Frequency, kinetics and determinants of viable SARS-CoV-2 in bioaerosols from ambulatory COVID-19 patients infected with the Beta, Delta or Omicron variants

Airborne transmission of SARS-CoV-2 aerosol remains contentious. Importantly, whether cough or breath-generated bioaerosols can harbor viable and replicating virus remains largely unclarified. We performed size-fractionated aerosol sampling (Andersen cascade impactor) and evaluated viral culturability in human cell lines (infectiousness), viral genetics, and host immunity in ambulatory participants with COVID-19. Sixty-one percent (27/44) and 50% (22/44) of participants emitted variant-specific culture-positive aerosols <10μm and <5μm, respectively, for up to 9 days after symptom onset. Aerosol culturability is significantly associated with lower neutralizing antibody titers, and suppression of transcriptomic pathways related to innate immunity and the humoral response. A nasopharyngeal Ct <17 rules-in ~40% of aerosol culture-positives and identifies those who are probably highly infectious. A parsimonious three transcript blood-based biosignature is highly predictive of infectious aerosol generation (PPV > 95%). There is considerable heterogeneity in potential infectiousness i.e., only 29% of participants were probably highly infectious (produced culture-positive aerosols <5μm at ~6 days after symptom onset). These data, which comprehensively confirm variant-specific culturable SARS-CoV-2 in aerosol, inform the targeting of transmission-related interventions and public health containment strategies emphasizing improved ventilation.

A rapid review of the effectiveness of screening practices at airports, land borders and ports to reduce the transmission of respiratory infectious diseases such as COVID-19

BACKGROUND

Travel screening for infectious diseases is often implemented to delay or prevent the entry of infected persons to a country/area.

OBJECTIVES

To evaluate the effectiveness of different point-of-entry screening strategies in achieving a reduction in imported COVID-19 transmission.

METHODS

A rapid evidence review was conducted, systematically searching PubMed and Google Scholar and grey literature on 27 March 2020.

RESULTS

We screened 1 194 records. Nine potential full-text articles were assessed for eligibility and included. Three articles investigated the effectiveness of entry-based thermal and body temperature scanning. Entry-based infrared thermal or body temperature scanning for COVID-19 was unlikely to be effective. Two systematic reviews found no additional benefit of travel restrictions/screening. In a COVID-19 modelling study, airport screening was not effective, with exit and entry thermal scanning identifying half and missing almost half of infected travellers. Two other modelling studies found that entry-based travel screening would achieve only modest delays in community transmission, while international travel quarantine could reduce case importations by 80%.

CONCLUSIONS

There is insufficient evidence to support entry and exit screening at points of entry, as these strategies detect just over half of the infected cases, missing almost half at entry points. The benefits of airport screening therefore need to be context specific and weighed against the resources and cost of implementation, the contribution of imported cases to total cases, and the benefits of identifying 50% of cases in the South African context with the country's high HIV and tuberculosis prevalence and limited resources to deal with a pandemic of this nature.

The role of serological testing in the SARS-CoV-2 outbreak

Antibody tests for the novel coronavirus, SARS-CoV2, have been developed both as rapid diagnostic assays and for high-throughput formal serology platforms. Although these tests may be a useful adjunct to a diagnostic strategy, they have a number of limitations. Because of the antibody and viral dynamics of the coronavirus, their sensitivity can be variable, especially at early time points after symptom onset. Additional data are required on the performance of the tests in the South African population, especially with regard to development and persistence of antibody responses and whether antibodies are protective against reinfection. These tests may, however, be useful in guiding the public health response, providing data for research (including seroprevalence surveys and vaccine initiatives) and development of therapeutic strategies.

Abstract P3-09-01: MYC dysregulates mitotic spindle function in triple-negative breast cancer creating a dependency on TPX2

Tumors that overexpress the MYC oncogene, including most receptor triple-negative breast cancers, frequently demonstrate aneuploidy, numerical chromosome alterations associated with highly aggressive cancers. Aneuploidy is also associated with rapid tumor evolution and poor patient outcome. We identify that MYC overexpression induces reversible defects in microtubule nucleation and mitotic spindle assembly, in TNBCs and other epithelial cells, promoting chromosome segregation defects, micronuclei and chromosomal instability (CIN). High TPX2 expression is permissive for mitotic spindle assembly and chromosome segregation in cells with MYC overexpression; whereas TPX2 depletion blocks mitotic progression, induces cell death and prevents tumor growth. Attenuating MYC expression reverses mitotic defects, even in established breast tumor cell lines, implicating an ongoing role for high MYC in the persistence of CIN in cancers. Our studies implicate the MYC oncogene as a regulator of spindle assembly and identify a new MYC-TPX2 synthetic-lethal interaction in TNBC that could represent a future therapeutic strategy in MYC-overexpressing cancers. Moreover, our studies suggest that blocking MYC activity can attenuate the emergence of CIN and tumor evolution.

Citation Format: Goga A, Rohrberg J, Corella A, Taileb M, Kilinc S, Jokisch M-L, Camarda R, Zhou A, Balakrishnan S, Chang AN, Klein-Connolly H. MYC dysregulates mitotic spindle function in triple-negative breast cancer creating a dependency on TPX2 [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-09-01.

Abstract OT3-04-04: A randomized phase II study of pembrolizumab in combination with carboplatin versus carboplatin alone in breast cancer patients with chest wall disease, with immunologic and genomic correlative studies

Background: Thirty percent of patients with breast cancer may experience chest wall recurrence, which is associated with a higher risk of developing distant metastases and a poor prognosis. Cancer cells may evade immune rejection through the programmed cell death 1 (PD-1) pathway. Pembrolizumab, an anti-PD-1 antibody, binds PD-1 and inhibits its interaction with the programmed death ligand 1 (PD-L1) to facilitate tumor immune rejection. We hypothesize that pembrolizumab may be an effective therapy in chest wall recurrence, given the inflammatory nature, and the high expression of PD-1 in tumors with lymphovascular invasion. Platinum agents may enhance anti-tumor immunity in a synergistic manner, and the combination of pembrolizumab and carboplatin has demonstrated efficacy in advanced lung cancer. In this study, the combination of pembrolizumab and carboplatin is being evaluated in breast cancer patients with chest wall disease.

Methods: This is a randomized phase II study of breast cancer patients with advanced, unresectable breast cancer involving the chest wall, being conducted through the Translational Breast Cancer Research Consortium (TBCRC). Patients may have hormone resistant disease (at least 2 prior lines of hormone therapy), triple negative breast cancer, or refractory HER2+ disease for enrollment. They may have other sites of distant metastases, have received any number of prior lines of therapy, have had prior surgery, but prior chest wall radiation is not necessary. Eighty-four patients at 7 TBCRC sites will be randomized 2:1 to treatment with pembrolizumab 200 mg IV and carboplatin AUC 5 IV every 3 weeks followed by pembrolizumab 200 mg IV alone every 3 weeks (Arm A, n=56) or carboplatin AUC 5 IV every 3 weeks (Arm B, n=28), with an option for patients in Arm B to cross-over to single agent pembrolizumab 200 mg IV every 3 weeks (arm Bx) on progression. Patients will undergo imaging with CT chest, abdomen, and pelvis at baseline and every 2 cycles of treatment for response evaluation. The primary endpoint is the disease control rate in the chest wall and distant sites at 18 weeks of treatment, and this study is powered to determine a 20% difference in disease control rates between arms A and B (hazard ratio of 0.52, α= 0.10, β= 0.20). After 18 patients are enrolled into Arm B, an interim analysis for futility will be conducted to enable early closure of that arm for lack of efficacy. Secondary endpoints in the study are toxicity, progression free survival, and response based on PD-L1 expression and irRECIST. Exploratory endpoints, which will be studied using peripheral blood testing and chest wall tumor biopsies at baseline and after 2 cycles of treatment, include determining associations of response with changes in tumor and peripheral blood immune composition, soluble PD-L1 expression, circulating tumor cells, cell free DNA, and tumor PD-L1 and MYC genomic expression. Ultimately this study promises to improve our understanding of checkpoint inhibition and chemotherapy for chest wall disease, and the underlying mechanism of action. This study is open for enrollment and 2 patients are currently enrolled. (NCT03095352).

Citation Format: Vidula N, Goga A, Hwang J, Liu MC, Park BH, Nanda R, Pohlmann PR, Storniolo AM, Brufsky A, Abramson V, Rugo HS. A randomized phase II study of pembrolizumab in combination with carboplatin versus carboplatin alone in breast cancer patients with chest wall disease, with immunologic and genomic correlative studies [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr OT3-04-04.

Abstract OT1-02-03: TBCRC 044: A randomized phase II study of pembrolizumab in combination with carboplatin versus carboplatin alone in breast cancer patients with chest wall disease

Background: Patients with breast cancer (BC) and chest wall disease have limited treatment options. We hypothesize that checkpoint inhibition may be an effective treatment approach due to the inflammatory nature of chest wall infiltration, and the association of PD-1 expression with lymphocytic infiltration. Platinum chemotherapy may facilitate anti-tumor immunity in a synergistic manner, and clinical studies of the PD-1 inhibitor pembrolizumab with platinum combinations have been effective in the treatment of advanced lung cancer. In this study, we will evaluate the combination of carboplatin and pembrolizumab in BC patients with chest wall disease.

Methods: This is a randomized phase II multicenter study in the TBCRC including patients with advanced, unresectable BC with hormone resistant or triple negative chest wall disease. Patients may have had prior surgery, prior chest wall radiation is not required, and other sites of distant metastases are allowed. Eighty-four patients at TBCRC sites will be randomized 2:1 to receive pembrolizumab and carboplatin (n=56, Arm A) or carboplatin alone (n=28, Arm B) until disease progression. Patients randomized to Arm B may cross-over following progression to pembrolizumab alone (Arm Bx). Patients in Arm A will be treated with pembrolizumab 200 mg IV and carboplatin AUC 5 IV every 3 weeks for at least 6 cycles followed by maintenance pembrolizumab 200 mg IV every 3 weeks if stable or responding disease. Patients in Arm B will be treated with carboplatin AUC 5 IV every 3 weeks until progression, then may cross-over to pembrolizumab 200 mg IV every 3 weeks alone (Arm Bx). An interim analysis for futility will be performed after 18 patients are enrolled into Arm B to allow early closure of that arm for lack of efficacy. The primary endpoint is disease control rate at 18 weeks of treatment; the study is powered to detect a 20% difference in disease control rates between arms (hazard ratio 0.52, α= 0.10, β= 0.20). Secondary endpoints include progression free survival, toxicity, and response based on PD-L1 expression and irRECIST. Exploratory endpoints include association of response with a number of biomarkers including tumor PD-L1 gene expression, tumor and peripheral blood immune composition and cytokine expression, peripheral T-cell PD-1 expression, circulating tumor DNA, circulating tumor cells, and tumor MYC genomic expression using tumor biopsy and peripheral blood testing before and after treatment. This study should be open to accrual by August of 2017. (NCT03095352)

Citation Format: Vidula N, Goga A, Krummel M, Hwang J, Liu M, Park BH, Nanda R, Pohlmann P, Storniolo AM, Van Poznak C, Brufsky A, Abramson V, Wolff A, Rugo HS. TBCRC 044: A randomized phase II study of pembrolizumab in combination with carboplatin versus carboplatin alone in breast cancer patients with chest wall disease [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr OT1-02-03.

Abstract PD3-05: Inhibition of fatty-acid oxidation as a therapy for triple-negative breast cancer

Expression of the oncogenic transcription factor MYC is disproportionately elevated in triple-negative breast cancer (TNBC) compared to estrogen, progesterone and/or human epidermal growth factor 2 receptor-positive (RP) breast tumors. We and others have shown that MYC alters metabolism during tumorigenesis. However, the role of MYC in TNBC metabolism remains largely unexplored. We hypothesized that pharmacologic inhibition of MYC-driven metabolic pathways may serve as a therapeutic strategy for this clinically challenging subtype of breast cancer. Using an unbiased metabolomics approach, we identified fatty acid oxidation intermediates as dramatically up-regulated in MYC-driven models of TNBC. A lipid metabolism gene signature was identified in patients with TNBC in the TCGA and multiple other clinical datasets, implicating fatty acid oxidation as a deregulated pathway critical for TNBC metabolism. We find that MYC-overexpressing TNBC, including transgenic models and patient-derived xenografts (PDX), display increased bioenergetic reliance upon fatty-acid oxidation (FAO). Pharmacologic inhibition of FAO catastrophically decreases energy metabolism of MYC over-expressing breast cancer, blocks growth of a MYC-driven transgenic TNBC model and MYC-overexpressing patient-derived xenografts. In vivo inhibition of FAO induced proliferation arrest and increased cell death in PDX models of TNBC. Our results demonstrate that inhibition of FAO is a novel therapeutic strategy against TNBCs that over-express MYC.

Citation Format: Goga A, Camarda R, Zhou AY, Kohnz RA, Balakrishnan S, Anderton B, Mahieu C, Eyob H, Krings G, Nomura DK. Inhibition of fatty-acid oxidation as a therapy for triple-negative breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr PD3-05.

Abstract P5-10-01: A systems approach to combine genomics and emerging therapeutics to discover new treatments for TNBC

Genomic analyses of patient tumors have unearthed an overwhelming number of recurrent somatic alterations in genes that undoubtedly have dramatic effects on breast tumor biology, clinical outcomes and responses to therapeutics. In triple negative breast cancer (TNBC), there is an urgent need to translate this emerging patient genomic data into new therapeutic paradigms. To develop genotype-specific therapies, we have established an isogenic cell-line drug screen that measures the impact of gene activation on a panel of emerging, clinically relevant compounds targeting a variety of cancer pathways. We seek to uncover genotype-dependent sensitivity to drugs, synthetic lethal relationships, and validate in breast cancer cells and in vivo mouse models. We focused on emerging compounds that are already approved or in testing for human use and we expect that this work will serve as a prelude to one or more clinical trials in TNBC.

RESULTS: We have developed a synthetic-lethal interaction mapping strategy to uncover the impact of gene activation on responses to a panel of emerging therapeutics. We believe that this approach can identify core synthetic lethal interactions which underlie drug sensitivity and can be used as a foundation to identify patient populations that will selectively respond to drug treatments. We have developed a high-throughput screening platform that can rapidly and accurately assess the proliferative capacity of cell lines in response to a panel of inhibitors. We have generated a panel of isogenic cell lines which closely mimic the impact of recurrent gene mutations and overexpression common in breast cancer. We have currently generated 23 cell lines expressing wild-type and/or mutant forms of these genes. To date we have screened 12 known oncogenes against our panel of 94 emerging therapeutic compounds. Using this systems approach, the resulting interaction map highlights both known and novel connections between oncogene activation and drug responses and provides a modular roadmap for the exploration of synthetic lethal relationships.

Based on the coordinate activity of multiple drugs targeting the same pathway, we identified oncogenes that can drive resistance to EGFR inhibitors and PI3K/AKT inhibitors. We also recapitulated a known synthetic lethal relationship between MYC and CDK inhibition which we have previously described and we are now evaluating in a clinical trial. Therefore, our approach identified both resistance and synthetic lethal relationships that have been independently described, indicating that this platform is a good model for predicting synthetic lethality in vivo. Several novel and previously uncharacterized connections were also uncovered, including novel connections between MYC activity and several promising drugs in clinical development. Synthetic-lethal interactions discovered using this chemical-genetic systems approach and their mechanistic validation will be described in the presentation. Furthermore, the important role of patient advocacy in developing this project will be described.

CONCLUSION: A novel systems biology approach that uses chemical-genetic maps of oncogenes and emerging therapeutics can define synthetic-lethal interactions and actionable therapeutics for breast cancer.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-10-01.

Is Option B+ the best choice?

This article is reprinted from The Lancet, with permission from Elsevier: Coutsodis A, Goga A, Desmond C, Barron P, Black V, Coovadia H. Is Option B+ the best choice? Lancet 2013;381(9863):269-271. [http://dx/doi.org/10.1016/S0140-6736(12)61807-8] The success of prevention of mother-to-child transmission (PMTCT) programmes (Options A and B) in middle-income countries, together with clinical trial data on antiretroviral (ARV) treatment as prophylaxis, has emboldened UN agencies to aggressively promote lifelong ARVs for PMTCT (Option B+). Unsubstantiated claims submit that Option B+ is cost-effective at population-level, will protect HIV-negative male partners, improve maternal and infant health, and increase ARV coverage. We provide counterfactual arguments about the ethics, medical safety, programme feasibility and economic benefits of Option B+. Option B+ offers no advantage to PMTCT and there are social hazards associated with privileging pregnant woman for treatment over men and non-pregnant women, especially with the absence of data to suggest that discordant relationships are more frequent among pregnant women or that they contribute disproportionately to the horizontal HIV transmission. The benefits and safety of long-term ARVs – including adherence and resistance – in mothers who do not need treatment for their own health, need to be considered, as well as, crucially, health service costs. The assumption that a decrease in efficiency caused by inappropriate targeting is compensated for by lower recruitment costs, is untested. Lives could be saved instead with appropriately targeted interventions. Countries should make individual decisions based on their HIV epidemiology, resources, priorities and local evidence. S Afr J HIV Med 2013;14(1):8-10. DOI:10.7196/SAJHIVMED.898

Survival of infants in the context of prevention of mother to child HIV transmission in South Africa

AIM

We sought to study the survival of newborn children according to HIV status of the mother, that of the child and the timing of infection.

METHODS

This is a prospective cohort study of 883 mothers (665 HIV-positive and 218 HIV-negative) and their infants. Data were collected using semi-structured questionnaires during home visits between the antenatal period and 36 weeks post-delivery. Infant HIV status was determined at 3, 24 and 36 weeks by HIV DNA PCR.

RESULTS

The majority (81.3%) of infected infants who died were infected by 3 weeks of age. Of the HIV-exposed infants who died, 19 (28.4%) died before 6 weeks and 38 (56.7%) died by 12 weeks. The hazard ratio (HR) of mortality at 36 weeks of age in HIV-infected infants compared with exposed but negative infants was 8.9 (95% CI: 6.7-11.8). There was no significant difference in 36 week survival rates between HIV-non-exposed and HIV-exposed but negative infants (HR: 0.7; 95% CI: 0.3-1.5). The infant being HIV-positive at age 3 weeks (HR: 32 95% CI: 14.0-73.1) and rural site (HR: 4.4 95% CI: 1.2-23.4) were the two independent risk factors for infant death amongst HIV-exposed infants.

CONCLUSION

The prognosis for infants with early HIV infection was very poor in this cohort. A greater focus on prevention of early infection, earlier screening for HIV infection and access to antiretrovirals for eligible infants is recommended.

The Role of microRNA Genes in Breast Cancer Progression and Metastasis

MicroRNAs influence tumor progression and metastasis via action as metastasis promoting genes (for example mir-373 and mir-520) or metastasis suppressor genes (for example mir-335). To identify additional microRNAs capable of driving breast cancer metastasis in vivo, we employed a functional genomics screen using the '4T1' syngeneic (Balb/c) mouse model of breast cancer progression. Approximately 400 microRNA sequences were subcloned into the pMSCV_puro retroviral expression vector and the resultant microRNA expression library transduced into fluorescent non-metastatic 67NR-pMSCV_mCherry and mildy-metastatic 66cl4-pMSCV_mCherry cells and selected with puromycin. The transduced cell lines were then implanted into the mammary fat pads of 20 syngeneic Balb/c mice per cell line. Following establishment of primary tumors, the primary tumors and secondary organs (lung, liver, spine, femurs), including any overt metastatic deposits, were dissected under a fluorescent stereomicroscope and the microRNA inserts amplified from the integrated retroviral genome by PCR.In an alternative approach, we have studied two populations of MDA-MB-231 human breast cancer cells that exhibit different behavior in vitro and in vivo to elucidate processes involved in cellular invasion and metastasis. A late-passage MDA-MB-231 (231variant) population formed invasive clusters when embedded in Matrigel, whereas early-passage (P3) MDA-MB-231 (231) cells did not. Moreover, 231v cells were more motile than 231 cells and showed enhanced tumorigenicity when grown as subcutaneous xenografts in nude mice, whereas proliferation and apoptotic response were indistinguishable between the two lines in vitro. Molecular characterization of the two cell populations cultured in 3D by array-based gene expression profiling revealed elevated expression of NF kappa B target genes in 231v cells (IL1A, IL1B, IL6, PTX3, NR4A2, HMOX1, FLIP). MicroRNA array analysis of 3D cultures also revealed marked induction (approximately 8-fold) of the related NF kappa B target microRNAs, mir-146a and mir-146b (Figure 1). Further evaluation of a panel of human breast cancer cell lines revealed that mir-146a and mir-146b were dramatically elevated in metastatic human breast cancer cell lines that display both epithelial-to-mesenchymal transition and high NF kappa B activity (MDA-MB-231, Hs578T, MDA-MB-436), compared to non-metastatic lines. Experiments to elucidate the function of mir-146a and mir-146b during breast cancer progression are underway. Figure 1: TaqMan qPCR analysis of mir-146a and mir-146b expression in 231 and 231v cells.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6142.

A functional screen for regulators of the c-Abl protein tyrosine kinase

c-Abl protein tyrosine kinase activity is tightly regulated in vertebrate cells. Several mutations can activate Abl and convert it into an oncogene. In man, chromosomal translocations result in fusion proteins associated with chronic myelogenous leukemias and some acute lymphocytic leukemias. In viral forms of abl, gag sequences are fused to Abl portions resulting in a deletion of N-terminal sequences. To study c-Abl activity in a cellular environment likely to lack specific regulators, we have expressed human c-Abl in Schizosaccharomyces pombe in an inducible fashion. c-Abl causes growth arrest followed by death of the cells. Mutations in the SH2 domain or in the autophosphorylation site dramatically reduce the ability of Abl to confer the growth arrest phenotype and to phosphorylate endogenous proteins, suggesting a fundamental role of these structures in the activity of the enzyme. An SH3 domain deletion mutant of Abl is as active as c-Abl in yeast indicating that there is no intrinsic regulation of c-Abl occurring via the SH3 domain and suggesting that the inhibitory effect of the SH3 domain observed in cells of vertebrate origin is mediated by a factor that is absent in fission yeast. We have used this assay to functionally screen a human cDNA library for molecules able to counteract the lethal effect of c-Abl expression. We are currently in the process of characterising the isolated clones. We hope to identify among them the molecule(s) responsible for regulating c-Abl activity in human cells.

Oligomerization of the ABL tyrosine kinase by the Ets protein TEL in human leukemia

TEL is a member of the Ets family of transcription factors which are frequently rearranged in human leukemia. The mechanism of TEL-mediated transformation, however, is unknown. We report the cloning and characterization of a chromosomal translocation associated with acute myeloid leukemia which fuses TEL to the ABL tyrosine kinase. The TEL-ABL fusion confers growth factor-independent growth to the marine hematopoietic cell line Ba/F3 and transforms Rat-1 fibroblasts and primary murine bone marrow cells. TEL-ABL is constitutively tyrosine phosphorylated and localizes to the cytoskeleton. A TEL-ABL mutant containing an ABL kinase-inactivating mutation is not constitutively phosphorylated and is nontransforming but retains cytoskeletal localization. However, constitutive phosphorylation, cytoskeletal localization, and transformation are all dependent upon a highly conserved region of TEL termed the helix-loop-helix (HLH) domain. TEL-ABL formed HLH-dependent homo-oligomers in vitro, a process critical for tyrosine kinase activation. These experiments suggest that oligomerization of TEL-ABL mediated by the TEL HLH domain is required for tyrosine kinase activation, cytoskeletal localization, and transformation. These data also suggest that oligomerization of Ets proteins through the highly conserved HLH domain may represent a previously unrecognized phenomenon.

Analysis of human c-Abl tyrosine kinase activity and regulation in S. pombe

c-Abl protein tyrosine kinase activity is tightly regulated in vertebrate cells. Several mutations, including deletions of the SH3 domain, can activate abl and convert it into an oncogene. To study c-Abl activity in a cellular environment likely to lack specific regulators, we have expressed human c-Abl in Schizosaccharomyces pombe in an inducible fashion. c-Abl, but not a kinase inactive form of the molecule, causes growth arrest followed by death of the cells. Concomitant to Abl expression we observed extensive phosphorylation of endogenous proteins on tyrosine. Mutations in the SH2 domain or in the autophosphorylation site dramatically reduce the ability of Abl to confer the growth arrest phenotype and to phosphorylate endogenous proteins, suggesting a fundamental role of these structures in the activity of the enzyme. An SH3 domain deletion mutant of Abl is equally active as wild type c-Abl in yeast, even under conditions allowing detection of subtle differences. These results demonstrate that there is no intrinsic regulation of c-Abl kinase activity via the SH3 domain and suggest that the inhibitory effect of the SH3 domain observed in mammalian cells is medicated by a factor that is absent in fission yeast. Expression of Ab1 S.pombe provides a novel quantitative assay for ab1 activity and regulation.

Alternative signals to RAS for hematopoietic transformation by the BCR-ABL oncogene

Biological function of the BCR-ABL oncogene is dependent on its activated tyrosine kinase. Mutations that inactivate the SRC homology 2 (SH2) domain, the GRB2-binding site in BCR, or the major autophosphorylation site of the kinase domain selectively disrupt downstream signaling but not tyrosine kinase activity. Despite a loss of fibroblast transformation activity, all three mutants retain the ability to render hematopoietic cell lines growth factor independent and transform primary bone marrow cells in vitro. In vivo tests of malignant potential reveal a most critical role for signals dependent on the BCR-ABL SH2 domain. The efficiency of both fibroblast and hematopoietic transformation by BCR-ABL is strongly affected by increased dosage of the SHC adapter protein, which can connect tyrosine kinase signals to RAS. The BCR-ABL oncogene activates multiple alternative pathways to RAS for hematopoietic transformation.

p53 dependent growth suppression by the c-Abl nuclear tyrosine kinase

Growth suppression by the Rb and p53 tumor suppressor proteins is mediated through effects on cell cycle regulatory proteins at the G1/S transition. Because overexpression of c-Abl induces G1 arrest in fibroblasts, we reasoned that c-Abl may also affect cell cycle proteins which regulate G1. We used fibroblasts containing disruptions of the Rb or p53 genes to genetically test the role of these proteins in c-Abl growth suppression. We find that c-Abl requires p53 but not Rb to suppress growth. c-Abl binds p53 in vitro and enhances p53 dependent transcription from a promoter containing p53 DNA binding sites. An Abl mutant which no longer binds p53 does not enhance p53 transcriptional activity and fails to suppress growth. These findings provide a novel link between a growth inhibitory tyrosine kinase and the p53 tumor suppressor protein.

Transcriptional activation of a ras-like gene (kir) by oncogenic tyrosine kinases

We report the characterization of a member of the ras gene family that is overexpressed in cells transformed by abl tyrosine kinase oncogenes. The gene, named kir (for kinase-inducible ras-like), is induced at the transcriptional level. kir mRNA has a rapid turnover and encodes a protein of 33 kDa with guanine nucleotide-binding activity but undetectable intrinsic GTPase activity. kir was cloned by differential screening of genes present in fully malignant versus growth factor-independent cell lines expressing wild-type or mutant forms of BCR/ABL. BCR/ABL and v-Abl induce transcription of the kir gene via specific signaling pathway(s), but kir overexpression alone is not sufficient to mediate transformation.

Differential complementation of Bcr-Abl point mutants with c-Myc

A complementation strategy was developed to define the signaling pathways activated by the Bcr-Abl tyrosine kinase. Transformation inactive point mutants of Bcr-Abl were tested for complementation with c-Myc. Single point mutations in the Src-homology 2 (SH2) domain, the major tyrosine autophosphorylation site of the kinase domain, and the Grb-2 binding site in the Bcr region impaired the transformation of fibroblasts by Bcr-Abl. Hyperexpression of c-Myc efficiently restored transformation activity only to the Bcr-Abl SH2 mutant. These data support a model in which Bcr-Abl activates at least two independent pathways for transformation. This strategy may be useful for discerning signaling pathways activated by other oncogenes.

The nuclear tyrosine kinase c-Abl negatively regulates cell growth

c-Abl is a tyrosine kinase localized primarily in the nucleus. Previous assays for abl function rely on cellular transformation by abl mutants, which are cytoplasmic. Using a conditional overexpression strategy, we have developed a functional assay for c-abl. Overexpression of c-abl inhibits growth by causing cell cycle arrest. Growth suppression requires tyrosine kinase activity, nuclear localization, and an intact SH2 domain. Overexpression of dominant negative c-abl disrupts cell cycle control and enhances transformation by tyrosine kinases, G proteins, and transcription factor oncogenes. These findings suggest that c-abl acts as a negative regulator of cell growth. This growth suppressive activity is functionally similar to that of tumor suppressor genes such as p53 and Rb.

Oncogenic activation of c-ABL by mutation within its last exon

The c-ABL proto-oncogene is a predominantly nuclear localized tyrosine kinase. A random mutagenesis scheme was used to isolate c-ABL mutants whose expression produced a transformed phenotype in rodent fibroblast cells. An in-frame deletion within the central region of the last exon was identified in one ABL mutant. The mechanism of c-ABL oncogenic activation by mutation within the last exon differs both functionally and structurally from those of v-ABL and BCR/ABL. This class of ABL mutants shows increased tyrosine phosphorylation of cellular proteins in vivo but low levels of autophosphorylation. Last-exon ABL mutants are distinguished from v-ABL or BCR/ABL by their inability to transform primary bone marrow cells or support the growth of transformed pre-B cells. These findings define a new mechanism of oncogenic activation for the ABL kinase through mutations in the last exon which do not require amino-terminal deletions or mutations within the src homology regions.