Case Report: Androgenetic/biparental chimera with two biparental cell lines leading to placental mesenchymal dysplasia: a possible novel mechanism of formation.

This article focuses on a rare case of placental mesenchymal dysplasia (PMD) associated with Beckwith–Wiedemann syndrome (BWS) in a fetus, characterized by the presence of an androgenetic/biparental chimera (ABC) involving three distinct fetal cell lines: two biparental lines sharing the same maternal but different paternal haplotypes, and one heterodisomic androgenetic line. The study presents comprehensive genetic, cytogenetic, and histopathological analyses supporting a novel mechanism of ABC formation through dispermic fertilization of a single ovum followed by heterogoneic division of a tripronucleated zygote. This mechanism contrasts with previously proposed models by requiring only a single fertilization error and explains the coexistence of multiple cell lines contributing to PMD and BWS phenotypes. The authors emphasize the importance of thorough genetic investigation in PMD pregnancies to detect complex chimerism that may be overlooked by standard analyses.