case of heterokaryotic monozygotic twins discordant for turner syndrome diagnosed with amniocentesis

ntroduction: Different karyotype in monozygotic twins is called heterokaryotic monozygosity. We describe a case of monochorial diamniotic twins both of whom have 45,X/46,XX mosaicism resulted from karyotype analysis on postnatal blood samples but differ in prenatal diagnosis of complete monosomy 45 X0 in twin B and a normal karyotype 46,XX in twin A resulted from amniocentesis. Case report: After a pregnancy characterized by maternal preeclampsia and amniocentesis demonstrating a complete monosomy 45X0 in twin B and a normal karyotype in twin A, the female twins were born at 33 weeks of gestation by caesarean section. Twin A had Apgar score of 7 at 1 min and 9 at 5 min, birth weight was 1540 g (10 th percentile), she had mild neonatal TS phenotype. Twin B had Apgar score of 7 at 1 min and 9 at 5 min, weight was 1710 g (10-50th percentile), length 42 cm (10-50th percentile) and she had neonatal TS phenotype. Echocardiography showed patent ductus arteriosus, patent foramen ovale and bicuspid aortic valve. Karyotype analysis on postnatal blood samples showed 45,X/46,XX mosaicism (25 analysed cells) in both twins: 45,X (8)/46,XX (17) in Twin A and 45,X (10)/46,XX (19) in Twin B. FISH with chromosome X-specific centromeric probes demonstrated the presence of specific signal in 32,5% nucleus in Twin A and 35% in Twin B. A total of 800 nucleus were analyzed. Conclusion: Karyotype analysis on peripheral lymphocytes in monozygotic and monochorial twins may be different from prenatal karyotype . This divergence is presumably due to anastomoses between the placentae resulting in a mixture of the two cell populations in the hematopoietic tissue. Therefore a regular clinic follow up and chromosomal analyses on skin fibroblasts and buccal smears could better clarify the phenotypical difference between twins, discrepance results of two karyotype and the therapeutic approch.