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The proportion of cells with functional X disomy is associated with the severity of mental retardation in mosaic ring X Turner syndrome femalesKubota T.a-c · Wakui K.a,b · Nakamura T.d · Ohashi H.e · Watanabe Y.f · Yoshino M.f · Kida T.g · Okamoto N.h · Matsumura M.c · Muroya K.i · Ogata T.j · Goto Y.c · Fukushima Y.a,b
aDepartment of Medical Genetics, Shinshu University School of Medicine, Nagano; bDivision of Clinical and Molecular Genetics, Shinshu University Hospital, Nagano; cNational Institute of Neuroscience, NCNP, Tokyo; dMitsubishi Kagaku Bio-Clinical Laboratories, Inc., Tokyo; eSaitama Children’s Medical Center, Saitama; fKurume University School of Medicine, Fukuoka; gHakodate Central Hospital, Hokkaido; hOsaka Medical Center and Research Institute for Maternal and Child Health, Osaka; iDepartment of Pediatrics, Tokyo Dental College, Ichikawa General Hospital, Chiba; and jDepartment of Endocrinology and Metabolism, National Research Institute for Child Health and Development, Tokyo (Japan)
Turner syndrome females (45,X) do not have mental retardation (MR), whereas some mosaic ring X Turner syndrome females, with 45,X/46,X,r(X), have severe MR. The MR is believed to be caused by a failure of X chromosome inactivation (XCI) of the small ring X chromosome, which leads to functional X disomy (FXD), To explore this hypothesis, we examined the proportion of FXD cells in the peripheral blood of four ring X Turner syndrome females with various levels of MR, using two newly developed XCI assays based on DNA methylation of X-linked genes. As a result, the two patients with extremely severe MR showed complete FXD patterns, whereas the remaining two patients with relatively milder MR showed partial FXD patterns. These results indicate that the proportion of FXD cells may be associated with the severity of MR in mosaic ring X Turner syndrome females, although this association should be confirmed by examining brain cells during development. One of the cases with severe MR and a complete FXD pattern neither lacked the XIST gene nor had uniparental X isodisomy, and we discuss the mechanism of the failure of XCI in this case.
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