Imprinting defects in mouse embryos: stochastic errors or polymorphic phenotype?

Defects in expression of imprinted genes are believed to cause developmental abnormalities and play a role in carcinogenesis. To determine whether spontaneous imprinting defects may occur in mouse embryos, we studied the expression of two imprinted genes H19 and Igf2 in individual postimplantation 7.5 d.p.c. and 8.5 d.p.c. embryos. Biallelic expression of H19 was found in 1.6% of the embryos, whereas biallelic expression of Igf2 was found in 0.5% of the embryos. The loss of H19 imprinting (LOI) observed in a small fraction of early postimplantation embryos may be purely stochastic. Alternatively, since we never observed it in an inbred background, it may depend on genetic factors acting in trans. Either mechanism could explain the occurrence of polymorphic imprinting as well as the genesis of sporadic imprinting defects, including cancer. The frequency of LOI of H19 was higher than the incidence of sporadic imprinting disorders in humans (about 1 in 20,000). This contradiction may be explained by different incidence of imprinting errors in different imprinted regions of the genome, in different species, or by loss of the majority of nonmosaic embryos with imprinting defects before birth.