In the hope of improving STS diagnosis and treatment, 32 STS, including 20 LMS and 12 DDLPS samples and the matched peripheral blood collected at Zhongshan Hospital, Fudan University were subjected to WES analysis. The patient characteristics are indicated in Table 1.
Table 1
Clinical information, TMB and MSI status of patients.
|
All
|
LMS
|
DDLPS
|
Sample size
|
32
|
20
|
12
|
Age ( years)
|
|
|
|
Median
|
51
|
49.5
|
54
|
Range
|
16–70
|
16–65
|
37–70
|
Gender (%)
|
|
|
|
Male
|
12 (37.5%)
|
3 (15%)
|
9 (75%)
|
Female
|
20 (62.5%)
|
17 (85%)
|
3 (25%)
|
TMB
|
|
|
|
Median
|
82.5
|
91.5
|
7.25
|
Range
|
42–563
|
55–563
|
42–158
|
MSI (%)
|
|
|
|
High
|
1 (3.1%)
|
1 (5%)
|
0 (0%)
|
Low
|
31 (96.9%)
|
19 (95%)
|
12 (100%)
|
Top Mutated Genes
We first investigated the mutation pattern of the patients. Overall, the tumor mutation burden (TMB) of STS is relatively low compared with other cancers[3]. The average TMB of the 20 LMS is significantly higher than that of the 12 DDLPS (162.35, vs. 123.58, P = 0.028). The top 20 genes in mutation frequency in the 32 STS samples are MUC16, PABPC3, MUC4, MUC6, TP53, AHNAK2, FLG, MUC17, MUC3A, DCHS1, HRNR, KRTAP10-6, NBPF10, MUC12, TTN, NBPF1, BIRC5, FLG2, PABPC1, and TPSAB1 (Fig. 1). Among them, TP53 (mutation frequency 34%) is a well-known tumor suppressor. The other genes are not listed in oncoKB (https://www.oncokb.org/cancerGenes) as common cancer genes. However, MUC16 is a type I transmembrane protein and is known to promote cancer cell proliferation and inhibit anti-cancer immune responses in multiple cancer types[23–25]. In a transgenic mouse model, p53 mutant animals which over-expressed MUC16 carboxy-terminal fragment developed more spontaneous tumors (sarcomas) when comparing to the animals which did not overexpress the MUC16 fragment [26].
When the samples are divided to LMS and DDLPS, the top20 genes with the highest difference in mutation frequency between the two groups are indicated in Fig. 2. Among these genes, TP53, FLG, AHNAK, ATRX, CSPG4, MKI67, PCMTD1and POTEJ is enriched in LMS, while HERC2, C12orf55, DNAJC16, PTPRQ, TIAM1, ARID1A, FLT4, FRS2, KDM6B, PLK1, PRKDC, and SLC2A3 (Fig. 2). TP53 mutations can only be found in the LMS group, with a mutation rate of 55%, which is similar to that of recently reported in TCGA LMS (50%)[3]. HERC2 belongs to E3 ubiquitin protein ligases, and can modulate p53 activity through regulating p53 oligomerization independent of MDM2 [27]. Interestingly, HERC2 mutation can only be found in the DDLPS group in our cohort (mutation rate 42% for DDLPS), probably because the mutations that affect p53 tetramerization disrupt the HERC2-p53 interaction, and therefore HERC2 mutations are redundant in LMS with mutant TP53.
Co-occurrent And Exclusive Mutations
We did somatic mutation interaction analysis using the somaticInteractions function in MAFtools. The results are shown in Fig. 3, and the full list of genes can be found in Table S1. TP53 is mutually exclusive with BRD9 (P < 0.1) and co-occurs with Filaggrin (FLG) (P < 0.1). BRD9 is a subunit of the human BAF (SWI/SNF) nucleosome remodeling complex[28] and has emerged as an attractive therapeutic target in cancer. Its bromodomain is highly homologues with that of BRD7 and the later is reported to interact with p53 and be required for p53 function[29]. Whether mutant BRD9 in STS functions through impairment of the p53 pathway is to be determined. FLG is a highly mutated driver gene. Its mutations are also found in several other cancer types such as non-melanoma skin cancer, head and neck cancer, lung cancer, colorectal cancer, uterine cancer, prostate cancer etc.[30]. There is no reported synergic interaction between FLG and TP53 mutations in cancer yet, including STS. The above somatic interaction analysis may provide hints for exploring genes with unspecified functions in STS.
Scna Results In Lms And Ddlps
A lack of general recurrent mutation in the LMS and DDLPS samples promoted us to look into SCNA in these tumors. Several recent studies have explored the molecular basis of sarcoma and identified multiple recurrent genomic alterations. To further reveal the somatic copy number alterations in Chinese sarcoma patients, we used GISTIC2.0 to detect SCNA in our cohort. The analysis revealed that chromosomal loss affecting tumor suppressor genes such as TP53, RB1, and PTEN, is a hallmark of LMS. On the other hand, focal amplification affecting chromosome 12q13–15 region which encodes MDM2 (12q15), CDK4 (12q14.1) and HMGA2 (12q14.3), is a feature of DDLPS (Fig. 4). Co-amplification of MDM2 and CDK4 occurs in more than 90% DDLPS. Such co-amplification combined with TP53 inactivation results in cell proliferation, and is thought to be the initiating factor to drive fat tumorigenesis[31].
Amplified and deleted genes detected by GISTIC 2.0 are listed in supplement Table S2-S5. In LMS, 96 genes are significantly amplified, while as much as 4532 genes are significantly deleted (confidence 0.95), indicating deletion is much more frequent than amplification. In DDLPS 1089 genes are significantly amplified and no genes are significantly deleted. Both DDLPS and LMS belong to complex-karyotype tumor in genetic classification of STS[32]. In DDLPS, amplification of oncogenes such as MDM2, CDK4, HMGA2 and JUN are more commonly seen, while deletion of tumor suppressor such as TP53, RB1, and PTEN are enriched in LMS, indicating genomic diversity of copy-number alterations of DDLPS and LMS.
Rna-seq Revealed Genes With Differential Expression
Eight each of the LMS and DDLPS samples that have high RNA quality were further subjected to RNA-Seq analysis. The significance threshold was set at adjusted P-value < 0.05 and Log2 fold change (LogFC) > 1. In total 2396 genes have significantly different expression levels in the two tumors (Table S6). The genes with the most variability were Z-score normalized and visualized in heatmap in Fig. 5. Unsupervised clustering was performed on all the samples to examine the discriminant effect of these genes. We found that all the LMS clustered together, and so did the DDLPS samples. MDM2 ranks the highest, with logFC = 4.12 (DDLPS over LMS) and adjusted P-value = 1.73E-52, consistent with previous studies [3, 33], indicating that the massive amplification of MDM2 in DDLPS is further enhanced at the mRNA level. JUN is upregulated in DDLPS and it has been known to play important roles in blocking adipocytic differentiation (logFC = 1.45, adjusted P-value = 0.0396). Correlation between SCNA and RNA expression is positive in all DDLPS in MDM2 and CDK4 (Fig. 6).
Pathway Analysis Of Genes With Differential Expression
To investigate the pathways affected by the significantly changed genes between LMS and DDLPS, Gene Set Enrichment Analysis (GSEA) was performed [17] [18] using NetworkAnalyst (https://www.networkanalyst.ca/) (Table 2 and Table S7). Pathways with negative EnrichmentScore are enriched in LMS, while pathways with positive EnrichmentScore are enriched in DDLPS. The pathways apparently enriched in LMS such as “Calcium signaling pathway", "Vascular smooth muscle contraction", and “Linoleic acid metabolism”(EnrichmentScores are − 0.37, -0.44, and − 0.60 respectively and P-values are 0.0002 for all) are probably due to the difference of the tissue origin rather than tumorgenesis [34, 35]. Therefore, the appearance of the above pathways in the enrichment should be interpreted cautiously. GSEA analysis revealed that in DDLPS, E3 ubiquitin ligase including MDM2 are up-regulated compared with that in LMS, indicating ubiquitin mediated proteolysis may play more important roles in DDLPS. Notably, GSEA revealed differential regulation of the spliceosome related genes between LMS and DDLPS (EnrichmentScore = 0.53, P = 0.003), which may result in different sets of neoantigens in the two tumors due to alternative splicing. This result may affect antigen selection in the two tumors used in CAR-T cell immunotherapy [36].
Table 2
Result of GSEA in LMS compare with DDLPS
| Name | Total | Hits | EnrichmentScore | Pval | Padj |
1 | Calcium signaling pathway | 68 | 58 | -0.37 | 0.00017 | 0.0028 |
2 | Vascular smooth muscle contraction | 30 | 22 | -0.44 | 0.000017 | 0.0028 |
3 | Linoleic acid metabolism | 30 | 26 | -0.60 | 0.00018 | 0.0028 |
4 | Ascorbate and aldarate metabolism | 34 | 28 | -0.67 | 0.00018 | 0.0028 |
5 | Chronic myeloid leukemia | 33 | 27 | 0.53 | 0.000022 | 0.0028 |
6 | p53 signaling pathway | 31 | 27 | 0.50 | 0.000022 | 0.0028 |
7 | Spliceosome | 27 | 22 | 0.52 | 0.000022 | 0.0028 |
8 | AGE-RAGE signaling pathway in diabetic complications | 18 | 16 | 0.47 | 0.000023 | 0.0028 |
9 | Ubiquitin mediated proteolysis | 27 | 25 | 0.47 | 0.000023 | 0.0028 |
10 | Osteoclast differentiation | 44 | 39 | 0.50 | 0.000023 | 0.0028 |
Rna-seq Revealed Distinct Fusion Patterns Between Ddlps And Lms
In the RNA-seq analysis, we identified four fusion transcripts in three LMS samples (out of 8 that undergone RNA-seq analysis) and 36 fusion transcripts in all the 8 DDLPS samples combined (P = 0.007). There is no recurrent fusion transcript identified. Fusion transcripts involving chromosome 12 are only found in DDLPS, including both interchromosomal and intrachromosomal rearrangements (Fig. 7A). MDM2 and RAB3IP are the most common fusion partners, and both locate in chromosome 12. There is also significant correlation between MDM2/CDK4 amplification and chromosome 12 rearrangement (P < 0.001, Fig. 7B). Gene fusions can occur as a result of translocation, interstitial deletion, or chromosomal inversion during DNA replication, which are more common in DDLPS than in LMS as indicated above. Therefore, it is unsurprised to see gene fusions more frequent in DDPLS, especially in chromosome 12 where ring or giant marker chromosomes occur [3, 37]. Fusion proteins generated from the identified fusion transcripts may generate neoepitopes, which can be targeted to produce safer CAR-T cells for immunotherapy [38].
LMS and DDLPS have different Profiles of Tumor Infiltrating Immune Cells
To investigate whether immune response plays different roles in LMS and DDLPS, MCP-counter was used to estimate the ratio of tumor infiltrating immune cells between the two tumors (Fig. 8) [22]. Among the eight immune populations (T cells, CD8 + T cells, cytotoxic lymphocytes, natural killer cells, B cell lineage, monocytic lineage, myeloid dendritic cells and neutrophils) and two stromal populations (endothelial cells and fibroblasts) we analyzed, DDLPS have higher fibroblasts signature score (P = 0.002) and higher endothelial cell signature score (P = 0.047) compared with LMS. Increased endothelial cell signature score are proved to be associated with high density of CD34 + endothelial cells and enhanced endothelial-driven angiogenesis in STS. The high fibroblasts signature score for DDLPS is consistent with the mesenchymal origin of STS. Previous TCGA study suggested that CD8 + T cells are higher in DDLPS compared with LMS (P < 0.01), but no such significance was observed in the current study (P = 0.76) probably due to its small sample size.
Unsupervised clustering analysis of tumors by Z-score standardized immune infiltration scores divide sixteen samples into two immune subtypes with different profiles. Most LMS samples were classified to immune class A, while most DDLPS samples were classified to immune class B. Previous studies revealed that STS immune subtypes are associated with response rate to PD1 blockade[22]. We cannot conduct survival analysis for the current cohort right now due to the relatively short follow-up time, but these factors can be potentially used for stratification in future study.