Imprinting at the mouse Ins2 locus: evidence for cis- and trans-allelic interactions

The mouse gene encoding preproinsulin 2 (Ins2) is located on the distal end of chromosome 7 in a region of several hundred kilobases that contains several imprinted genes. The exclusive expression of the Ins2 paternal allele in the visceral yolk sac during the last part of gestation indicates that Ins2 also is imprinted. However, in other tissues in which Ins2 is expressed, both alleles are active at all developmental stages. Taking advantage of two mouse strains carrying different null mutations introduced at the Ins2 locus via homologous recombination in ES cells, we examined whether genes inserted at the Ins2 locus become imprinted and have the same restricted pattern of monoallelic expression. 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 genes was monitored by RT-PCR performed on various reciprocal crosses involving the two mutants and the wildtype alleles. In (Zneo x wildtype) F1 embryos, the pattern of LacZ expression was similar to that of Ins2; i.e., LacZ is expressed in the yolk sac only when paternally inherited, while its expression in the embryo proper is independent of its paternal or maternal origin. For both of the mutant alleles, Neo was transcribed only when paternally inherited, in the yolk sac as well as in the embryo. Unexpectedly, we found that LacZ transcription on the maternal chromosome varied depending on the nature of the allele on the paternal chromosome. While fully expressed in the embryo when the paternal chromosome carries the wildtype allele, the maternally inherited LacZ is extinguished when the paternal allele is the Neo allele. The major conclusion from our results is that individual genes introduced into an imprinted chromosomal domain can 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 other and influence the transcription at the locus.