Biallelic expression of imprinted genes in the mouse germ line: implications for erasure, establishment, and mechanisms of genomic imprinting.

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Abstract
Genomic imprinting in mammals determines parental-specific (monoallelic) expression of a relatively small number of genes during development. Imprinting must logically be imparted in the germ line, where inherited maternal and paternal imprinting is erased and new imprinting established according to the individual's sex. We have assessed the allele-specific expression of four imprinted genes, two of which exhibit maternal-specific (H19 and Igf2r) and two of which exhibit paternal-specific (Igf2 and Snrpn) monoallelic somatic expression, in the germ line of F1 hybrid mice utilizing quantitative RT-PCR single-nucleotide primer extension assays. The expression of each gene was biallelic in the female and male germ line from the time that migratory mitotic PGCs entered the embryonic genital ridge and throughout gametogenesis, except that H19 RNA was not detected late in gametogenesis. These findings demonstrate that inherited imprinting is erased, or not recognized, in germ cells by the time of genital ridge colonization; also that new imprinting may not be established until late in gametogenesis, or that it is incomplete or not recognized at this stage. Regardless of imprinting status, a generalized neutralization of imprinting is evident in the germ line, associated with the totipotent state of this unique cell lineage.