title: An AXIN2 Mutant Allele Associated With Predisposition to Colorectal Neoplasia Has Context-Dependent Effects on AXIN2 Protein Function.
abstract: Heterozygous, germline nonsense mutations in AXIN2 have been reported in two families with oligodontia and colorectal cancer (CRC) predisposition, including an AXIN2 1989G>A mutation. Somatic AXIN2 mutations predicted to generate truncated AXIN2 (trAXIN2) proteins have been reported in some CRCs. Our studies of cells from an AXIN2 1989G>A mutation carrier showed that the mutant transcripts are not significantly susceptible to nonsense-mediated decay and, thus, could encode a trAXIN2 protein. In transient transfection assays, trAXIN2 was more abundant than wild-type AXIN2 protein, and in contrast to AXIN2, glycogen synthase kinase 3β inhibition did not increase trAXIN2 levels. Like AXIN2, the trAXIN2 protein interacts with β-catenin destruction complex proteins. When ectopically overexpressed, trAXIN2 inhibits β-catenin/T-cell factor-dependent reporter gene activity and SW480 CRC cell colony formation. These findings suggest the trAXIN2 protein may retain some wild-type functions when highly expressed. However, when stably expressed in rat intestinal IEC-6 cells, the trAXIN2 protein did not match AXIN2's activity in inhibiting Wnt-mediated induction of Wnt-regulated target genes, and SW480 cells with stable expression of trAXIN2 but not AXIN2 could be generated. Our data suggest the AXIN2 1989G>A mutation may not have solely a loss-of-function role in CRC. Rather, its contribution may depend on context, with potential loss-of-function when AXIN2 levels are low, such as in the absence of Wnt pathway activation. However, given its apparent increased stability in some settings, the trAXIN2 protein might have gain-of-function in cells with substantially elevated AXIN2 expression, such as Wnt pathway-defective CRC cells.
authors.affiliation: Department of Human Genetics, University of Michigan Medical School, Ann Arbor, MI, USA.
authors.forename: Serina M
authors.initials: SM
authors.lastname: Mazzoni
authors.affiliation: Department of Pediatrics, University of Wisconsin School of Medicine and Public Health, Madison, WI, USA.
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Petty
authors.affiliation: Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, MI, USA.
authors.forename: Elena M
authors.initials: EM
authors.lastname: Stoffel
authors.affiliation: Department of Human Genetics, University of Michigan Medical School, Ann Arbor, MI, USA; Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, MI, USA; Department of Pathology, University of Michigan Medical School, Ann Arbor, MI, USA. Electronic address: fearon@umich.edu.
authors.forename: Eric R
authors.initials: ER
authors.lastname: Fearon
mesh_terms: D000483:Alleles; D000818:Animals; D060466:Axin Protein / Q000235:genetics* / Q000378:metabolism*; D018389:Codon, Nonsense; D015179:Colorectal Neoplasms / Q000235:genetics*; D020022:Genetic Predisposition to Disease / Q000235:genetics*; D018095:Germ-Line Mutation; D006801:Humans; D015151:Immunoblotting; D047468:Immunoprecipitation; D016297:Mutagenesis, Site-Directed; D016133:Polymerase Chain Reaction; D051381:Rats; D014162:Transfection
chemical_list: C491761:AXIN2 protein, human; D060466:Axin Protein; D018389:Codon, Nonsense