**3. DNA methyltransferases**

The methylation process is catalyzed by the DNA methyltransferases. There are currently four known DNMTs; DNMT1, 2, 3A and 3B (Okano et al., 1998). DNMT3A and DNMT3B are the de novo methyltransferases while DNMT1 maintains the methylation patterns during DNA replication (mitosis) (Bestor, 2000). The actual function of DNMT2 is not clear. It has been shown that DNMT2 possesses weak methyltransferase activity, and its deletion in the embryonic cells caused no detectible effect on global methylation (Okano et al., 1998). DNMT1 has a 5-30 fold preference for hemimethylated DNA (Goyal et al., 2006;Yoder et al., 1997). As well as to the epigenetic silencing of particular genes, DNMT1 supports the long term silencing of non-coding DNA, including most of the repetitive elements (Brannan & Bartolomei, 1999;Fuks, 2005;Jaenisch & Bird, 2003;Jones & Takai, 2001). DNMT1 exist as a component of the DNA replication complex, and thus methylates the newly synthesized DNA strand in correspondence to the template strand (Vertino et al., 2002). DNMT1 has different isoforms, the somatic tissue isoform DNMT1S, the oocyte specific isoform DNMT1o and the spermatocyte isoform DNMT1p. DNMT1o is responsible for maintaining maternal imprints during cleavage (Howell et al., 2001). In addition to that, over expression of DNMT1 has been reported in human tumours and many contribute to the global methylation abnormalities seen in cancer cells although increased expression of the DNMTs likely to be only partially responsible for the observed methylation abnormalities since not all tumours overexpress these enzymes (Robertson & Jones, 2000).

On the other hand, de novo DNA methylation is catalyzed by DNMT3a, DNMT3b and DNMT3L (Okano et al.,1999; Chedin et al., 2002). DNMT3L lacks the ability to bind to SAM, and is responsible for increasing the binding of DNMT3a to SAM (Chedin et al., 2002; Aapola et al., 2000). DNMT2, a small 391-amino-acid protein, is reported to possess weak DNA methyltransferase activity, but its biological function is not yet elucidated (Dong et al, 2001). Very recent studies have shown that Dicer-mediated microRNA biogenesis is involved in modulation of DNA methylation by indirectly regulating the expression of DNMT3 genes (Sinkkonen et al., 2008; Benetti et al., 2008). Dicer belongs to the RNase III family enzymes and is implicated in processing the biosynthesis of small interfering RNAs (siRNAs) and microRNAs (miRNAs) (Kim et al., 2005). In dicer−/− cells, the microRNAs of the miR-290 cluster are depleted and expression levels of their target Rbl2 protein (retinoblastoma-like protein) are increased, leading to downregulation of DNMT3 gene expression through Rbl2-

human cancers (Feinberg & Vogelstein, 1983; Narayan et al., 1998; Jones & Baylin, 2002). Although less well studied than DNA hypermethylation, several lines of investigation indicate that the global DNA hypomethylation identified in cancer cells might contribute to structural changes in chromosomes, loss of imprinting (LOI), micro satellite and chromosome instability through aberrant DNA recombination, aberrant activation of protooncogene expression and increased mutagenesis (Chen et al.,1998; Eden et al., 2003; Kaneda & Feinberg, 2005; Jones & Baylin, 2002). Global genomic hypomethylation in breast cancer has been known to correlate with some clinical features such as disease stage, tumor size and histological grade (Soares et al., 1999). Some proto-oncogenes implicated in proliferation and metastasis (e.g., synuclein γ and urokinase genes) or drug resistance to endocrine therapy (e.g., N-cadherin, ID4, annexin A4, β-catenin and WNT11 genes) have been found to be upregulated in breast cancer through the hypomethylation of their promoters (Fan et al.,

The methylation process is catalyzed by the DNA methyltransferases. There are currently four known DNMTs; DNMT1, 2, 3A and 3B (Okano et al., 1998). DNMT3A and DNMT3B are the de novo methyltransferases while DNMT1 maintains the methylation patterns during DNA replication (mitosis) (Bestor, 2000). The actual function of DNMT2 is not clear. It has been shown that DNMT2 possesses weak methyltransferase activity, and its deletion in the embryonic cells caused no detectible effect on global methylation (Okano et al., 1998). DNMT1 has a 5-30 fold preference for hemimethylated DNA (Goyal et al., 2006;Yoder et al., 1997). As well as to the epigenetic silencing of particular genes, DNMT1 supports the long term silencing of non-coding DNA, including most of the repetitive elements (Brannan & Bartolomei, 1999;Fuks, 2005;Jaenisch & Bird, 2003;Jones & Takai, 2001). DNMT1 exist as a component of the DNA replication complex, and thus methylates the newly synthesized DNA strand in correspondence to the template strand (Vertino et al., 2002). DNMT1 has different isoforms, the somatic tissue isoform DNMT1S, the oocyte specific isoform DNMT1o and the spermatocyte isoform DNMT1p. DNMT1o is responsible for maintaining maternal imprints during cleavage (Howell et al., 2001). In addition to that, over expression of DNMT1 has been reported in human tumours and many contribute to the global methylation abnormalities seen in cancer cells although increased expression of the DNMTs likely to be only partially responsible for the observed methylation abnormalities since not

On the other hand, de novo DNA methylation is catalyzed by DNMT3a, DNMT3b and DNMT3L (Okano et al.,1999; Chedin et al., 2002). DNMT3L lacks the ability to bind to SAM, and is responsible for increasing the binding of DNMT3a to SAM (Chedin et al., 2002; Aapola et al., 2000). DNMT2, a small 391-amino-acid protein, is reported to possess weak DNA methyltransferase activity, but its biological function is not yet elucidated (Dong et al, 2001). Very recent studies have shown that Dicer-mediated microRNA biogenesis is involved in modulation of DNA methylation by indirectly regulating the expression of DNMT3 genes (Sinkkonen et al., 2008; Benetti et al., 2008). Dicer belongs to the RNase III family enzymes and is implicated in processing the biosynthesis of small interfering RNAs (siRNAs) and microRNAs (miRNAs) (Kim et al., 2005). In dicer−/− cells, the microRNAs of the miR-290 cluster are depleted and expression levels of their target Rbl2 protein (retinoblastoma-like protein) are increased, leading to downregulation of DNMT3 gene expression through Rbl2-

2006; Gupta et al., 2003; Pakneshan et al., 2004).

all tumours overexpress these enzymes (Robertson & Jones, 2000).

**3. DNA methyltransferases** 

mediated transcriptional repression, and in turn causing the DNA methylation defect (global hypomethylation) (Sinkkonen et al., 2008; Benetti et al., 2008). Regarding the role of DNMTs in breast tumorigenesis, it has been reported that DNMT3b mRNA is overexpressed in breast cancer, a finding that correlates well with the hypermethylator phenotype and poor prognosis in breast tumors (Girault et al., 2003; Roll et al.,2008).
