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BACKGROUND: Hydatidiform moles are associated with an excess of paternal genome set(s) or abnormal imprinting of multiple imprinted genes. However, it appears unlikely that all imprinted genes are relevant for the pathogenesis of a hydatidiform mole.CASES: We studied two rare cases of triploid, diandric moles that were tetrasomic for chromosome 6. DNA-marker analysis showed that in one mole the fourth chromosome 6 originated in the father, and in the other mole the fourth chromosome 6 originated in the mother. Histopathologic revision disclosed that both moles were partial moles with no significant difference in the phenotypes.CONCLUSION: It is unlikely that a major gene involved in the pathogenesis of hydatidiform mole, or a major gene involved in determining the severity of the molar phenotype, is located on chromosome 6.

Hydatidiform moles (HMs) are abnormal human pregnancies with vesicular chorionic villi, imposing two clinical challenges; miscarriage and a risk of gestational trophoblastic neoplasia (GTN). The parental type of most HMs are either diandric diploid (PP) or diandric triploid (PPM). We consecutively collected 154 triploid or near-triploid samples from conceptuses with vesicular chorionic villi. We used analysis of DNA markers and/or methylation sensitive-MLPA and collected data from registries and patients records. We performed whole genome SNP analysis of one case of twinning (PP+PM).In all 154 triploids or near-triploids we found two different paternal contributions to the genome (P1P2M). The ratios between the sex chromosomal constitutions XXX, XXY, and XYY were 5.7: 6.9: 1.0. No cases of GTN were observed. Our results corroborate that all triploid human conceptuses with vesicular chorionic villi have the parental type P1P2M. The sex chromosomal ratios suggest approximately equal frequencies of meiosis I and meiosis II errors with selection against the XYY conceptuses or a combination of dispermy, non-disjunction in meiosis I and meiosis II and selection against XYY conceptuses. Although single cases of GTN after a triploid HM have been reported, the results of this study combined with data from previous prospective studies estimate the risk of GTN after a triploid mole to 0% (95% CI: 0-1,4%).