Abstract
BackgroundNon-Hodgkin lymphoma is of high prevalence among HIV-infected people. In particular, the incidence of HIV-associated Burkitt lymphoma (BL) remains high despite the advent of Highly Active Anti-Retroviral Therapy. Recent evidence shows that serum-soluble HIV proteins can enhance oncogenesis, particularly in lymphoid tissues. This study sought to define the role of HIV protein Negative regulatory factor (Nef) in BL development by assessing its effect on key lymphoma driver genes.MethodsA recombinant Nef protein was used to assess changes in expressions of activation-induced cytidine deaminase (AICDA/AID) and c-MYC in B lymphocytes exposed extracellularly to the protein. Additionally, changes in the promoter activities of these genes were measured using a Nef-expressing cellular model and reporter assays. Confocal microscopy was used to observe c-MYC and AID expression and localization, and genomic integrity via the recruitment of phosphorylated γ-H2AX, in Nef-exposed cells.ResultsmRNA transcription of c-MYC and AICDA were significantly enhanced in lymphoma cells, up to 2-fold for c-MYC and up to 4-fold for AICDA, when exposed to varying concentrations of Nef (0–1000 ng/ml) and for different periods of time (3, 6 and 12 h). The protein expressions of AID and c-MYC followed a similar pattern and these effects were specific to BL but not lymphoblastoid cells. While the promoter activity of c-MYC was enhanced in the presence of Nef in a dose-dependent manner, the same was not observed for AICDA. Both AID and c-MYC accumulated within the cytoplasmic and nuclear spaces of Nef-exposed lymphoma cells, with a concomitant increase in DNA double strand breaks within the genome.ConclusionsExposure to HIV Nef leads to significant increases in AID and c-MYC, leading to genomic instability, potentially enhancing the oncogenic potential of Burkitt lymphoma. Our findings align with that of others to show that HIV proteins can directly contribute to the development and pathogenesis of HIV-associated lymphoma and accounts for the elevated incidence of BL observed in the HIV-infected population.
Highlights
Non-Hodgkin lymphoma is of high prevalence among Human Immunodeficiency Virus (HIV)-infected people
Extracellular exposure to recombinant HIV Negative regulatory factor (Nef) leads to upregulation of c-MYC and Activation-Induced cytidine Deaminase (AID) in lymphoma cells To mimic an in vivo scenario where B cells are exposed to soluble HIV proteins found in the serum of HIV infected individuals, B lymphocytes were extracellularly exposed to varying concentrations of recombinant Nef protein for various lengths of time
The amount of Nef present in the serum of a patient is dependent on several factors including viral load and studies have reported up to 1000 ng/ml of Nef being detected in the serum of HIV positive individuals, and in vitro studies have used up to 1000 ng/ml recombinant Nef protein in exogenous assays [30, 31]
Summary
The incidence of HIV-associated Burkitt lymphoma (BL) remains high despite the advent of Highly Active Anti-Retroviral Therapy. The c-MYC translocation event has been shown to be dependent on expression of the B-cell specific protein Activation-Induced cytidine Deaminase (AID), an essential enzyme in antibody diversification in B cell immune responses [5, 6]. Both somatic hypermutation and class switching require AID which converts deoxycytidines to deoxyuracils in single-stranded DNA. The deregulation of AID activity has been observed especially in cancers of the B cell lineage, and interestingly a study in Diffuse Large B Cell Lymphoma patients who underwent CHOP based chemotherapy (cyclophosphamide, doxorubicin, vincristine, and prednisone) found that elevated AID expression can be used as a marker of unfavourable outcome [7, 8]
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