Mosaic trisomy 9

Most cases of mosaic trisomy 9 occur due to a random event during the formation of the reproductive cells (egg and sperm) or after fertilization has taken place. An error in cell division (called nondisjunction) may cause some eggs or sperm to have an abnormal number of chromosomes. If an egg or sperm with an extra chromosome 9 contributes to the genetic makeup of an embryo, the embryo will have an extra copy of chromosome 9 in each cell. As the embryo grows and divides, an attempt may be made to correct the mistake by eliminating one extra chromosome 9. In people with mosaic trisomy 9, this attempt may be partly successful, leaving some cells with an extra chromosome 9 and some cells with the extra chromosome deleted (the usual chromosome number). This correction process is called trisomy rescue.

In other cases, the egg and sperm may have a normal number of chromosomes, but an error of cell division (nondisjunction) occurs when the fertilized egg is growing and dividing. If an error occurs during one of the divisions, it can cause some cells to have an abnormal number of chromosomes. In people affected by mosaic trisomy 9, some of the body's cells have the usual two copies of chromosome 9, and other cells have three copies of this chromosome (trisomy). The percentage of cells with trisomy 9 and which parts of the body are affected vary from person to person. This leads to variability in the range and severity of symptoms.

In rare cases, mosaic trisomy 9 is inherited from a parent with a chromosomal rearrangement called a "pericentric inversion." This occurs when a segment of chromosome 9 has broken off in two places, swiveled round 180 degrees and reinserted itself into the chromosome. If this rearrangement is considered "balanced," meaning the piece of chromosome is in a different order but no genetic material is gained or lost, it usually does not cause any symptoms or health problems. However, it can be associated with an increased risk of having children with an abnormal number or chromosomes.

In some cases, mosaic trisomy 9 is diagnosed before birth. A pregnancy ultrasound may reveal signs and symptoms that are suggestive of a chromosomal or developmental disorder. Additional tests, such as chorionic villus sampling (CVS) or an amniocentesis, may be offered to further investigate these features. During a CVS, a tissue sample from a portion of the placenta is removed and analyzed, while amniocentesis involves the removal of a sample of fluid that surrounds the developing baby. In both tests, the fluid or tissue sample is used to obtain a picture of the baby's chromosomes, which is called a karyotype. This may reveal mosaic trisomy 9.

In other cases, the child is not diagnosed until after birth. Mosaic trisomy 9 may be suspected after characteristic signs and symptoms are identified on physical exam. A diagnosis can be confirmed by examining the child's chromosomes from a sample of blood.

Mosaic trisomy 9 is usually not inherited. It often occurs sporadically as a random event during the formation of the reproductive cells (egg and sperm) or as the fertilized egg divides.

In rare cases, mosaic trisomy 9 may be inherited from a parent with a chromosomal rearrangement called a "pericentric inversion." This occurs when a segment of chromosome 9 has broken off in two places, swiveled round 180 degrees and reinserted itself into the chromosome. In these cases, the parent has a "balanced" rearrangement, meaning the piece of chromosome is in a different order but no genetic material is gained or lost. Carriers of a balanced rearrangement typically to not have any symptoms or health problems. However, they may be at an increased risk of having children with an abnormal number or chromosomes.

The long-term outlook (prognosis) for people with mosaic trisomy 9 largely depends on the degree to which the condition has affected any major organs, such as the heart and/or brain. In people affected by mosaic trisomy 9, some of the body's cells have the usual two copies of chromosome 9, and other cells have three copies of this chromosome (trisomy). The percentage of cells with trisomy 9 and which parts of the body are affected vary from person to person. This leads to variability in the range and severity of symptoms.

Because mosaic trisomy 9 affects many different systems of the body, medical management is often provided by a team of doctors and other healthcare professionals. Treatment for this condition varies based on the signs and symptoms present in each person. For example, children with bone or muscle abnormalities and/or delayed motor milestones (i.e. walking) may be referred for physical or occupational therapy. Depending on the degree of intellectual disability, a child may require special education classes. Heart defects and cleft lip and/or palate may need to be surgically repaired. Children with hydrocephalus may be treated with certain medications and/or shunting (placement of a specialized device that drains excess fluid away from the brain). Other surgeries may be recommended depending on the nature and severity of the other features (i.e. craniofacial, muscular, skeletal, kidney, and/or reproductive system problems) and their associated symptoms.

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