|Classification and external resources|
XXXXX syndrome (also called pentasomy X or 49, XXXXX) is a type of aneuploidy (an abnormal number of chromosomes) which results in the presence of three additional X chromosomes. The condition was first described in 1963. It is characterized by severe intellectual disability, short stature and abnormalities to the head and face. As these features can be seen in other conditions, karyotyping must be carried out to confirm diagnosis. There have been cases of XXXXX syndrome being misdiagnosed as Down syndrome.
It is an extremely rare condition with no more than 30 patients reported in medical literature. The exact incidence is not known but it may be similar to the rate of 1 in 85,000 seen in males with 49, XXXXY syndrome.
Signs and symptoms
- 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 5th finger
- Deformities of the feet
- Heart defects
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.
- Cho YG, Kim DS, Lee HS, Cho SC, Choi SI (Sep 2004). "A case of 49,XXXXX in which the extra X chromosomes were maternal in origin.". J Clin Pathol. 57 (9): 1004–6. doi:10.1136/jcp.2004.017475. PMC 1770429. PMID 15333671.
- Zhang RH, Pan NH, Li XF, Wang XQ, Wu M (Dec 1982). "A case of 49, XXXXX syndrome.". Chin Med J (Engl). 95 (12): 891–4. PMID 6819931.
- Moraes LM, Cardoso LC, Moura VL, Moreira MA, Menezes AN, Llerena JC Jr, Seuánez HN (Oct 2009). "Detailed analysis of X chromosome inactivation in a 49,XXXXX pentasomy.". Mol Cytogenet. 2: 20. doi:10.1186/1755-8166-2-20. PMC 2766382. PMID 19811657.
- Lee NR, Wallace GL, Adeyemi EI, Lopez KC, Blumenthal JD, Clasen LS, Giedd JN (Oct 2012). "Dosage effects of X and Y chromosomes on language and social functioning in children with supernumerary sex chromosome aneuploidies: implications for idiopathic language impairment and autism spectrum disorders.". J Child Psychol Psychiatry. 53 (10): 1072–81. doi:10.1111/j.1469-7610.2012.02573.x. PMC 3480208. PMID 22827287.
- Monheit A, Francke U, Saunders B, Jones KL (Oct 1980). "The penta-X syndrome.". J Med Genet. 17 (5): 392–6. doi:10.1136/jmg.17.5.392. PMC 1048607. PMID 7218280.