Genetic Considerations in the Patient with Turner Syndrome 45 X with or without Mosaicism

Turner syndrome, which combines short stature with a 45,X cell line (pure or mosaic), usually results from a male meiotic error. Chromosomal complement does not accurately predict the phenotype.

Quincy Zhong, B.S. and Lawrence Layman, M.D.

Volume 98, Issue 4, Pages 775-779, October 2012

Turner syndrome (TS) is a complex developmental disorder in individuals with short stature who possess a 45,X cell line, with or without mosaicism. Since the single X chromosome is maternally derived in 80%, the genesis of the 45,X karyotype is due to instability of the Y chromosome leading to its loss during meiosis. Phenotypic features vary depending upon the mode of ascertainment, with postnatal presentation usually generating a more severe phenotype than a prenatal one. Although patients with pure 45,X present with delayed puberty more often than those with 46,XX or 47,XXX cell lines, the chromosomal complement cannot reliably predict the clinical presentation. Most living TS patients are mosaics, while nearly all first trimester TS fetuses have single 45,X cell line. Exclusion of a Ycell line, the presence of which increases the risk of gonadoblastomas and subsequent gonadal germ cell tumors, is best accomplished by karyotype, fluorescent in situ hybridization, and DNA analysis if necessary. The precise genetic etiology of TS has not been elucidated, but it does appear that deletion of the short arm of the X chromosome is sufficient to result in the TS phenotype, thereby implicating haploinsufficiency of multiple genes including SHOX.

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