Clinical presentation and histopathological considerations
A 67-year-old woman, of whom an intracranial lesion was detected and a closer follow-up observation was prescribed in June 2010. In October 2013, a low-density area with a suborbicular shadow was observed at the right frontal lobe. Attributed to a variety of reasons including no deteriorating symptoms, she didn’t receive any formal medical care. In May 2018, she suffered aggravated symptoms, imaging examination showed the tumor at the right frontal lobe increased, while a new enhanced shadow appeared at the posterior of the primary tumor, which indicated the occurrence of a secondary higher-grade glioma there (Fig.1A). The primary and secondary tumors were resected and diagnosed as grade II diffuse astrocytoma (G2 in abbreviation) and grade IV glioblastoma (G4 in abbreviation), respectively. The immunohistochemical result showed that Ki67 expression was higher in G4 than in G2, Olig-2 (oligodendrocyte transcription factor 2) expression was positive in both samples and was higher in G4 (Fig.1B).
The molecular origin of G4
Clinical trajectory suggested that the G4 originated from G2. We explored it at the molecular level. Tumor purity of G2 and G4 were both 100% evaluated histologically, and those were 97% and 96% separately estimated by ESTIMATE(36), which guaranteed the comparability of them. A total of 117 and 96 somatic mutations were detected in G2 and G4, respectively. Among them, 53 mutations were shared between the two samples (Fig.2A). Of note, Minor allele frequencies (MAF) of glioma driver genes IDH(p.R132S), TP53(p.R234C), and ATRX(p.T1582NfsTer19) were approximately equal between the two samples (Fig. 2B). TERT promoter mutations, EGFR VIII fusion, and 1p/19q co-deletion were detected in neither samples. Therefore, the mutation subtypes of these two specimens were both IDH mutant and 1p/19q non-codel, which were consistent with the characteristics of diffuse astrocytoma.
Transcriptionally, IDH mutations suggested these two tumors were of proneural subtype. ssGSEA analysis using the signature defined by Verhaak et al.5 verified the result, proneural got the highest score among the four transcription subtypes in both samples, which was 32.4% in G2 and 32.8% in G4. However, ssGSEA scores of the rest three transcriptional subtypes were also comparable, which implied the mixing subtypes (Fig.2C). And cell of origin estimated by ssGSEA using signature defined by Cahoy et al.39 also favored that both samples were of mixing cell origination. Oligodendrocytes accounted for 28.7% and 29.4% in G2 and G4 separately, while astrocytes accounted for 26.6% and 28.3% in G2 and G4 separately (Fig.2C).
At the methylation level, beta values at all CpG loci of G2 and G4 were highly correlated, with R2 equals 0.991. And only 4,035 out of 761,759 CpGs showed a difference in beta value greater than 0.2 between these two samples (Fig.2D). MGMG promoter status of G2, as well as that of G4, was methylated, with the probability of methylated status equals 0.99 estimated by MGMT-STP27 mode for both samples. Epigenetic subtype analysis revealed that both samples were CIMP+ as the probability density of beta-values of probes used to define CIMP+ reached a peak when the beta-value was about 0.76, which was in accordant with that both samples were IDH mutant (Fig.2E). Furthermore, both samples were CIMP-high as the majority of beta-values of probes utilized to define CIMP-high distributed around 0.75 (Fig.2F).
Tumor evolution trajectory from G2 to G4
43 mutations were identified only in G4 (private mutations, Fig.3A). Among them, CIC mutation (p.Thr761LeufsTer163) had a MAF of 23.3%, and at the transcription level, mutant CIC reads accounted for 10% of overall CIC reads detected in RNA sequencing. Two genes RPA4 (p.S2IfsTer50) and BRCA2 (p.M3217V), which were related to homologous recombinations (HR) pathway, had MAFs of 24% and 15.4%, respectively.
Seven clones were identified (Fig.3B, Table 1). Driver genes with high MAF including IHD, TP53, and NAF1 were all assigned to clone 1, which was the earliest event. Of note, the N219K mutation of NAF1 has never been reported before. MAF of it increased from 44.4% in G2 to 75.0% in G4, and the proportion of the mutant RNA reads increased from 37.5% to 70.0%, correspondingly. Clone 2, which was also an early event, composed the second-largest cell population in G4. NOTCH1(p.A1967V) mutation, of which MAF increased from 20.5% in G2 to 37.5% in G4, was assigned to this clone. Clone 4 was the biggest in G4, and private mutations of G4 including CIC, BRCA2, and MMRN1 were all assigned to this clone, indicating their role in tumor progression. Clone 5 was a new event in G4, MAF of all genes assigned to this clone was less than 10%, except FFAR1 (p.A17V). Contradictorily, clone 6 and clone 7, which occurred in G2, were almost eliminated in G4.
Oncogenic pathways of the two samples were further compared by ssGSEA. Pathways of which the absolute difference of normalized enrichment score (NES) calculated between G4 and G2 was greater than 20% of the NES of G2 were analyzed. (Fig.3C). The highest up-regulated pathway in G4 was the MSP-RON pathway, activation of which can stimulate the Ras-PI3K signaling pathway(46) which was reported to be altered in 68% of recurrent glioma patients(21). The second up-regulated pathway in G4 was the nuclear factor-κB (NF-κB) pathway, activation of which could enhance cancer cell proliferation and induce immunosuppression. Contrarily, the down-regulated pathways included the dendritic cell pathway, which can stimulate Th1 and Th2 cell differentiation, IL17 pathway, and cytotoxic T cell pathway, indicating the immunosuppression in G4.
Immune microenvironment alteration concomitant with tumor evolution
Immune cell fraction in the two tumors was deconstructed using CIBERSORT(47) (Fig.4A). The infiltration levels of activated mast cells, memory B cell, and follicular helper T cell were higher in G4 (Fig.4A), suggesting their role in defending glioma progression. However, The infiltrations of resting CD4+ memory and CD8+ T cells were decreased in G4 noteworthily, which was consistent with the down-regulation of Th1, Th2, Th17, and cytotoxic T cell pathway aforementioned and implied the evasion of the adaptive immune response in G4.
TCR sequencing of the two tissue samples and peripheral blood was performed to profile epitope recognition patterns during glioma evolution. Of the 1926 clonotypes discovered in peripheral blood, only 42 (2.13%) were detected in G2 or G4 (Fig.4B). Diversity estimated by Shannon-Index was higher in G4 than in G2, correspondingly, clonality was lower in G4 than in G2, which suggested the occurrence of new epitopes during glioma evolution. 15 clonotypes were identified in common between G2 and G4 (Fig.4C), which accounted for 13.2% and 11.4% of all clonotypes of G2 and G4 separately. clone frequencies of all these shared clonotypes were decreased from G2 to G4. Among them, two clonotypes (CASGDRVSWGYTF and CASSLSRGQLNGYTF) had the highest clone frequency in all of G2, G4, and peripheral blood (Fig.4C-D).