ALLELE-SPECIFIC EXPRESSION AND TOTAL EXPRESSION LEVELS OF IMPRINTED GENES DURING EARLY MOUSE DEVELOPMENT - IMPLICATIONS FOR IMPRINTING MECHANISMS

Genomic imprinting determines the monoallelic expression of a small nu mber of genes during at least later stages of development. To obtain i nformation necessary for the elucidation of imprinting mechanisms, we assessed the allele-specific expression and total expression level of four imprinted genes during early stages of development of normal F-1 hybrid mice utilizing quantitative allele-specific reverse transcripti on-PCR (RT-PCR) single-nucleotide primer extension assays. The Igf2r a nd Snrpn genes were activated by the early 4-cell stage and exhibited biallelic and monoallelic expression, respectively, throughout preimpl antation development. Thus, with respect to different imprinted genes, epigenetic systems determining monoallelic expression are not uniform in their time of establishment. Biallelic expression of Igf2r was obs erved in single blastomeres, discounting the possibility of random all elic inactivation at this stage. The closely linked H19 and Igf2 genes were activated after the blastocyst stage and often exhibited biallel ic and monoallelic expression respectively in tissues of pregastrulati on postimplantation-stage embryos, rather than reciprocal monoallelic modes as observed at later stages. This raises the possibility that im printing of H19 is involved only in the maintenance and not in the ini tiation of monoallelic expression of Igf2. Monoallelic expression of S nrpn was observed in each blastomere at the 4-cell stage, demonstratin g that the germ line, which exhibits biallelic expression of imprinted genes, must be derived from cells in which imprinting was once manife st.