Abstract
The immune checkpoint HLA-G prevents maternal rejection of the fetus and contributes in cancer invasion and acceptance of allografts. The 5’ and 3’ regulatory regions of the HLA-G gene are polymorphic and balancing selection probably maintains this variability. It is proposed that nucleotide variations may affect the level of HLA-G expression. To investigate this issue we aimed to analyze how haplotypes of the 3’ untranslated region (3’UTR) with highest worldwide frequencies, namely UTR-1, UTR-2, UTR-3, UTR-4, UTR-5, UTR-18 and UTR-7, impact the expression of a luciferase reporter gene in vitro. Experiments performed with the HLA-G positive cell lines JEG-3 (choricarcinoma) and FON (melanoma), and with the HLA-G negative cell lines M8 (melanoma) and U251MG (glioblastoma) showed that the HLA-G 3’UTR polymorphism influences the response to endogenous cellular factors and may vary according to the cell type. UTR-5 and UTR-7 impact the activity of luciferase the most whereas UTR-2, UTR-3, UTR-4, and UTR-18 have intermediate impact, and UTR-1 has the lowest impact. These results corroborate the previous associations between amounts of plasma sHLA-G levels and 3’UTR haplotypes in healthy individuals and reinforce that 3’UTR typing may be a predictor of the genetic predisposition of an individual to express different levels of HLA-G.
Highlights
HLA-G is a major immune checkpoint molecule
Considering the untranslated region (UTR)-1, UTR-2, UTR-3, UTR-4, UTR-5, UTR-18 and UTR-7 haplotypes of HLA-G 3’ untranslated region (3’UTR) segment, we first investigated the influence of the absence or the presence of the three putative polyadenylation signals identified in this region, on the expression of the luciferase reporter gene
These results suggest that the segment downstream position +3165 up to position +3280 may not influence the action of endogenous factors of each cell line and that the location of polyadenylation signals presumably do not affect this action
Summary
The earliest studies on HLA-G demonstrated that this non-classical HLA is expressed on the cell surface of cytotrophoblasts, where it plays a key function in maternal-fetal tolerance [1]. In contrast to normal conditions in which the expression of HLA-G is very restricted to few tissues, ectopic HLA-G expression is common in pathological situations such as cancer, and favors the tumor escape from the patient’s immune surveillance. The mechanisms underlying the normal and ectopic HLA-G expression are partially elucidated. Regulation of HLA-G expression differs from regulation of classical HLA-class I genes [6]. HLA-G expression is regulated both at the transcriptional and post-transcriptional levels [7] and involves at least four alternative transcripts for membrane-bound HLA-G forms (HLA-G1 to HLA-G4) [8, 9] and three alternative transcripts for soluble forms (HLA-G5 to HLA-G7) [10,11,12]
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