A murine transgene, HRD, is methylated only when carried in certain inbred strain backgrounds. A locus on distal chromosome 4, Ssm1 (strain-specific modifier), controls this phenomenon. In order to characterize the activity of Ssm1, we have investigated developmental acquisition of methylation over the transgene. Analysis of postimplantation embryos revealed that strain-specific methylation is initiated prior to embryonic day (E) 6.5. Strain-specific transgene methylation is all-or-none in pattern and occurs exclusively in the primitive ectoderm lineage. A strain-independent pattern of partial methylation occurs in the primitive endoderm and trophectoderm lineages. To examine earlier stages, embryonic stem (ES) cells were derived from E3.5 blastocysts and examined for transgene methylation before and after differentiation. Though the transgene had already acquired some methylation in undifferentiated ES cells, differentiation induced further, de novo methylation in a strain-dependent manner. Analysis of methylation in ES cultures suggests that the transgene and endogenous genes (such as immunoglobulin genes) are synchronously methylated during early development. These results are interpreted in the context of a model in which Ssm1-like modifier genes produce alterations in chromatin structure during and/or shortly after implantation, thereby marking target loci for de novo methylation with the rest of the genome during gastrulation.

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