Stop codon readthrough generates a C-terminally extended variant of the human vitamin D receptor with reduced calcitriol response

• DOI: 10.1074/jbc.m117.818526
• Publication Date: 2018-02-01T00:45:15Z
• Authors: Gary Loughran; Irwin Jungreis; Ioanna Tzani; Michael Power; Ruslan I. Dmitriev; Ivaylo P. Ivanov; Manolis Kellis; John F. Atkins
• Keywords: 0301 basic medicine; 570; 572; PhyloCSF; Stop codon; Open Reading Frames; 03 medical and health sciences; Calcitriol; Humans; Translation release factor; RNA, Messenger; Vitamin D; VDR; 3. Good health; transfer RNA (tRNA); Calcium Channel Agonists; HEK293 Cells; Gene Expression Regulation; Nuclear receptor; Protein Biosynthesis; Codon, Terminator; RNA; Receptors, Calcitriol; Transcription factor; Readthrough; HeLa Cells

Although stop codon readthrough is used extensively by viruses to expand their gene expression, verified instances of mammalian readthrough have only recently been uncovered by systems biology and comparative genomics approaches. Previously, our analysis of conserved protein coding signatures that extend beyond annotated stop codons predicted stop codon readthrough of several mammalian genes, all of which have been validated experimentally. Four mRNAs display highly efficient stop codon readthrough, and these mRNAs have a UGA stop codon immediately followed by CUAG (UGA_CUAG) that is conserved throughout vertebrates. Extending on the identification of this readthrough motif, we here investigated stop codon readthrough, using tissue culture reporter assays, for all previously untested human genes containing UGA_CUAG. The readthrough efficiency of the annotated stop codon for the sequence encoding vitamin D receptor (VDR) was 6.7%. It was the highest of those tested but all showed notable levels of readthrough. The VDR is a member of the nuclear receptor superfamily of ligand-inducible transcription factors, and it binds its major ligand, calcitriol, via its C-terminal ligand-binding domain. Readthrough of the annotated VDR mRNA results in a 67 amino acid-long C-terminal extension that generates a VDR proteoform named VDRx. VDRx may form homodimers and heterodimers with VDR but, compared with VDR, VDRx displayed a reduced transcriptional response to calcitriol even in the presence of its partner retinoid X receptor.