We report isolation of a novel tumor-reversing gene, tentatively named SVS-1, encoding a protein of 820 amino acids with localization on the plasma membrane as a type I transmembrane protein.

(1-4) Class I enzymes (topo I, EC 5.99.1.2) act by making a transient break in one DNA strand, allowing the DNA to swivel and release torsional strain, changing the linking number by steps of one.(2,4) Class II enzymes (topo II, EC 5.99.1.3) make transient breaks in both strands of one DNA molecule, allowing the passage of another DNA duplex through the gap, changing the linking number by steps of two.(1-3) These enzymes are crucial for cellular genetic processes such as DNA replication, transcription, recombination, and chromosome segregation at mitosis.It has long been accepted that topos are valuable targets of cancer chemotherapeutics.(2-5) Several classes of topo inhibitors have been introduced into cancer clinics as potent anticancer drugs, including camptothecin (CPT) derivatives (e.g. irinotecan and topotecan) inhibiting topo I (4) and anthracyclines (e.g. doxorubicin and mitoxantorone), epipodophyllotoxins (e.g. etoposide , aminoacridines (e.g. m-AMSA) and ellipticines targeting topo II.(4,5) These agents are active in both hematological and solid malignancies. The activity of these agents is thought to result from stabilization of the DNA /topo cleavable complex (CC), an intermediate in the catalytic cycle of the enzymes, (2,5,6) resulting ultimately in apoptosis. A number of new topo inhibitors have recently been reported that do not stabilize CC. Thus, two general mechanistic classes of topo inhibitors, especially for topo II, have recently been described: (7) (1) classical topo 'poisons' that stabilize CC and stimulate single-or double-strand cleavage of DNA, such as CPT and its derivatives, indolocarbazoles for topo I, and for topos I and II; and (2) catalytic inhibitors that prevent the catalytic cycle of the enzymes at steps other than cleavage intermediates, such as dioxopiperazines, (7,9,10) aclarubicin, (11) intoplicin, (12) and F11782.(13) Some of these compounds are dual inhibitors of topos I and II. The catalytic inhibitors of topo II, merbarone (14,15) and dioxopiperazines, (9,10) have been extensively studied and have been shown to inhibit the reopening of the closed clamp formed by the enzyme around DNA by inhibiting the ATPase activity of the enzyme, thus sequestering the enzyme within the cell. (7,16,17)

Self-renewal of mouse embryonic stem cells (mESCs) is maintained by leukemia inhibitory factor (LIF)/signal transducer and activator of transcription (STAT3) signaling. However, this signaling control does not function in neither mouse epiblast stem cells (mEpiSCs) nor human ESCs (hESCs) or human induced pluripotent stem cells (hiPSCs). To date, the underlying molecular mechanisms that determine this differential LIF-responsiveness have not been clarified. Here, we show that the cell surface glycan LacdiNAc (GalNAcb1-4GlcNAc) is required for LIF/ STAT3 signaling. Undifferentiated state mESCs expressed LacdiNAc at a higher level than differentiated state cells. Knockdown of b4GalNAc-T3 reduced LacdiNAc expression and caused a decrease in LIF/STAT3 signaling that lessened the rate of self-renewal of mESCs. A biochemical analysis showed that LacdiNAc expression on LIF receptor (LIFR) and gp130 was required for the stable localization of the receptors with lipid raft/caveolar components, such as caveolin-1. This localization is required for transduction of a sufficiently strong LIF/STAT3 signal. In primed state pluripotent stem cells, such as hiPSCs and mEpiSC-like cells produced from mESCs, LacdiNAc expression on LIFR and gp130 was extremely weak and the level of localization of these receptors on rafts/caveolae was also low. Furthermore, knockdown of b4GalNAc-T3 decreased LacdiNAc expression and reduced the efficiency of reversion of primed state mEpiSC-like cells into naïve state mESCs. These findings show that the different LIF-responsiveness of naïve state (mESCs) and primed state (mEpiSCs, hESCs, and hiPSCs) cells is dependent on the expression of LacdiNAc on LIFR and gp130 and that this expression is required for the induction and maintenance of the naïve state. STEM CELLS 2011;29:641-650 Disclosure of potential conflicts of interest is found at the end of this article.

SVS-1/SUSD2 is a novel gene, which inhibits growth and reverses