Genomic imprinting is an epigenetic phenomenon that causes a differential expression of paternally and maternally inherited alleles of a subset of genes (the so-called imprinted genes). Imprinted genes are distributed throughout the genome and it is predicted that about 1% of the human genes may be imprinted. It is recognized that the allelic expression of imprinted genes varies between tissues and developmental stages. The current study represents the first attempt to estimate a prevalence of imprinted genes within the total human transcriptome. In silico analysis of the normalized expression profiles of a comprehensive panel of 173 established and candidate human imprinted genes was performed, in 492 publicly available SAGE libraries. The latter represent human cell and tissue samples in a variety of physiological and pathological conditions. Variations in the prevalence of imprinted genes within the total transcriptomes (ranging from 0.08% to 4.36%) and expression profiles of the individual imprinted genes are assessed. This paper thus provides a useful reference on the size of the imprinted transcriptome and expression of the individual imprinted genes. 1. Introduction Genomic imprinting is an epigenetic phenomenon that causes a differential expression of paternally and maternally inherited alleles of a minor subset of genes (the so-called imprinted genes). Genomic imprinting was first discovered in 1984 [1, 2], and in 1991 the first imprinted genes (IGF2, paternally expressed; IGF2R and H19, maternally expressed) were identified in the mouse [3–5]. Since then, the imprinting status was confirmed for numerous genes in Homo sapiens and Mus musculus genomes, less for Bos taurus, Rattus norvegicus, Sus scrofa, Canis lupus familiaris, and Ovis aries; many more genes are considered candidates [6]. Functional significance of the genomic imprinting is not yet fully understood [7–9], while alterations in the expression of imprinted genes are linked to certain pathologies, including Angelman syndrome, Prader-Willi syndrome, and particular cancer subtypes. Genomic imprinting varies between species and tissues. Furthermore, it is a dynamic process and may vary depending on the developmental stage [10]. The goal of the study was to estimate a prevalence of imprinted genes within the total human transcriptome, in cell and tissue samples in a variety of physiological and pathological conditions. Serial analysis of gene expression (SAGE) is a sequence-based technique to study mRNA transcripts quantitatively in cell populations [11]. Two major principles underline
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