DNA methylation involves the addition of a methyl group to the carbon-5 position of the cytosine ring in the CpG dinucleotide and converting it to methylcytosine.  This process is catalyzed by DNA methyltransferase.  In numerous cancers, the CpG islands of selected genes are aberrantly methylated (hypermethylated) which results in transcriptional repression.  This may be an alternate mechanism of gene inactivation.
Multiple genes have been discovered to be frequently methylated in cancers and leukemias.  More specifically, the deregulation of the Wnt signaling pathway has been implicated in a wide array of cancers that is mainly seen as a result of loss-of-function mutations of APC and axin or as a gain-of-function mutation of CTNNB1 (B-catenin). The GC content of the SFRP1 promoter in humans is 56.3%.
It has been found that the overexpression of B-catenin may lead to enhanced proliferation in myeloma plasma cells; thus, soluble Wnt inhibitors are potential tumor suppressor genes and, if inactivated, may contribute to myeloma pathogenesis.  This led Chim et al. to investigate the role of aberrant gene methylation of a panel of soluble Wnt antagonists, including SFRP1.  Complete methylation led to silencing of respective genes (no transcripts), whereas absence of gene methylation was associated with constitutive gene expression.  Methylation of soluble Wnt antagonists would be important in the pathogenesis of multiple myeloma if Wnt signaling was regulated by an autocrine loop by Wnt and Fz.  If an autocrine loops exists, then both the ligand (Wz) and receptor (Fzd) should be simultaneously expressed in myeloma cells and growth of tumour cells should be inhibited upon addition of SFRP1.  Chim et al. demonstrated simultaneous expression of Wz and Fzd in myeloma plasma cells.  Moreover, treatment with recombinant SFRP1 inhibited the growth of myeloma cells in a dose-dependent manner.  These findings implicate soluble Wnt inhibitors as tumor suppressors that could be inactivated by methylation.
Veeck and colleagues found all of their eight breast cancer cell lines had complete methylation in the SFRP1 promoter region, while no methylation was detectable in non-malignant cell lines.  After treatment with 5-Aza-2’-deoxycytidine (DAC), an inhibitor of DNA methyltransferase, SFRP1 expression was restored in all four treated breast cancer cell lines, supporting the hypothesis of methylation-mediated SFRP1 gene silencing in breast cancer.
Furthermore, the transcriptional silencing mechanism underlying DNA methylation which is brought about through the hypermethylation of CpG-rich islands present in the promoter region of genes, can cooperate with histone deacetylation to change chromatin structure to a repressed form.  Lo and colleagues looked at the effects of DAC and trichostatin A (TSA, selectively inhibits the mammalian histone deacetylase family of enzymes) on cancer cells.  In 4 breast cancer cell lines, SFRP1 expression was significantly restored after treatment with DAC alone.  TSA, only in combination with DAC, had a slightly enhanced effect on SFRP1 expression in these cell lines.  A different breast cancer cell line (SKBR3, showed loss of SFRP1 expression without significant methylation of the SFRP1 promoter.  Lo et al. hypothesized that this may be due to silencing via histone deacetylation.  After SKBR3 cells were treated with TSA, SFRP1 expression was restored in a dose- and time-dependent manner.  Yet another breast cancer cell line (T47D) required both DAC and TSA to upregulate SFRP1 expression.  This indicates that T47D cells are tightly regulated by two layers of epigenetic control (DNA methylation and histion deacetylation) and relieving inhibition by both mechanisms is necessary for reactivation of SFRP1.  This study shows that both the epigenetic mechanisms, DNA methylation and histone deacetylation, are involved in silencing of SFRP1.