Molecular analysis of a ring chromosome X in a family with fragile X syndrome |
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Authors: | Etienne Mornet Any Bogyo Christiane Deluchat Brigitte Simon-Bouy Michèle Mathieu François Thépot Marie -Claude Grisard Eric Leguern Joelle Boué André Boué |
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Affiliation: | (1) Centre d'Etudes de Biologie Prénatale (CEBIOP), Château de Longchamp, Bois de Boulogne, F-75016 Paris, France;(2) Laboratoire de Cytogénétique et Génétique Moléculaire Humaine, Université de Versailles St-Quentin, 45 Avenue des Etats-Unis, F-78000 Versailles, France;(3) Laboratoire de Cytogénétique et de Biologie de la Reproduction, Centre Hospitalier Universitaire d'Amiens, 124 rue Camille Desmoulins, F-80054 Amiens Cedex 1, France;(4) CEBIOP, Château de Longchamp, Bois de Boulogne, F-75016 Paris, France |
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Abstract: | The phenotypically normal sister of a patient affected by fragile X syndrome was referred for genetic counselling and was found to carry a mosaic karyotype 46,X,r(X)/45,X. Because the probability of the simultaneous chance occurrence of fragile X syndrome and a ring chromosome X in the same family is very low, we postulated that the breakpoint of the ring chromosome X originated in the cytogenetic break in Xq27.3 responsible for fragile X syndrome. In order to determine the relative positions of the breakpoint on the ring chromosome X and the (CGG)n unstable sequence responsible for the fragile X mutation, we used molecular markers to analyse the telomeric regions of chromosome X in this family. The results showed that the ring chromosome X was the maternal fragile X chromosome and that the telomeric deletion on the long arm encompassed the (CGG)n sequence. This suggests that the cytogenetic break in Xq27.3 is distinct from the unstable (CGG)n sequence, or that the break followed by the end-to-end fusion creating the ring chromosome was not completely conservative. Analysis of DNA markers on the short arm of chromosome X evidenced a deletion of a large part of the pseudoautosomal region, allowing us to position the genes involved in stature and in some syndromes associated with telomeric deletions of Xp on the proximal side of the pseudoautosomal region. |
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