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Peutz-Jeghers syndrome is caused by mutations in a gene on chromosome 19 known as STK11. Mutations in STK11 are identified in about 70% of familial cases. Peutz-Jeghers syndrome is associated with up to a 93% chance of developing one of the associated cancers by age 64. The risks of breast and ovarian cancer fall within the ranges associated with mutations in the BRCA1 and BRCA2 genes.
The STK11 gene is a tumor suppressor gene, which usually controls cell growth and cell death. Both copies of a tumor suppressor gene must be altered, or mutated, before a person will develop cancer. With Peutz-Jeghers syndrome, the first mutation is inherited from either the mother or the father and is therefore present in all cells of the body. This 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 breast, then breast cancer may develop. If it is in the colon, then colon cancer may develop. The process of tumor development actually requires mutations in multiple growth control genes. Loss of both copies of STK11 is just the first step in the process. What causes these additional mutations to be acquired is unknown. Possible causes include chemical, physical, or biological environmental exposures or chance errors in cell replication.
Some individuals who have inherited a germline STK11 mutation never develop cancer because they never get the second mutation necessary to knock out the function of the gene and to 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 STK11 gene is not located on the sex chromosomes. Therefore, mutations can be inherited from the mother or the father's side of the family.