SOX11 expression is restricted to EBV-negative Burkitt lymphoma and associates with molecular genetic features

SRY-related HMG-box gene 11 (SOX11) is a transcription factor overexpressed in mantle cell lymphoma (MCL), a subset of Burkitt lymphomas (BL) and precursor lymphoid cell neoplasms but is absent in normal B-cells and other B-cell lymphomas. SOX11 has an oncogenic role in MCL but its contribution to BL pathogenesis remains uncertain. Here, we observed that the presence of Epstein-Barr virus (EBV) and SOX11 expression were mutually exclusive in BL. SOX11 expression in EBV- BL was associated with an IG∷MYC translocation generated by aberrant class switch recombination, while in EBV-/SOX11- tumors the IG∷MYC translocation was mediated by mistaken somatic hypermutations. Interestingly, EBV- SOX11 expressing BL showed higher frequency of SMARCA4 and ID3 mutations compared to EBV-/SOX11- cases. By RNA-sequencing, we identified a SOX11-associated gene expression profile, with functional annotations showing partial overlap with the SOX11 transcriptional program of MCL. Contrary to MCL, no differences on cell migration or BCR signaling were found between SOX11- and SOX11+ BL cells. However, SOX11+ BL showed higher adhesion to VCAM-1 than SOX11- BL cell lines. Here we demonstrate that EBV- BL comprises two subsets of cases based on SOX11 expression. The mutual exclusion of SOX11 and EBV, and the association of SOX11 with a specific genetic landscape suggest a role of SOX11 in the early pathogenesis of BL.

SOX11/PRDX2 axis modulates redox homeostasis and chemoresistance in aggressive mantle cell lymphoma

Mantle cell lymphoma (MCL) is an incurable B-cell neoplasm characterized by an aggressive behavior, short responses to conventional therapies and SOX11 overexpression, which is associated with aggressive disease features and inferior clinical outcome of patients. Oxidative stress is known to induce tumorigenesis and tumor progression, whereas high expression levels of antioxidant genes have been associated with chemoresistance in different cancers. However, the role of oxidative stress in MCL pathogenesis and the involvement of SOX11 regulating redox homeostasis in MCL cells are largely unknown. Here, by integrating gene set enrichment analysis of two independent series of MCL, we observed that SOX11+ MCL had higher reactive oxygen species (ROS) levels compared to SOX11- MCL primary tumors and increased expression of Peredoxine2 (PRDX2), which upregulation significantly correlated with SOX11 overexpression, higher ROS production and worse overall survival of patients. SOX11 knockout (SOX11KO) significantly reduced PRDX2 expression, and SOX11KO and PRDX2 knockdown (PRDX2KD) had increased ROS levels and ROS-mediated tumor cell death upon treatment with drugs, compared to control MCL cell lines. Our results suggest an aberrant redox homeostasis associated with chemoresistance in aggressive MCL through SOX11-mediated PRDX2 upregulation, highlighting PRDX2 as promising target for new therapeutic strategies to overcome chemoresistance in aggressive MCLs.

SOX11 is a novel binding partner and endogenous inhibitor of SAMHD1 ara-CTPase activity in mantle cell lymphoma

The sterile alpha motif and histidine-aspartate (HD) domain containing protein 1 (SAMHD1) is a deoxynucleoside triphosphate triphosphohydrolase with ara-CTPase activity that confers cytarabine (ara-C) resistance in several haematological malignancies. Targeting SAMHD1's ara-CTPase activity has recently been demonstrated to enhance ara-C efficacy in acute myeloid leukemia. Here, we identify the transcription factor SRY-related HMG-box containing protein 11 (SOX11) as a novel direct binding partner and first known endogenous inhibitor of SAMHD1. SOX11 is aberrantly expressed not only in mantle cell lymphoma (MCL), but also in some Burkitt lymphomas. Co-immunoprecipitation of SOX11 followed by mass spectrometry in MCL cell lines identified SAMHD1 as the top SOX11 interaction partner which was validated by proximity ligation assay. In vitro, SAMHD1 bound to the HMG box of SOX11 with low-micromolar affinity. In situ crosslinking studies further indicated that SOX11-SAMHD1 binding resulted in a reduced tetramerization of SAMHD1. Functionally, expression of SOX11 inhibited SAMHD1 ara-CTPase activity in a dose-dependent manner resulting in ara-C sensitization in cell lines and in a SOX11-inducible mouse model of MCL. In SOX11-negative MCL, SOX11-mediated ara-CTPase inhibition could be mimicked by adding the recently identified SAMHD1 inhibitor hydroxyurea. Taken together, our results identify SOX11 as a novel SAMHD1 interaction partner and its first known endogenous inhibitor with potentially important implications for clinical therapy stratification.

Tumorigenic role of Musashi-2 in aggressive mantle cell lymphoma

SOX11 overexpression has been associated with aggressive behavior of mantle cell lymphomas (MCL). SOX11 is overexpressed in embryonic and cancer stem cells (CSC) of some tumors. Although CSC have been isolated from primary MCL, their relationship to SOX11 expression and contribution to MCL pathogenesis and clinical evolution remain unknown. Here, we observed enrichment in leukemic and hematopoietic stem cells gene signatures in SOX11+ compared to SOX11- MCL primary cases. Musashi-2 (MSI2) emerged as one of the most significant upregulated stem cell-related genes in SOX11+ MCLs. SOX11 is directly bound to the MSI2 promoter upregulating its expression in vitro. MSI2 intronic enhancers were strongly activated in SOX11+ MCL cell lines and primary cases. MSI2 upregulation was significantly associated with poor overall survival independently of other high-risk features of MCL. MSI2 knockdown decreased the expression of genes related to apoptosis and stem cell features and significantly reduced clonogenic growth, tumor cell survival and chemoresistance in MCL cells. MSI2-knockdown cells had reduced tumorigenic engraftment into mice bone marrow and spleen compared to control cells in xenotransplanted mouse models. Our results suggest that MSI2 might play a key role in sustaining stemness and tumor cell survival, representing a possible novel target for therapeutic interventions in MCL.

Structural Basis for the SUMO2 Isoform Specificity of SENP7

SUMO proteases or deSUMOylases regulate the lifetime of SUMO-conjugated targets in the cell by cleaving off the isopetidic bond between the substrate and the SUMO modifier, thus reversing the conjugation activity of the SUMO E3 ligases. In humans the deSUMOylating activity is mainly conducted by the SENP/ULP protease family, which is constituted of six members sharing a homologous catalytic globular domain. SENP6 and SENP7 are the most divergent members of the family and they show a unique SUMO2/3 isoform preference and a particular activity for dismantling polySUMO2 chains. Here, we present the crystal structure of the catalytic domain of human SENP7 bound to SUMO2, revealing structural key elements for the SUMO2 isoform specificity of SENP7. In particular, we describe the specific contacts between SUMO2 and a unique insertion in SENP7 (named Loop1) that is responsible for the SUMO2 isoform specificity. All the other interface contacts between SENP7 and SUMO2, including the SUMO2 C-terminal tail interaction, are conserved among members of the SENP/ULP family. Our data give insight into an evolutionary adaptation to restrict the deSUMOylating activity in SENP6 and SENP7 for the SUMO2/3 isoforms.

Abstract 777: Deciphering the role of MSI2 as a regulator of mantle cell lymphoma stem-like properties

Mantle cell lymphoma (MCL) is a mature B cell neoplasm with two distinct biological subtypes regarding clinical, molecular and pathological features. Conventional MCL (cMCL) subtype is characterized by high aggressiveness and poor outcome with frequent relapses after initial response to treatment, suggesting the presence of a population of cancer stem cells (CSC) able to self-renew and responsible for drug resistance. SOX11 embryonic transcription factor is aberrantly overexpressed mostly in cMCL and has an oncogenic role in MCL. MCL-CSC have been isolated from MCL cases by different groups. However, its relationship to SOX11 expression and contribution to MCL clinical features and aggressive behavior remains unknown. We hypothesized that SOX11 may promote stem cell-like properties through the activation of stem cell-related genes, leading to an aggressive and incurable tumor in MCL patients. Here, we integrated the differential gene expression profile (GEP) between SOX11+ and SOX11- MCL primary cases with stem cell-related genes, and specific SOX11 ChIP-chip and epigenetic data in MCL cell lines and primary cases, to identify potential mediators of CSC directly regulated by SOX11 in MCL. We observed an enrichment of hematopoietic (HSC) and leukemic stem cells (LSC) gene signatures in SOX11+ compared to SOX11- MCL primary cases. MSI2, an RNA binding protein that maintains self-renewal and prevents differentiation in HSC, emerged as one of the most significant upregulated CSC-related gene in SOX11+ compared to SOX11- MCLs, positively correlating with SOX11 expression in MCL primary cases. Our in vitro experiments showed that SOX11 binds to MSI2 regulatory regions increasing its expression in MCL cells. MSI2 enhancers located in intronic regions were activated in SOX11+ but not in SOX11- MCL cell lines and primary cases. MSI2 upregulation significantly associated with poor overall survival, independently of common risk factors (SOX11 expression, high copy number alterations, 17p/TP53, 11q/ATM and 9p/CDKN2A alterations), in MCL patients. MSI2 knockdown MCL cell lines had a gene expression profile enriched in proapoptotic-related genes; whereas HSC and LSC gene signatures were downregulated compared to its control cells. MSI2 knockdown or MSI2 inhibition with Ro 08-2750 treatment decreased clonogenic growth, tumor survival and chemoresistance in MCL cells. SOX11+/MSI2 high MCL primary leukemic samples showed higher percentage of ALDH+ cells compared to SOX11-/MSI2 low samples, which decreased after Ro 08-2750 treatment. Besides, MSI2 silencing delayed tumor growth in vivo in MCL xenograft mice models. Overall, our results suggest that SOX11+ MCL cases acquire stemness features through the upregulation of MSI2 expression, which promotes chemoresistance, self-renewal and tumor survival. MSI2 represents a novel biomarker for MCL-CSC and therapeutic target for relapsed MCL.

Citation Format: Marta Sureda-Gómez, Patricia Balsas, Marta Leonor Rodríguez, Ferran Nadeu, Anna De Bolòs, Álvaro Eguileor, Marta Kulis, Giancarlo Castellano, José Ignacio Martin-Subero, Santiago Demajo, Pedro Jares, Eva Giné, Elias Campo, Virginia Amador. Deciphering the role of MSI2 as a regulator of mantle cell lymphoma stem-like properties [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 777.

Impaired ribosome biogenesis checkpoint activation induces p53-dependent MCL-1 degradation and MYC-driven lymphoma death

MYC-driven B-cell lymphomas are addicted to increased levels of ribosome biogenesis (RiBi), offering the potential for therapeutic intervention. However, it is unclear whether inhibition of RiBi suppresses lymphomagenesis by decreasing translational capacity and/or by p53 activation mediated by the impaired RiBi checkpoint (IRBC). Here we generated Eμ-Myc lymphoma cells expressing inducible short hairpin RNAs to either ribosomal protein L7a (RPL7a) or RPL11, the latter an essential component of the IRBC. The loss of either protein reduced RiBi, protein synthesis, and cell proliferation to similar extents. However, only RPL7a depletion induced p53-mediated apoptosis through the selective proteasomal degradation of antiapoptotic MCL-1, indicating the critical role of the IRBC in this mechanism. Strikingly, low concentrations of the US Food and Drug Administration-approved anticancer RNA polymerase I inhibitor Actinomycin D (ActD) dramatically prolonged the survival of mice harboring Trp53+/+;Eμ-Myc but not Trp53-/-;Eμ-Myc lymphomas, which provides a rationale for treating MYC-driven B-cell lymphomas with ActD. Importantly, the molecular effects of ActD on Eμ-Myc cells were recapitulated in human B-cell lymphoma cell lines, highlighting the potential for ActD as a therapeutic avenue for p53 wild-type lymphoma.

Selective BTK inhibition improves bendamustine therapy response and normalizes immune effector functions in chronic lymphocytic leukemia

The Bruton's tyrosine kinase (BTK) inhibitor ibrutinib has been shown to be highly effective in patients with chronic lymphocytic leukemia (CLL) and is approved for CLL treatment. Unfortunately, resistance and intolerance to ibrutinib has been observed in several studies, opening the door for more specific BTK inhibitors. CC-292 (spebrutinib) is a BTK inhibitor with increased specificity for BTK and less inhibition of other kinases. Our in vitro studies showed that CC-292 potently inhibited B-cell receptor signaling, activation, proliferation and chemotaxis of CLL cells. In in vivo studies using the adoptive transfer TCL1 mouse model of CLL, CC-292 reduced tumor load and normalized tumor-associated expansion of T cells and monocytes, while not affecting T cell function. Importantly, the combination of CC-292 and bendamustine impaired CLL cell proliferation in vivo and enhanced the control of CLL progression. Our results demonstrate that CC-292 is a specific BTK inhibitor with promising performance in combination with bendamustine in CLL. Further clinical trials are warranted to investigate the therapeutic efficacy of this combination regimen.

A quaternary tetramer assembly inhibits the deubiquitinating activity of USP25

USP25 deubiquitinating enzyme is a key member of the ubiquitin system, which acts as a positive regulator of the Wnt/β-catenin signaling by promoting the deubiquitination and stabilization of tankyrases. USP25 is characterized by the presence of a long insertion in the middle of the conserved catalytic domain. The crystal structure of USP25 displays an unexpected homotetrameric quaternary assembly that is directly involved in the inhibition of its enzymatic activity. The tetramer is assembled by the association of two dimers and includes contacts between the coiled-coil insertion domain and the ubiquitin-binding pocket at the catalytic domain, revealing a distinctive autoinhibitory mechanism. Biochemical and kinetic assays with dimer, tetramer and truncation constructs of USP25 support this mechanism, displaying higher catalytic activity in the dimer assembly. Moreover, the high stabilization of tankyrases in cultured cells by ectopic expression of a constitutive dimer of USP25 supports a biological relevance of this tetramerization/inhibition mechanism.

USP25 is a deubiquitinating enzyme and a positive regulator of Wnt/β-catenin signaling. Here the authors present the crystal structure of USP25 in a tetrameric inactive state and their biochemical and kinetic assays support an USP25 autoinhibitory mechanism that is mediated through a dimer to tetramerization transition.

Abstract 2369: SOX11 promotes protective microenviromental nixes in agressive MCL

Mantle cell lymphoma (MCL) is considered one of the most aggressive lymphoid neoplasms and althought MCL represents only a 5-10% of all non-Hodgkins lymphomas, the aggressive clinical evolution with short responses to treatment, frequent relapses and the poor response to current therapies of the MCL patients, makes this lymphoma one of the tumors most studied internationally. Recent clinical observations have identified some patients with a more indolent form of this disease that have a long survival even without the need for treatment. Gene expression profiling (GEP) studies have identified the aberrant expression of the transcription factor SOX11 in aggressive MCL and absent in MCL with an indolent outcome, other lymphomas or normal B-cells, suggesting that SOX11 might play a relevant pathogenic role in the development and progression of this tumor. We have recently demonstrated in experimental studies that SOX11 blocks the maturation of the MCL cells and promotes the growth and angiogenesis of these tumors confirming the clinical observations that SOX11 may play an important role in the aggressive behavior of MCL. We have now generated preliminary results suggesting that SOX11 may regulate the migration and adhesion of the tumor cells increasing tumor dissemination and homing to different tissues compartments, protective microenvironment niches, the proliferation of new vessels bringing nutrients to the tumor and modulating an immature cellular status with more stemness capacity and drugs resistance known as cancer stem cell status. The understanding of these mechanisms will provide new potential candidates for therapeutic interventions.

Citation Format: Virginia Amador. SOX11 promotes protective microenviromental nixes in agressive MCL. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2369. doi:10.1158/1538-7445.AM2015-2369

Update on the molecular pathogenesis and targeted approaches of mantle cell lymphoma: summary of the 12th annual conference of the European Mantle Cell Lymphoma Network

Mantle cell lymphoma (MCL) is a distinct subtype of malignant lymphoma which is characterized by the chromosomal translocation t(11;14)(q13;q32) resulting in constitutional overexpression of cyclin D1 and cell cycle dysregulation in almost all cases. Clinically, MCL shows an aggressive clinical course with a continuous relapse pattern and a median survival of only 3-5 years. However, recently a subset of 15% long-term survivors has been identified with a rather indolent clinical course. Targeted strategies include the proteasome inhibitors, immune modulatory drugs (IMiDs), mammalian target of rapamycin (mTOR) inhibitors and especially inhibitors of the B-cell receptor pathway. Our recent annual conference focused on the molecular pathogenesis of the disease and how these underlying molecular alterations may guide the selection and integration of innovative approaches for therapy. This review of the meeting covers in particular the identification of indolent cases, and deals with the role of the B-cell receptor pathway in MCL, as well as the detection of minimal residual disease and implementation of molecular approaches in current clinical trials.

microRNA expression profiles identify subtypes of mantle cell lymphoma with different clinicobiological characteristics

PURPOSE

microRNAs (miRNA) are posttranscriptional gene regulators that may be useful as diagnostic and/or prognostic biomarkers. We aim to study the expression profiles of a high number of miRNAs and their relationship with clinicopathologic and biologic relevant features in leukemic mantle cell lymphomas (MCL).

EXPERIMENTAL DESIGN

Expression profiling of 664 miRNAs was investigated using a high-throughput quantitative real-time PCR platform in 30 leukemic MCLs. Statistical and bioinformatic analyses were conducted to define miRNAs associated with different clinicopathologic parameters. Gene expression profiling was investigated by microarrays in 16 matching cases to study the potential genes and pathways targeted by selected miRNAs. The prognostic value of miR-34a was investigated in 2 independent series of 29 leukemic and 50 nodal MCLs.

RESULTS

Robust consensus clustering defined 2 main MCL subgroups with significant differences in the immunoglobulin (IGHV) mutational status, SOX11 expression, genomic complexity, and nodal clinical presentation. Supervised analyses of IGHV and SOX11 categories identified 17 and 22 miRNAs differentially expressed, respectively. Enriched targets of these miRNAs corresponded to relevant pathways in MCL pathogenesis such as DNA stress response, CD40 signaling, and chromatin modification. In addition, we found 7 miRNAs showing prognostic significance independently of IGHV status and SOX11 expression. Among them, miR-34a was also associated with poor prognosis in 2 independent series of leukemic and nodal MCL, and in cooperation with high expression of the MYC oncogene.

CONCLUSION

We have identified miRNAs and target pathways related to clinical and biologic variants of leukemic MCL, and validated miR-34a as a prognostic marker in MCL.

Genomic and gene expression profiling defines indolent forms of mantle cell lymphoma

Mantle cell lymphoma (MCL) is typically a very aggressive disease with poor outcomes, but some cases display an indolent behavior that might not necessitate treatment at diagnosis. To define molecular criteria that might permit recognition of such cases, we compared the clinicopathologic features, gene expression, and genomic profile of patients who had indolent or conventional disease (iMCL or cMCL). Patients with iMCL displayed nonnodal leukemic disease with predominantly hypermutated IGVH and noncomplex karyotypes. iMCL and cMCL shared a common gene expression profile that differed from other leukemic lymphoid neoplasms. However, we identified a signature of 13 genes that was highly expressed in cMCL but underexpressed in iMCL. SOX11 was notable in this signature and we confirmed a restriction of SOX11 protein expression to cMCL. To validate the potential use of SOX11 as a biomarker for cMCL, we evaluated SOX11 protein expression in an independent series of 112 cases of MCL. Fifteen patients with SOX11-negative tumors exhibited more frequent nonnodal presentation and better survival compared with 97 patients with SOX11-positive MCL (5-year overall survival of 78% versus 36%, respectively; P = 0.001). In conclusion, we defined nonnodal presentation, predominantly hypermutated IGVH, lack of genomic complexity, and absence of SOX11 expression as qualities of a specific subtype of iMCL with excellent outcomes that might be managed more conservatively than cMCL.

PCNA-dependent regulation of p21 ubiquitylation and degradation via the CRL4Cdt2 ubiquitin ligase complex

The DNA polymerase delta processivity factor Proliferating Cell Nuclear Antigen (PCNA) promotes the DNA damage-induced degradation of the replication initiation factor Cdt1 via the CRL4(Cdt2) E3 ubiquitin ligase complex. Here we demonstrate that PCNA promotes the ubiquitylation and degradation of the CDK inhibitor p21 in cells irradiated with low dose of ultraviolet (UV) by a similar mechanism. Human cells that are depleted of Cul4, DDB1 (damage-specific DNA-binding protein-1), or the DCAF Cdt2, are deficient in the UV-induced ubiquitylation and degradation of p21. Depletion of mammalian cells of PCNA by siRNA, or mutations in p21 that abrogate PCNA binding, prevent UV-induced p21 ubiquitylation and degradation, indicating that physical binding with PCNA is necessary for the efficient ubiquitylation of p21 via the CRL4(Cdt2) ubiquitin ligase. Cdt2 functions as the substrate recruiting factor for p21 to the rest of the CRL4 ubiquitin ligase complex. The CRL4(Cdt2) E3 ubiquitin ligase ubiquitylates p21 both in vivo and in vitro, and its activity is dependent on the interaction of p21 with PCNA. Finally, we show that the CRL4(Cdt2) and the SCF(Skp2) ubiquitin ligases are redundant with each other in promoting the degradation of p21 during an unperturbed S phase of the cell cycle.

APC/C(Cdc20) controls the ubiquitin-mediated degradation of p21 in prometaphase

During the G1/S transition, p21 proteolysis is mediated by Skp2; however, p21 reaccumulates in G2 and is degraded again in prometaphase. How p21 degradation is controlled in mitosis remains unexplored. We found that Cdc20 (an activator of the ubiquitin ligase APC/C) binds p21 in cultured cells and identified a D box motif in p21 necessary for APC/C(Cdc20)-mediated ubiquitylation of p21. Overexpression of Cdc20 or Skp2 destabilized wild-type p21; however, only Skp2, but not Cdc20, was able to destabilize a p21(D box) mutant. Silencing of Cdc20 induced an accumulation of p21, increased the fraction of p21 bound to Cdk1, and inhibited Cdk1 activity in p21(+/+) prometaphase cells, but not in p21(-/-) cells. Thus, in prometaphase Cdc20 positively regulates Cdk1 by mediating the degradation of p21. We propose that the APC/C(Cdc20)-mediated degradation of p21 contributes to the full activation of Cdk1 necessary for mitotic events and prevents mitotic slippage during spindle checkpoint activation.

Control of the SCFSkp2–Cks1 ubiquitin ligase by the APC/CCdh1 ubiquitin ligase

Skp2 and its cofactor Cks1 are the substrate-targeting subunits of the SCF(Skp2-Cks1) (Skp1/Cul1/F-box protein) ubiquitin ligase complex that regulates entry into S phase by inducing the degradation of the cyclin-dependent kinase inhibitors p21 and p27 (ref. 1). Skp2 is an oncoprotein that often shows increased expression in human cancers; however, the mechanism that regulates its cellular abundance is not well understood. Here we show that both Skp2 and Cks1 proteins are unstable in G1 and that their degradation is mediated by the ubiquitin ligase APC/C(Cdh1) (anaphase-promoting complex/cyclosome and its activator Cdh1). Silencing of Cdh1 by RNA interference in G1 cells stabilizes Skp2 and Cks1, with a consequent increase in p21 and p27 proteolysis. Depletion of Cdh1 also increases the percentage of cells in S phase, whereas concomitant downregulation of Skp2 reverses this effect, showing that Skp2 is an essential target of APC/C(Cdh1). Expression of a stable Skp2 mutant that cannot bind APC/C(Cdh1) induces premature entry into S phase. Thus, the induction of Skp2 and Cks1 degradation in G1 represents a principal mechanism by which APC/C(Cdh1) prevents the unscheduled degradation of SCF(Skp2-Cks1) substrates and maintains the G1 state.

Proteasome-Mediated Degradation of p21 via N-Terminal Ubiquitinylation

We examined the mechanism responsible for the degradation of p21, a negative regulator of the cell division cycle. We found that p21 proteolysis requires functional ubiquitin and Nedd8 systems. Ubiquitinylated forms of p21 and p21(K0), a p21 mutant missing all lysines, are detected in vivo and in vitro, showing that the presence of lysines is dispensable for p21 ubiquitinylation. Instead, the free amino group of the N-terminal methionine of p21 is a site for ubiquitinylation in vivo. Although wild-type p21 is more abundantly ubiquitinylated than p21(K0) mutant due to the presence of internal lysine residues, their rates of proteolysis are indistinguishable. These results demonstrate that proteasomal degradation of p21 is regulated by the ubiquitin pathway and suggest that the site of the ubiquitin chain is critical in making p21 a competent substrate for the proteasome.

Gibberellins signal nuclear import of PHOR1, a photoperiod-responsive protein with homology to Drosophila armadillo

S. tuberosum ssp. andigena potato plants require short days (SD) for tuberization. We have isolated PHOR1 (photoperiod-responsive 1), which shows upregulated expression in induced leaves (SD). PHOR1 encodes an arm repeat protein with homology to the Drosophila segment polarity protein armadillo. Antisense inhibition of PHOR1 produces a semidwarf phenotype similar to that of GA-deficient plants, and the antisense lines show reduced GA responsiveness combined with a higher endogenous GA content than wild-type plants. Feedback regulation of GA biosynthetic genes is also altered in these lines. Conversely, transgenic lines overexpressing PHOR1 show an enhanced response to GA. GA application induces rapid migration of PHOR1-GFP protein to the nucleus. Thus, PHOR1 appears to be a general component of GA signaling pathways that relocalizes to the nucleus in the presence of GA.

[Microscopic characteristics of epidural filter pores]

OBJECTIVE

The increased use of the epidural route for administering opioids to treat chronic pain and the need to reduce complications as much as possible, has led some authors to recommend using micro filters to reduce catheter contamination. This study was motivated by the lack of technical information documenting epidural filters used routinely, as well as by the scarcity of literature describing their characteristics. Our aim was to investigate the true nature of the membrane pores, their characteristics and dimensions.

MATERIAL AND METHOD

Samples from 30 epidural filters labelled "Porosity: 0.2 microns" from three different manufacturers were studied. Filters from Vygon, Braun and Abbot were labelled A, B and C, respectively. The samples were placed in six groups of five filters each, and 15 random studies were made of each sample. Three of the six groups were used to study prefiltration surfaces and the others to study postfiltration surfaces. Each sample was metalized with gold and its center was then studied by scanning electron microscope. Given that the pores were anfractuous, they were measured by taking the diameter of the largest circle fitting inside that could predict the size of the smallest spherical non elastic body that might be retained. The samples for measuring thickness were cryofractured for determining the number of filtration planes in the 15 filters.

RESULTS

Prefiltration surface: Pore diameters were 0.70 (0.66 to 0.74), 0.45 (0.41 to 0.49), and 2.077 (2.01 to 2.15) microns on the filtration surfaces of manufacturers A, B and C, respectively. The differences were significant (p < 0.01) and the pore shapes were also different. Postfiltration surface: The function pores of filters from manufacturers A and B measured 0.26 (0.25 to 0.28) and 0.26 (0.24 to 0.28) microns, and the differences were not significant. The pores of filters from company C were significantly larger (p < 0.01), measuring 0.46 (0.43 to 0.49) microns. There were significant differences (p < 0.001) in pore size on the pre- and postfiltration surfaces from all three manufacturers. Gauge: The five A, B and C filters averaged 130, 118 and 165 microns thick, respectively, with an average number of 140, 220 and 210 filtration planes, respectively.

CONCLUSION

The pores of filters for epidural use labelled "0.2 microns" actually had much larger pores on their prefiltration surfaces and throughout the membrane thickness. On the postfiltration surface, however, the diameters of pores on filters manufactured by Vygon and Braun approached 0.2 microns. Pores on filters manufactured by Abbot, however, were approximately 0.46 microns. We believe that in the future manufacturers should include more information in the documentation accompanying their filters.

[Skin fragments carried by spinal needles in cadavers]

Epidural or intradural puncture with inappropriately stiffened or improperly placed needles can carry cells or fragments of epithelial tissue into the epidural or intradural space. These skin fragments feed by imbibition, possibly leading to the development of epidermoid cysts. We aimed to study the ability of today's needles to transport cells or epithelial fragments. We studied 120 needles in 6 groups of 20, as follows: group 1, Touhy G-16; group 2, Touhy G-17; group 3, Quincke G-22; group 4, Quincke G-26; group 5, Sprotte G-22, and group 6, Sprotte G-24. These needles were used to make intradural and epidural insertions, as indicated, with stiffeners fully in place. Insertions were made into 3 cadavers, epidermal cells or skin fragments were then isolated from the solutions used to wash the needles, and the samples were studied under an optical microscope. We identified groups of cells or epidermal tissues in 45% of the Touhy G-16 samples and in 30% of the Touhy G-17 samples. Squamous epithelial cells were found in 15% of the Quincke G-22 samples and in 30% of the Sprotte G-22 samples. There was a significant difference between the amount of tissue transported by the Touhy needles in comparison with the Quincke (p < 0.01) and Sprotte (p < 0.05) needles. Needles from some manufacturers transport epithelial fragments during lumbar puncture. We believe that better quality control during manufacture of epidural and intradural needles can help to eradicate the rare neurological complications derived from the removal of epithelial cells and their subsequent deposit inside the spinal channel.