Differential genomic imprinting of major histocompatibility complex class I antigens in the placenta of the rat.

The RT1.A and Pa (class I) loci in the major histocompatibility complex (MHC) of the rat are genomically imprinted in the placenta, since only the paternally derived antigens are expressed. To determine whether the RT1.E (class I) locus, which defines the end of the MHC opposite RT1.A, is genomically imprinted in the placenta, two different types of crosses were studied: all possible mating combinations of the DA (AaE-) and WF (AuEu) inbred strains and various matings of the R16 (AaE-), R33 (AaEu), and R34 (AaE-) congenic recombinant strains. The results showed that the Eu locus was not imprinted. Thus, genomic imprinting can be different at two endogenous loci that are part of a multigene family in tight-linkage disequilibrium. The lack of Eu imprinting is not due to the locus' being monomorphic, since the monomorphic Pa locus is imprinted, and it is most probably not due to its distance from the imprinted RT1.A and Pa loci (0.4 cM). We suggest that the control of the imprinting may lie in the physical structure of the DNA proximate to the imprinted locus.