We searched for novel imprinted genes in a region of human chromosome 11p15.5, which contains several known imprinted genes. Here we describe the cloning and characterization of the IPL ( I mprinted in P lacenta and L iver) gene, which shows tissue-specific expression and functional imprinting, with the maternal allele active and the paternal allele relatively inactive, in many human and mouse tissues. Human IPL is highly expressed in placenta and shows low but detectable expression in fetal and adult liver and lung. Mouse Ipl maps to the region of chromosome 7 which is syntenic with human 11p15.5 and this gene is expressed in placenta and at higher levels in extraembryonic membranes (yolk sac), fetal liver and adult kidney. Mouse and human IPL show sequence similarity to TDAG51 , a gene which was shown to be essential for Fas expression and susceptibility to apoptosis in a T lymphocyte cell line. Like several other imprinted genes, mouse and human IPL genes are small and contain small introns. These data expand the repertoire of known imprinted genes and will be helpful in testing the mechanism of genomic imprinting and the role of imprinted genes in growth regulation.
Human chromosome 11p15.5 and distal mouse chromosome 7 include a megabase-scale chromosomal domain with multiple genes subject to parental imprinting. Here we describe mouse and human versions of a novel imprinted gene, IMPT1 , which lies between IPL and p57 KIP2 and which encodes a predicted multi-membrane-spanning protein similar to bacterial and eukaryotic polyspecific metabolite transporter and multi-drug resistance pumps. Mouse Impt1 and human IMPT1 mRNAs are highly expressed in tissues with metabolite transport functions, including liver, kidney, intestine, extra-embryonic membranes and placenta, and there is strongly preferential expression of the maternal allele in various mouse tissues at fetal stages. In post-natal tissues there is persistent expression, but the allelic bias attenuates. An allelic expression bias is also observed in human fetal and post-natal tissues, but there is significant interindividual variation and rare somatic allele switching. The fact that Impt1 is relatively repressed on the paternal allele, together with data from other imprinted genes, allows a statistical conclusion that the primary effect of human chromosome 11p15.5/mouse distal chromosome 7 imprinting is domain-wide relative repression of genes on the paternal homolog. Dosage regulation of the metabolite transporter gene(s) by imprinting might regulate placental and fetal growth.
There is increasing evidence for chromosomal domains containing multiple imprinted genes and for domain-wide disruption of imprinting in certain diseases. In a majority of Wilms' tumors (WTs) there is an abnormal bipaternal pattern of expression at three imprinted loci, H19, IGF2 and KIP2, clustered on chromosome 11p15.5. We previously described biallelic expression of L23MRP, 40 kb downstream of H19. Here we map two additional genes, the first encoding a ubiquitously expressed RNA, 2G7, and the second encoding the fast isoform of skeletal muscle troponin-T (TNNT3), in the 55 kb of DNA downstream of L23MRP. 2G7 RNA is spliced and polyadenylated but lacks long open reading frames. 2G7 and TNNT3 are biallelically expressed in mid-fetal and adult human tissues and 2G7 shows persistent expression in WTs. The rat homologue of L23MRP is highly conserved and lies within 85 kb of H19 in a region of rat chromosome 1 which also contains IGF2 and TNNT3. Parallel expression of H19 and TNNT3 in different adult skeletal muscle types suggests that these genes may share an enhancer. These data outline multiple contiguous loci downstream of H19 which escape functional imprinting in humans. The rodent-human synteny of this region may facilitate a search for an imprinting domain boundary.
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