1. ¹Department of Pathology, Queen's University, Kingston, ON, Canada K7L 3N6
2. ²Department of Medicine, University of Western Ontario, London, ON, Canada
3. ³Department of Biochemistry, University of Western Ontario, London, ON, Canada

Correspondence: LM Mulligan, Department of Pathology, 20 Barrie St, Queen's University, Kingston, ON, Canada K7L 3N6. E-mail: mulligal@post.queensu.ca

Received 25 May 2003; Revised 19 April 2004; Accepted 21 April 2004; Published online 13 December 2004.

Abstract

Mutations of the RET proto-oncogene are found in the majority of patients with the inherited cancer syndrome multiple endocrine neoplasia type 2 (MEN 2). A minority of cases, however, have no detectable RET mutation and there is considerable phenotypic variation within and among MEN 2 families with the same RET mutation, suggesting a role for other loci in this disease. A candidate for such a gene is glial cell line-derived neurotrophic factor receptor alpha 4 (GFRA4), which encodes a cell surface-bound co-receptor (GFR4) required for interaction of RET with its ligand persephin. The GFRA4 gene has multiple alternative splices leading to three distinct protein isoforms that are prominently expressed in thyroid. We postulated that mutations of GFRA4 contribute to MEN 2 in the absence of RET mutations or modify the RET mutation phenotype. We screened patients with MEN 2 or MEN 2-like phenotypes, with and without RET mutations, for variants of GFRA4. We identified 10 variants, one of which was over represented in, and two of which were found exclusively in, our patient populations. One of these was a single-base substitution upstream of the GFR4 coding region, where it may alter gene expression. The second was a 7 bp insertion, which results in a change in reading frame for all three GFR4 isoforms. This would cause a relative shift in membrane bound and soluble forms of GFR4, which would significantly alter the formation of RET signalling complexes. Our data suggest a model of wild-type GFR4 isoform expression that includes both activating and inhibiting co-receptors for RET.