What is hereditary mixed polyposis syndrome?
Hereditary mixed polyposis syndrome (HMPS) is a hereditary condition that is associated with an increased risk of developing polyps in the digestive tract, most commonly in the colon and/or rectum. A polyp is a growth of normal tissue that forms a lump. As the name suggests, a variety of polyps may occur. People with HMPS are thought to have an increased risk of developing colorectal cancer, since some of these polyps may turn into cancer over time, unless they are removed.
The same types of polyps that develop in HMPS can also develop in individuals without HMPS. Compared to the general population, people with HMPS typically have higher numbers of polyps, develop them at younger ages, and often develop them more quickly. Examples of the types of polyps that can be seen in HMPS include:
Adenomas, or adenomatous polyps – these have a potential to turn into cancer, unless they are removed
Hyperplastic polyps – these typically do not have a risk of turning into cancer
Hamartomatous or Juvenile polyps – these may have a risk of turning into cancer; the term “juvenile” does not refer to the age of the individual
Serrated polyps – these may have a risk of turning into cancer
What causes HMPS?
HMPS is a genetic condition that predisposes a person to an increased risk of developing cancer and polyps. This means that the condition can be passed from generation to generation in a family. For most families with HMPS, a specific gene mutation causing the syndrome cannot be identified, although some families will have an inherited mutation in the GREM1 gene, but this is rare.
How is HMPS inherited?
Normally, every cell has 2 copies of each gene: 1 inherited from the mother and 1 inherited from the father. Rarely, a family with HMPS will be found to carry a mutation in the GREM1 gene. In these families, genetic testing can help identify which individuals are at risk for developing polyps and cancer, and which individuals are not. For HMPS families without an identifiable genetic mutation, all individuals should undergo screening to test for possible polyps and/or cancer.
Although most families with HMPS do not have a specific gene mutation that can be identified, HMPS is believed to follow an autosomal dominant inheritance pattern, in which a mutation (alteration) needs to happen in only 1 copy of the gene for the person to have an increased risk of getting that disease. This means that a parent with a gene mutation may pass along a copy of their normal gene or a copy of the gene with the mutation. Therefore, a child who has a parent with a mutation has a 50% chance of inheriting that mutation. A brother, sister, or parent of a person who has a mutation also has a 50% chance of having the same mutation.
Options exist for people interested in having a child when a prospective parent carries a GREM1 gene mutation that increases the risk for this hereditary cancer syndrome. Preimplantation genetic diagnosis (PGD) is a medical procedure done in conjunction with in-vitro fertilization (IVF). It allows people who carry a specific known genetic mutation to have children who do not carry the mutation. A woman’s eggs are removed and fertilized in a laboratory. When the embryos reach a certain size, 1 cell is removed and is tested for the hereditary condition in question. The parents can then choose to transfer embryos which do not have the mutation. PGD has been in use for over 2 decades, and has been used for several hereditary cancer predisposition syndromes. However, this is a complex procedure with financial, physical, and emotional factors to consider before starting. For more information, talk with an assisted reproduction specialist at a fertility clinic.
How common is HMPS?
HMPS is considered to be rare.
How is HMPS diagnosed?
There are no specific diagnostic criteria for HMPS. HMPS is suspected in individuals and families with a history of multiple colon polyps of different types. It is also important to consider the possibility of 1 of the other hereditary polyposis syndromes, such as familial adenomatous polyposis (FAP), Peutz-Jeghers syndrome, and juvenile polyposis syndrome. There is currently no blood test that can help diagnose HMPS.
What are the estimated cancer risks associated with HMPS?
People with HMPS are considered to be at increased risk for developing colorectal cancer, but the amount of risk is not currently known.
What are the screening options for HMPS?
A colonoscopy is a test that allows a doctor to view the entire large intestine. Individuals with GREM1 mutations should begin having screening colonoscopies at age 25 to 30 or earlier if someone in the family had a diagnosis of polyps and/or colorectal cancer before the age of 30. Individuals with GREM1 mutations who are found to have polyps at their initial colonoscopy should continue having colonoscopies every 1 to2 years. Individuals with GREM1 mutations who do not have polyps at their initial colonoscopy should continue having colonoscopies every 2 to 3 years.
There are no specific screening guidelines developed for HMPS individuals who lack a GREM1 mutation, although regular screening colonoscopies are appropriate. Many doctors would suggest beginning screening 5 to 10 years earlier than the youngest age at which polyps or colorectal cancer were identified in a family member. If polyps are found, screening may need to be done every 1 to 2 years. Learn more about what to expect during a colonoscopy.
Screening options may change over time as new technologies are developed and more is learned about HMPS. It is important to talk with your doctor about appropriate screening tests.
Learn more about what to expect when having common tests, procedures, and scans.
Questions to ask the doctor
If you are concerned about your risk of cancer, talk with your doctor. Consider asking the following questions of your doctor:
What is my risk of developing colorectal cancer?
How many colon polyps have I had in total?
What types of colon polyps have I had? The most common types are adenomatous, hyperplastic, juvenile, hamartomatous, and serrated polyps.
What is my risk of other types of cancer?
What can I do to reduce my risk of cancer?
What are my options for cancer screening?
If you are concerned about your family history and think your family may have HMPS, consider asking the following questions:
Does my family history increase my risk of colorectal cancer or other types of cancer?
Will you refer me to a genetic counselor or other genetics specialist?
Should I consider genetic testing?
To find a genetic counselor in your area, ask your doctor or visit these websites:
National Society of Genetic Counselors
National Cancer Institute: Cancer Genetics Services Directory