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Basal cell nevus syndrome is caused by a tumor suppressor gene, called PTCH, located on chromosome 9. Mutations in this gene may increase the risk of ovarian cancer.
Tumor suppressor genes usually control cell growth and cell death. Both copies of a tumor suppressor gene must be altered, or mutated, before a person will develop cancer. With basal cell nevus syndrome, the first mutation is inherited from either the mother or the father in 60-80% of cases. In 20-40% of cases, the first mutation is not inherited and arises de novo (for the first time) in the fertilized egg from which the person with symptoms was conceived. Whether de novo or inherited, this first mutation is present in all of the cells of the body and, as such, is called a germline mutation.
Whether a person who has a germline mutation will develop cancer and where the cancer(s) will develop depends upon where (which cell type) the second mutation occurs. For example, if the second mutation is in the skin, then skin cancer may develop. If it is in the ovary, then ovarian cancer may develop. The process of tumor development actually requires mutations in multiple growth control genes. Loss of both copies of PTCH is just the first step in the process. What causes of these additional mutations to be acquired is unknown. Possible causes include chemical, physical, or biological environmental exposures (such as sunlight) or chance errors in cell replication.
Some individuals who have inherited a germline tumor suppressor gene mutation may never develop cancer because they never get the second mutation necessary to knock out the function of the gene and start the process of tumor formation. This can make the cancer appear to skip generations in a family, when, in reality the mutation is present. Persons with a mutation, regardless of whether they develop cancer, however, have a 50/50 chance to pass the mutation on to the next generation.
It is also important to remember that the gene responsible for basal cell nevus syndrome is not located on the sex chromosomes. Therefore, mutations can be inherited from the mother or the father's side of the family.
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