A therapeutic strategy to target distinct sources of IgE and durably reverse allergy.

Immunoglobulin E (IgE) is a key driver of type 1 hypersensitivity reactions and allergic disorders, which are globally increasing in number and severity. Although eliminating pathogenic IgE may be a powerful way to treat allergy, no therapeutic strategy reported to date can fully ablate IgE production. Interleukin-4 receptor α (IL-4Rα) signaling is required for IgE class switching, and IL-4Rα blockade gradually reduces, but does not eliminate, IgE. The persistence of IgE after IL-4Rα blockade may be due to long-lived IgE+ plasma cells that maintain serological memory to allergens and thus may be susceptible to plasma cell-targeted therapeutics. We demonstrate that transient administration of a B cell maturation antigen x CD3 (BCMAxCD3) bispecific antibody markedly depletes IgE, as well as other immunoglobulins, by ablating long-lived plasma cells, although IgE and other immunoglobulins rapidly rebound after treatment. Concomitant IL-4Rα blockade specifically and durably prevents the reemergence of IgE by blocking IgE class switching while allowing the restoration of other immunoglobulins. Moreover, this combination treatment prevented anaphylaxis in mice. Together with additional cynomolgus monkey and human data, our studies demonstrate that allergic memory is primarily maintained by both non-IgE+ memory B cells that require class switching and long-lived IgE+ plasma cells. Our combination approach to durably eliminate pathogenic IgE has potential to benefit allergy in humans while preserving antibody-mediated immunity.

Garetosmab in fibrodysplasia ossificans progressiva: a randomized, double-blind, placebo-controlled phase 2 trial.

Fibrodysplasia ossificans progressiva (FOP) is a rare disease characterized by heterotopic ossification (HO) in connective tissues and painful flare-ups. In the phase 2 LUMINA-1 trial, adult patients with FOP were randomized to garetosmab, an activin A-blocking antibody (n = 20) or placebo (n = 24) in period 1 (28 weeks), followed by an open-label period 2 (28 weeks; n = 43). The primary end points were safety and for period 1, the activity and size of HO lesions. All patients experienced at least one treatment-emergent adverse event during period 1, notably epistaxis, madarosis and skin abscesses. Five deaths (5 of 44; 11.4%) occurred in the open-label period and, while considered unlikely to be related, causality cannot be ruled out. The primary efficacy end point in period 1 (total lesion activity by PET-CT) was not met (P = 0.0741). As the development of new HO lesions was suppressed in period 1, the primary efficacy end point in period 2 was prospectively changed to the number of new HO lesions versus period 1. No placebo patients crossing over to garetosmab developed new HO lesions (0% in period 2 versus 40.9% in period 1; P = 0.0027). Further investigation of garetosmab in FOP is ongoing. ClinicalTrials.gov identifier NCT03188666 .

Association of complement pathways with COVID-19 severity and outcomes.

OBJECTIVES: Complement activation has been implicated in COVID-19 pathogenesis. This study aimed to assess the levels of complement activation products and full-length proteins in hospitalized patients with COVID-19, and evaluated whether complement pathway markers are associated with outcomes. METHODS: Longitudinal measurements of complement biomarkers from 89 hospitalized adult patients, grouped by baseline disease severity, enrolled in an adaptive, phase 2/3, randomized, double-blind, placebo-controlled trial and treated with intravenous sarilumab (200 mg or 400 mg) or placebo (NCT04315298), were performed. These measurements were then correlated with clinical and laboratory parameters. RESULTS: All complement pathways were activated in hospitalized patients with COVID-19. Alternative pathway activation was predominant earlier in the disease course. Complement biomarkers correlated with multiple variables of multi-organ dysfunction and inflammatory injury. High plasma sC5b-9, C3a, factor Bb levels, and low mannan-binding lectin levels were associated with increased mortality. Sarilumab treatment showed a modest inhibitory effect on complement activation. Moreover, sera from patients spontaneously deposited C5b-9 complex on the endothelial surface ex vivo, suggesting a microvascular thrombotic potential. CONCLUSION: These results advance our understanding of COVID-19 disease pathophysiology and demonstrate the importance of specific complement pathway components as prognostic biomarkers in COVID-19.

UGT1A1 genetic variants are associated with increases in bilirubin levels in rheumatoid arthritis patients treated with sarilumab.

Sarilumab is a human monoclonal antibody against interleukin (IL)-6Rα that has been approved for the treatment of adult patients with moderately to severely active rheumatoid arthritis (RA) and an inadequate response or intolerance to one or more disease-modifying antirheumatic drugs (DMARDs). Mild liver function test abnormalities have been observed in patients treated with sarilumab. We describe a genome-wide association study of bilirubin elevations in RA patients treated with sarilumab. Array genotyping and exome sequencing were performed on DNA samples from 1075 patients. Variants in the UGT1A1 gene were strongly associated with maximum bilirubin elevations in sarilumab-treated patients (rs4148325; p = 2.88 × 10-41) but were not associated with aminotransferase elevations. No other independent loci showed evidence of association with bilirubin elevations after sarilumab treatment. These findings suggest that most bilirubin increases during sarilumab treatment are related to genetic variation in UGT1A1 rather than underlying liver injury.

Efficacy and Safety of Sarilumab in Hospitalized Patients With Coronavirus Disease 2019: A Randomized Clinical Trial.

BACKGROUND: Open-label platform trials and a prospective meta-analysis suggest efficacy of anti-interleukin (IL)-6R therapies in hospitalized patients with coronavirus disease 2019 (COVID-19) receiving corticosteroids. This study evaluated the efficacy and safety of sarilumab, an anti-IL-6R monoclonal antibody, in the treatment of hospitalized patients with COVID-19. METHODS: In this adaptive, phase 2/3, randomized, double-blind, placebo-controlled trial, adults hospitalized with COVID-19 received intravenous sarilumab 400 mg or placebo. The phase 3 primary analysis population included patients with critical COVID-19 receiving mechanical ventilation (MV). The primary outcome was proportion of patients with ≥1-point improvement in clinical status from baseline to day 22. RESULTS: There were 457 and 1365 patients randomized and treated in phases 2 and 3, respectively. In phase 3, patients with critical COVID-19 receiving MV (n = 298; 28.2% on corticosteroids), the proportion with ≥1-point improvement in clinical status (alive, not receiving MV) at day 22 was 43.2% for sarilumab and 35.5% for placebo (risk difference, +7.5%; 95% confidence interval [CI], -7.4 to 21.3; P =.3261), a relative risk improvement of 21.7%. In post hoc analyses pooling phase 2 and 3 critical patients receiving MV, the hazard ratio for death for sarilumab vs placebo was 0.76 (95% CI, .51 to 1.13) overall and 0.49 (95% CI, .25 to .94) in patients receiving corticosteroids at baseline. CONCLUSIONS: This study did not establish the efficacy of sarilumab in hospitalized patients with severe/critical COVID-19. Post hoc analyses were consistent with other studies that found a benefit of sarilumab in patients receiving corticosteroids. CLINICAL TRIALS REGISTRATION: NCT04315298.

The Activin/FLRG Pathway Associates with Poor COVID-19 Outcomes in Hospitalized Patients.

A subset of hospitalized COVID-19 patients, particularly the aged and those with comorbidities, develop the most severe form of the disease, characterized by acute respiratory disease syndrome (ARDS), coincident with experiencing a "cytokine storm." Here, we demonstrate that cytokines which activate the NF-κB pathway can induce activin A. Patients with elevated activin A, activin B, and FLRG at hospital admission were associated with the most severe outcomes of COVID-19, including the requirement for mechanical ventilation, and all-cause mortality. A prior study showed that activin A could decrease viral load, which indicated there might be a risk to giving COVID-19 patients an inhibitor of activin. To evaluate this, the role for activin A was examined in a hamster model of SARS-CoV-2 infection, via blockade of activin A signaling. The hamster model demonstrated that use of an anti-activin A antibody did not worsen the disease and there was no evidence for increase in lung viral load and pathology. The study indicates blockade of activin signaling may be beneficial in treating COVID-19 patients experiencing ARDS.

Baseline Severe Acute Respiratory Syndrome Viral Load Is Associated With Coronavirus Disease 2019 Severity and Clinical Outcomes: Post Hoc Analyses of a Phase 2/3 Trial.

BACKGROUND: Elucidating the relationship between severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral load and clinical outcomes is critical for understanding coronavirus disease 2019 (COVID-19). METHODS: The SARS-CoV-2 levels were analyzed by quantitative real-time polymerase chain reaction (RT-qPCR) of nasopharyngeal or oropharyngeal swab specimens collected at baseline, and clinical outcomes were recorded over 60 days from 1362 COVID-19 hospitalized patients enrolled in a multicenter, randomized, placebo-controlled phase 2/3 trial of sarilumab for COVID-19 (ClinicalTrials.gov NCT04315298). RESULTS: In post hoc analyses, higher baseline viral load, measured by both RT-qPCR cycle threshold and log10 copies/mL, was associated with greater supplemental oxygenation requirements and disease severity at study entry. Higher baseline viral load was associated with higher mortality, lower likelihood of improvement in clinical status and supplemental oxygenation requirements, and lower rates of hospital discharge. Viral load was not impacted by sarilumab treatment over time versus placebo. CONCLUSIONS: These data support viral load as an important determinant of clinical outcomes in hospitalized patients with COVID-19 requiring supplemental oxygen or assisted ventilation.

Pharmacokinetics and Pharmacodynamics of Subcutaneous Sarilumab and Intravenous Tocilizumab Following Single-Dose Administration in Patients With Active Rheumatoid Arthritis on Stable Methotrexate.

We assessed pharmacokinetics (PK), pharmacodynamics (PD), and PK/PD relationships of interleukin-6 (IL-6), soluble IL-6 receptor, and C-reactive protein (CRP) in serum, and absolute neutrophil count (ANC) in blood following single doses of subcutaneous sarilumab versus intravenous tocilizumab (NCT02097524) from patients with rheumatoid arthritis (RA) who are inadequate responders to methotrexate (MTX) and on a stable dose of MTX. Patients with RA randomized (1:1:1:1) to single-dose sarilumab (150 or 200 mg subcutaneously) or tocilizumab (4 or 8 mg/kg intravenously) were included (n = 101), and PK, PD, and PK/PD relationships and safety were assessed over 6 weeks postdose. PK profiles for both drugs are described by parallel linear and nonlinear target-mediated clearance pathways. PD markers showed similar onset of effect during the first week postdose, regardless of dose or route of administration. CRP and ANC decreased, with median postdose nadirs at 7-15 days for CRP and 3-5 days for ANC. Both drugs at low and high doses achieved the same nadir for ANC and a similar return toward baseline within 2 weeks postdose, suggesting a saturation of effect. Safety profiles of sarilumab and tocilizumab were generally similar. In conclusion, despite differences in PK, the onset of the decrease in CRP (efficacy) and ANC (safety) after a single dose were similar for subcutaneous sarilumab and intravenous tocilizumab. PD effects and safety were consistent with previous studies.

High levels of interleukin-6 in patients with rheumatoid arthritis are associated with greater improvements in health-related quality of life for sarilumab compared with adalimumab.

BACKGROUND: Increased levels of cytokines, including interleukin-6 (IL-6), reflect inflammation and have been shown to be predictive of therapeutic responses, fatigue, pain, and depression in patients with rheumatoid arthritis (RA), but limited data exist on associations between IL-6 levels and health-related quality of life (HRQoL). This post hoc analysis of MONARCH phase III randomized controlled trial data evaluated the potential of baseline IL-6 levels to differentially predict HRQoL improvements with sarilumab, a fully human monoclonal antibody directed against both soluble and membrane-bound IL-6 receptor α (anti-IL-6Rα) versus adalimumab, a tumor necrosis factor α inhibitor, both approved for treatment of active RA. METHODS: Baseline serum IL-6 levels in 300/369 randomized patients were categorized into low (1.6-7.1 pg/mL), medium (7.2-39.5 pg/mL), and high (39.6-692.3 pg/mL) tertiles. HRQoL was measured at baseline and week (W)24 and W52 by Short Form 36 (SF-36) physical/mental component summary (PCS/MCS) and domain scores, Functional Assessment of Chronic Illness Therapy -fatigue, and duration of morning stiffness visual analog scale (AM-stiffness VAS). Linear regression of changes from baseline in HRQoL (IL-6 tertile, treatment, region as a stratification factor, and IL-6 tertile-by-treatment interaction as fixed effects) assessed predictivity of baseline IL-6 levels, with low tertile as reference. Pairwise comparisons of improvements between treatment groups were performed by tertile; least squares mean differences and 95% CIs were calculated. Similar analyses evaluated W24 patient-level response on minimum clinically important differences (MCID). RESULTS: At baseline, patients with high versus medium or low IL-6 levels (n = 100, respectively) reported worse (nominal p < 0.05) SF-36 MCS and role-physical, bodily pain, social functioning, role-emotional domain, and AM-stiffness VAS scores. There was a greater treatment effect with sarilumab versus adalimumab in high tertile versus low tertile groups in SF-36 PCS, physical functioning domain, and AM-stiffness VAS (nominal interaction p < 0.05). PCS improvements ≥MCID were higher in high (odds ratio [OR] 6.31 [2.37, 16.81]) versus low (OR 0.97 [0.43, 2.16]) tertiles with sarilumab versus adalimumab (nominal interaction p < 0.05). Adverse events between IL-6 tertiles were similar. CONCLUSIONS: Patients with high baseline IL-6 levels reported better improvements in PCS, physical functioning domain, and AM-stiffness scores with sarilumab versus adalimumab and safety consistent with IL-6R blockade. TRIAL REGISTRATION: NCT02332590 . Registered on 5 January 2015.

Population Pharmacodynamic Model of Neutrophil Margination and Tolerance to Describe Effect of Sarilumab on Absolute Neutrophil Count in Patients with Rheumatoid Arthritis.

Evidence suggests that effects of interleukin-6 pathway inhibitors sarilumab, tocilizumab, and sirukumab on absolute neutrophil count (ANC) are due to margination of circulating neutrophils into rapidly mobilizable noncirculating pools. We developed a population pharmacodynamic model using compartments for neutrophil margination and ANC-specific tolerance to describe rapid, transient ANC changes in blood following administration of subcutaneous sarilumab and intravenous/subcutaneous tocilizumab based on data from 322 patients with rheumatoid arthritis in two single-dose (NCT02097524 and NCT02404558) and one multiple-dose (NCT01768572) trials. The model incorporated a tolerance compartment to account for ANC nadir and beginning of recovery before maximal drug concentration after subcutaneous dosing, and absence of a nadir plateau when the ANC response is saturated after subcutaneous or intravenous dosing. The model effectively describes the ANC changes and supports neutrophil margination and tolerance as an explanation for the absence of increased infection risk associated with low ANC due to interleukin-6 pathway inhibitor treatment.

Association of High Serum Interleukin-6 Levels With Severe Progression of Rheumatoid Arthritis and Increased Treatment Response Differentiating Sarilumab From Adalimumab or Methotrexate in a Post Hoc Analysis.

OBJECTIVE: The development of biomarkers to guide treatment decisions is a major research focus in rheumatoid arthritis (RA). Patients with RA have elevated interleukin-6 (IL-6) levels; however, the utility of IL-6 as a predictor of treatment response is unclear. This study was undertaken to investigate, by post hoc analysis, whether baseline IL-6 levels are predictive of sarilumab treatment responses in 2 phase III studies. METHODS: Serum IL-6 concentrations were measured in patients with RA prior to receiving sarilumab 200 mg (n = 148) or adalimumab 40 mg (n = 152) every 2 weeks (in the MONARCH trial; ClinicalTrials.gov identifier: NCT02332590) or sarilumab 150 mg, sarilumab 200 mg, or placebo every 2 weeks plus methotrexate (MTX) (n = 401, n = 396, and n = 397, respectively) (in the MOBILITY trial; ClinicalTrials.gov identifier: NCT01061736). Efficacy and patient-reported outcomes were compared between and within groups according to IL-6 tertile using linear and logistic regression. RESULTS: In MONARCH, patients with high baseline IL-6 levels (all ≥3 times the upper limit of normal; n = 100) had higher disease activity at baseline than those with low IL-6 levels (n = 100). The magnitude of clinical improvement over 24 weeks with sarilumab versus adalimumab was greater in patients with high compared to those with low baseline IL-6 levels. In MOBILITY, compared to patients with low IL-6 levels (n = 397), patients with high IL-6 levels (n = 398) had higher disease activity and joint damage at baseline, were more likely to have joint progression, and had less clinical improvement over 52 weeks' treatment with placebo plus MTX compared to sarilumab 150 mg or 200 mg plus MTX. Baseline IL-6 and C-reactive protein levels were both predictive of outcomes. Safety profiles were similar between defined IL-6 tertiles. CONCLUSION: IL-6 may be a prognostic marker of disease progression and severity, and patients with high IL-6 levels may be likely to benefit from sarilumab compared to adalimumab or MTX. Prospective validation is warranted to confirm the results of these post hoc analyses.

Sarilumab and adalimumab differential effects on bone remodelling and cardiovascular risk biomarkers, and predictions of treatment outcomes.

BACKGROUND: Interleukin-6 (IL-6) is a pleiotropic cytokine that plays a key role in the pathogenesis of rheumatoid arthritis. Sarilumab is a human monoclonal antibody that binds membrane-bound and soluble IL-6 receptor-α to inhibit IL-6 signalling. The aim of this study was to compare the effects of sarilumab and adalimumab (a tumour necrosis factor alpha inhibitor) monotherapy on levels of circulating biomarkers associated with the acute-phase response, bone remodelling, atherothrombosis, anaemia of chronic disease and markers purported to reflect synovial lymphoid and myeloid cell infiltrates, as well as the potential of these biomarkers to differentially predict clinical and patient-reported outcomes with sarilumab vs. adalimumab. METHODS: In this post hoc analysis, serum samples were analysed at baseline and prespecified post-treatment timepoints up to week 24 in adults with moderate-to-severe active rheumatoid arthritis intolerant of or inadequate responders to methotrexate from the MONARCH trial (NCT02332590). RESULTS: Greater reductions in C-reactive protein (CRP; - 94.0% vs. -24.0%), serum amyloid A (SAA; - 83.2% vs. -17.4%), total receptor activator of nuclear factor-κB ligand (RANKL; - 18.3% vs. 10.5%) and lipoprotein (a) (- 41.0% vs. -2.8%) were observed at week 24 with sarilumab vs. adalimumab, respectively (adjusted p < 0.0001). Greater increases in procollagen type 1 N-terminal propeptide (P1NP) were observed with sarilumab vs. adalimumab at week 24 (22.8% vs. 6.2%, p = 0.027). Patients with high baseline SAA, CRP and matrix metalloproteinase-3 (MMP-3) were more likely to achieve clinical efficacy, including American College of Rheumatology 20% improvement criteria and Disease Activity Score (28 joints)-CRP < 3.2, and report improvements in patient-reported outcomes, including Health Assessment Questionnaire-Disability Index and pain visual analogue scale, with sarilumab than adalimumab. CONCLUSION: Sarilumab was associated with greater positive effects on bone remodelling and decreases in biomarkers of the acute-phase response, synovial inflammation and cardiovascular risk vs. adalimumab. High baseline concentrations of SAA, CRP and MMP-3 are predictive of clinical and patient-reported outcome responses to sarilumab treatment and prospective validation is warranted to confirm these results. TRIAL REGISTRATION: ClinicalTrials.gov, NCT02332590. Registered on 5 January 2015.

Identification of sarilumab pharmacodynamic and predictive markers in patients with inadequate response to TNF inhibition: a biomarker substudy of the phase 3 TARGET study.

INTRODUCTION: Interleukin-6 (IL-6) orchestrates formation of an inflammatory pannus, leading to joint damage in rheumatoid arthritis (RA). Sarilumab is a human monoclonal antibody blocking the IL-6Rα. In TARGET (NCT01709578), a phase 3 study in adults with moderate-to-severe RA and inadequate response or intolerance to tumour necrosis factor inhibitors, subcutaneous sarilumab 200 mg or 150 mg every 2 weeks (q2w) plus conventional synthetic disease-modifying antirheumatic drugs (csDMARDs) significantly reduced disease activity versus placebo plus csDMARDs. METHODS: Circulating levels of biomarkers associated with synovial inflammation (matrix metalloproteinase 3 (MMP-3), collagen type I MMP-cleaved fragment (C1M), collagen type III MMP-cleaved fragment (C3M)), myeloid (soluble intercellular adhesion molecule 1 (sICAM-1), IL-8 and calprotectin) and lymphoid activation (chemokine, CXC motif, ligand 13 (CXCL13), CXCL10, B cell-activating factor) and bone remodelling (receptor activator of nuclear factor-κB ligand (RANKL), osteoprotegerin and osteocalcin) were evaluated in patients from a TARGET substudy. RESULTS: Sarilumab significantly decreased C1M, C3M, CXCL13, MMP-3 and total RANKL levels at week 24 versus placebo; some markers were significantly suppressed at week 2 and normalised to levels in healthy controls. Levels of sICAM-1 were predictive of disease activity score by C-reactive protein and clinical disease activity index low disease activity (LDA) response in the sarilumab 200 mg q2w group at week 12. A trend was observed in which patients with lower sICAM-1 levels at baseline had better response compared with patients with higher sICAM-1. CONCLUSIONS: Sarilumab plus csDMARDs decreased circulating biomarkers of synovial inflammation and bone resorption; sICAM-1 was predictive of achieving LDA with sarilumab. TRIAL REGISTRATION NUMBER: NCT01709578; Post-results.

Sarilumab plus methotrexate suppresses circulating biomarkers of bone resorption and synovial damage in patients with rheumatoid arthritis and inadequate response to methotrexate: a biomarker study of MOBILITY.

BACKGROUND: Interleukin 6 (IL-6) signaling plays a key role in the pathophysiology of rheumatoid arthritis (RA) and is inhibited by sarilumab, a human monoclonal antibody blocking the IL-6 receptor alpha (IL-6Rα). The effects of sarilumab plus methotrexate (MTX) on serum biomarkers of joint damage and bone resorption were assessed in two independent studies (phase II (part A) and phase III (part B)) of patients with RA with a history of inadequate response to MTX from the MOBILITY study (NCT01061736). METHODS: Serum samples were analyzed at baseline and prespecified posttreatment time points. Biomarkers of tissue destruction, cartilage degradation, and synovial inflammation were measured in part A; assessment of these markers was repeated in part B and included additional analysis of biomarkers of bone formation and resorption (including soluble receptor activator of nuclear factor-kB ligand (sRANKL)). A mixed model for repeated measures was used to compare treatment effects on change in biomarkers. Additionally, changes from baseline in biomarkers were compared between American College of Rheumatology 50 % responders and nonresponders and between patients who achieved or did not achieve low disease activity (LDA), separately by treatment group, at week 24. RESULTS: In part A, sarilumab 150 and 200 mg every 2 weeks (q2w) significantly reduced biomarkers of tissue destruction, cartilage degradation, and synovial inflammation at both 2 and 12 weeks posttreatment (p < 0.05 vs placebo). These results were replicated in part B, with markers of these damaging processes reduced at weeks 2 and 24 (p < 0.05 vs placebo). Additionally, sarilumab 200 mg q2w significantly reduced both sRANKL and sRANKL/osteoprotegerin ratio at week 24 (p < 0.01 vs placebo). Trends for reduction were noted for several biomarkers in patients who achieved LDA compared with those who did not. CONCLUSIONS: Sarilumab plus MTX significantly suppressed biomarkers of bone resorption and joint damage, as compared with placebo plus MTX, in patients with RA. Additional work is needed to determine whether differences in biomarker profiles at baseline or posttreatment can identify patients who achieve improvement in disease activity. TRIAL REGISTRATION: ClinicalTrials.gov, NCT01061736 , February 2, 2010.

SERPINB13 is a novel RUNX1 target gene.

RUNX1 is a critical transcription factor during embryogenesis and neoplastic disease. To identify novel transcriptional targets of RUNX1 in the context of chromatin, we performed genome wide location analysis (ChIP-on-chip). Here we report that SERPINB13, a gene downregulated in head and neck cancers, is a novel RUNX1transcriptional target. RUNX1 binds the SERPINB13 promoter in chromatin to repress its transcription. Mutation of either RUNX1 binding site in the SERPINB13 promoter increased the activity of the promoter. Finally, overexpression of RUNX1 and concomitant decrease in SERPINB13 expression led to increased activity of cathepsin K, an enzyme inhibited by SERPINB13. These data demonstrate that RUNX1 is an important regulator of SERPINB13 and cathepsin K activity.

SON controls cell-cycle progression by coordinated regulation of RNA splicing.

It has been suspected that cell-cycle progression might be functionally coupled with RNA processing. However, little is known about the role of the precise splicing control in cell-cycle progression. Here, we report that SON, a large Ser/Arg (SR)-related protein, is a splicing cofactor contributing to efficient splicing of cell-cycle regulators. Downregulation of SON leads to severe impairment of spindle pole separation, microtubule dynamics, and genome integrity. These molecular defects result from inadequate RNA splicing of a specific set of cell-cycle-related genes that possess weak splice sites. Furthermore, we show that SON facilitates the interaction of SR proteins with RNA polymerase II and other key spliceosome components, suggesting its function in efficient cotranscriptional RNA processing. These results reveal a mechanism for controlling cell-cycle progression through SON-dependent constitutive splicing at suboptimal splice sites, with strong implications for its role in cancer and other human diseases.

3-phosphoinositide-dependent protein kinase-1 regulates proliferation and survival of cancer cells with an activated mitogen-activated protein kinase pathway.

Engagement of cell surface receptor tyrosine kinases by insulin and growth factors activates phosphatidylinositol 3-kinase (PI3K) and generates the second messenger, phosphatidylinositol 3,4,5-trisphosphate. This second messenger leads to the recruitment of 3-phosphoinositide-dependent protein kinase-1 (PDK1) to the proximal side of the plasma membrane, which results in the activation of AKT kinase. In addition, PDK1 can phosphorylate numerous other kinases, including p90RSK, a kinase downstream of mitogen-activated protein kinase (MAPK) that is important for cell proliferation and survival. Previous studies have shown that the loss of PDK1 sensitizes tumor cells to chemotherapeutic agents and radiation but have not focused on delineating the contribution of PDK1 to pathway-specific mutations associated with various cancers other than the PI3K/AKT pathway. In this study, we show that the reduction of PDK1 by RNAi in melanoma and colon cancer cell lines activated in the MAPK pathway results in significant cell growth inhibition and apoptosis. Furthermore, PDK1 reduction in tumor cells resulted in impaired PAK kinase signaling, altered actin polymerization, and reduced cell migration. These studies show that PDK1 plays a pivotal role in MAPK and PI3K signaling in tumor cells.

Disruption of the NHR4 domain structure in AML1-ETO abrogates SON binding and promotes leukemogenesis.

AML1-ETO is generated from t(8;21)(q22;q22), which is a common form of chromosomal translocation associated with development of acute myeloid leukemia (AML). Although full-length AML1-ETO alone fails to promote leukemia because of its detrimental effects on cell proliferation, an alternatively spliced isoform, AML1-ETO9a, without its C-terminal NHR3/NHR4 domains, strongly induces leukemia. However, full-length AML1-ETO is a major form of fusion product in many t(8;21) AML patients, suggesting additional molecular mechanisms of t(8;21)-related leukemogenesis. Here, we report that disruption of the zinc-chelating structure in the NHR4 domain of AML1-ETO by replacing only one critical amino acid leads to rapid onset of leukemia, demonstrating that the NHR4 domain with the intact structure generates inhibitory effects on leukemogenesis. Furthermore, we identified SON, a DNA/RNA-binding domain containing protein, as a novel NHR4-interacting protein. Knock-down of SON by siRNA resulted in significant growth arrest, and disruption of the interaction between AML1-ETO and endogenous SON rescued cells from AML1-ETO-induced growth arrest, suggesting that SON is an indispensable factor for cell growth, and AML1-ETO binding to SON may trigger signals inhibiting leukemogenesis. In t(8;21) AML patient-derived primary leukemic cells and cell lines, abnormal cytoplasmic localization of SON was detected, which may keep cells proliferating in the presence of full-length AML1-ETO. These results uncovered the crucial role of the NHR4 domain in determination of cellular fate during AML1-ETO-associated leukemogenesis.

t(8;21)(q22;q22) Fusion proteins preferentially bind to duplicated AML1/RUNX1 DNA-binding sequences to differentially regulate gene expression.

Chromosome abnormalities are frequently associated with cancer development. The 8;21(q22;q22) chromosomal translocation is one of the most common chromosome abnormalities identified in leukemia. It generates fusion proteins between AML1 and ETO. Since AML1 is a well-defined DNA-binding protein, AML1-ETO fusion proteins have been recognized as DNA-binding proteins interacting with the same consensus DNA-binding site as AML1. The alteration of AML1 target gene expression due to the presence of AML1-ETO is related to the development of leukemia. Here, using a 25-bp random double-stranded oligonucleotide library and a polymerase chain reaction (PCR)-based DNA-binding site screen, we show that compared with native AML1, AML1-ETO fusion proteins preferentially bind to DNA sequences with duplicated AML1 consensus sites. This finding is further confirmed by both in vitro and in vivo DNA-protein interaction assays. These results suggest that AML1-ETO fusion proteins have a selective preference for certain AML1 target genes that contain multimerized AML1 consensus sites in their regulatory elements. Such selected regulation provides an important molecular mechanism for the dysregulation of gene expression during cancer development.

Cell type dependent regulation of multidrug resistance-1 gene expression by AML1-ETO.

The AML1-ETO fusion protein is generated from the 8;21 chromosome translocation that is commonly identified in acute myeloid leukemia. AML1-ETO is a DNA binding transcription factor and has been demonstrated to play a critical role in promoting leukemogenesis. Therefore, it is important to define the molecular mechanism of AML1-ETO in the regulation of gene expression. Here, we report that the effect of AML1-ETO on the promoter of multidrug resistance-1 (MDR1) gene, a known AML1-ETO target, is highly cell type specific. Besides observing repression of the MDR1 promoter in C33A and CV-1 cells as reported previously, AML1-ETO strongly activated the promoter in K562 and B210 cells. More importantly, this activation required both the AML1 and ETO portions of the fusion protein, but did not depend on the AML1 binding site in MDR1 promoter. Furthermore, results from promoter deletion analysis and chromatin immunoprecipitation assays suggested that this activation effect was likely through the influence of the general transcription machinery rather than promoter-specific factors. Based on these data, we propose that AML1-ETO may have opposing effects on gene expression depending on the various conditions of the cellular environment.