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Neoplasia. 2015 May;17(5):463-72. doi: 10.1016/j.neo.2015.04.006.

An AXIN2 Mutant Allele Associated With Predisposition to Colorectal Neoplasia Has Context-Dependent Effects on AXIN2 Protein Function.

Author information

1
Department of Human Genetics, University of Michigan Medical School, Ann Arbor, MI, USA.
2
Department of Pediatrics, University of Wisconsin School of Medicine and Public Health, Madison, WI, USA.
3
Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, MI, USA.
4
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.

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.

PMID:
26025668
PMCID:
PMC4468370
DOI:
10.1016/j.neo.2015.04.006
[Indexed for MEDLINE]
Free PMC Article

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