PMC6330521_01
Karyotype results for the fetus and both parents. 1A) Fetus karyotype showing the presence of an extra chromosome X and an sSMC in fetal cells. 1B) Normal karyotype results for the mother. 1C) Normal karyotype results for the father.
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The determined karyotype was 47,XXY,+mar (figure 1A), which was compatible with apparently Klinefelter syndrome male with an additional marker chromosome. To investigate further, karyotype was performed for both parents on PHA stimulated lymphocyte cultures; both were normal (figures 1B and 1C).
QF-PCR was performed for the fetus and the result is shown by electrophoretogram. 2A) Chromosomes X and Y. AMELXY, XY2, and DXYS267 markers show trisomic diallelic pattern with peak ratio 2:1, 1:2, and 1:2, respectively, confirming the presence of an extra X chromosome. Relative dosage comparison between chromosomes X and 3 (3.X) shows two peaks of identical size, indicating the presence of two chromosomes X and two chromosomes 3. STR markers containing DXS1187, DXS981, and DXS6803 show one peak due to the presence of homozygous alleles. 2B) Electrophoretograms of chromosome 18 multiplex PCR products. D18S391 and GATA markers demonstrate trisomic triallelic pattern and D18S535 marker shows normal diallelic pattern. Relative dosage comparison between chromosomes 18 and 1 shows two peaks of identical size, indicating the presence of two chromosomes 18 and two chromosomes 1. D18S386 marker shows one peak due to the presence of homozygous alleles.
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QF-PCR analysis revealed normal results for chromosomes 21 and 13 but it demonstrated the presence of two chromosomes X and one chromosome Y (figure 2A). The other markers on the long arm of chromosome 18 (D18S535, D18S386, and STS18/1; locations: 18q12.3, 18q22.1, and 18q12.1/1p32, respectively) showed normal patterns (figure 2B).
The result of QF-PCR for the father, mother, and fetus shown by electrophoretogram. A) Paternal, maternal, and fetal allelic sizes of some amplified markers on chromosome 18 showed that the origin of sSMC 18 is maternal and nondisjunction occurred during maternal meiosis I. B) Paternal, maternal, and fetal allelic sizes of some amplified markers on chromosome X showed that the origin of the extra X chromosome is maternal and nondisjunction occurred during maternal meiosis II.
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Considering amplicon sizes, the origin of sSMC 18 and extra X chromosome was identified as maternal and caused by nondisjunction during maternal meiosis I and II, respectively (figures 3A and 3B). 
Characterization of sSMC using chromosome 18 centromere (red) and short arm subtelomeric region (green) fluorescent probes. The result demonstrated that the sSMC is a derivative of chromosome 18.
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For a more precise characterization, FISH analysis was performed using POSEIDON centromeric and subtelomeric probes containing chromosome 18 centromere probe, SE18 (D18Z1), and a probe for subtelomeric region of the short arm of chromosome 18 in 40 metaphase spreads (figure 4).