ACKNOWLEDGEMENTSWe thank M. Nomura, T. Kobayashi, S. Roeder and S. Liebman for strains and plasmids; Micheal Lichten and A. Hinnebusch for critical reading of the manuscript; K. Bloom and A. Arnaoutov for time-lapse microscopy advice; R. Mehta and J. Panebianco for technical help. This research was supported by the Intramural Research Program of the NIH at the NICHD and at the Center for Cancer Research, NCI. NOTESupplemental information can be found at: http://www.landesbioscience.com/journals/cc/ supplement/wangCC5-19-sup.pdf. http://www.landesbioscience.com/journals/cc/ supplement/wangCC5-16-sup_2A.mov. http://www.landesbioscience.com/journals/cc/ supplement/wangCC5-16-sup_2B.mov. Report Condensin Function in Mitotic Nucleolar Segregation is Regulated by rDNA Transcription ABSTRACTChromosome condensation is established and maintained by the condensin complex. The mechanisms governing loading of condensin onto specific chromosomal sites remain unknown. To elucidate the molecular pathways that determine condensin binding to the nucleolar organizer, a key condensin binding site, we analyzed the properties of condensin-bound sites within the rDNA repeat in budding yeast and demonstrated that the bulk of mitotic condensin binding to rDNA is reduced or eliminated when Pol I transcription is elevated. Conversely, when Pol I transcription is repressed either by rapamycin treatment or by promoter shut-off, condensin binding to rDNA is increased. This novel potential role for constitutive and/or periodic repression of Pol I transcription in rDNA condensin loading is an important factor in determining the segregation proficiency of NOR-containing chromosomes.

The condensin complex is essential for sister chromatid segregation in eukaryotic mitosis. Nevertheless, in budding yeast, condensin mutations result in massive mis-segregation of chromosomes containing the nucleolar organizer, while other chromosomes, which also contain condensin binding sites, remain genetically stable. To investigate this phenomenon we analyzed the mechanism of the cell-cycle arrest elicited by condensin mutations. Under restrictive conditions, the majority of condensin-deficient cells arrest in metaphase. This metaphase arrest is mediated by the spindle checkpoint, particularly by the spindle-kinetochore tension-controlling pathway. Inactivation of the spindle checkpoint in condensin mutants resulted in frequent chromosome non-disjunction, eliminating the bias in chromosome mis-segregation towards rDNA-containing chromosomes. The spindle tension defect in condensin-impaired cells is likely mediated by structural defects in centromere chromatin reflected by the partial loss of the centromere histone Cse4p. These findings show that, in addition to its essential role in rDNA segregation, condensin mediates segregation of the whole genome by maintaining the centromere structure in Saccharomyces cerevisiae .

Introduction. Overall survival (OS) of patients in chronic phase (CP) chronic myeloid leukemia (CML) dramatically increased in the era of tyrosine kinase inhibitors (TKI). Meanwhile nearly half of patients discontinue 1-st line Imatinib due to resistance or intolerance. Half of them subsequently failure treatment with second line TKI. It seems that 20-25% of CP CML patients need 3-d line therapy (TKI-3l). There are a few reports regarding durable outcome of TKI-3l. Materials and Methods. In our retrospective study 53 patients (20 male, 33 female) with CML CP treated either by Nilotinib 400 mg BID (n=18), Dasatinib 100 mg QD (n=33) or Bosutinib 500 mg QD (n=5) as TKI-3l were included. The median age at the time of diagnosis was 46 years (23-88 years). The main reason for previous TKIs discontinuation was resistance: 48/53 (91%) had failure of one and 42/53 (79%) patients had failure of both previous TKIs treatment. Median CML duration before TKI-3l was 55 months (2-314 months). Before TKI-3l mutation analysis was performed in 35 patients: 18 mutations were revealed in 16 (46%) patients including T315I mutation in 3 cases. At the moment of 3-d line TKIs therapy initiation, all patients were in CP and 43/53 (81%) had at least complete hematologic response (CHR), 8/53 (15%) patients had major cytogenetic response (MCyR) including 1 patient with complete cytogenetic response (CCyR). Results. At the time of analysis, the median duration of TKI-3l therapy was 21 months (1-67 months). No additional patients achieved CHR, but during observational time CHR was maintained nearly in all 40/43 (93%) patients with CHR at baseline. New cases of MCyR and CCyR were observed in 15/45(33%) and 11/52(21%) of patients, respectively. Median time to MCyR and CCyR was 3.4 (3-8) and 5.2 (3-13) months, respectively. Median duration of MCyR and CCyR was 9.3 (1-43) months and 4.5 (3-6) months, respectively. Patients with resistant mutations did not obtain any cytogenetic response. TKI-3l treatment was discontinued in 21 patients. Intolerance was the reason of treatment discontinuation in 5/53(10%) cases. Progression to accelerated or blastic phases during therapy or after discontinuation occurred in 8/53 (15%) patients. Median time to progression was 14.7 months (1-46 months). There were 13 deaths in overall group of 53 patients. Two-year OS in TKI-3l was 67%. All patients with MCyR were alive and preserved CP phase. Concluson: Our results showed that 20% of patients might obtain at least CCyR and benefit with TKIs as third line. Therefore, after two TKI lines patients, who are not eligible for allogeneic transplantation and without resistant mutations should be treated with one more line of TKI therapy. Disclosures Lomaia: Novartis: Consultancy. Zaritskey:University of Heidelberg: Research Funding; Novartis: Consultancy.

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