Mirror syndrome and placental ectopic liver in association with de novo SOS1 variant

Mirror syndrome is a rare obstetric condition characterized by maternal fluid retention mirroring fetal hydrops. Placental ectopic liver tissue is an extremely rare non-trophoblastic placental tumor, potentially arising from aberrant hepatoblast migration. While its association with fetal hydrops has been reported, the clinical significance remains uncertain. We present a patient of maternal mirror syndrome linked to fetal hydrops due to a de novo SOS1 variant, with histopathological identification of placental ectopic liver tissue. A 32-year-old woman was admitted at 31 weeks' gestation with fetal hydrops, presenting with bilateral pleural effusions, ascites, and subcutaneous edema. Due to worsening maternal pleural effusion, she was transferred to our hospital at 31 weeks and 6 days. Given progressive maternal and fetal deterioration, an emergency cesarean section was performed at 32 weeks due to concerns regarding maternal mirror syndrome. The female infant was delivered with severe respiratory distress and succumbed at 9 days of age. Trio-based exome sequencing identified a de novo heterozygous SOS1 variant (NM_005633.4:c.512T > A [p.V171G]), confirming a postmortem diagnosis of Noonan syndrome. Histopathological analysis of the placenta revealed ectopic liver tissue within the villi, confirmed by positive immunostaining for hepatocyte markers. A recent report of RIT1-associated mirror syndrome and non-immune hydrops fetalis (NIHF) further supports the role of Rasopathies in the pathogenesis of mirror syndrome. Our findings confirm that mirror syndrome is a potential manifestation of Rasopathies, while the role of ectopic liver tissue in the placenta remains uncertain. Future research should focus on genetic factors underlying mirror syndrome rather than incidental placental anomalies.