Research Article: The Imprinted Retrotransposon-Like Gene PEG11 (RTL1) Is Expressed as a Full-Length Protein in Skeletal Muscle from Callipyge Sheep

Date Published: January 8, 2010

Publisher: Public Library of Science

Author(s): Keren Byrne, Michelle L. Colgrave, Tony Vuocolo, Roger Pearson, Christopher A. Bidwell, Noelle E. Cockett, David J. Lynn, Jolena N. Fleming-Waddell, Ross L. Tellam, Rodolfo Aramayo.

Abstract: Members of the Ty3-Gypsy retrotransposon family are rare in mammalian genomes despite their abundance in invertebrates and some vertebrates. These elements contain a gag-pol-like structure characteristic of retroviruses but have lost their ability to retrotranspose into the mammalian genome and are thought to be inactive relics of ancient retrotransposition events. One of these retrotransposon-like elements, PEG11 (also called RTL1) is located at the distal end of ovine chromosome 18 within an imprinted gene cluster that is highly conserved in placental mammals. The region contains several conserved imprinted genes including BEGAIN, DLK1, DAT, GTL2 (MEG3), PEG11 (RTL1), PEG11as, MEG8, MIRG and DIO3. An intergenic point mutation between DLK1 and GTL2 causes muscle hypertrophy in callipyge sheep and is associated with large changes in expression of the genes linked in cis between DLK1 and MEG8. It has been suggested that over-expression of DLK1 is the effector of the callipyge phenotype; however, PEG11 gene expression is also strongly correlated with the emergence of the muscling phenotype as a function of genotype, muscle type and developmental stage. To date, there has been no direct evidence that PEG11 encodes a protein, especially as its anti-sense transcript (PEG11as) contains six miRNA that cause cleavage of the PEG11 transcript. Using immunological and mass spectrometry approaches we have directly identified the full-length PEG11 protein from postnatal nuclear preparations of callipyge skeletal muscle and conclude that its over-expression may be involved in inducing muscle hypertrophy. The developmental expression pattern of the PEG11 gene is consistent with the callipyge mutation causing recapitulation of the normal fetal-like gene expression program during postnatal development. Analysis of the PEG11 sequence indicates strong conservation of the regions encoding the antisense microRNA and in at least two cases these correspond with structural or functional domains of the protein suggesting co-evolution of the sense and antisense genes.

Partial Text: More than 45% of the mammalian genome is composed of repetitive elements, representing DNA transposons, long terminal repeat (LTR) retrotransposons, LINEs (Long Interspersed Nuclear Elements) and SINEs (Short Interspersed Nuclear Elements) [1]. Many of these are genetic relics of ancient transposition events and are generally thought to be inactive due to accumulated mutations and silencing by epigenetic genome ‘defense’ mechanisms, particularly DNA methylation [2]. However, in some instances these repetitive elements may influence the transcription of adjacent protein encoding genes [3]. DNA methylation is also involved in the regulation of genomic imprinting, causing genes to be mono-allelically expressed in a parent of origin specific manner; hence a relationship between the retroelements and the evolution of genomic imprinting mechanisms has been suggested [4], [5], [6], [7].

The conservation of a long open reading frame in the PEG11 gene and evidence for purifying selection suggested that it encodes a protein. The current study has demonstrated that PEG11 is transcribed and produces a full length 151,028 Da protein in callipyge skeletal muscle. This is the first definitive demonstration that this mammalian retrotransposon related gene produces a corresponding protein. There has been one report, using a mouse genetic model of RTL1 over-expression, showing perinuclear immunolocalisation of RTL1 in capillary endothelial cells of the placenta [32]. Gene targeting of RTL1 combined with paternal or maternal inheritance of the disrupted allele resulted in loss or over-expression of RTL1 expression, respectively [32]. Both circumstances generated placental abnormalities contributing to late-fetal and /or neonatal lethality phenotypes, respectively.



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