Genomic imprinting is an epigenetic modification of the gamete or zygote leading to parental origin-specific differential expression of the two alleles of a gene in somatic cells of the offspring. We previously reported that the human K(v)LQT1 gene is imprinted and disrupted in patients with germline balanced chromosomal rearrangements and Beckwith-Wiedemann syndrome. In human, the gene is imprinted in most fetal tissues except the heart, and K(v)LQT1 is part of a 1-Mb cluster of imprinted genes on human chromosome 11p15.5. We sought to determine whether the mouse K(v)lqt1 gene is imprinted, by performing interspecific crosses of 129/SvEv mice with CAST/E(i) (Mus musculus castaneus). We identified a transcribed polymorphism that distinguishes the two parental alleles in F1 offspring. Examination of embryonic, neonatal, and postnatal tissues revealed that K(v)lqt1 is imprinted in mouse early embryos, in both female 129 x male CS and female CS x male 129 offspring, with preferential expression of the maternal allele, like the human homologue. Surprisingly, imprinting was developmentally relaxed, and the developmental stage and tissue specificity of relaxation of imprinting was strain-dependent. To our knowledge, this is the first example of an endogenous gene that shows strain-dependent developmental relaxation of imprinting.
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