Chromosome 18p tetrasomy

Alternate names

Tetrasomy 18p; Tetrasomy chromosome 18p; Isochromosome 18p

Definition

Chromosome 18p tetrasomy is a chromosomal disorder that affects many parts of the body. It occurs when the short arm of chromosome 18 (18p) appears four times (tetrasomy) rather than the normal two times in cells of the body.

Epidemiology

Tetrasomy 18p is a rare disorder. Only 250 families were affected worldwide.

Cause

• Tetrasomy 18p results from the presence of an abnormal extra chromosome, called an isochromosome 18p, in each cell.
• Cells normally have two copies of each chromosome, one inherited from each parent.
• In people with tetrasomy 18p, cells have the usual two copies of chromosome 18 plus an isochromosome 18p.
• As a result, each cell has four copies of the short arm of chromosome 18. (The word "tetrasomy" is derived from "tetra," the Greek word for "four.")
• The extra genetic material from the isochromosome disrupts the normal course of development, causing the characteristic features of this disorder.

Inheritance

In most cases, chromosome 18p tetrasomy is the result of a spontaneous (de novo) genetic change (mutation) early in fetal development during pregnancy.

Signs and symptoms

The symptoms of chromosomy 18p tetrasomy vary from case to case but may include abnormalities of the head and face (craniofacial) area, malformations of the spine, hands, and/or feet, neuromuscular abnormalities, kidney malformations, intellectual disability, speech delays, and behavioral abnormalities.

Clinical presentation

For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed.

80%-99% of people have these symptoms

• Abnormality of neuronal migration
• Long philtrum
• Microcephaly(Abnormally small skull)

30%-79% of people have these symptoms

• Epicanthus(Eye folds)
• Low-set, posteriorly rotated ears

5%-29% of people have these symptoms

• Achalasia
• Downslanted palpebral fissures(Downward slanting of the opening between the eyelids)
• Facial asymmetry(Asymmetry of face)
• Gait disturbance(Abnormal gait)
• Intellectual disability(Mental deficiency)
• Large hands(large hand)
• Narrow mouth(Small mouth)
• Scoliosis
• Seizure
• Short nose(Decreased length of nose)
• Syncope(Fainting spell)
• Thin vermilion border(Decreased volume of lip)

Diagnosis

Chromosome disorders may be suspected in people who have developmental delays, intellectual disabilities and/or physical abnormalities.

Several types of genetic tests can identify chromosome disorders:

• Karyotype - a karyotype is a laboratory test that produces an image of a person's chromosomes. This test can be used to diagnose large deletions.
• FISH - a laboratory technique that is used to detect and locate a specific DNA sequence on a chromosome. During FISH, a chromosome is exposed to a small DNA sequence called a probe that has a fluorescent molecule attached to it. The probe sequence binds to its corresponding sequence on the chromosome. This test can be used in combination with karyotyping for deletions that are too small to be seen on karyotype, alone. However, FISH is only useful if the person ordering the test suspects there is a deletion of a specific region of 4q.
• Array CGH - a technology that detects deletions that are too small to be seen on karyotype.

Treatment

• Although there is no specific treatment or cure for chromosome 18p tetrasomy, there are ways to manage the symptoms.
• A team of doctors or specialists is often needed to figure out the treatment options for each person.

NIH genetic and rare disease info

• NIH info on Chromosome 18p tetrasomy

Chromosome 18p tetrasomy is a rare disease.

Chromosome 18p tetrasomy Resources
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