Pentasomy X
Pentasomy X, also known as 49,XXXXX, is a chromosomal abnormality in which a female has five X chromosomes instead of the normal two. Signs may include intellectual disability, short height, low-set ears, decreased muscle tone, and developmental delay. Complications may include congenital heart disease.
The condition is due to problems during the formation of the reproductive cells in a person's parents. Risk factors include older parents at the time of conception. Diagnosis is suspected based on symptoms and confirmed by chromosomal analysis.
Treatment is based on symptoms. The condition is extremely rare, with fewer than forty reported cases as of 2011. The condition was first described in 1963.
Signs and symptoms
The main characteristics of pentasomy X are intellectual disability, short stature and craniofacial abnormalities. Other physical traits include the following:
- Small head
- Ear abnormalities
- Widely spaced eyes with upward slanting palpebral fissures and epicanthal folds
- Short neck
- Broad nose with a depressed nasal bridge
- Hyperextension of the elbows
- Dental abnormalities and cleft palate
- Clinodactyly of the fifth finger
- Deformities of the feet
- Heart defects
Causes
The aneuploidy is thought to be caused by problems occurring during meiosis, either in the mother or in both the mother and father. Successive nondisjunctions have been observed in the mother of at least one patient.
The features of the syndrome likely arise due to failure of X-inactivation and the presence of multiple X chromosomes from the same parent causing problems with parental imprinting. In theory, X-inactivation should occur and leave only one X chromosome active in each cell. However, failure of this process has been observed in one individual studied. The reason for this is thought to be the presence of an unusually large, and imbalanced, number of X chromosomes interfering with the process.
See also
- Trisomy X
- Tetrasomy X