AN ATYPICAL TURNER SYNDROME PATIENT WITH RING X-CHROMOSOME MOSAICISM

Small marker chromosomes (SMC) associated with severe Turner syndrome (TS) variants often represent reduced X chromosomes lacking the X inac tivation center (XIC), perturbed dosage compensation, and unbalanced g ene expression. A TS patient with mental retardation (MR), unusually s hort stature, facial and limb malformations, and karyotypic mosaicism involving SMCs is described. Cytogenetic and fluorescence in situ hybr idization (FISH) studies of blood and lymphoblastoid cells showed that the SMC was X-chromosome derived, contained a functional centromere, and had ring formation. Karyotypes of 45/46,X,r(X) in blood cells and 45,X/46,XX/46,X,r(X)/47,X,r(X),+r(X) in fibroblasts were found. Late-r eplication of the SMC was inconclusive, but the X inactivation specifi c transcript (XIST) locus within XIC was demonstrated by fluorescent i n situ hybridization (FISH). Mechanisms are reviewed that can account for our patient's unusual TS phenotype.