1. ¹Department of Medical and Molecular Genetics, King's College London School of Medicine, Guy's Hospital, London, UK
2. ²Department of Pathology, Sapporo Medical University School of Medicine, Chuo-ku, Sapporo, Japan

Correspondence: Professor CG Mathew, Department of Medical and Molecular Genetics, King's College London School of Medicine, 8th Floor Guy's Tower, Guy's Hospital, London SE1 9RT, UK. Tel: +44 207 188 3713; Fax: +44 207 188 2585; E-mail: Christopher.Mathew@Genetics.kcl.ac.uk

Received 31 August 2007; Revised 16 November 2007; Accepted 5 December 2007; Published online 23 January 2008.

Abstract

CARD8 (TUCAN) is implicated in the regulation of apoptosis and inflammation, and is a positional and functional candidate gene for inflammatory bowel disease (IBD). Recent investigations have reported conflicting results of association between a CARD8 nonsynonymous SNP, rs2043211, and IBD. SNP rs2043211 results in an A>T transversion in the CARD8 template strand, which introduces a stop codon polymorphism (Cys10Stop), and genotyping of the Cys10Stop variant revealed that 9% of the control population was homozygous for the 'Stop' allele. The effect of the Stop allele on mRNA and protein expression of the two known isoforms of this gene was investigated. IBD patients homozygous for the Stop allele showed somewhat reduced expression of CARD8 mRNA, but, contrary to expectation, expressed a 48 kDa protein isoform. A search of the EST database and reverse transcription-PCR analysis revealed a novel coding exon and three novel CARD8 mRNA isoforms that are conserved in primates. The isoforms of CARD8 differ in their N-termini, resulting in diverse predicted molecular weights (47, 48, 51, 54 and 60 kDa) and multiple outcomes for the variant including Cys10Stop, Cys34Stop, Phe52Ile and Phe102Ile; one isoform may arise through transcription and translation initiated downstream of rs2043211 to yield a novel protein isoform of 47 kDa. The multiple isoforms and differing consequences for a predicted stop codon polymorphism underline the importance of detailed analysis of the effects of proposed functional variants on gene expression.