Prenatal diagnosis analysis of three cases of Turner syndrome fetuses with complex mosaic small supernumerary marker chromosomes

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To explore the value of applying multiple genetic testing techniques for the prenatal diagnosis of Turner syndrome fetuses with complex mosaic small supernumerary marker chromosomes (sSMC). Chromosomal karyotypes of amniotic fluid samples from 5 030 pregnant women who had undergone amniocentesis at the Prenatal Diagnosis Center of the Third Affiliated Hospital of Zhengzhou University from January to December 2022 were retrospectively reviewed. Three fetuses with complex mosaicism fetuses (carrying 2 types of sSMC) were selected as the study subjects. Genetic tests including G-banded chromosomal karyotyping analysis, fluorescence in situ hybridization (FISH), chromosomal microarray analysis (CMA), and copy number variation sequencing (CNV-seq) were used to clarify the origin and mosaic status of the sSMC. This study has been approved by the Medical Ethics Committee of the Third Affiliated Hospital of Zhengzhou University (No. 2023-159-01). G-banded chromosomal analysis of fetus 1 showed a karyotype of 45,X[64]/46,X,+mar1[13]/46,X,+mar2[3]. FISH results showed that 52% of of its cells had contained one X chromosome signal, whilst 48% contained two X chromosome signals. CMA results revealed the fetus had harbored a 32.32 Mb and a 50.93 Mb deletion in Xp22.33p21.1 and Xq22.2q28 regions, respectively, in addition with mosaic deletions of approximately 1.43 copies, 1.78 copies and 1.43 copies in the Xp21.1p11.1, Xq11.1q21.1 and Xq21.2q22.2 regions, respectively. The fetus 2 had a karyotype of 45,X[27]/46,X,+mar1[14]/46,X,+mar2[12]. FISH results indicated that 88% of its cells contained one X chromosomes signal and two Y chromosome signals, and 12% contained signals for one X chromosomes signal and one Y chromosome signal. CNV-seq results revealed a deletion of 7.74 Mb in the Yq11.222q11.23 region and a mosaic duplication of approximately 1.738 copies in the Yp11.31q11.221 region. The fetus 3 had a karyotype of 45,X[60]/46,X,+mar1[11]/46,X,+mar2[6]. FISH results showed that 28% of its cells contained one X chromosome signal, and 72% contained tow X chromosome signals. CNV-seq results revealed deletions of 55.60 Mb and 53.50 Mb in the Xp22.33p11.1 and Xq22.1q28 regions, respectively, along with a mosaic deletion of approximately 1.85 copies in the Xp11.1q13.2 region and a mosaic repeats of approximately 2.66 copies in the Xq13.2q22.1 region. The sSMCs in the 3 fetuses had all originated from sex chromosomes and were of complex mosaic type. After genetic counseling, the three couples had all opted to terminate the pregnancy. The combined use of multiple genetic testing techniques has determined the origin and structure of complex mosaic sSMCs and provided a basis for prenantal diagnosis and genetic counseling.