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Developmental consequences of imprinted transcription at the Mest locus MacIsaac, Julia Lynn

Abstract

The Mest locus is regulated by genomic imprinting in mammals and only the paternally inherited allele is expressed. A targeted mutation at this locus revealed that it plays an important role in the regulation of embryonic growth and adult behavior. The Mest locus is located in a conserved imprinted domain on mouse chromosome 6 where it is thought to play a key role in the regulation of neighboring maternally expressed genes Copg2 and Klf14 since it contains the only potential imprinting center (IC) identified thus far in this domain, a differentially methylated region (DMR) methylated in oogenesis. Here we describe new larger isoforms of the Mest mRNA, referred to as MestXL, that are generated via alternative polyadenylation and transcribed more than 10kb into the adjacent antisense gene Copg2 exclusively in the developing central nervous system. The MestXL isoforms appear to regulate the allelic usage at Copg2, but not at Klf14, in embryonic neural tissues, as Copg2 is preferentially maternally expressed only in these tissues presumably due to transcriptional interference from MestXL on the paternal chromosome. Our results therefore establish the Mest DMR as an IC and propose a new mechanism to regulate allelic usage and imprinting at sense-antisense gene pairs in mammalian genomes, via tissue-specific alternative polyadenylation and transcriptional interference. Imprinted transcription at the Mest locus also produces a microRNA, miR-335, that acts to down-regulate target genes via binding to their 3’UTRs and ultimately repressing their translation. Here we show that production of miR-335 is imprinted and that its levels are reduced from the mutant MestKO allele. Additionally, we identify several candidate target genes of miR-335 by RNA-seq analysis on primary mouse fibroblasts that under-express miR-335. Our investigation of MestXL and miR-335, two unique alternative functions of the Mest locus, demonstrates that the Mest locus is involved in two types of RNA-mediated regulation and ultimately contributes to the understanding of genomic imprinting and microRNAs in mammalian biology.

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