In the era of the diseasomes and interactome networks, linking genetics with phenotypic traits in Turner syndrome should be studied thoroughly. As a part of this stratagem, mosaicism of both X and Y chromosome which is a common finding in TS and an evaluation of congenital heart diseases in the different situations of mosaic TS types, can be helpful in the identification of disturbed sex chromosomes, genes and signaling pathway actors. Here we report the case of a mosaic TS associated to four left-sided CHD, including BAV, COA, aortic aneurysms and dissections at an early age. The mosaicism included two cell lines, well-defined at the cytogenetic and molecular levels: a cell line which is monosomic for Xp and Xq genes (45,X) and another which is trisomic for pseudoautosomal genes that are present on the X and Y chromosomes and escape X inactivation: 45,X[8]/46,X,idic(Y)(pter→q11.2::q11.2→pter)[42]. This case generates two hypotheses about the contribution of genes linked to the sex chromosomes and the signaling pathways involving these genes, in left-sided heart diseases. The first hypothesis suggests the interaction between X chromosome and autosomal genes or loci of aortic development, possibly dose-dependent, and which could be in the framework of TGF-β-SMAD signaling pathways. The second implies that left-sided congenital heart lesions involve sex chromosomes loci. The reduced dosage of X chromosome gene(s), escaping X inactivation during development, contributes to this type of CHD. Regarding our case, these X chromosome genes may have homologues at the Y chromosome, but the process of inactivation of the centromeres of the isodicentric Y spreads to the concerned Y chromosome genes. Therefore, this case emerges as an invitation to consider the mosaics of Turner syndrome and to study their phenotypes in correlation with their genotypes to discover the underlying developmental and genetic mechanisms, especially the ones related to sex chromosomes.

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