This work was principally motivated by three questions: (1) What genes are differentially expressed in FISS, and what are their biological pathways and functions? (2) In which genomic regions are genes coherently differentially expressed in FISS vs. normal tissue? and (3) To what extent does the FISS transcriptome overlap with those of dog and human sarcomas?
The 3049 genes that we detected with altered transcript abundances in FISS tumors vs. normal skin comprise genes in multiple pathways that could plausibly play a role in the pathogenesis or progression of FISS.
Sarcoma-upregulated genes that are common to sarcomas in the three species
Among the genes that are upregulated in sarcoma in all three species, several genes stand out:
FAP (fibroblast activation protein α) is known to be expressed in soft tissue sarcomas and is involved in control of fibroblast growth [
59],
WT1 (Wilms tumor suppressor) is expressed in over half of soft tissue sarcomas and has prognostic significance [
60], and
PRAME (preferentially expressed antigen in melanoma) promotes cell growth, is expressed in tumors including soft tissue sarcomas, and has been proposed as a potential immunotherapy target for sarcoma [
61]. Additionally, several enriched gene annotations are notable. Many of the upregulated genes are associated with the immune system or complement system (
TLR2,
CD14,
C3AR1,
ITGA4, CXCR4,
IFI44,
CSF3R,
CSF1R,
CD53,
CD48,
IL6,
CCL13,
CD86, LY86, LCP1, OAS, SRGN, CTSS, TREM1) or the extracellular matrix (ECM) (
TNC, SPP1, MMP13, LRRC15, LAMA4, FN1, FBN2, FAP, COL6A3, COCH, ADAM28). Other cellular functions or biological processes with which multiple consistently upregulated genes are annotated include cell proliferation (
PTH1R, PRAME, MCM5, IGFBP4, GPC3), cytoskeleton (
MAP1B, SEPT11, KRT7, ASAP1)
, gene regulation (
WT1, BICC1, BARX1), lysosome (
LAPTM5, CTSS, CTSK), signaling (
GPC3, RGS1, RAB31), trafficking (
TGONL2, SNX, COPB1), and metabolism (
FFAR2, ALDH18A1).
The finding of enrichment of ECM-related genes among the 53 genes that are upregulated in sarcoma is consistent with the abundant ECM typically seen in histopathological analysis of FISS tumors [
3] and the upregulation of both ECM structural constituents (
COL6A3, FBN2, LAMA4), as well as degradative enzymes (
MMP13, ADAM28) is suggestive of ECM remodeling. Of note, increased expression of orthologous genes regulating ECM remodeling was also seen in human and canine sarcomas (Fig.
6a), which may suggest that some molecular mechanisms of FISS invasiveness are shared with soft-tissue sarcomas in dogs and humans.
At the level of specific cytokine pathways, we found enrichments for genes associated with two specific cytokines (interleukin-2 and interleukin-12) among the 53 genes that are upregulated in sarcoma. Genes annotated as having functions related to regulation of production of interleukin-2 (IL-2) (e.g.,
CD86, CD28, PRKCQ, CD80, IL1A, LAG3, IL1B, IL17F, CD276, GLMN) were enriched at FDR < 0.05 with a GSEA enrichment score of 2.0. Interleukin-2 is primarily produced by activated CD4+ T lymphocytes, and it promotes the differentiation of immature T cells into regulatory T cells. Lymphocytic aggregates have been reported in approximately 59% of FISS cases, and these are typically enriched for T cells [
62]. IL-2 has antitumor activity and, in humans, recombinant IL-2 immunotherapy has been used clinically for treating melanoma. In the context of feline fibrosarcoma, adjuvant immunotherapy with local injection of recombinant human IL-2 extended survival times [
63] and local canarypox-vectored recombinant IL-2 immunotherapy significantly reduced recurrence rates in cat fibrosarcomas [
64,
65]. Genes annotated as having functions related to the regulation of production of the cytokine interleukin-12 (IL-12) (e.g.,
ACP5, CCR7, MAST2, IDO1, RIPK2, TLR4, IL10, MEFV, TLR2, IRF8, NFKB1, IFNG, JAK3, TIRAP, IL12B) were enriched among genes upregulated in FISS at FDR < 0.02 with a GSEA enrichment score of 2.2. Interleukin-12 is produced by activated phagocytic cells such as macrophages, dendritic cells, and neutrophils; it influences both innate and adaptive arms of the immune system, including augmenting cytotoxic CD8+ T-cell activation [
66], reprogramming T helper 17 (T
H17) cells into a T
H1 phenotype, and reprogramming tumor-associated antigen-presenting cells to prime cytotoxic T-cells for antitumor activity [
66,
67]. Inflammation is a hallmark of FISS, with lymphoplasmacytic infiltrates always present in the tumor as well as macrophages and neutrophils; furthermore, significant aggregates of macrophages are present in or at the periphery of approximately 25% of tumors [
62]. In a mouse xenograft model of human rhabdomyosarcoma, administration of anti-histone antibody-conjugated IL-12 caused long-term remission and increased survival [
68]. In cats, systemic IL-12 administration for treatment of spontaneous soft-tissue sarcoma has been evaluated in a phase-one clinical trial and achieved dose-dependent delivery of IL-12 to the tumor [
69]. Overall and together with our transcriptional findings for genes related to IL-12 regulation, these results are suggestive that IL-12 immunotherapy may prove beneficial in the treatment of FISS.
Sarcoma-downregulated genes that are common to sarcomas in the three species
Among the genes that are downregulated in sarcoma in all three species, there are three known tumor suppressors:
TP63 [
70],
EPHA1 [
71]
, and DUSP26 (which may alternately function as an oncogene depending on the cancer context) [
72]. Many of the downregulated genes are associated with membrane transport (
UNC93A, SLC9A2, SLCL5A1, SLC39A2, SLC38A4, PRSS8, CLDN8, CLDN4, AQP3, ABCD2, SMPD3) or metabolism (
MAOB, HSD17B2, HMGCS2, FA2H, DCT, ALOX12B). Other cellular functions or biological processes for which multiple consistently downregulated genes are annotated include gene regulation (
TFCP2L1, RORC, CPEB3, BARX2), signaling (
PTPN3, LY6G6C), immune function (
MBP,
AIM1L), cytoskeleton (
SNTB1, PACSIN3, MAPT, KRT16, GDPD2, COBL; see also Fig.
1d, “intermediate filament”), and cell adhesion (
ITGB6, CDH15, CARD14; see also Fig.
1d, “cell-cell adherens junction”). The latter gene category is perhaps unsurprising given that there is altered and often reduced cell-cell adhesion and cell-extracellular matrix (cell-ECM) interaction in many cancers [
73] and given the invasive nature of FISS. Among the cross-species downregulated genes, there are two additional genes that particularly noteworthy:
SERPINB13 is an angiogenesis inhibitor [
74] that is downregulated in head and neck cancers [
75], and the neutral sphingomyelinase
SMPD3 is a cell cycle regulator [
76].
The finding that many genes with functions in fatty acid (FA) metabolism, such as multiple FA elongases (Additional file
3: Table S2), fatty acid 2-hydroxylase (
FA2H), and arachidonate 12-lipoxygenase (
ALOX12B, which produces an eicosanoid signaling molecule that stimulates epidermal lipid envelope synthesis; see Fig.
6c) are downregulated in sarcoma vs. normal skin (see also Fig.
1d, “long-chain fatty acid metabolism”) is interesting. Many tumors are lipogenic in order to sustain cell proliferation and tumor growth [
77] and skin has a modest overall rate of FA synthesis, on a per-gram basis, compared to other normal tissues [
78]. On the other hand, the finding that the angiogenesis-inhibiting gene
SERPINB13 is expressed at lower levels in soft-tissue sarcoma vs. normal tissue is not surprising (Fig.
6c), given the strong evidence for the role of the neovasculature in supporting sarcoma tumor progression [
79].
Parallel examination of tissue samples and cultured cells reveals that there is an overall high degree of concordance in differential expression of genes in FISS tissue versus normal skin compared to FISS-derived cell lines versus cultured fibroblasts. Overall, this finding strengthens the utility of this in vitro model with one caveat. Our cross-species analysis of tumor suppressor gene (TSG)/oncogene expression suggests that bias in differential expression of feline orthologs to known oncogenes and TSG is not as strong in cultured cells as it is in primary tissue (Table
2); this could be a reflection of shared selective pressure imposed by environmental parameters of the cell culture systems.
Over 158 recurring focal SCNAs have been identified in a recent survey of copy number alterations in human cancers and cancer cell lines [
80]. Although not all of these SCNAs contain known cancer-target genes, most occur in more than one human cancer type. Human soft tissue sarcoma is known to have many recurrent SCNAs, including deletions and amplifications, that range from focal to broad in scale [
81]. Our region-based analysis of tumor mRNA-seq data for coherently up- or down-regulated genes in FISS versus skin identified nine putative recurring SCNAs. By our mRNA-seq-based method, we found one putative SCNA (a probable deletion on
Fc-D3) that overlapped with a recurrent SCNA detected in a previous DNA-based analysis of feline sarcoma [
4]. Overall, this modest amount of overlap is not unexpected given the limited size of the primary tissue dataset used in this study and the differing measurement modality. However, our finding does bolster the likely functional significance of the recurrent
Fc-D3 deletion in FISS. One of the nine putative SCNA regions,
Fc-C1 (70–80 Mbp) which had the highest-magnitude overall effect on transcription of any of the putative SCNA regions in FISS, is particularly noteworthy. Its human syntenic region,
Hs-18q23, is recurrently deleted in sarcomas and contains two genes,
GALR1 and
CYB5A, that are annotated as tumor suppressors [
82].
The eight genes whose differential expression in FISS vs. normal skin we chose to validate by qPCR include three transcription factors (
BARX1 and
BARX2 [
83], which encode BarH-like homeobox transcription factors 1 and 2 which are normally expressed in gastrointestinal tissue; and
LEF1, which encodes lymphoid enhancer binding factor 1, a transcription factor downstream of the Wnt / β-catenin pathway [
84]); two genes encoding extracellular proteins (
MMP13, which encodes matrix metallopeptidase 13, which is involved in degrading extracellular matrix proteins such as fibrillar collagen and fibronectin and which has a role in controlling angiogenesis in some cancers [
85]; and
FBN2, which encodes an extracellular protein, fibrillin-2, that is a structural component of microfibrils in the extracellular matrix [
86]); and a tumor suppressor (
LATS1, which encodes large tumor suppressor kinase 1, a serine/threonine kinase that complexes with cell division cycle protein 1 (CDC1) in early mitosis [
87]). Overall, we validated seven out of the eight genes originally identified as differentially expressed through mRNA-seq that were assayed by qPCR, with the exception being
LATS1 (notably, it was the only gene for which a custom hydrolysis probe-based qPCR assay had to be designed due to the absence of an off-the-shelf commercial assay). Among all eight genes, the expression ratios as measured by mRNA-seq and qPCR were highly concordant, with
r ≠ 0 significant at
P < 0.0001. Given that
BARX1 transcript abundance is nearly forty-fold higher in FISS than in skin, and given our interest in the FISS-upregulated gene
FN1 (a five-fold FISS-upregulated gene that encodes fibronectin-1, an ECM protein that has an important role in cell adhesion, growth, migration, and differentiation and whose expression is associated with more advanced disease in renal cancer [
88]), we selected BARX1 and FN1 to assay at the protein level. The findings that the gastrointestinal transcription factor BARX1 is upregulated (three-fold) in FISS vs. normal skin and that the ECM protein FN1 is upregulated (two-fold) in
some FISS-derived cell lines vs. normal fibroblasts are worthy of further focused investigation to determine their specific functions in the pathogenesis of FISS.
Of the fifteen drugs and drug targets that were identified through the screen of drug-to-cell-line databases using the sets of genes that are coherently upregulated or downregulated in sarcoma (across studies in three species), the compound GSK-1059615 is particularly interesting. GSK-1059615 inhibits PI3K and mTOR and it has been investigated as a potential antineoplastic in humans [
89]. The finding (Fig.
7) that GSK-1059615 potently inhibits growth of FISS-derived cells suggests that dual PI3K and mTOR targeting is worthy of further study as a potential therapeutic approach for FISS. Additionally, the platelet-derived growth factor BB monomer (PDGF-BB) is noteworthy as a potential target. Members of the platelet derived growth factor family, including PDGF-BB, act as mitogens on mesenchymal cells, often in an autocrine fashion [
90]. Depending on the cell type, PDGF-BB signaling can activate cell proliferation in the absence of inflammation. Tumor-derived PDGF-BB promotes pericyte recruitment and leads to increased stability of intratumoral vasculature, tumor cell proliferation, and survival. Notably, the anti-PDGF-A antibody olaratumab has shown promise in treatment of advanced human soft tissue sarcoma [
91]. Together, these previous and new findings suggest that PDGF-BB inhibition may be worthy of study in the treatment of FISS. Also intriguing is the identification of the drugs AS601245, a Janus N-terminal kinase inhibitor, and rosiglitazone, a PPARɣ selective agonist, as their co-administration to colon cancer cells has been reported to synergistically reduce cell migration [
54].