Manipulation of gene expression in early mouse embryo

Maternal (oocyte) and embryonic programmes of hypoxanthine phosphoribosyl transferase (HPRT) and adenine phosphoribosyl transferase (APRT) gene expression have been investigated in mouse oocytes and during preimplantation development. The onset of the embryonic HPRT gene occurs following fertilization, or parthenogenetic activation, before the 4-cell stage. The oocyte-determined increase in HPRT activity due to preformed mRNA continues in aging oocytes, as well as in fertilized or activated eggs, i.e. irrespective of the initiation of embryonic development. Attempts were made to determine whether there is a detectable time difference in the onset of maternal and paternal genomes by assaying the onset of embryo- coded HPRT activity in embryos of different maternal and paternal X-chromosome constitution, but due to difficulties in comparable staging of embryos, no definite conclusion could be drawn. The expression of an exogenously introduced HPRT minigene has been monitored throughout preimplantation development. The embryos injected with supercoiled HPRT minigene showed an approximately twofold increase in HPRT activity at the 2-cell stage compared with control uninjected embryos. Linear minigene DNA was less efficient in giving active enzyme. The efficacy of three different promoters were studied in 2-cell mouse embryos using the expression of the HPRT minigene as a reporter function. The mouse HPRT promoter and the uninduced mouse metallothionein-1 (MT-1) promoter functioned equally well whereas the viral SV40 promoter did not allow HPRT expression. The mouse MT-1 promoter linked to the HPRT minigene allowed induction of HPRT gene expression in mouse embryos cultured in the presence of cadmium. The inhibition of enzyme expression from injected minigene DNA is mediated by simultaneous injection of a fivefold molar excess of HPRT antisense DNA. The same negation of exogenous HPRT activity was observed with simultaneous injection of HPRT exon-1 antisense DNA. The use of an inducible HPRT antisense construct achieved repression of gene activity with equivalent molar amounts of antisense to the sense molecules. Transgenic mice were produced with an antisense HPRT minigene construct attached to the inducible mouse MT-l promoter. The prospect of "cancelling" the endogenous "sense" gene activity in these mice at a specific stage of development by applying the induction stimulus is discussed