Regulation of translational efficiency by different splice variants of the Disc large 1 oncosuppressor 5′‐UTR

Abstract

Human Disc large (DLG1) has been demonstrated to be involved in thecontrol of cell polarity and maintenance of tissue architecture, and is fre-quently lost in human tumours. However, the mechanisms controllingDLG1 expression are poorly understood. To further examine the regulationof DLG1 expression, we analysed the 5¢ ends of DLG1 transcripts by rapidamplification of cDNA ends polymerase chain reaction. We identified analternative splicing event in the 5¢ region of DLG1 mRNA that generatestranscripts with two different 5¢ untranslated regions (5¢-UTRs). We showby reporter assays that the DLG1 5¢-UTR containing an alternativelyspliced exon interferes with the translation of a downstream open readingframe (ORF). However, no significant differences in mRNA stabilityamong the DLG1 5¢-UTR variants were observed. Sequence analysis of theadditional exon present in the larger DLG1 5¢-UTR showed the presenceof an upstream short ORF which is lost in the short version of the 5¢-UTRDLG1. By mutagenesis and luciferase assays, we analysed the contributionof this upstream short ORF in reducing translation efficiency, and showedthat its disruption can revert, to some extent, the negative regulation oflarge 5¢-UTR. Using computational modelling we also show that the largeDLG1 5¢-UTR isoform forms a more stable structure than the short ver-sion, and this may contribute to its ability to repress translation. This rep-resents the first analysis of the 5¢ region of the DLG1 transcripts andshows that differential expression of alternatively spliced 5¢-UTRs with dif-ferent translational properties could result in changes in DLG1 abundance.