Chromosome 22, Trisomy Mosaic is a rare chromosomal disorder in which chromosome 22 appears three times (trisomy) rather than twice in some cells of the body. The term "mosaic" indicates that some cells contain the extra chromosome 22, whereas others have the normal chromosomal pair.
The range and severity of associated symptoms and findings may vary, depending upon the percentage of cells with the chromosomal abnormality. However, characteristic features typically include growth delays, mental retardation, unequal development of the two sides of the body (hemidystrophy), and webbing of the neck. Affected individuals may also have abnormal outward deviation of the elbows upon extension (cubitus valgus), multiple pigmented moles or birthmarks (nevi), distinctive malformations of the head and facial (craniofacial) area, and other physical abnormalities.
The symptoms and physical findings associated with Chromosome 22, Trisomy Mosaic may depend on the percentage of cells containing the extra 22nd chromosome. However, the disorder is often characterized by mental retardation, severe growth and developmental delays, and other physical abnormalities.
In many individuals with the disorder, there is unequal development of the two sides of the body (hemidystrophy), causing the body to appear dissimilar from one side to the other. For example, one leg may appear shorter than the other. In addition, in many individuals with hemidystrophy, there is associated hearing loss affecting one ear (unilateral hearing impairment).
According to reports in the medical literature, individuals with Chromosome 22, Trisomy Mosaic typically have additional symptoms and findings that are also often seen in Turner syndrome, another chromosomal disorder. (For further information on Turner syndrome, please see the “Related Disorders” section of this report below.) Such abnormalities include short stature; abnormal outward deviation of the elbows upon extension (cubitus valgus); multiple pigmented birthmarks (nevi); and malformations of the heart and its major blood vessels (cardiovascular defects), particularly defects affecting the major artery (aorta) that carries oxygen-rich blood from the lower left chamber (ventricle) of the heart throughout the body. Chromosome 22, Trisomy Mosaic is also associated with additional malformations seen in Turner syndrome, such as drooping of the upper eyelids (ptosis); webbing of the neck; a low hairline at the back of the neck; malformed (dysplastic) nails; and defective development of the ovaries (ovarian dysgenesis) in affected females. The ovaries are the paired glands that produce female reproductive cells (eggs) and certain female hormones. Therefore, ovarian dysgenesis may be associated with delayed or failed development of secondary sexual characteristics during puberty (e.g., breast development, the appearance of pubic hair, menstruation) and infertility.
Some individuals with Chromosome 22, Trisomy Mosaic also have additional malformations of the head and facial (craniofacial) area. In addition to drooping of the upper eyelids (ptosis), such malformations may include widely spaced eyes (ocular hypertelorism); vertical skin folds covering the eyes’ inner corners (epicanthal folds); and abnormal pits or indentations in front of the ears (preauricular pits). Some affected individuals have additional physical abnormalities, such as underdeveloped fingers and toes (digits); abnormal skin creases on the palms of the hands; and kidney (renal) malformations.
In individuals with Chromosome 22, Trisomy Mosaic, chromosome 22 is present three times (trisomy) rather than twice in some cells of the body. Chromosomes are found in the nucleus of all body cells. They carry the genetic characteristics of each individual. Pairs of human chromosomes are numbered from 1 through 22, with an unequal 23rd pair of X and Y chromosomes for males and two X chromosomes for females. Each chromosome has a short arm designated as “p” and a long arm identified by the letter “q”.
The same chromosomal (karyotypical) makeup is usually present in all body cells (i.e., one cell line). However, those with a chromosomal mosaicism have two or more cell lines. Chromosome 22, Trisomy Mosaic is characterized by an extra chromosome 22 in one of an individual’s cell lines, with at least one unaffected cell line also present. The presence of the additional chromosome is responsible for the symptoms and physical findings that characterize the disorder. Individuals with a low percentage of affected cells may have fewer, less severe symptoms than those with a high percentage of affected cells.
In most individuals with Chromosome 22, Trisomy Mosaic, the disorder appears to result from errors (e.g., nondisjunction) during the division of reproductive cells in one of the parents (parental meiosis) or during cellular division after fertilization (fetal mitosis). There have also been reports in which the disorder has occurred in association with uniparental disomy, an abnormality in which affected individuals have inherited both copies of a chromosomal pair from one parent, rather than one copy from each parent.
Chromosome 22, Trisomy Mosaic appears to affect females more frequently than males. Approximately 15 cases have been reported in the medical literature.
A diagnosis of Chromosome 22, Trisomy Mosaic may be suggested before birth (prenatally) by specialized tests, such as ultrasound, amniocentesis, or chorionic villus sampling (CVS). Fetal ultrasonography is a noninvasive diagnostic procedure during which reflected sound waves are used to create an image of the developing fetus. During amniocentesis, a sample of fluid that surrounds the fetus is removed and analyzed, whereas CVS involves the removal of tissue samples from a portion of the placenta. Chromosomal analysis performed on the fluid or tissue samples may reveal the presence of Chromosome 22, Trisomy Mosaic.
Other testing may also sometimes be conducted to help confirm the diagnosis. For example, according to reports in the medical literature, fetal skin biopsy may be recommended in some cases in which mosaicism has been detected during amniocentesis. Fetal skin biopsy involves the removal and microscopic examination of certain cells (e.g., fibroblasts) from the developing fetus.
A diagnosis of Chromosome 22, Trisomy Mosaic may also be made or confirmed after birth (postnatally) based upon a thorough clinical evaluation, detection of characteristic physical findings, and chromosomal analysis. In addition, specialized testing may be conducted to detect or characterize certain abnormalities that may be associated with the disorder (e.g., hearing impairment, cardiovascular defects, renal abnormalities, ovarian dysgenesis, etc.).
The treatment of Chromosome 22, Trisomy Mosaic is directed toward the specific symptoms that are apparent in each individual. Such treatment may require the coordinated efforts of a team of medical professionals, such as pediatricians; surgeons; physicians who diagnose and treat heart abnormalities (cardiologists); hearing specialists; and/or other health care professionals.
In some infants and children with Chromosome 22, Trisomy Mosaic, treatment may include the surgical repair of certain craniofacial, cardiovascular, or other abnormalities potentially associated with the disorder. The surgical procedures performed will depend upon the severity of the anatomical abnormalities, their associated symptoms, and other factors. In addition, for some individuals with unilateral hearing loss, the use of artificial devices (prostheses) may be recommended, such as specialized hearing aids. Other treatment for the disorder is symptomatic and supportive.
Early intervention may be important in ensuring that children with Chromosome 22, Trisomy Mosaic reach their potential. Special services that may be beneficial include special education, speech therapy, physical therapy, and/or other medical, social, and/or vocational services. Genetic counseling will also be of benefit for affected individuals and their families.
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