Two-Step Imprinted X Inactivation: Repeat versus Genic Silencing in the Mouse

Abstract

ABSTRACT Mammals compensate for unequal X-linked gene dosages between the sexes by inactivating one X chromosome in the female. In marsupials and in the early mouse embryo, X chromosome inactivation (XCI) is imprinted to occur selectively on the paternal X chromosome (X P ). The mechanisms and events underlying X P imprinting remain unclear. Here, we find that the imprinted X P can be functionally divided into two domains, one comprising traditional coding genes (genic) and the other comprising intergenic repetitive elements. X P repetitive element silencing occurs by the two-cell stage, does not require Xist , and occurs several divisions prior to genic silencing. In contrast, genic silencing initiates at the morula-to-blastocyst stage and absolutely requires Xist . Genes translocate into the presilenced repeat region as they are inactivated, whereas active genes remain outside. Thus, during the gamete-embryo transition, imprinted XCI occurs in two steps, with repeat silencing preceding genic inactivation. Nucleolar association may underlie the epigenetic asymmetry of X P and X M . We hypothesize that transgenerational information (the imprint) is carried by repeats from the paternal germ line or that, alternatively, repetitive elements are silenced at the two-cell stage in a parent-of-origin-specific manner. Our model incorporates aspects of the so-called classical, de novo , and preinactivation hypotheses and suggests that Xist RNA functions relatively late during preimplantation mouse development.