HTLV-1 persistence and leukemogenesis: A game of hide-and-seek with the host immune system

The HTLV-I oncoprotein Tax inactivates the tumor suppressor FBXW7

Human T-cell leukemia virus type 1 (HTLV-I) is the etiological agent of adult T-cell leukemia (ATL). Mutational analysis has demonstrated that the tumor suppressor, F-box and WD repeat domain containing 7 (FBXW7/FBW7/CDC4), is mutated in primary ATL patients. However, even in the absence of genetic mutations, FBXW7 substrates are stabilized in ATL cells, suggesting additional mechanisms can prevent FBXW7 functions. Here, we report that the viral oncoprotein Tax represses FBXW7 activity, resulting in the stabilization of activated Notch intracellular domain, c-MYC, Cyclin E, and myeloid cell leukemia sequence 1 (BCL2-related) (Mcl-1). Mechanistically, we demonstrate that Tax directly binds to FBXW7 in the nucleus, effectively outcompeting other targets for binding to FBXW7, resulting in decreased ubiquitination and degradation of FBXW7 substrates. In support of the nuclear role of Tax, a non-degradable form of the nuclear factor kappa B subunit 2 (NFκB2/p100) was found to delocalize Tax to the cytoplasm, thereby preventing Tax interactions with FBXW7 and Tax-mediated inhibition of FBXW7. Finally, we characterize a Tax mutant that is unable to interact with FBXW7, unable to block FBXW7 tumor suppressor functions, and unable to effectively transform fibroblasts. These results demonstrate that HTLV-I Tax can inhibit FBXW7 functions without genetic mutations to promote an oncogenic state. These results suggest that Tax-mediated inhibition of FBXW7 is likely critical during the early stages of the cellular transformation process.

IMPORTANCE

F-box and WD repeat domain containing 7 (FBXW7), a critical tumor suppressor of human cancers, is frequently mutated or epigenetically suppressed. Loss of FBXW7 functions is associated with stabilization and increased expression of oncogenic factors such as Cyclin E, c-Myc, Mcl-1, mTOR, Jun, and Notch. In this study, we demonstrate that the human retrovirus human T-cell leukemia virus type 1 oncoprotein Tax directly interacts with FBXW7, effectively outcompeting other targets for binding to FBXW7, resulting in decreased ubiquitination and degradation of FBXW7 cellular substrates. We further demonstrate that a Tax mutant unable to interact with and inactivate FBXW7 loses its ability to transform primary fibroblasts. Collectively, our results describe a novel mechanism used by a human tumor virus to promote cellular transformation.

Trans-Activation of the Coactivator-Associated Arginine Methyltransferase 1 (Carm1) Gene by the Oncogene Product Tax of Human T-Cell Leukemia Virus Type 1

Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia/lymphoma. The oncogene product Tax of HTLV-I is thought to play crucial roles in leukemogenesis by promoting proliferation of the virus-infected cells through activation of growth-promoting genes. These genes code for growth factors and their receptors, cytokines, cell adhesion molecules, growth signal transducers, transcription factors and cell cycle regulators. We show here that Tax activates the gene coding for coactivator-associated arginine methyltransferase 1 (CARM1), which epigenetically enhances gene expression through methylation of histones. Tax activated the Carm1 gene and increased protein expression, not only in human T-cell lines but also in normal peripheral blood lymphocytes (PHA-PBLs). Tax increased R17-methylated histone H3 on the target gene IL-2Rα, concomitant with increased expression of CARM1. Short hairpin RNA (shRNA)-mediated knockdown of CARM1 decreased Tax-mediated induction of IL-2Rα and Cyclin D2 gene expression, reduced E2F activation and inhibited cell cycle progression. Tax acted via response elements in intron 1 of the Carm1 gene, through the NF-κB pathway. These results suggest that Tax-mediated activation of the Carm1 gene contributes to leukemogenic target-gene expression and cell cycle progression, identifying the first epigenetic target gene for Tax-mediated trans-activation in cell growth promotion.

Complete Rescue of HTLV-1p12KO Infectivity by Depletion of Monocytes Together with NK and CD8+ T Cells

The transient depletion of monocytes alone prior to exposure of macaques to HTLV-1 enhances both HTLV-1WT (wild type) and HTLV-1p12KO (Orf-1 knockout) infectivity, but seroconversion to either virus is not sustained over time, suggesting a progressive decrease in virus expression. These results raise the hypotheses that either HTLV-1 persistence depends on a monocyte reservoir or monocyte depletion provides a transient immune evasion benefit. To test these hypotheses, we simultaneously depleted NK cells, CD8+ T cells, and monocytes (triple depletion) prior to exposure to HTLV-1WT or HTLV-1p12KO. Remarkably, triple depletion resulted in exacerbation of infection by both viruses and complete rescue of HTLV-1p12KO infectivity. Following triple depletion, we observed rapid and sustained seroconversion, high titers of antibodies against HTLV-1 p24Gag, and frequent detection of viral DNA in the blood and tissues of all animals when compared with depletion of only CD8+ and NK cells, or monocytes alone. The infection of macaques with HTLV-1WT or HTLV-1p12KO was associated with higher plasma levels of IL-10 after 21 weeks, while IL-6, IFN-γ, IL-18, and IL-1β were only elevated in animals infected with HTLV-1WT. The repeat depletion of monocytes, NK, and CD8+ cells seven months following the first exposure to HTLV-1 did not further exacerbate viral replication. These results underscore the contribution of monocytes in orchestrating anti-viral immunity. Indeed, the absence of orf-1 expression was fully compensated by the simultaneous depletion of CD8+ T cells, NK cells, and monocytes, underlining the primary role of orf-1 in hijacking host immunity.

Gene-to-gene coordinated regulation of transcription and alternative splicing by 3D chromatin remodeling upon NF-κB activation

The NF-κB protein p65/RelA plays a pivotal role in coordinating gene expression in response to diverse stimuli, including viral infections. At the chromatin level, p65/RelA regulates gene transcription and alternative splicing through promoter enrichment and genomic exon occupancy, respectively. The intricate ways in which p65/RelA simultaneously governs these functions across various genes remain to be fully elucidated. In this study, we employed the HTLV-1 Tax oncoprotein, a potent activator of NF-κB, to investigate its influence on the three-dimensional organization of the genome, a key factor in gene regulation. We discovered that Tax restructures the 3D genomic landscape, bringing together genes based on their regulation and splicing patterns. Notably, we found that the Tax-induced gene-gene contact between the two master genes NFKBIA and RELA is associated with their respective changes in gene expression and alternative splicing. Through dCas9-mediated approaches, we demonstrated that NFKBIA-RELA interaction is required for alternative splicing regulation and is caused by an intragenic enrichment of p65/RelA on RELA. Our findings shed light on new regulatory mechanisms upon HTLV-1 Tax and underscore the integral role of p65/RelA in coordinated regulation of NF-κB-responsive genes at both transcriptional and splicing levels in the context of the 3D genome.

Clinical characteristics and outcomes of infection with human T-lymphotropic virus in a non-endemic area: a single institution study

Introduction

Understanding of human T-lymphotropic virus (HTLV) remains largely based on epidemiologic and clinical data from endemic areas. Globalization has resulted in migration of persons living with HTLV (PLHTLV) from endemic to non-endemic areas, and a rise of HTLV infection in the United States. Yet, due to the historical rarity of this disease, affected patients are often under- and mis-diagnosed. Thus, we sought to characterize the epidemiology, clinical features, comorbidities, and survival of HTLV-1- or HTLV-2-positive individuals identified in a non-endemic area.

Methods

Our study was a single institution, retrospective case-control analysis of HTLV-1 or HTLV-2 patients between 1998 and 2020. We utilized two HTLV-negative controls, matched for age, sex, and ethnicity, for each HTLV-positive case. We evaluated associations between HTLV infection and several hematologic, neurologic, infectious, and rheumatologic covariates. Finally, clinical factors predictive of overall survival (OS) were assessed.

Results

We identified 38 cases of HTLV infection, of whom 23 were HTLV-1 and 15 were HTLV-2 positive. The majority (~54%) of patients in our control group received HTLV testing for transplant evaluation, compared to ~24% of HTLV-seropositive patients. Co-morbidities associated with HTLV, hepatitis C seropositivity were higher in HTLV-seropositive patients compared to controls (OR 10.7, 95% CI = 3.2-59.0, p < 0.001). Hepatitis C and HTLV co-infection resulted in decreased OS, compared to no infection, hepatitis C infection alone, or HTLV infection alone. Patients with any cancer diagnosis and HTLV infection had worse OS compared to patients with cancer or HTLV alone. HTLV-1 positive patients had lower median OS compared to HTLV-2 patients (47.7 months vs. 77.4 months). In univariate analysis, the hazard for 1-year all-cause mortality was increased among patients with HTLV-seropositivity, adult T-cell leukemia, acute myelogenous leukemia, and hepatitis C infection. When corrected, multivariate analysis showed that HTLV seropositivity was no longer associated with 1 year all-cause mortality; however association with AML and hepatitis C infection remained significant.

Conclusion

HTLV-seropositivity was not associated with increased 1 year mortality in multivariate analysis. However, our study is limited by our small patient sample size, as well as the biased patient control population due to selection factors for HTLV testing.