These four lectures focused on a series of diseases in which the epigenetic phenomenon of parental imprinting plays a key role: Prader-Willi and Angelman syndromes, and Beckwith-Wiedemann and Silver-Russell syndromes. The possible and less established role of parental imprinting in the phenotypic variability of Turner and Klinefelter syndromes was also discussed, as was work suggesting a modest but real effect of medically-assisted reproduction on the risk of diseases linked to genomic imprinting. In my introduction, I recalled that the notion of parental imprinting is derived from experimental embryology and genetics work on mice, conducted by A. Surani, D. Solter and B. Cattanach, published in 1984-85, showing the non-equivalence of paternal and maternal autosomal genomes. This field of research became very active in the 1990s, with the identification of homologous genome regions subject to parental imprinting in mice and humans, and their involvement in human genetic pathologies. Fundamental research aims to study the mechanisms of imprinting and deletion, notably involving DNA methylation enzymes (Dnml1, Dnmt3a and 3L). Other, more theoretical works analyze the evolutionary role of this mechanism in mammals, including the hypothesis of Darwinian parental conflict, first put forward in 1991 by T. Moore and D. Haig, concerning embryo growth, nutrient exchange between mother and fetus, and perhaps also maternal behavior towards newborns. The bulk of this series of lectures focused on the mechanisms involved in the four pathologies mentioned above, involving two regions of the genome subject to parental imprinting in humans and mice.
Prader-Willi and Angelman syndromes are very dissimilar neurodevelopmental diseases, yet they may be caused by the same mechanisms (uniparental deletion or disomy), but leading in the case of Prader-Willi to the absence of a paternal copy of the involved region of chromosome 15 (15q11.2-q13), and, in the case of Angelman, to the absence of the maternal copy. But while Angelman syndrome is linked to a single gene expression anomaly in this region (UBE3A, coding for an ubiquitin protein ligase), the situation is more complex for Prader-Willi syndrome, involving a cluster of genes coding for snoRNAs(small nucleolar RNAs), but perhaps also the gene coding for the necdin protein, all present in this region. Work in both humans and mice has highlighted the major importance of an imprinting control center (ICR) in this region, and of the expression of large antisense RNAs. Major results concerning these mechanisms have been obtained by comparing specific genomic lesions in patients, and specific mouse models obtained by homologous recombination. Here too, the recent possibility of creating pluripotent stem cells (iPSCs) from patient fibroblasts offers highly promising new tools.