Cell cycle genes and ovarian cancer susceptibility: a tagSNP analysis

Ovarian cancer is the seventh most common cancer and the fourth leading cause of cancer death in women worldwide (Parkin et al, 2005). With the highest mortality of all gynaecological malignancies, 15 520 deaths were estimated in the US in 2008 (American Cancer Society, 2008). The pathogenesis and progression of ovarian cancer is not well understood, which contributes to its poor survival, along with difficulties in early detection among asymptomatic women. Modifiable risk factors, which are few, include oral contraceptives, family history and age at menarche. Known genetic risk factors are restricted to mutations inherited in the high risk, high penetrant genes (e.g. BRCA1/2 and DNA mismatch repair genes), which are rare in the general population and estimated to account for no greater than 10–15% of ovarian cancer (Chen et al, 2006; Lancaster et al, 2007). Owing to a consensus that genetic factors have a function in susceptibility to ovarian cancer, studies targeting specific pathways in ovarian cancer case–control studies have emerged (Dicioccio et al, 2004; Auranen et al, 2005; Beesley et al, 2007; Song et al, 2007; Mann et al, 2008; Pearce et al, 2008; Quaye et al, 2008) and some report nominally significant associations with ovarian cancer risk (Buller et al, 1997; Berchuck et al, 2004; Dicioccio et al, 2004; Kelemen et al, 2008; Pearce et al, 2008; Sellers et al, 2008). Dysregulation of the cell cycle is a hallmark of many cancers (Pharoah et al, 2007; Butt et al, 2008; Nam and Kim, 2008) and control and timing of the cell cycle involves checkpoints and regulatory pathways that ensure the fidelity of DNA replication and chromosome segregation (Elledge, 1996). These processes involve a large collection of key molecules, which are excellent candidates for ovarian cancer susceptibility variants. These include the cyclins (CCNA1, CCNA2, CCNB1, CCNB2, CCND1, CCND2, CCND3, CCNE1, CCNE2, CCNG1, CCNG2), cyclin-dependent kinases (CDKS: CDK2, CDK4, CDK6, CDK7, CDC2), CDK inhibitors (CDKN1A, CDKN1B, CDKN2A, CDKN2B, CDKN2C, CDKN2D) and CDC2 regulators (CDC25A, CDC25B). The catalytic subunit of CDKs is activated by one of many activating subunits, the cyclins. Cyclin levels oscillate during the cell cycle, and cyclin–CDK complexes finely regulate progression through the cell cycle. Inhibitors of CDK promote cell cycle arrest and may affect response to mitogenic stimuli. In addition to the cyclins, CDKs and CDK inhibitors, the E2 family of transcription factors is a critical element as well as the E2F family's dimerization partners TFDP1, TFDP2, CUL1 and SKP2, which are involved in the SCF ubiquitin ligase complex. In addition, Rb (and two Rb-like genes) regulates progression of cells from G1 to S to G2 phases. CCND, CCNE and E2F are over-expressed in a variety of cancer, including ovarian cancer (D′Andrilli et al, 2004), and data emanating from an immunohistochemical study of ovarian cancer (Hashiguchi et al, 2004) reveals alteration of G2 in ovarian cancer specimens. The SCF ubiquitin ligases are well-characterized mammalian cullin RING ubiquitin ligases (Frescas and Pagano, 2008), and this complex is an essential element in the CDKNA–CDK2 S phase. SKP2 activates CDK2 and CDK1 by directing the degradation of CDKN1 (p27) and CDKN1B (p21). SKP2 is also known to target tumour suppressor proteins p21 and CDKN1C, resulting in protein degradation (Frescas and Pagano, 2008). Activation and inactivation of CDKs is an additional crucial process, and dysregulation may be involved in cell transformation. Other important kinases include ABL1, a non-tyrosine kinase, that may regulate the CDC2 kinase (Lin et al, 2004), and PLK1, a cell cycle regulated kinase (Yuan et al, 2002). As cell cycle abnormalities have been observed in ovarian cancer (Milde-Langosch and Riethdorf, 2003; De Meyer et al, 2009), we hypothesized that common genetic variation in genes altering the functionality of the molecules may influence the ovarian carcinogenic process. An earlier study of 13 genes (88 informative single nucleotide polymorphisms, SNPs) involved in regulation of the G1–S phase of the cell cycle (CCNDA, CCND2, CCND3, CCNE1, CDK2, CDK4, CDK6, CDKN1A, CDKN1B, CDKN2A, CDKN2B, CDKN2C and CDKN2D) found nominally significant associations between SNPs in CDKN2A and CDKN1B [rs3731257 homozygous minor vs homozygous major odds ratio, ORBB vs AA, 0.87 (95% confidence interval, 95% CI, 0.73–1.03) P-value=0.021; rs2066827 ORBB vs AA 0.79 (0.66–0.95) P-value=0.04] (Gayther et al, 2007). In addition, a combined analyses of 6 studies and 12 genes including imputed genotypes found evidence of association with selected SNPs in CDKN2A, CCND1, CDK2 and CCNE1, but not in CDKN2C, CDKN1A, CCND3, CCND2, CDKN1B, CDK4, RB1, CDKN2D or CDKN2B (Goode et al, 2009) Here, we report on a more comprehensive two-stage analysis of the association of ovarian cancer risk at 39 genes (288 SNPs) involved in G1/S and G2/M phases of the cell cycle and transcription- and ubiquitin-mediated degradation (Table 1). Table 1 Cell cycle related genes