Epigenetic inactivation of SFRP genes allows constitutive WNT signaling in colorectal cancer

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Abstract
Aberrant WNT pathway signaling is an early progression event in 90% of colorectal cancers1. It occurs through mutations mainly of APC and less often of CTNNB1 (encoding β-catenin)1,2,3 or AXIN2 (encoding axin-2, also known as conductin)4. These mutations allow ligand-independent WNT signaling that culminates in abnormal accumulation of free β-catenin in the nucleus1,2,3. We previously identified frequent promoter hypermethylation and gene silencing of the genes encoding secreted frizzled-related proteins (SFRPs) in colorectal cancer5. SFRPs possess a domain similar to one in the WNT-receptor frizzled proteins and can inhibit WNT receptor binding to downregulate pathway signaling during development6,7,8,9,10. Here we show that restoration of SFRP function in colorectal cancer cells attenuates WNT signaling even in the presence of downstream mutations. We also show that the epigenetic loss of SFRP function occurs early in colorectal cancer progression and may thus provide constitutive WNT signaling that is required to complement downstream mutations in the evolution of colorectal cancer.