(D) Coimmunoprecipitation of BRCA2 with RAD51 from lysates of HT1080 cells transfected with a non-targeting small interfering RNA (siRNA) control or siRNA forSYCP3. mechanism for chromosomal instability in cancer and extend the range of PARP-inhibitor sensitive tumours to those expressing SYCP3. Keywords:chromosomal instability, homologous NSHC recombination, meiosis-specific protein == Introduction == SYCP3 is a component of the synaptonemal complex, a meiosis-specific supramolecular proteinaceous structure essential for synapsis of the maternal and paternal homologous chromosomes (Page & Hawley, 2004). Although SYCP3 was first considered to be a meiosis-specific protein, it has been reported to be aberrantly expressed in human leukaemia and primary cervical cancers (Niemeyer et al, 2003;Kang et al, 2010), suggesting that SYCP3 is a member of cancer/testis antigens whose expression is normally limited to the germ cells but abnormally activated in cancer (Simpson et al, 2005). However, although the meiotic role of SYCP3 is well characterized, its mitotic role is entirely unknown. Homologous recombination (HR) is one of the main pathways in the repair of DNA double-strand breaks (DSBs) in mitotic cells (Hartlerode & Scully, 2009). RAD51 has a central role in the early stages of HR. The breast cancer susceptibility protein BRCA2 binds to RAD51 and recruits it to the sites of DSBs and promotes RAD51 filament formation, which is essential for the subsequent homologous DNA pairing and strand-exchange steps of HR (Gudmundsdottir & Ashworth, 2006). In this study, we investigated the role for SYCP3 in mitotic cells. We show that SYCP3 inhibits the mediator role of BRCA2 in HR and induces chromosomal instability by impairing the intrinsic repair pathway. As SYCP3 is expressed in various tumour types, our finding suggests that inactivation of HR by SYCP3 might be prevalent in cancer. == Results == == SYCP3 is expressed in various tumours == We first examined the expression profiles of SYCP3 in two non-cancerous normal human cells and 16 human cancer cell lines by western blot analysis. Although no manifestation of SYCP3 was recognized in the normal retinal pigmented epithelial cell collection RPE and the normal human being mammary epithelial cell collection HME, SYCP3 was aberrantly indicated in various cancer cell lines (supplementary Fig S1Aonline). The hepatocellular carcinoma cell line HepG2 and the prostate cancer cell line DU145 showed moderate manifestation levels, whereas most of additional cancer cells showed low manifestation Hordenine levels compared with the high manifestation levels observed in normal testis. AlthoughSYCP3manifestation was not recognized in the colorectal carcinoma cell collection DLD1, its manifestation was induced in DLD1 cells after treatment with the demethylating agent 5-azacytidine (supplementary Fig S1Bonline), indicating thatSYCP3manifestation in mitotic cells is regulated by a demethylation-dependent process, which is in agreement with the previous statement (Maatouk et al, 2006). To evaluate the manifestation pattern ofSYCP3in human being main tumours, we analysed its manifestation by using an mRNA array that contained two duplicated spots of mRNA from 47 different tumours and 47 normal tissues from unequaled donors (supplementary Fig S2A and Table S1online). The array was probed for manifestation ofSYCP3and-actin, and the signal ratios ofSYCP3/-actinfrom the two duplicated spots were averaged. Whereas almost all the normal samples Hordenine showed low levels of signal ratios, significantly increased levels ofSYCP3/-actinsignal ratios were observed in one adrenal tumour, three liver tumours, one belly tumour and one kidney tumour (supplementary Fig S2Bonline). Although one normal liver sample showed a high level of the signal ratio, its biological significance is unfamiliar because the profile of the donor is not available. These findings indicate thatSYCP3manifestation is not specific to particular tumour types but is usually observed in tumours of various tissue origins. == DNA damage is accumulated by manifestation of SYCP3 == To investigate the part of SYCP3 manifestation in mitotic cells, we founded two impartial RPE clones stably expressing SYCP3 at low levels similar with endogenous levels in cancer such as the fibrosarcoma cell collection HT1080 (supplementary Fig S3Aonline). Immunofluorescence detection of nuclear foci of H2AX, the phosphorylated form of histone H2AX, which is recruited to DSBs in Hordenine response to DNA damage, revealed an increase in the rate of recurrence of foci-positive cells after pressured SYCP3 manifestation from 5.51.5% (means.d.) in mock cells to 17.50.5% and 19.50.5% in the two SYCP3-expressing RPE clones, respectively (Fig 1A,B). These results.