The purpose of this meta-analysis was to better characterize and compare the in vitro and in vivo human intracellular transcriptional response to infection with various hantaviruses. Specifically, we identified unique sets of DEGs in both comparisons that have been identified previously, as well as novel DEGs that could provide additional knowledge into the underlying mechanisms of viral pathogenesis. We determined statistically significant signaling pathways that were enriched with the identified DEGs for each set of samples, and subsequently used these pathways to predict potential therapeutics that could be relevant to natural infections.
We believe that this analysis was somewhat hindered by the relatively small number of samples in the public datasets. Specifically, the public data that we used came from HTNV infected tissue, rather than a variety of several hantaviruses. This is somewhat unsurprising due to various logistical reasons including the difficulty and danger of culturing hantavirus in a laboratory setting, or a perceived lack of interest in or access to hantavirus. Such studies can be quite useful in identifying potential causes or compounding factors in HFRS and HCPS. Other hantaviruses, such as Andes virus and Sin nombre virus, may cause far fewer infections each year, but infections by these pathogens have a higher mortality rate. Much research is still needed to determine whether the differential expression patterns vary between species of hantaviruses and between infected human tissue types, but this study attempts to address hantaviruses as a whole in an attempt to identify common expression patterns between different viral species. Our findings could potentially be applied across multiple hantaviruses.
In vivo, we found that of the top 10 genes found, only the NUSAP1 gene, which plays a role in microtubule organization [30], was upregulated. MYBL1 was the top hit, which is a protooncogene associated with promoting piRNA expression, was found to be strongly downregulated.
As stated earlier, of the top 100 identified genes, only OLFM1, CCR7, KLRG1, and PTGDR2 have been identified in previous hantavirus publications. OLFM1, which produces olfactomedin and was found to be associated with hantavirus in a previous study, is also found to be highly expressed in neuronal tissue and may aid in nervous system development; however, relatively little is understood about many of its functions in other cell types [31]. Interestingly, while this gene is expressed in monocytes, its function is unknown and may warrant future investigation [32]. Additionally, three of the top 10 genes found in this analysis, ZNF365, NOG and AMIGO1, regulate proper neuronal development [33–36]. Hantavirus rarely affects the nervous system and has not been observed to directly infect nervous tissue, but these genes may also play a role in development in other cell types which can be affected during hantavirus infection.
CCR7, or C-C Motif Chemokine Receptor seven, is upregulated in lymphocytes and monocytes during a variety of viral infections. A previous study showed that CCR7 induces monocyte egress during hantavirus infection, and therefore a decrease in the number of CCR7-expressing monocytes occurs in the bloodstream [27]. While this may account for the decrease in overall expression, it is unknown if lymphocyte expression of this gene is modulated by hantavirus infection. Similarly, KLRG1 is upregulated in T-cells during viral infection, but it is unclear if it is modulated by hantavirus infection [28].
PTGDR2, or the prostaglandin D2 receptor, promotes inflammation during a viral infection [37]. While no studies of this gene in association with hantavirus have been performed in humans, a 2014 study found that deer mice, a natural non-symptomatic host for the virus, upregulate this gene during active infection of Andes virus, a species of hantavirus [29]. Our analysis showed that this gene is strongly downregulated in humans during infection of HTNV, another hantavirus. Although our observation could be due to a difference in viral species or strain, the gene product may play a role in human pathogenesis of the virus that is not seen in murine species, and likely warrants additional investigation.
The most significant upregulated genes in vivo NUSAP1, KIF20A, CDC25C, and DEPDC1 were associated with cellular division, especially in rapidly dividing cells, which is expected during active viral infection and tissue repair [38–42]. In addition, CDC25C has been shown to play a role in viral replication of HSV, however there is no indication if this is true in other viruses such as hantavirus [43].
In vitro, of all the DEGs, only SLC27A3, AC090527.2 (a novel, uncharacterized gene not shown on figure STRING), and TSEN34 were not associated with interferon stimulation and are not part of the antiviral response. SLC27A3, which is a Very Long-Chain Acyl-CoA Synthetase, is both a synthetase and a transport protein. It has a role in brain development but has not previously been associated with viral pathogenesis or the immune response [44].The tRNA Splicing Endonuclease 34 (TSEN34) gene, and novel gene AC090527.2 similarly have no immune associated function. Finally, Immunoglobulin Heavy Constant Gamma 2 (IGHG2), is shown to be strongly downregulated. This is unexpected since the endothelial cells used in this analysis normally do not produce immunoglobulin. This may be a result of the cells being immortalized, or as an artifact of the analysis. However, at least one prior study has shown that primary endothelial cells can produce antibodies in vitro [45].
The pathway analysis revealed a much wider variety of genes and perturbed pathways in the in vivo dataset as compared to the in vivo dataset. We believe that one of the primary causes of this difference is likely the higher content and complexity of cellular signals that are present in vivo. As the putative cause for the disease is a cytokine storm, we expected to see cytokine signaling along with the interferon response in addition to repair pathways that would not be seen in vitro [46].
Using data from the pathway analysis, medications identified for the in vivo dataset generally treat rapidly dividing and surviving cells, such as in cancer. Specifically, the drugs we predicted would affect the highest number of in vivo pathways in this analysis included those that target cyclin-dependent kinases and JAK kinases such as dinaciclib, alvocidib, and roniciclib. All three of these therapeutics have shown promise as a host-based anti-viral in other pathogens [47–50], but none have been identified or tested for hantaviruses previously. Interestingly, our approach ranked these drugs higher than other potential therapeutics that were further down on the list that have previously been used as host-based anti-virals, such as vorinostat [51–53]. In contrast, the therapeutic prediction analysis from the in vitro dataset identified baricitinib and interferon derivatives, which are commonly used for viral infections [54–56]. As of now, there are no effective treatments for hantavirus infection, and the current suggested remedy is supportive therapy such as bed rest and IV fluids [9]. We believe that testing a subset of these drugs in a laboratory setting would be justified, and that laboratory validation of these findings could improve treatment options for patients in the clinic.