A skin sample from a 17-year-old female was received for routine karyotyping with a set of clinical features including clonic seizures, cardiomyopathy, hepatic adenomas, and skeletal dysplasia. Conventional karyotyping revealed a mosaic Turner syndrome karyotype with a cell line containing a small marker of X chromosome origin. This was later confirmed on peripheral blood cultures by conventional G-banding, fluorescence in situ hybridisation and microarray analysis. Similar Turner mosaic marker chromosome cases have been previously reported in the literature, with a variable phenotype ranging from the mild “classic” Turner syndrome to anencephaly, agenesis of the corpus callosum, complex heart malformation, and syndactyly of the fingers and toes. This case report has a phenotype that is largely discordant with previously published cases as it lies at the severe end of the Turner variant phenotype scale. The observed cytogenetic abnormalities in this study may represent a coincidental finding, but we cannot exclude the possibility that the marker has a nonfunctioning X chromosome inactivation locus, leading to functional disomy of those genes carried by the marker. 1. Introduction Turner syndrome (TS) presents with a characteristic mild phenotype with some degree of variability [1]. The majority of patients have short stature, are infertile, and do not develop secondary sexual characteristics. Less consistent abnormalities include webbed neck, renal malformations (>50%), and cardiac defects (10%), while intelligence is considered normal. Approximately three-quarters of TS females inherit their X chromosome maternally [2]. Turner syndrome mosaics are also well documented and can be subcategorised according to whether the second cell line contains a whole or part of a sex chromosome. Jacobs et al. (1997) showed that, of 84 Turner syndrome cases with a standard karyotype of 45,X, 16% were mosaic, with a second cell line containing a ring X chromosome (45,X/46,X,r(X)) [3]. The phenotypic variability of these mosaics is largely dependent on the size of the ring and the presence of a functioning XIST. XIST is a cis-acting gene in the X-inactivation centre (XIC), located in band Xq13. As a general rule, when one X chromosome has an imbalance that does not involve an autosome, the XIC on the abnormal X chromosome is activated. This activation leads to nonrandom skewing of X chromosome inactivation, with the XIST transcript inactivating the abnormal chromosome. The phenotype of this group of patients is generally that of a mild Turner variant phenotype [2].
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