A faulty gene (the TNSALP or ALPL gene) is the cause of HPP. This defective gene limits the body’s production of an enzyme called alkaline phosphatase (ALP). Healthy bones are made strong and rigid when minerals, like calcium and phosphorus, are added to the bones through a process called mineralization. Without enough ALP, certain chemicals build up and prevent calcium and phosphorus from binding together and depositing in the bones, leading the bones to be soft, curved and fragile and teeth to become loose or fall out prematurely.
The affected gene can be passed from generation to generation, but it is possible for people to carry the gene and not exhibit any symptoms. These people are referred to as “healthy carriers”. In the severe perinatal and infantile forms of HPP, one defective gene is inherited from each parent, and some milder childhood or adult HPP patients may develop it this way as well. Other mild adult and odontohypophosphatasia cases seem to be inherited when there is only one defective gene from one parent. Individuals with HPP and parents of children with HPP are encouraged to seek genetic counseling to explain the likelihood and severity of HPP recurring in their families.