Mosaic trisomy 9

Alternate names

Mosaic trisomy chromosome 9; Trisomy 9 mosaicism

Definition

Mosaic trisomy 9 is a chromosomal abnormality that can affect may parts of the body. In people affected by this condition, some of the body's cells have three copies of chromosome 9 (trisomy), while other cells have the usual two copies of this chromosome.

Cause

• 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.

Inheritance

• 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.

Signs and symptoms

The signs and symptoms of mosaic trisomy 9 vary but may include: Different degrees of developmental delay and intellectual disability

• Abnormal growth including low birth weight, failure to thrive, hypotonia (low muscle tone), and short stature
• Characteristic craniofacial features such as microcephaly (unusually small head); a sloping forehead with narrow temples; a broad nose with a bulbous tip and "slitlike" nostrils; a small jaw; abnormally wide fontanelles at birth; cleft lip and/or palate; low-set, misshapen ears; microphthalmia (unusually small eyes) and/or short, upwardly slanting eyelid folds (palpebral fissures)
• Vision problems
• Congenital heart defects
• Abnormalities of the muscles and/or bones such as congenital dislocation of the hips; abnormal position and/or limited function of the joints; underdevelopment of certain bones; and/or abnormal curvature of the spine
• Unusually formed feet, such as club foot or "rocker bottom" feet
• Abnormalities of the male reproductive system, including undescended testes, a small penis, and/or abnormal placement of the urinary opening
• Kidney problems
• Brain malformations such as hydrocephalus and/or Dandy-Walker malformation

Diagnosis

• 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.

Treatment

• 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.

Prognosis

• 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.

NIH genetic and rare disease info

• NIH info on Mosaic trisomy 9

Mosaic trisomy 9 is a rare disease.

Mosaic trisomy 9 Resources
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