Interspecies approaches for the analysis of parental imprinting during mouse development

Genomic imprinting results in the functional specialization of the maternal and paternal genomes during development whereby offspring inherit only one active copy of a gene. Although the reason for its evolutionary genesis remains speculative, the consequences of genomic imprinting are evidenced by the failure of parthenogenetic and androgenetic development in mammals, and parental-specific effects in the etiology of a number of human diseases. While the precise molecular mechanism of imprinting is unknown, evidence suggests that the specialization of the parental genomes is established during gametogenesis when the parental genomes are epigenetically modified to reflect the parent of origin. To examine the epigenetic modification of specific imprinted genes and subsequent differential expression, an assay is required that can distinguish between the maternal and paternal alleles and their respective transcripts. During the past 5 years several strategies have been used to identify imprinted genes, ranging from the fortuitous disruption of specific parental alleles to subtraction hybridization between cDNAs from normal and parthenogenetic embryos. To study the developmental regulation of these imprinted genes during mammalian development, we describe a relatively simple interspecies "mRNA phenotyping" approach applicable to the analysis of allele-specific expression as well as the identification of candidate imprinted genes.