In mammals, parental-specific methylation imprinting, which occurs independently during male and female gametogenesis, regulates monoallelic expression of imprinted genes. It is well known that many imprinted genes are involved in embryonic development. In mice, the Dlk1-Gtl2 domain spanning 1 Mb and located in the distal region of chromosome 12 contains a cluster of imprinted genes: the paternally expressed genes Dlk1, Rtl1, and Dio3 and the maternally expressed genes Gtl2, antiRtl1, Rian, and Mirg. The observations that maternal and paternal duplications in this region resulted in perinatal origin-dependent lethality, indicate that the Dlk1-Gtl2 domain plays an essential role in peri- and postnatal development. Furthermore, studies using gene knockout mice have shown that Dlk1 is responsible for growth and skeletal formation, and that Dio3 contributes to the metabolism of thyroid hormones. However, little is known regarding the function of the maternally expressed imprinted genes Gtl2, antiRtl1, Rian, and Mirg, except that they are non-coding RNA. Here, we generated mice harboring a deletion that spanned exons 1-5 by using a neomycin-resistance cassette and carried out mating tests in order to elucidate the function of Gtl2. Mice inheriting the mutation from either the father (Gtl2 Pat-KO) or the mother (Gtl2 Mat-KO) were produced by mating WT female mice × heterozygous mutant male mice (n = 12) and heterozygous female mice × WT male mice (n = 4), respectively. In the Gtl2 Pat-KO mice, the mean number of offspring per litter was significantly lower (2.7/litter, n = 12) than that of WT mice (4.9/litter, n = 12), suggesting loss of the embryos before birth. Furthermore, the pups showed severe pre- and postnatal growth retardation, and half the pups died within 1 day of birth. However, the survivors grew up to be fertile adults. On the other hand, in the Gtl2 Mat-KO mice, the pups were born with normal phenotypes, showing normal litter size and body weight. However, they died within 4 weeks of birth and showed severe growth retardation at this time. In order to understand these parental-origin-specific phenotypes, we examined the expression of the imprinted genes in the 1-day-old pups by using quantitative RT-PCR. In the Gtl2 Pat-KO mice, the expression of Dlk1 and Dio3, which are involved in growth, was reduced to half the levels observed in the controls. In the Gtl2 Mat-KO mice, in addition to the lack of Gtl2 expression, the expression levels of Rian and Mirg were significantly reduced. It is known that these 2 maternal transcripts contain a number of snoRNAs and miRNAs, although the function of these remains unknown. The present data, however, suggested that non-coding RNA, including Gtl2, is involved in postnatal development. The present results implicate Gtl2, for the first time, in the regulation of gene expression of the Dlk1-Gtl2 domain and causation of parental-origin-specific fetal and postnatal lethality in mice.
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