A Genomic Imprinting Defect in Mice Traced to a Single Gene

Mammalian androgenones have two paternally or sperm-derived genomes. In mice (Mus musculus) they die at peri-implantation due to the misexpression of imprinted genes-the genes that are expressed monoallelically according to the parent of origin. The misexpressions involved are poorly defined. To gain further insight, we examined the causes of midgestation death of embryos with paternal duplication (PatDp) of distal chromosome 7 (dist7), a region replete with imprinted genes. PatDp(dist7) embryos have a similar phenotype to mice with a knockout of a maternally expressed imprinted gene, Ascl2 [achaete-scute complex homolog-like 2 (Drosophila)], and their death at midgestation could result from two inactive paternal copies of this gene. However, other dist7 misexpressions could duplicate this phenotype, and the potential epistatic load is undefined. We show that an Ascl2 transgene is able to promote the development of PatDp(dist7) embryos to term, providing strong evidence that Ascl2 is the only imprinted gene in the genome for which PatDp results in early embryonic death. While some of the defects in perinatal transgenic PatDp(dist7) fetuses were consistent with known misexpressions of dist7 imprinted genes, the overall phenotype indicates a role for additional undefined misexpressions of imprinted genes. This study provides implications for the human imprinting-related fetal overgrowth disorder, Beckwith-Wiedemann syndrome.