2-COP, a Novel Imprinted Gene on Chromosome 7q32, Defines a New Imprinting Cluster in the Human Genome

Abstract

We describe a novel imprinted gene, γ2-COP (nonclathrin coat protein), identified in a search for expressed sequences in human chromosome 7q32 where the paternally expressed MEST gene is located. γ2-COP contains 24 exons and spans >50 kb of genomic DNA. Like MEST, γ2-COP is ubiquitously transcribed in fetal and adult tissues. In fetal tissues, including skeletal muscle, skin, kidney, adrenal, placenta, intestine, lung, chorionic plate and amnion, γ2-COP is imprinted and expressed from the paternal allele. In contrast to the monoallelic expression observed in these fetal tissues, biallelic expression was evident in fetal brain and liver and in adult peripheral blood. Biallelic expression in blood is supported by the demonstration of γ2-COP transcripts in lymphoblastoid cell lines with maternal uniparental disomy 7. Absence of paternal γ2-COP transcripts during embryonic development may contribute to Silver-Russell syndrome. However, on mutation scanning the only γ2-COP mutation detected was maternally derived. Amino acid comparison of γ2-COP protein revealed close relation to γ-COP, a subunit of the coatomer complex COPI, suggesting a role of γ2-COP in cellular vesicle traffic. The existence of distinct coatomer complexes could be the basis for the functional heterogeneity of COPI vesicles in retrograde and anterograde transport and/or in cargo selection. Together, γ2-COP and MEST constitute a novel imprinting cluster in the human genome that may contain other, as yet unknown, imprinted genes.