Post-Transplantation Cyclophosphamide-Based Graft-versus-Host Disease Prophylaxis.

BACKGROUND: In patients undergoing allogeneic hematopoietic stem-cell transplantation (HSCT), a calcineurin inhibitor plus methotrexate has been a standard prophylaxis against graft-versus-host disease (GVHD). A phase 2 study indicated the potential superiority of a post-transplantation regimen of cyclophosphamide, tacrolimus, and mycophenolate mofetil. METHODS: In a phase 3 trial, we randomly assigned adults with hematologic cancers in a 1:1 ratio to receive cyclophosphamide-tacrolimus-mycophenolate mofetil (experimental prophylaxis) or tacrolimus-methotrexate (standard prophylaxis). The patients underwent HSCT from an HLA-matched related donor or a matched or 7/8 mismatched (i.e., mismatched at only one of the HLA-A, HLA-B, HLA-C, and HLA-DRB1 loci) unrelated donor, after reduced-intensity conditioning. The primary end point was GVHD-free, relapse-free survival at 1 year, assessed in a time-to-event analysis, with events defined as grade III or IV acute GVHD, chronic GVHD warranting systemic immunosuppression, disease relapse or progression, and death from any cause. RESULTS: In a multivariate Cox regression analysis, GVHD-free, relapse-free survival was significantly more common among the 214 patients in the experimental-prophylaxis group than among the 217 patients in the standard-prophylaxis group (hazard ratio for grade III or IV acute GVHD, chronic GVHD, disease relapse or progression, or death, 0.64; 95% confidence interval [CI], 0.49 to 0.83; P = 0.001). At 1 year, the adjusted GVHD-free, relapse-free survival was 52.7% (95% CI, 45.8 to 59.2) with experimental prophylaxis and 34.9% (95% CI, 28.6 to 41.3) with standard prophylaxis. Patients in the experimental-prophylaxis group appeared to have less severe acute or chronic GVHD and a higher incidence of immunosuppression-free survival at 1 year. Overall and disease-free survival, relapse, transplantation-related death, and engraftment did not differ substantially between the groups. CONCLUSIONS: Among patients undergoing allogeneic HLA-matched HSCT with reduced-intensity conditioning, GVHD-free, relapse-free survival at 1 year was significantly more common among those who received cyclophosphamide-tacrolimus-mycophenolate mofetil than among those who received tacrolimus-methotrexate. (Funded by the National Heart, Lung, and Blood Institute and others; BMT CTN 1703 ClinicalTrials.gov number, NCT03959241.).

Utility of targeted gene sequencing to differentiate myeloid malignancies from other cytopenic conditions.

The National Heart, Lung, and Blood Institute-funded National MDS Natural History Study (NCT02775383) is a prospective cohort study enrolling patients with cytopenia with suspected myelodysplastic syndromes (MDS) to evaluate factors associated with disease. Here, we sequenced 53 genes in bone marrow samples harvested from 1298 patients diagnosed with myeloid malignancy, including MDS and non-MDS myeloid malignancy or alternative marrow conditions with cytopenia based on concordance between independent histopathologic reviews (local, centralized, and tertiary to adjudicate disagreements when needed). We developed a novel 2-stage diagnostic classifier based on mutational profiles in 18 of 53 sequenced genes that were sufficient to best predict a diagnosis of myeloid malignancy and among those with a predicted myeloid malignancy, predict whether they had MDS. The classifier achieved a positive predictive value (PPV) of 0.84 and negative predictive value (NPV) of 0.8 with an area under the receiver operating characteristic curve (AUROC) of 0.85 when classifying patients as having myeloid vs no myeloid malignancy based on variant allele frequencies (VAFs) in 17 genes and a PPV of 0.71 and NPV of 0.64 with an AUROC of 0.73 when classifying patients as having MDS vs non-MDS malignancy based on VAFs in 10 genes. We next assessed how this approach could complement histopathology to improve diagnostic accuracy. For 99 of 139 (71%) patients (PPV of 0.83 and NPV of 0.65) with local and centralized histopathologic disagreement in myeloid vs no myeloid malignancy, the classifier-predicted diagnosis agreed with the tertiary pathology review (considered the internal gold standard).

Secondary Neoplasms After Hematopoietic Cell Transplant for Sickle Cell Disease.

PURPOSE: To report the incidence and risk factors for secondary neoplasm after transplantation for sickle cell disease. METHODS: Included are 1,096 transplants for sickle cell disease between 1991 and 2016. There were 22 secondary neoplasms. Types included leukemia/myelodysplastic syndrome (MDS; n = 15) and solid tumor (n = 7). Fine-Gray regression models examined for risk factors for leukemia/MDS and any secondary neoplasm. RESULTS: The 10-year incidence of leukemia/MDS was 1.7% (95% CI, 0.90 to 2.9) and of any secondary neoplasm was 2.4% (95% CI, 1.4 to 3.8). After adjusting for other risk factors, risks for leukemia/MDS (hazard ratio, 22.69; 95% CI, 4.34 to 118.66; P = .0002) or any secondary neoplasm (hazard ratio, 7.78; 95% CI, 2.20 to 27.53; P = .0015) were higher with low-intensity (nonmyeloablative) regimens compared with more intense regimens. All low-intensity regimens included total-body irradiation (TBI 300 or 400 cGy with alemtuzumab, TBI 300 or 400 cGy with cyclophosphamide, TBI 200, 300, or 400 cGy with cyclophosphamide and fludarabine, or TBI 200 cGy with fludarabine). None of the patients receiving myeloablative and only 23% of those receiving reduced-intensity regimens received TBI. CONCLUSION: Low-intensity regimens rely on tolerance induction and establishment of mixed-donor chimerism. Persistence of host cells exposed to low-dose radiation triggering myeloid malignancy is one plausible etiology. Pre-existing myeloid mutations and prior inflammation may also contribute but could not be studied using our data source. Choosing conditioning regimens likely to result in full-donor chimerism may in part mitigate the higher risk for leukemia/MDS.

Bringing Patient and Caregivers Voices to the Clinical Trial Chorus: A Report From the BMT CTN Patient and Caregiver Advocacy Task Force.

The Blood and Marrow Transplant Clinical Trials Network (BMT CTN), funded by the National Heart, Lung, and Blood Institute and the National Cancer Institute for more than 2 decades, is focused on improving the outcomes of hematopoietic cell transplantation (HCT) and other cellular therapies. It answered critical questions about conditioning intensity, donor choice, graft-versus-host disease prevention and treatment, and relapse mitigation strategies in a manner made possible by an extensive network of centers that have enrolled more than 16,000 patients to more than 55 trials. Although the BMT CTN has engaged patients in a variety of ways since its establishment, there is a growing realization that increasing that engagement and including caregivers offers many additional benefits to patients and investigators alike. Bringing the voice of patients and caregivers to clinical trial design is likely to enhance trial participation and reduce barriers to recruitment/retention. Unless clinical trials are designed with unique considerations of patients who have socioeconomic and access challenges, these populations will remain under-represented in HCT trials with limited generalizability of results. The next main frontier in our field is patient and caregiver access to high-quality HCT facilities coupled with the opportunity to participate in relevant, meaningful clinical research. In 2021, the BMT CTN Executive Committee convened a Patient and Caregiver Advocacy Task Force with a diverse membership representing a variety of stakeholders. The charge to the Task Force was to provide achievable recommendations for incorporating patient input at all steps of clinical trial development from initial concept to dissemination of results. Four focus areas were identified: (1) patient and caregiver input in research portfolio; (2) patient engagement in informed consent, protocol development and trial conduct; (3) communication to patients about trial progress, primary outcomes and secondary analyses; and (4) increased awareness among patients who may be considering BMT or cell therapy about BMT CTN trials. Three specific initiatives were considered as high priority by the Task Force: Fostering patient and caregiver engagement in development of the research portfolio and protocols; Developing communication plans for ongoing studies; and Simplifying the process for informed consent to make it more patient friendly. The BMT CTN has established a patient and caregiver advocacy committee that is tasked with developing concrete steps to incorporate recommendations of the BMT CTN Task Force in its current and future work. The BMT CTN recognizes patient and caregivers are valuable partners whose voice will enhance the conduct of impactful trials in BMT and cell therapy.

Pulmonary Complications of Pediatric Hematopoietic Cell Transplantation. A National Institutes of Health Workshop Summary.

Approximately 2,500 pediatric hematopoietic cell transplants (HCTs), most of which are allogeneic, are performed annually in the United States for life-threatening malignant and nonmalignant conditions. Although HCT is undertaken with curative intent, post-HCT complications limit successful outcomes, with pulmonary dysfunction representing the leading cause of nonrelapse mortality. To better understand, predict, prevent, and/or treat pulmonary complications after HCT, a multidisciplinary group of 33 experts met in a 2-day National Institutes of Health Workshop to identify knowledge gaps and research strategies most likely to improve outcomes. This summary of Workshop deliberations outlines the consensus focus areas for future research.

Priorities for Improving Outcomes for Nonmalignant Blood Diseases: A Report from the Blood and Marrow Transplant Clinical Trials Network.

Nonmalignant blood diseases such as bone marrow failure disorders, immune dysregulation disorders, and hemoglobinopathies often lead to shortened life spans and poor quality of life. Many of these diseases can be cured with allogeneic hematopoietic cell transplantation, but patients are often not offered the procedure because of perceived insufficient efficacy and/or excess toxicity. In 2018, the Blood and Marrow Transplant Clinical Trials Network convened a task force to identify the most urgently needed yet feasible clinical trials with potential to improve the outcomes for patients with nonmalignant diseases. This report summarizes the task force discussions and specifies the network plans for clinical trial development for nonmalignant blood diseases.

The National MDS Natural History Study: design of an integrated data and sample biorepository to promote research studies in myelodysplastic syndromes.

Myelodysplastic syndromes (MDS), a spectrum of heterogeneous hematopoietic stem cell diseases, vary in clinical severity, response to therapy, and propensity toward progression to acute myeloid leukemia. These are acquired clonal disorders resulting from somatic mutations within the hematopoietic stem or progenitor cell population. Understanding the natural history and the risk of developing leukemia and other adverse outcomes is dependent on access to well-annotated biospecimens linked to robust clinical and molecular data. To facilitate the acquisition and distribution of MDS biospecimens to the wider scientific community and support scientific discovery in this disease, the National MDS Natural History study was initiated by the National Heart, Lung, and Blood Institute (NHLBI) and is being conducted in collaboration with community hospitals and academic medical centers supported by the National Cancer Institute (NCI). The study will recruit up to 2000 MDS patients or overlapping myeloproliferative neoplasms (MDS/MPN) and up to 500 cases of idiopathic cytopenia of undetermined significance (ICUS). The National MDS Natural History Study (NCT02775383) will offer the world's largest disease-focused tissue biobank linked to longitudinal clinical and molecular data in MDS. Here, we report on the study design features and describe the vanguard phase of 200 cases. The study assembles a comprehensive clinical database, quality of life results, laboratory data, histopathology slides and images, genetic information, hematopoietic and germline tissues representing high-quality biospecimens and data from diverse centers across the United States. These resources will be available to the scientific community for investigator-initiated research.

A trial of unrelated donor marrow transplantation for children with severe sickle cell disease.

Children with sickle cell disease experience organ damage, impaired quality of life, and premature mortality. Allogeneic bone marrow transplant from an HLA-matched sibling can halt disease progression but is limited by donor availability. A Blood and Marrow Transplant Clinical Trials Network (BMT CTN) phase 2 trial conducted from 2008 to 2014 enrolled 30 children aged 4 to 19 years; 29 were eligible for evaluation. The primary objective was 1-year event-free survival (EFS) after HLA allele-matched (at HLA-A, -B, -C, and -DRB1 loci) unrelated donor transplant. The conditioning regimen included alemtuzumab, fludarabine, and melphalan. Graft-versus-host disease (GVHD) prophylaxis included calcineurin inhibitor, short-course methotrexate, and methylprednisolone. Transplant indications included stroke (n = 12), transcranial Doppler velocity >200 cm/s (n = 2), ≥3 vaso-occlusive pain crises per year (n = 12), or ≥2 acute chest syndrome episodes (n = 4) in the 2 years preceding enrollment. Median follow-up was 26 months (range, 12-62 months); graft rejection was 10%. The 1- and 2-year EFS rates were 76% and 69%, respectively. The corresponding rates for overall survival were 86% and 79%. The day 100 incidence rate of grade II-IV acute GVHD was 28%, and the 1-year incidence rate of chronic GVHD was 62%; 38% classified as extensive. There were 7 GVHD-related deaths. A 34% incidence of posterior reversible encephalopathy syndrome was noted in the first 6 months. Although the 1-year EFS met the prespecified target of ≥75%, this regimen cannot be considered sufficiently safe for widespread adoption without modifications to achieve more effective GVHD prophylaxis. The BMT CTN #0601 trial was registered at www.clinicaltrials.gov as #NCT00745420.

Cyclophosphamide conditioning in patients with severe aplastic anaemia given unrelated marrow transplantation: a phase 1-2 dose de-escalation study.

BACKGROUND: The optimum preparative regimen for unrelated donor marrow transplantation in patients with severe aplastic anaemia remains to be established. We investigated whether the combination of fludarabine, anti-thymocyte globulin, and total body irradiation (TBI) would enable reduction of the cyclophosphamide dose to less than 200 mg/kg while maintaining engraftment and having a survival similar to or better than that with standard regimens using a cyclophosphamide dose of 200 mg/kg (known to be associated with significant organ toxicity) for unrelated donor transplantation for severe aplastic anaemia. We have previously shown that cyclophosphamide at 150 mg/kg resulted in excess toxicity and its omission (0 mg/kg) resulted in unacceptable graft failure (three of three patients had secondary graft failure). Here we report results for the 50 mg/kg and 100 mg/kg cohorts. METHODS: In a multicentre phase 1-2 study, patients (aged ≤65 years) with severe aplastic anaemia, adequate organ function, and an unrelated adult marrow donor HLA matched at the allele level for HLA A, B, C, and DRB1 or mismatched at a single HLA locus received bone marrow grafts from unrelated donors. All patients received anti-thymocyte globulin (rabbit derived 3 mg/kg per day, intravenously, on days -4 to -2, or equine derived 30 mg/kg per day, intravenously, on days -4 to -2), fludarabine (30 mg/m(2) per day, intravenously, on days -5 to -2), and TBI (2 Gy). Cyclophosphamide dosing started at 150 mg/kg and was de-escalated in steps of 50 mg/kg (to 100 mg/kg, 50 mg/kg, and 0 mg/kg). The primary endpoint was the selection of the optimum cyclophosphamide dose based on assessments of graft failure (primary or secondary), toxicity, and early death during 100 days of follow-up after the transplant; this is the planned final analysis for the primary endpoint. This trial is registered with ClinicalTrials.gov, number NCT00326417. FINDINGS: 96 patients had bone marrow transplant. At day 100, 35 (92%) of 38 patients were engrafted and alive in the cyclophosphamide 50 mg/kg cohort and 35 (85%) of 41 in the 100 mg/kg cohort. Cyclophosphamide 50 mg/kg and 100 mg/kg resulted in posterior means for fatality without graft failure of 0·7% (credible interval 0-3·3) and 1·4% (0-4·9), respectively. Three patients (8%) had graft failure with cyclophosphamide 50 mg/kg and six (15%) with cyclophosphamide 100 mg/kg. Four (11%) patients had major regimen-related toxicity with cyclophosphamide 50 mg/kg and nine (22%) with cyclophosphamide 100 mg/kg. The most common organ toxicity was pulmonary (grade 3 or 4 dyspnoea or hypoxia including mechanical ventilation), and occurred in three (8%) and four (10%) patients given cyclophosphamide 50 mg/kg and 100 mg/kg, respectively. INTERPRETATION: Cyclophosphamide at 50 mg/kg and 100 mg/kg with TBI 2 Gy, fludarabine, and anti-thymocyte globulin results in effective conditioning and few early deaths after unrelated donor transplantation for severe aplastic anaemia. These doses of cyclophosphamide provide a framework for further regimen optimisation strategies. FUNDING: US National Heart, Lung, and Blood Institute and National Cancer Institute.

Randomized, double-blind, placebo-controlled trial of soluble tumor necrosis factor receptor: enbrel (etanercept) for the treatment of idiopathic pneumonia syndrome after allogeneic stem cell transplantation: blood and marrow transplant clinical trials network protocol.

Idiopathic pneumonia syndrome (IPS) is a diffuse, noninfectious lung injury that occurs acutely after allogeneic hematopoietic cell transplantation (HCT). IPS-related mortality has been historically high (>50%) despite treatment with systemic corticosteroids and supportive care measures. We have now examined the role of tumor necrosis factor inhibition in a randomized, double-blind, placebo-controlled trial of corticosteroids with etanercept or placebo. Thirty-four subjects (≥18 years) with IPS after HCT were randomized to receive methylprednisolone (2 mg/kg/day) plus etanercept (0.4 mg/kg twice weekly × 4 weeks; n = 16) or placebo (n = 18). No active infections and a pathogen-negative bronchoscopy were required at study entry. Response (alive, with complete discontinuation of supplemental oxygen support) and overall survival were examined. This study, originally planned to accrue 120 patients, was terminated prematurely due to slow accrual. In the limited number of patients examined, there were no differences in response rates at day 28 of study. Ten of 16 patients (62.5% [95% confidence interval {CI}, 35.4% to 84.8%]) receiving etanercept and 12 of 18 patients (66.7% [95% CI, 41.0% to 86.7%]) receiving placebo met the day 28 response definition (P = 1.00). The median survival was 170 days (95% CI, 11 to 362) with etanercept versus 64 days (95% CI, 26 to 209) with placebo (P = .51). Among responders, the median time to discontinuation of supplemental oxygen was 9 days (etanercept) versus 7 days (placebo). Therapy was well tolerated, with 1 toxicity-related death from infectious pneumonia in the placebo arm. The treatment of IPS with corticosteroids in adult HCT recipients was associated with high early response rates (>60%) compared with historical reports, with poor overall survival. The addition of etanercept did not lead to further increases in response, although the sample size of this truncated trial preclude a definitive conclusion.

Fludarabine-based conditioning for marrow transplantation from unrelated donors in severe aplastic anemia: early results of a cyclophosphamide dose deescalation study show life-threatening adverse events at predefined cyclophosphamide dose levels.

Excessive adverse events were encountered in a Phase I/II study of cyclophosphamide (CY) dose deescalation in a fludarabine-based conditioning regimen for bone marrow transplantation from unrelated donors in patients with severe aplastic anemia. All patients received fixed doses of antithymocyte globulin, fludarabine, and low-dose total body irradiation. The starting CY dose was 150 mg/kg, with deescalation to 100 mg/kg, 50 mg/kg, or 0 mg/kg. CY dose level 0 mg/kg was closed due to graft failure in 3 of 3 patients. CY dose level 150 mg/kg was closed due to excessive organ toxicity (n = 6) or viral pneumonia (n = 1), resulting in the death of 7 of 14 patients. CY dose levels 50 and 100 mg/kg remain open. Thus, CY at doses of 150 mg/kg in combination with total body irradiation (2 Gy), fludarabine (120 mg/m(2)), and antithymocyte globulin was associated with excessive organ toxicity.

Optimization of therapy for severe aplastic anemia based on clinical, biologic, and treatment response parameters: conclusions of an international working group on severe aplastic anemia convened by the Blood and Marrow Transplant Clinical Trials Network, March 2010.

Although recent advances in therapy offer the promise for improving survival in patients with severe aplastic anemia (SAA), the small size of the patient population, lack of a mechanism in North America for longitudinal follow-up of patients, and inadequate cooperation among hematologists, scientists, and transplant physicians remain obstacles to conducting large studies that would advance the field. To address this issue, the Blood and Marrow Transplant Clinical Trials Network (BMT CTN) convened a group of international experts in March 2010 to define the most important questions in the basic science, immunosuppressive therapy (IST), and bone marrow transplantation (BMT) of SAA and propose initiatives to facilitate clinical and biologic research. Key conclusions of the working group were: (1) new patients should obtain accurate, expert diagnosis and early identification of biologic risk; (2) a population-based SAA outcomes registry should be established in North America to collect data on patients longitudinally from diagnosis through and after treatment; (3) a repository of biologic samples linked to the clinical data in the outcomes registry should be developed; (4) innovative approaches to unrelated donor BMT that decrease graft-versus-host disease are needed; and (5) alternative donor transplantation approaches for patients lacking HLA-matched unrelated donors must be improved. A partnership of BMT, IST, and basic science researchers will develop initiatives and partner with advocacy and funding organizations to address these challenges. Collaboration with similar study groups in Europe and Asia will be pursued.

Etanercept, mycophenolate, denileukin, or pentostatin plus corticosteroids for acute graft-versus-host disease: a randomized phase 2 trial from the Blood and Marrow Transplant Clinical Trials Network.

Acute graft-versus-host disease (aGVHD) is the primary limitation of allogeneic hematopoietic cell transplantation. Corticosteroids remain the standard initial therapy, yet only 25% to 41% of patients completely respond. This randomized, 4-arm, phase 2 trial was designed to identify the most promising agent(s) for initial therapy for aGVHD. Patients were randomized to receive methylprednisolone 2 mg/kg per day plus etanercept, mycophenolate mofetil (MMF), denileukin diftitox (denileukin), or pentostatin. Patients (n = 180) were randomized; their median age was 50 years (range, 7.5-70 years). Myeloablative conditioning represented 66% of transplants. Grafts were peripheral blood (61%), bone marrow (25%), or umbilical cord blood (14%); 53% were from unrelated donors. Patients who received MMF for prophylaxis (24%) were randomized to a non-MMF arm. At randomization, aGVHD was grade I to II (68%), III to IV (32%), and (53%) had visceral organ involvement. Day 28 complete response rates were etanercept 26%, MMF 60%, denileukin 53%, and pentostatin 38%. Corresponding 9-month overall survival was 47%, 64%, 49%, and 47%, respectively. Cumulative incidences of severe infections were as follows: etanercept 48%, MMF 44%, denileukin 62%, and pentostatin 57%. Efficacy and toxicity data suggest the use of MMF plus corticosteroids is the most promising regimen to compare against corticosteroids alone in a definitive phase 3 trial. This study is registered at http://www.clinicaltrials.gov as NCT00224874.

Dexamethasone-mediated repression of MUC5AC gene expression in human lung epithelial cells.

Glucocorticoids regulate gene expression via binding of the ligand-activated glucocorticoid receptor (GR) to glucocorticoid-responsive elements (GRE) in target gene promoters. The MUC5AC gene, which encodes the protein backbone of an abundant secreted airway mucin, has several putative GRE cis-elements in its 5' sequence. Mechanism(s) whereby glucocorticoids regulate mucin genes have not previously been described. In this study, the glucocorticoid dexamethasone (Dex) decreased MUC5AC mRNA abundance in A549 and NCI-H292 cell lines and primary differentiated normal bronchial epithelial cells by 50-80%, suggesting a common mechanism of MUC5AC gene repression in human lung epithelial cells. Kinetic analyses showed that MUC5AC mRNA was not significantly decreased until 6 h after Dex exposure, and that nuclear translocation of GR was biphasic, suggesting that Dex-mediated cis-repression of MUC5AC gene expression was a delayed response of GR translocation. Transfection analyses demonstrated that Dex transcriptionally repressed the MUC5AC promoter. Electrophoretic mobility shift assays with wild-type and mutant oligonucleotide probes showed that GR bound to two GRE cis-sites (nucleotides -930 to -912 and -369 to -351) in the MUC5AC promoter. Analyses of mutated MUC5AC promoter constructs demonstrated that NF-kappaB cis-sites were not involved in Dex-mediated repression of MUC5AC. Dex did not alter mRNA stability of MUC5AC transcripts. Taken together, the data indicate that Dex transcriptionally mediates repression of MUC5AC gene expression in human lung epithelial cells at quiescent states after binding of GR to one or more GRE cis-elements in the MUC5AC promoter.

Convergence of RNA cis elements and cellular polyadenylation factors in the regulation of human cytomegalovirus UL37 exon 1 unspliced RNA production.

The human cytomegalovirus (HCMV) UL36-38 immediate early (IE) locus encodes proteins required for its growth. The UL37 promoter drives production of an unspliced and several alternatively spliced RNAs. The UL37 exon 1 (UL37x1) unspliced RNA is abundant from IE to late times of HCMV infection, whereas the UL37 spliced RNAs are markedly less abundant. Production of the UL37x1 unspliced RNA requires polyadenylation (PA) at nucleotide 50998, which lies within intron 1, upstream of the UL37 exon 2 (UL37x2) acceptor. The physical proximity of its cis elements suggests steric hindrance between PA and splicing machineries for UL37 pre-mRNA. To test this possibility, we generated site-specific mutants in Target 1 PA and RNA splicing cis elements and compared the PA and splicing efficiencies of mutant RNAs with those of wild-type RNA. The mutually exclusive processing events of UL37x1 PA and UL37x1-UL37x2 splicing have been accurately recapitulated in transfected permissive human fibroblasts (HFFs) expressing a Target 1 minigene RNA, which contains the required splicing and PA cis elements. Two mutants in the invariant PA signal dramatically decreased UL37x1 PA as expected and, concomitantly, increased the efficiency of UL37x1-UL37x2 RNA splicing. Consistent with these results, changes to consensus UL37x1 donor and UL37x2 acceptor sites increased the efficiency of UL37x1-UL37x2 RNA splicing but decreased the efficiency of UL37x1 PA. Moreover, HCMV infection of HFFs increased the abundance of the PA cleavage stimulatory factor CstF-64, the potent splicing suppressor PTB, and the hypophosphorylated form of the splicing factor SF2 at 4 h postinfection. Induction of these factors further favors production of the UL37x1 unspliced RNA over that of the spliced RNAs. Taken together, these results suggest that there is a convergence in UL37 RNA regulation by cis elements and cellular proteins which favors production of the UL37x1 unspliced RNA during HCMV infection at the posttranscriptional level.

Duplication of U3 sequences in the long terminal repeat of mink cell focus-inducing viruses generates redundancies of transcription factor binding sites important for the induction of thymomas.

The ability of mink cell focus-inducing (MCF) viruses to induce thymomas is determined, in part, by transcriptional enhancers in the U3 region of their long terminal repeats (LTRs). To elucidate sequence motifs important for enhancer function in vivo, we injected newborn mice with MCF 1dr (supF), a weakly pathogenic, molecularly tagged (supF) MCF virus containing only one copy of a sequence that is present as two copies (known as the directly repeated [DR] sequence) in the U3 region of MCF 247 and analyzed LTRs from supF-tagged proviruses in two resulting thymomas. Tagged proviruses integrated upstream and in the reverse transcriptional orientation relative to c-myc provided the focus of our studies. These proviruses are thought to contribute to thymoma induction by enhancer-mediated deregulation of c-myc expression. The U3 region in a tagged LTR in one thymoma was cloned and sequenced. Relative to MCF 1dr (supF), the cloned U3 region contained an insertion of 140 bp derived predominantly from the DR sequence of the injected virus. The inserted sequence contains predicted binding sites for transcription factors known to regulate the U3 regions of various murine leukemia viruses. Similar constellations of binding sites were duplicated in two proviral LTRs integrated upstream from c-myc in a second thymoma. We replaced the U3 sequences in an infectious molecular clone of MCF 247 with the cloned proviral U3 sequences from the first thymoma and generated an infectious chimeric virus, MCF ProEn. When injected into neonatal AKR mice, MCF ProEn was more pathogenic than the parental virus, MCF 1dr (supF), as evidenced by the more rapid onset and higher incidence of thymomas. Molecular analyses of the resultant thymomas indicated that the U3 region of MCF ProEn was genetically stable. These data suggest that the arrangement and/or redundancy of transcription factor binding sites generated by specific U3 sequence duplications are important to the biological events mediated by MCF proviruses integrated near c-myc that contribute to transformation.

Ikaros, a lymphoid-cell-specific transcription factor, contributes to the leukemogenic phenotype of a mink cell focus-inducing murine leukemia virus.

Mink cell focus-inducing (MCF) viruses induce T-cell lymphomas in AKR/J strain mice. MCF 247, the prototype of this group of nonacute murine leukemia viruses, transforms thymocytes, in part, by insertional mutagenesis and enhancer-mediated dysregulation of cellular proto-oncogenes. The unique 3' (U3) regions in the long terminal repeats of other murine leukemia viruses contain transcription factor binding sites known to be important for enhancer function and for the induction of T-cell lymphomas. Although transcription factor binding sites important for the biological properties of MCF 247 have not been identified, pathogenesis studies from our laboratory suggested to us that binding sites for Ikaros, a lymphoid-cell-restricted transcriptional regulator, affect the biological properties of MCF 247. In this report, we demonstrate that Ikaros binds to predicted sites in U3 sequences of MCF 247 and that site-directed mutations in these sites greatly diminish this binding in vitro. Consistent with these findings, ectopic expression of Ikaros in murine cells that do not normally express this protein significantly increases transcription from the viral promoter in transient gene expression assays. Moreover, site-directed mutations in specific Ikaros-binding sites reduce this activity in T-cell lines that express Ikaros endogenously. To determine whether the Ikaros-binding sites are functional in vivo, we inoculated newborn mice with a variant MCF virus containing a mutant Ikaros-binding site. The variant virus replicated in thymocytes less efficiently and induced lymphomas with a delayed onset compared to the wild-type virus. These data are consistent with the hypothesis that the Ikaros-binding sites in the U3 region of MCF 247 are functional and cooperate with other DNA elements for optimal enhancer function in vivo.