IMPRINTING AT THE MOUSE INS2 LOCUS - EVIDENCE FOR CIS-ALLELIC AND TRANS-ALLELIC INTERACTIONS

The mouse gene encoding preproinsulin 2 (Ins2) is located on the dista l end of chromosome 7 in a region of several hundred kilobases that co ntains several imprinted genes. The exclusive expression of the Ins2 p aternal allele in the visceral yolk sac during the last part of gestat ion indicates that Ins2 also is imprinted. However, in other tissues i n which Ins2 is expressed, both alleles are active at all developmenta l stages. Taking advantage of two mouse strains carrying different nai l mutations introduced at the Ins2 locus via homologous recombination in ES cells, me examined whether genes inserted at the Ins2 locus beco me imprinted and have the same restricted pattern of monoallelic expre ssion, In the first: null allele, Ins2 was replaced by LacZ, under the control of the endogenous Ins2 promoter, and a Neo cassette with its own promoter was inserted 3' to LacZ (Zneo allele), In the second null allele, Ins2 and its promoter were replaced by the same Neo cassette (Neo allele). Expression of the maternally and paternally inherited ge nes was monitored by RT-PCR. performed on various reciprocal crosses i nvolving the two mutants and the wildtype alleles. In (Zneo x wildtype ) F1 embryos, the pattern of LacZ expression was similar to that of In s2; i.e., LacZ is expressed in the yolk sac only when paternally inher ited, while its expression in the embryo proper is independent of its paternal or materna origin. For both of the mutant alleles, Neo was tr anscribed only when paternally inherited, in the yolk sac as well as i n the embryo, Unexpectedly, we found that LacZ transcription on the ma ternal chromosome varied depending on the nature of the allele on the paternal chromosome. While fully expressed in the embryo when the pate rnal chromosome carries the wildtype allele, the maternally inherited LacZ is extinguished when the parental allele is the Net, allele. The major conclusion from our results is that individual genes introduced into an imprinted chromosomal domain call become imprinted, indicating the influence of long-range cis-acting effects. In addition, our data suggest that the two parental alleles may ''communicate'' with each o ther and influence the transcription at the locus. (C) 1998 Academic P ress.