expression) of the JAG1 gene has been linked to certain cancers, including breast cancer and head and neck tumors. The increased expression of the JAG1 gene may promote the development of new blood vessels that nourish a growing tumor. The altered gene expression may also enhance other cancer-related events such as cell division (proliferation) and the inflammatory response.