Figure 5.

A model for the function of the 15q11-q13 IC. The locations and target chromosomes for the putative germ line imprint switch elements are represented by vertical arrows marked oogenesis-responsive element (ORE) (responsible for switching the paternal to a maternal imprint in the female germ line) and spermatogenesis-responsive element (SRE) (responsible for maternal to paternal switching in the male germ line). We propose that the maternal chromosome is inherited with a heterochromatic organization of the MAR sequences identified in this study. The function of the SRE (SNURF-SNRPN promoter) as a switch element in this model is to displace the heterochromatin constituents, whereas the ORE is directing the formation of heterochromatin at the adjacent MARs on the paternal chromosome. This model does not require imprinted expression of the SNRPN U transcript or SNURF-SNRPN during gametogenesis but does require that SNURF-SNRPN be expressed on the maternal chromosome in the male germ line, for which there is evidence in mouse (50). The presence of the heterochromatin on the maternal chromosome is likely to be associated with suppressive effects in cis, allowing the proposal of a position effect model for imprint spreading to regulate the 2-Mb domain.