[Molecular tumor board prostate cancer]

In modern oncology, molecular tumor boards are the interface between the public healthcare system and clinical research institutions. An interdisciplinary team of medical and scientific experts assesses if extensive molecular testing for tumor profiling is appropriate and discusses therapeutic options for patients with newly diagnosed treatable alterations. In the field of metastatic prostate cancer, patients especially with a strong family history, young age of diagnosis and those who have exhausted standard treatments may benefit from molecular profiling. Expression of the androgen receptor splice variant 7 (AR-V7) predicts nonresponse to next-generation AR-directed therapy like abiraterone or enzalutamide. Different blood tests for AR-V7 detection are now commercially available. Mutations in the DNA repair pathway are another frequent event in metastatic prostate cancer. Homologous recombination defects sensitize cancer cells to poly(ADP-ribose) polymerase (PARP) inhibitors. In the TOPARP-A trial, the PARP inhibitor olaparib led to high response rates (88%) in patients with mutated DNA repair genes. Furthermore, patients with DNA mismatch repair deficiency and/or microsatellite instability seem to benefit from PD-1 inhibitors, particularly pembrolizumab. At this time neither PARP inhibitors nor PD-1 inhibitors are approved for metastatic prostate cancer treatment in Germany. Therefore, a recommendation of a molecular tumor board for biomarker-matched off-label use of approved drugs across entity barriers will support coverage by health insurance.

Clinical experience with 100 consecutive patients treated with Lu-177-labeled PSMA-I&T radioligand therapy for metastatic castration-resistant prostate cancer

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Background: To report our clinical experience with 177Lutetium-labeled prostate-specific membrane antigen-ligand (177Lu-PSMA-I&T) for systemic radioligand therapy in 100 consecutive patients with metastatic castration-resistant prostate cancer (mCRPC). Methods: All patients were treated under a review board-approved compassionate use protocol. Eligibility criteria for 177Lu-PSMA-I&T therapy included previous treatment with abiraterone or enzalutamide, previous taxane-based chemotherapy or unsuitability for taxanes as well as positive 68Ga-PSMA tracer uptake of metastases in a prior PET-scan. Intravenous treatment with 177Lu-PSMA-I&T was given 6- to 8-weekly with an activity of 7.4GBq up to 6 cycles in patients without clinical or radiographic progression. We report prostate-specific antigen (PSA) decline, PSA progression-free survival (PSA-PFS), clinical progression-free survival (cPFS) and overall survival (OS) as well as toxicity. Results: Median age was 72 years (range 46-85) and median PSA level was 164 ng/ml (range 0-6178). Bone, lymph node and visceral metastases were present in 94%, 85% and 33% of patients, respectively. The median number of previous treatment regimens for mCRPC was 3 (range 1-6) and 84% of patients were pretreated with chemotherapy. At the time of evaluation, 286 cycles with 177Lu-PSMA-I&T were applied (median 2 cycles per patient, range 1-6), while treatment was still ongoing in 27% of patients. Overall, 4 and 6 cycles were applied in 33 and 15 patients, respectively. PSA decline ≥30%, ≥50% and ≥90% was achieved in 40%, 32% and 9% of patients, respectively. Median PSA-PFS was 3.4 months (95%CI 2.7-4.0), median cPFS was 4.1 months (95%CI 2.5-5.7) and median OS was 12.2 months (95%CI 8.8-15.7). Treatment-emergent hematologic grade 3/4 toxicities were anemia in 7%, thrombocytopenia in 5% and neutropenia in 4% of patients. Grade 3/4-non-hematologic toxicities were not observed. The main non-hematologic grade 1/2 toxicities were dry mouth in 18%, fatigue in 16% and loss of appetite in 9/% of patients. Conclusions: Radioligand therapy with 177Lu-PSMA I&T appears to be safe and active in late-stage mCRPC.

Oncological and postoperative outcome of salvage PSMA-radioguided surgery in recurrent prostate cancer

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Background: Positron-emission tomography (PET) directed against PSMA allows detection of even small metastatic prostate cancer (PC) lesions at low PSA values. In a subset of patients (pts) with recurrent PC salvage surgery might be beneficial. To facilitate removal during salvage surgery, we recently introduced PSMA-targeted radioguided surgery (PSMA-RGS). Methods: 121 consecutive patients with recurrent PC (PSA median: 1.13 ng/ml, range: 0.00 – 13.90 ng/ml; 9 pts under androgen-deprivation therapy) and soft-tissue lesions on 68Ga-PSMA PET after radical prostatectomy underwent 111In- or 99mTc-based PSMA-RGS between April 2014 and May 2017. The rate of complete biochemical response (cBR; PSA < 0.2ng/ml) was determined 6-16 weeks following PSMA-RGS. Biochemical recurrence-free survival (bRFS), PC-specific treatment-free survival and postoperative complications were evaluated. Results: Metastatic soft-tissue lesions from PC metastases could be removed in 120/121 pts (99.2%). One patient died six days postoperatively from a pulmonary embolism, five patients were lost to follow-up. Eleven pts suffered from Clavien grade III complications within 90d from surgery. In 75 out of 115 (65.2%) pts cBR was achieved. cBR was more likely in patients with a preoperative PSA level < 1.13 (76.3% vs. 52.7%) or a single anatomical location of recurrence (71.9% vs. 58.8%). Median bRFS was 5.1 months. At the time of analysis, 15 pts exhibited an ongoing cBR for at least 12 months (range: 12-32 months). A significantly longer bRFS was achieved in pts with a preoperative PSA < 1.13 ng/ml (median 14.9 vs 3.2 months, p = 0.02). In pts with a single compared to multiple location of recurrence we observed a trend towards a longer median bRFS (8.2 months vs. 3.5 months, p = 0.08). After one year of follow-up, 61.6 % of pts did not receive any further PC-directed treatment. Conclusions: PSMA-RGS is a promising tool to enhance intraoperative detection of metastatic lesions in PC with an acceptable complication rate. It leads to a high number of biochemical response with substantial duration in a subset of pts. Our data showed that bRFS was highest in patients with a low preoperative PSA and a single anatomical site of recurrence.

[PSMA-targeted radioligand therapy in prostate cancer]

Radioligand therapy (RLT) directed against prostate-specific membrane antigen (PSMA) enables tumor-specific treatment directed against PSMA-overexpressing prostate cancer cells. Several PSMA ligands such as PSMA-617 or PSMA-I&T have been developed that can be labeled with β‑radiating lutetium-177. These are currently applied in compassionate use programs to treat metastatic castration-resistant prostate cancer (mCRPC). PSMA-directed RLT is currently being offered in several nuclear medicine departments throughout Germany. Several retrospective case series demonstrate its activity with a prostate-specific antigen (PSA) decrease >50% in 30-60% of mCRPC patients. The toxicity seems to be low. Hematologic grade 4 toxicity has not been observed and grade 3 toxicities rarely occur. The main nonhematologic adverse events are intermittent dry mouth because of unspecific PSMA expression in the salivary glands as well as fatigue and nausea. Currently there are no prospective studies available for evaluation of PSMA-targeted RLT and a survival benefit over approved standard therapies such as abiraterone, enzalutamide, radium-223-dichloride, docetaxel or cabazitaxel has not been shown. PSMA-targeted RLT should therefore currently only be offered after critical evaluation in patients who exhausted the approved standard therapies.

A robust blood gene expression-based prognostic model for castration-resistant prostate cancer

BACKGROUND

Castration-resistant prostate cancer (CRPC) is associated with wide variations in survival. Recent studies of whole blood mRNA expression-based biomarkers strongly predicted survival but the genes used in these biomarker models were non-overlapping and their relationship was unknown. We developed a biomarker model for CRPC that is robust, but also captures underlying biological processes that drive prostate cancer lethality.

METHODS

Using three independent cohorts of CRPC patients, we developed an integrative genomic approach for understanding the biological processes underlying genes associated with cancer progression, constructed a novel four-gene model that captured these changes, and compared the performance of the new model with existing gene models and other clinical parameters.

RESULTS

Our analysis revealed striking patterns of myeloid- and lymphoid-specific distribution of genes that were differentially expressed in whole blood mRNA profiles: up-regulated genes in patients with worse survival were overexpressed in myeloid cells, whereas down-regulated genes were noted in lymphocytes. A resulting novel four-gene model showed significant prognostic power independent of known clinical predictors in two independent datasets totaling 90 patients with CRPC, and was superior to the two existing gene models.

CONCLUSIONS

Whole blood mRNA profiling provides clinically relevant information in patients with CRPC. Integrative genomic analysis revealed patterns of differential mRNA expression with changes in gene expression in immune cell components which robustly predicted the survival of CRPC patients. The next step would be validation in a cohort of suitable size to quantify the prognostic improvement by the gene score upon the standard set of clinical parameters.

Influence of prostate cancer disease state and therapeutic selection on peripheral whole-blood RNA signature

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Background: Prostate cancer is a heterogeneous disease with differences in tumor stromal interactions contributing to variability in treatment response and outcome. Gene expression of peripheral blood cells is altered by interactions with neoplastic tissue. We previously developed a peripheral whole blood six-gene signature prognostic for survival in mCRPC. Here we evaluate how different clinical disease states and treatment with different therapeutic agents impact this signature. Methods: Whole blood was collected in PAXgene Blood RNA tubes in two cohorts of prostate cancer patients, one at Mount Sinai (n=135), the other in Munich (n=59), in the context of prospective clinical studies. Whole blood RNA was extracted and the six target genes were amplified using qPCR. Scores were derived using normalized cycle threshold (ΔCT) values of the six genes, according to the model: 2 x ABL2 + SEMA4D + ITGAL – C1QA – TIMP1 – CDKN1A. Patients were categorized by disease state in the Mount Sinai cohort, and by treatment received in the Munich cohort, for data analysis. Results: CRPC is the only disease state with a mean six-gene score (18.06) above the high-risk cutoff (17.9), and is significantly higher than localized or hormone sensitive advanced disease (16.07, 16.52, respectively; p=0.0002). Among patients with localized disease, there was no significant difference in the mean six-gene scores for patients with low-, intermediate-, and high-risk disease (16.07, 15.33, 16.66, respectively; p=0.27). In CRPC patients treated with docetaxel, there are significant changes to the six-gene score over the course of treatment (p=0.002), with a notable percentage decrease (-6.2%) at the 2-8 week timepoint that is not observed in patients treated with abiraterone or enzalutamide. Conclusions: Gene expression profiling of whole blood is influenced by the clinical state of prostate cancer as seen by differences to the six-gene score from localized to castrate resistant disease. Cytotoxic chemotherapy appears to modulate the six-gene score, something not seen with AR-directed therapies. Further investigation will be needed to understand the significance of these changes.

Circulating tumor cells versus objective response assessment predicting survival in metastatic castration-resistant prostate cancer patients treated with docetaxel chemotherapy

PURPOSE

Circulating tumor cell (CTC) counts might display a superior prognostic value for overall survival (OS) compared to objective response criteria (OR) in metastatic castration-resistant prostate cancer (mCRPC) patients.

METHODS

CTCs were detected using the CellSearch™ System out of 122 samples during docetaxel chemotherapy (75 mg/m(2)) at baseline (q0) and after 1 (q1), 4 (q4) and 10 (q10) cycles, in mCRPC patients (n = 33). OR was evaluated by morphologic RECIST and clinical criteria after 4 (q4) and 10 (q10) cycles.

RESULTS

For OS, analyses revealed a significant prognostic value for categorical (<5 vs. ≥5) CTC counts (q0, p = 0.005; q1, p = 0.001; q4, p < 0.001; q10, p = 0.002), RECIST (q4, p < 0.001; q10, p = 0.02) and clinical criteria (q4, p < 0.001; q10, p = 0.02). Concordance of CTC counts with OR revealed a sensitivity of 83.3-87.5 % and a specificity of 68.0-76.5 % with complementary discriminatory power for OS. Comparing CTC counts with concomitant OR at q4 in multivariate analyses, an independent prognostic value for OS was found for CTC counts (HR 3.3; p = 0.02) similar to clinical (HR 4.9; p = 0.02) and radiologic response (HR 3.4; p = 0.051). Comparing the predictive value for death, early post-treatment CTC counts at q1 demonstrated significant accuracy with an area under the curve of 79.5 % (p = 0.004) similar to CTC counts at q4 (76.7 %; p = 0.009). Radiologic and clinical response at q4 displayed accuracy similar to early CTC counts at q1 (72.2 %; p = 0.03 and 75.0 %; p = 0.02) despite low sensitivities.

CONCLUSIONS

CTC counts appear to be an earlier and more sensitive predictor for survival and treatment response than current OR approaches and may provide complementary information toward individualized treatment strategies.

Mutant PIK3CA controls DUSP1-dependent ERK 1/2 activity to confer response to AKT target therapy

Background:

Alterations in the phosphoinositide 3-kinase/AKT/mammalian target of rapamycin (PI3K/AKT/mTOR) signalling pathway are frequent in urothelial bladder cancer (BLCA) and thus provide a potential target for novel therapeutic strategies. We investigated the efficacy of the AKT inhibitor MK-2206 in BLCA and the molecular determinants that predict therapy response.

Methods:

Biochemical and functional effects of the AKT inhibitor MK-2206 were analysed on a panel of 11 BLCA cell lines possessing different genetic alterations. Cell viability (CellTiter-Blue, cell counts), apoptosis (caspase 3/7 activity) and cell cycle progression (EdU incorporation) were analysed to determine effects on cell growth and proliferation. cDNA or siRNA transfections were used to manipulate the expression of specific proteins such as wild-type or mutant PIK3CA, DUSP1 or CREB. For in vivo analysis, the chicken chorioallantoic membrane model was utilised and tumours were characterised by weight and biochemically for the expression of Ki-67 and AKT phosphorylation.

Results:

Treatment with MK-2206 suppressed AKT and S6K1 but not 4E-BP1 phosphorylation in all cell lines. Functionally, only cell lines bearing mutations in the hotspot helical domain of PIK3CA were sensitive to the drug, independent of other genetic alterations in the PI3K or MAPK signalling pathway. Following MK-2206 treatment, the presence of mutant PIK3CA resulted in an increase in DUSP1 expression that induced a decrease in ERK 1/2 phosphorylation. Manipulating the expression of mutant or wild-type PIK3CA or DUSP1 confirmed that this mechanism is responsible for the induction of apoptosis and the inhibition of tumour proliferation in vitro and in vivo, to sensitise cells to AKT target therapy.

Conclusion or interpretation:

PIK3CA mutations confer sensitivity to AKT target therapy in BLCA by regulating DUSP1 expression and subsequent ERK1/2 dephosphorylation and can potentially serve as a stratifying biomarker for treatment.

[Compassionate use of abiraterone and cabazitaxel: first experiences in docetaxel-pretreated castration-resistant prostate cancer patients]

OBJECTIVE

First clinical experiences with abiraterone and cabazitaxel for the treatment of metastatic castration-resistant prostate cancer patients following docetaxel chemotherapy are reported.

PATIENTS AND METHODS

We describe PSA response rates and disease control rates determined by imaging studies at 3 months as well as side effects in the daily routine. All patients were treated within the"compassionate use" programs of cabazitaxel and abiraterone or treated according to their inclusion and exclusion criteria at the "Technische Universität München".

RESULTS

Of 54 patients, 15 were treated with cabazitaxel and 39 with abiraterone. In patients treated with cabazitaxel, after 3 months of therapy the PSA reduction rate > 50% was 46.2%, the PSA progression rate was 15.4%, and the disease control rate was 83.3%. Main grade 3/4 hematotoxicities were neutropenia (40%) and anemia (20%). Febrile neutropenia was observed in 2 of 15 (13.3%) patients. Main non-hematological grade 3/4 toxicities were diarrhea (13.3%) and polyneuropathy (13.3%). In patients treated with abiraterone, after 3 months of therapy the PSA reduction rate >50% was 35.1%, the PSA progression rate was 46.0%, and the disease control rate was 47.1%. Main grade 3/4 hematotoxicities were anemia (5.1%) and thrombocytopenia (5.1%). Main non-hematological toxicities were fatigue (20.5%), sweating (17.9%), and constipation (10.3%).

CONCLUSION

Utilization of cabazitaxel and abiraterone in the daily routine show response rates comparable to their approval studies with acceptable side effects.

Prediction of favorable outcome in a docetaxel rechallenge setting in metastatic castration-resistant prostate cancer

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Background: To identify predictors of favourable oncological outcome in metastatic castration-resistant prostate cancer (mCRPC) patients who are treated with docetaxel rechallenge following first-line chemotherapy with docetaxel.

Methods: We retrospectively evaluated the oncological outcome of mCRPC patients who were treated with 3-weekly docetaxel (75mg/m2) at first-line chemotherapy and rechallenge plus prednisone/ prednisolone. The endpoints of oncological outcome were PSA-progression-free survival (PSA-PFS) and overall survival (OS) after initiation of docetaxel rechallenge. The effect of clinical variables on PSA-PFS and OS was statistically analysed by a log-rank test or Cox regression with hazard ratios. All analyses were performed using a 0.05 level of significance.

Results: 47 patients were included on analysis. At a median follow-up of 25.8 months (range 9.8-89.8 months) after the first administration of docetaxel, 27 (57.4%) patients had died. Median PSA-PFS was 5.9 months (95% CI 3.5-6.8 months) and median OS was 21.4 months (95% CI 18.9-23.9 months) after initiation of docetaxel rechallenge. PSA-reduction ≥ 30% was the only pre-treatment variable that correlated significantly with prolonged PSA-PFS (p=0.03) and OS (p=0.002). Patients with PSA-reduction ≥ 30% at first-line chemotherapy showed a median OS of 21.8 months since initiation of docetaxel rechallenge in comparison to 4.5 months in patients with < 30% PSA-reduction.

Conclusions: Docetaxel rechallenge represents an active treatment option in selected docetaxel-pretreated patients with mCRPC. In this retrospective study, PSA-reduction ≥ 30% at first-line chemotherapy with docetaxel predicted superior PSA-PFS and OS in the rechallenge setting and might, therefore, present a rational indication for docetaxel rechallenge.

Loss of cell cycle checkpoint control in Drosophila Rfc4 mutants

Two alleles of the Drosophila melanogaster Rfc4 (DmRfc4) gene, which encodes subunit 4 of the replication factor C (RFC) complex, cause striking defects in mitotic chromosome cohesion and condensation. These mutations produce larval phenotypes consistent with a role in DNA replication but also result in mitotic chromosomal defects appearing either as premature chromosome condensation-like or precocious sister chromatid separation figures. Though the DmRFC4 protein localizes to all replicating nuclei, it is dispersed from chromatin in mitosis. Thus the mitotic defects appear not to be the result of a direct role for RFC4 in chromosome structure. We also show that the mitotic defects in these two DmRfc4 alleles are the result of aberrant checkpoint control in response to DNA replication inhibition or damage to chromosomes. Not all surveillance function is compromised in these mutants, as the kinetochore attachment checkpoint is operative. Intriguingly, metaphase delay is frequently observed with the more severe of the two alleles, indicating that subsequent chromosome segregation may be inhibited. This is the first demonstration that subunit 4 of RFC functions in checkpoint control in any organism, and our findings additionally emphasize the conserved nature of RFC's involvement in checkpoint control in multicellular eukaryotes.

A role for Drosophila SMC4 in the resolution of sister chromatids in mitosis

BACKGROUND

Faithful segregation of the genome during mitosis requires interphase chromatin to be condensed into well-defined chromosomes. Chromosome condensation involves a multiprotein complex known as condensin that associates with chromatin early in prophase. Until now, genetic analysis of SMC subunits of the condensin complex in higher eukaryotic cells has not been performed, and consequently the detailed contribution of different subunits to the formation of mitotic chromosome morphology is poorly understood.

RESULTS

We show that the SMC4 subunit of condensin is encoded by the essential gluon locus in Drosophila. DmSMC4 contains all the conserved domains present in other members of the structural-maintenance-of-chromosomes protein family. DmSMC4 is both nuclear and cytoplasmic during interphase, concentrates on chromatin during prophase, and localizes to the axial chromosome core at metaphase and anaphase. During decondensation in telophase, most of the DmSMC4 leaves the chromosomes. An examination of gluon mutations indicates that SMC4 is required for chromosome condensation and segregation during different developmental stages. A detailed analysis of mitotic chromosome structure in mutant cells indicates that although the longitudinal axis can be shortened normally, sister chromatid resolution is strikingly disrupted. This phenotype then leads to severe chromosome segregation defects, chromosome breakage, and apoptosis.

CONCLUSIONS

Our results demonstrate that SMC4 is critically important for the resolution of sister chromatids during mitosis prior to anaphase onset.

The Drosophila RAD21 cohesin persists at the centromere region in mitosis

'Cohesin' is a highly conserved multiprotein complex thought to be the primary effector of sister-chromatid cohesion in all eukaryotes. Cohesin complexes in budding yeast hold sister chromatids together from S phase until anaphase, but in metazoans, cohesin proteins dissociate from chromosomes and redistribute into the whole cell volume during prophase, well before sister chromatids separate (reviewed in [1,2]). Here we address this apparent anomaly by investigating the cell-cycle dynamics of DRAD21, the Drosophila orthologue of the Xenopus XRAD21 and Saccharomyces cerevisiae Scc1p/Mcd1p cohesins [3]. Analysis of DRAD21 in S2 Drosophila tissue culture cells and live embryos expressing a DRAD21-green fluorescent protein (GFP) fusion revealed the presence of four distinct subcellular pools of DRAD21: a cytoplasmic pool; a chromosome-associated pool which dissociates from chromatin as chromosomes condense in prophase; a short-lived centrosome-associated pool present during metaphase-anaphase; and a centromere-proximal pool which remains bound to condensed chromosomes, is found along the junction of sister chromatids between kinetochores, and persists until the metaphase-anaphase transition. We conclude that in Drosophila, and possibly all metazoans, a minor pool of cohesin remains bound to centromere-proximal chromatin after prophase and maintains sister-chromatid cohesion until the metaphase-anaphase transition.

Review: SMCs in the world of chromosome biology- from prokaryotes to higher eukaryotes

The study of higher order chromosome structure and how it is modified through the course of the cell cycle has fascinated geneticists, biochemists, and cell biologists for decades. The results from many diverse technical avenues have converged in the discovery of a large superfamily of chromosome-associated proteins known as SMCs, for structural maintenance of chromosomes, which are predicted to have ATPase activity. Now found in all eukaryotes examined, and numerous prokaryotes as well, SMCs play crucial roles in chromatid cohesion, chromosome condensation, sex chromosome dosage compensation, and DNA recombination repair. In eukaryotes, SMCs exist in five subfamilies, which appear to associate with one another in particular pairs to perform their specific functions. In this review, we summarize current progress examining the roles these proteins, and the complexes they form, play in chromosome metabolism. We also present a twist in the SMC story, with the possibility of one SMC moonlighting in an unpredicted location.

Dr. Dolittle and the making of the mitotic spindle

The intrinsic polarity of microtubules within cells is exploited each time cells divide. Kinesins, microtubule-associated motor proteins, are required to execute the dramatic events of mitosis: bipolar spindle assembly, metaphase chromosome alignment, anaphase chromosome segregation, and separation of spindle poles prior to cytokinesis. Surprisingly, kinesin-related proteins have been found to move in either "plus-ward" or "minus-ward" directions along microtubules. Evidence from genetic analyses of simple eukaryotes and in vitro activity assays supports the notion that certain subfamilies of kinesin-related proteins provide antagonistic activities necessary to balance mitotic forces. A recent study by Sharp et al.((1)) sheds further light on the subject by exploiting the genetics and cytology of the fruit fly embryo.

The kinesin-like protein KLP61F is essential for mitosis in Drosophila

We report here that disruption of a recently discovered kinesin-like protein in Drosophila melanogaster, KLP61F, results in a mitotic mutation lethal to the organism. We show that in the absence of KLP61F function, spindle poles fail to separate, resulting in the formation of monopolar mitotic spindles. The resulting phenotype of metaphase arrest with polyploid cells is reminiscent of that seen in the fungal bimC and cut7 mutations, where it has also been shown that spindle pole bodies are not segregated. KLP61F is specifically expressed in proliferating tissues during embryonic and larval development, consistent with a primary role in cell division. The structural and functional homology of the KLP61F, bimC, cut7, and Eg5 kinesin-like proteins demonstrates the existence of a conserved family of kinesin-like molecules important for spindle pole separation and mitotic spindle dynamics.

Multiple replication origins are used during Drosophila chorion gene amplification

DNA from Drosophila egg chambers undergoing chorion gene amplification was analyzed using the two-dimensional gel technique of Brewer and Fangman. At stage 10, 34% of DNA molecules from the maximally amplified region of the third chromosome chorion gene cluster contained replication forks or bubbles. These nonlinear forms were intermediates in the process of amplification; they were confined to follicle cells, and were found only within the replicating region during the time of amplification. Multiple origins gave rise to these intermediates, since three separate regions of the third chromosome chorion locus contained replication bubbles. However, initiation was nonrandom; the majority of initiations appeared to occur near the Bgl II site located between the s18 and s15 chorion genes. The P[S6.9] chorion transposon also contained abundant replication intermediates in follicle cells from a transformed line. Initiation within P[S6.9] occurred near two previously defined cis-regulatory elements, one near the same Bgl II site (in the AER-d region) and one near the ACE3 element.

In vivo phosphorylation of the 170-kDa form of eukaryotic DNA topoisomerase II. Cell cycle analysis

We have examined the level of incorporation of 32P into DNA topoisomerase II in vivo in chicken lymphoblastoid cells that were fractionated into the various cell cycle phases by centrifugal elutriation. We find that topoisomerase II is phosphorylated in vivo, with the level of incorporation being approximately 3.5-fold higher in the G2 + M fraction than earlier in the cell cycle. Our antibody studies have revealed that topoisomerase II antigen exists as a number of discrete polypeptide species in these cells. Of these, the 170-kDa intact polypeptide is phosphorylated approximately 4.5-fold more than several antigenic fragments that actually comprise the bulk of the topoisomerase II antigen in these cells at mitosis. Phosphorylation of the 170-kDa form of the enzyme may be involved in activation of the enzyme for its role in the disjunction of sister chromatids at anaphase.

Human autoantibody to topoisomerase II

The rheumatic diseases are characterized by the production of autoantibodies that are usually directed against components of the cell nucleus. In this communication, we describe autoantibodies that recognize DNA topoisomerase II (anti-topoII) present in the serum of a patient with systemic lupus erythematosus. Several lines of evidence indicate that this antibody recognizes topoisomerase II. First, it binds to the native enzyme in soluble extracts prepared from isolated chromosomes and effectively depletes such extracts of active enzyme. Second, the serum binds to topoisomerase II in immunoblots of mitotic chromosomes and chromosome scaffolds. Finally, the antiserum binds strongly to a fusion protein encoded by a cloned cDNA and expressed in Escherichia coli that (based on immunological evidence) represents the carboxy-terminal portion of chicken topoisomerase II. Autoantibodies such as the one described here may provide useful reagents for the study of human topoisomerase II.

Differential expression of DNA topoisomerases I and II during the eukaryotic cell cycle

We have utilized antibody probes to examine the expression of DNA topoisomerases I and II and chromosome scaffold protein Sc-2 in normal and transformed cells. Neither topoisomerase I nor Sc-2 shows significant fluctuations in content or stability across the cell cycle. In contrast, topoisomerase II undergoes significant cell cycle-dependent alterations in both amount and stability. As cells progress from mitosis into G1, much of the topoisomerase II is degraded. During the first 2 hr of G1, the half life of topoisomerase II is decreased from that measured in asynchronous cell populations by a factor of 7. This suggests that the chromosome condensation/decondensation cycle is coupled to a parallel cycle of synthesis and degradation of topoisomerase II. In control experiments, we also found that the half-life of topoisomerase II is shorter in normal cells than in transformed cells by a factor of 4. Since the number of copies of topoisomerase II per cell is also lower in normal cells, this suggests that control of topoisomerase II stability is altered upon transformation. The stability of topoisomerase I and Sc-2 does not differ significantly between normal and transformed cells.

The inner centromere protein (INCENP) antigens: movement from inner centromere to midbody during mitosis

We describe a novel set of polypeptide antigens that shows a dramatic change in structural localization during mitosis. Through metaphase these antigens define a new chromosomal substructure that is located between the sister chromatids. Because the antigens are concentrated in the pericentromeric region, we have provisionally termed them the INCENPs (inner centromere proteins). The INCENPs (two polypeptides of 155 and 135 kD) were identified with a monoclonal antibody that was raised against the bulk proteins of the mitotic chromosome scaffold fraction. These two polypeptides are the most tightly bound chromosomal proteins known. When scaffolds are prepared, 100% of the detectable INCENPs remain scaffold associated. We were therefore unprepared for the fate of the INCENPs at anaphase. As the sister chromatids separate, the INCENPs dissociate fully from them, remaining behind at the metaphase plate as the chromatids migrate to the spindle poles. During anaphase the INCENPs are found on coarse fibers in the central spindle, and also in close apposition to the cell membrane in the region of the forming contractile ring. During telophase, the INCENPs gradually become focused onto the forming midbody, together with which they are ultimately discarded. Several possible in vivo roles for the INCENPs are suggested by these data: regulation of sister chromatid pairing, stabilization of the plane of cleavage, and separation of spindle poles at anaphase.

Topoisomerase II: A specific marker for cell proliferation

We have used an antibody probe to measure the levels of topoisomerase II in several transformed and developmentally regulated normal cell types. Transformed cells contain roughly 1 X 10(6) copies of the enzyme. During erythropoiesis in chicken embryos the enzyme level drops from 7.8 X 10(4) copies per erythroblast to less than 300 copies per erythrocyte concomitant with the cessation of mitosis in the blood. Cultured myoblasts also lose topoisomerase II upon fusion into nonproliferating myotubes. When peripheral blood lymphocytes (which lack detectable topoisomerase II) commence proliferation, they express topoisomerase II de novo. Appearance of the enzyme exactly parallels the onset of DNA replication. These results suggest that topoisomerase II is not required for transcription in higher eukaryotes, but that it may function during DNA replication. Furthermore, topoisomerase II is a sensitive and specific marker for proliferating cells.

Topoisomerase II is a structural component of mitotic chromosome scaffolds

We have obtained a polyclonal antibody that recognizes a major polypeptide component of chicken mitotic chromosome scaffolds. This polypeptide migrates in SDS PAGE with Mr 170,000. Indirect immunofluorescence and subcellular fractionation experiments confirm that it is present in both mitotic chromosomes and interphase nuclei. Two lines of evidence suggest that this protein is DNA topoisomerase II, an abundant nuclear enzyme that controls DNA topological states: anti-scaffold antibody inhibits the strand-passing activity of DNA topoisomerase II; and both anti-scaffold antibody and an independent antibody raised against purified bovine topoisomerase II recognize identical partial proteolysis fragments of the 170,000-mol-wt scaffold protein in immunoblots. Our results suggest that topoisomerase II may be an enzyme that is also a structural protein of interphase nuclei and mitotic chromosomes.

Localization of topoisomerase II in mitotic chromosomes

In the preceding article we described a polyclonal antibody that recognizes cSc-1, a major polypeptide component of the chicken mitotic chromosome scaffold. This polypeptide was shown to be chicken topoisomerase II. In the experiments described in the present article we use indirect immunofluorescence and immunoelectron microscopy to examine the distribution of topoisomerase II within intact chromosomes. We also describe a simple experimental protocol that differentiates antigens that are interspersed along the chromatin fiber from those that occupy restricted domains within the chromosome. These experiments indicate that the distribution of the enzyme appears to be independent of the bulk chromatin. Our data suggest that topoisomerase II is bound to the bases of the radial loop domains of mitotic chromosomes.