Structure and expression of the mouse L23mrp gene downstream of the imprinted H19 gene: biallelic expression and lack of interaction with the H19 enhancers.

The human L23 (mitochondrial)-related protein gene, located 40 kb downstream of the imprinted H19 gene, is biallelically expressed. We have cloned and characterized its mouse homolog, L23mrp, which maps to the conserved syntenic region on mouse chromosome 7. The promoter of L23mrp is a CpG island that is transcribed ubiquitously, but at different levels, in different fetal tissues. Allele-specific expression analysis revealed that both parental alleles are equally active. Since the enhancers located between H19 and L23mrp had been shown to be involved in the imprinted expression of Ins-2, Igf-2, and H19, we asked whether they also influence L23mrp. Analysis of mice with a targeted deletion of the enhancers demonstrated that they were not disrupted in the expression of L23mrp. These findings indicate that L23mrp is functionally insulated from the Ins-2/Igf-2/H19 domain in terms of both imprinting and enhancer action.