Tumour compartment transcriptomics demonstrates the activation of inflammatory and odontogenic programmes in human adamantinomatous craniopharyngioma and identifies the MAPK/ERK pathway as a novel therapeutic target
Adamantinomatous craniopharyngiomas (ACPs) are clinically challenging tumours, the majority of which have activating mutations in CTNNB1. They are histologically complex, showing cystic and solid components, the latter comprised of different morphological cell types (e.g. β-catenin-accumulating cluster cells and palisading epithelium), surrounded by a florid glial reaction with immune cells. Here, we have carried out RNA sequencing on 18 ACP samples and integrated these data with an existing ACP transcriptomic dataset. No studies so far have examined the patterns of gene expression within the different cellular compartments of the tumour. To achieve this goal, we have combined laser capture microdissection with computational analyses to reveal groups of genes that are associated with either epithelial tumour cells (clusters and palisading epithelium), glial tissue or immune infiltrate. We use these human ACP molecular signatures and RNA-Seq data from two ACP mouse models to reveal that cell clusters are molecularly analogous to the enamel knot, a critical signalling centre controlling normal tooth morphogenesis. Supporting this finding, we show that human cluster cells express high levels of several members of the FGF, TGFB and BMP families of secreted factors, which signal to neighbouring cells as evidenced by immunostaining against the phosphorylated proteins pERK1/2, pSMAD3 and pSMAD1/5/9 in both human and mouse ACP. We reveal that inhibiting the MAPK/ERK pathway with trametinib, a clinically approved MEK inhibitor, results in reduced proliferation and increased apoptosis in explant cultures of human and mouse ACP. Finally, we analyse a prominent molecular signature in the glial reactive tissue to characterise the inflammatory microenvironment and uncover the activation of inflammasomes in human ACP. We validate these results by immunostaining against immune cell markers, cytokine ELISA and proteome analysis in both solid tumour and cystic fluid from ACP patients. Our data support a new molecular paradigm for understanding ACP tumorigenesis as an aberrant mimic of natural tooth development and opens new therapeutic opportunities by revealing the activation of the MAPK/ERK and inflammasome pathways in human ACP.
KeywordsCraniopharyngioma Odontogenesis Inflammasome IL1-β MAPK/ERK pathway Trametinib Paracrine signalling
Adamantinomatous craniopharyngioma (ACP) is the most common tumour of the sellar region in children . Despite being cytologically benign, ACPs display clinically aggressive behaviour such as destruction of the pituitary gland and invasion of the hypothalamus and visual pathways, altogether leading to chronic severe morbidity and increased mortality during long-term follow-up [41, 43].
ACPs are histologically complex tumours with variable cystic, calcified and solid components. They frequently invade adjacent tissues with infiltrating finger-like structures surrounded by a florid glial and inflammatory reactive tissue . A pathognomonic feature of ACP is the presence of anuclear ghost cells, also known as “wet keratin”, while other common features include calcification and cholesterol clefts [35, 39]. The majority of ACPs have somatic activating mutations in CTNNB1, the gene encoding β-catenin [8, 10, 31, 35, 50]. However, nucleo-cytoplasmic accumulation of β-catenin is limited to only a small proportion of cells, often correlating with epithelial whorl-like structures (referred to as β-catenin-accumulating cell clusters), or in single cells throughout the tumour [8, 23, 25, 31, 41]. As expected, these regions correlate with WNT pathway activation, evidenced by the expression of pathway target genes (e.g. AXIN2 and LEF1) [25, 51]. In contrast, in other histological compartments such as the palisading epithelium or stellate reticulum, β-catenin expression is limited to the cell membrane and activation of the WNT pathway is less pronounced . The similarities between ACP and both normal tooth development and odontogenic tumours have been recognised for decades, on the basis of their comparable histopathology and co-expression of enamel proteins, proteinases and some keratins, as well as the occasional identification of fully formed teeth in ACP [6, 42, 49]. However, the molecular mechanisms underlying these commonalities are not well understood [7, 10, 19, 29, 32, 42, 45, 51, 54].
Insight into the functional significance of the β-catenin-accumulating cell clusters has been provided by studies conducted in murine ACP models, where a degradation-resistant (activated) form of β-catenin is expressed in either Rathke’s pouch derivatives (Hesx1Cre/+/Ctnnb1lox(ex3)/+ mouse line; ACP embryonic model) or Sox2-expressing adult pituitary stem cells (Sox2CreERT2/+/Ctnnb1lox(ex3)/+ mouse line; ACP inducible model). Evidence gathered from both models suggests these clusters act in a paracrine manner, driving tumour growth and/or invasion into surrounding tissues through the secretion of a wide range of factors (e.g. SHH, FGFs, BMPs, TGFB1; as well as pro-inflammatory mediators such IL1, IL6 and other CXC and CC chemokines) [1, 2, 14, 17, 41, 53]. This hypothesis is consistent with their location at the leading edge of tissue invasion in human tumours [3, 9, 53]. These findings have raised interest in discovering therapeutic approaches targeting these clusters and highlighted a need to better understand the pathways and cellular processes active both within clusters and the responding tissues.
Two recent publications have analysed mRNA expression microarray profiles of ACPs, comparing either with other tumour types and control tissues, or with the BRAF mutation-driven papillary craniopharyngioma (PCP) subtype. These studies have highlighted the differences between ACP and PCP and identified a number of potential therapeutic targets using differential expression analysis, many of these previously anticipated by studies using the murine models [21, 26]. However, correlation between dysregulated gene pathways in human ACP and tumour architecture has been limited to the analyses of specific proteins by immunostaining. Revealing which pathways are dysregulated in specific tumour cellular compartments is important to increase our understanding of the pathogenesis of human ACP. Additionally, it is a necessary step to determine which cells are likely to be susceptible to specific targeted therapies, helping predict possible outcomes. For instance, pancreatic ductal cell carcinoma (PDCA) epithelial cells secrete SHH and activate the pathway in the tumours, but its targeted inhibition in mouse models and patients causes disruption of the host-derived stroma with little effect on the cancer epithelial cells, resulting in rapid disease progression and increased aggressiveness [36, 46].
In this manuscript, we have performed a comprehensive gene expression study of 18 human ACPs combining whole-tumour RNA-Seq with transcriptomics of laser capture microdissected tumour cellular components to reveal the molecular signatures of specific tumour cell compartments. We complement this gene expression study with proteome analysis and ELISA of both tumour and cystic fluid from ACP patients. Our data provide a molecular rationale for the resemblance of ACP and tooth development and highlight a complex signalling cascade orchestrated by the cluster cells. Of clinical relevance, we identify the MAPK/ERK pathway and inflammasome signalling as potentially targetable pathways and provide preclinical evidence supporting the use of MEK inhibitors against human ACP.
Materials and methods
Human tumour and pituitary samples
Anonymised archival frozen and formalin-fixed paraffin-embedded (FFPE) specimens of ACP and non-functioning pituitary adenoma (NFPA) were identified in the local pathology archive, through the Childhood’s Cancer and Leukaemia Group Tissue Bank, Brain UK and from collaborators. Fetal pituitary tissue (19 weeks corrected gestational age) was accessed through the Human Developmental Biology Resource (HDBR). Further details can be found in Suppl. Materials and Methods (Online Resource 1).
Laser capture microdissection (LCM)
Two cases (JA004, JA029) were used for LCM because they contained easily definable histological features (clusters, palisading epithelium and glial reaction) and cryopreserved tissue of sufficient quality was available. Further details can be found in Suppl. Materials and Methods (Online Resource 1).
For human ACP samples sequencing was performed by UCL Genomics. Murine ACP samples were sequenced by the Oxford Wellcome Trust Centre for Human Genetics. Further details can be found in Suppl. Materials and Methods (Online Resource 1).
Immunostaining of histological sections
Ex vivo culture of mouse and human tumours
Ex vivo culture of neoplastic pituitaries was performed as described  in the presence of either trametinib at 2 or 20 nM (Mekinist) or DMSO (vehicle control). Histological analysis was performed after 18 h. Small pieces of human ACP (around 1–2 mm3) were cultured in identical conditions. The proportion of Ki67 positive and cleaved caspase-3 cells in the ex vivo culture experiments was calculated as an index out of the total DAPI-stained nuclei. Over 150,000 DAPI nuclei were counted from six histological sections per pituitary in a total of six neoplastic pituitaries. Three human tumours were used in the ex vivo experiments and the Ki-67 and cleaved caspase-3 index calculated as described above from four histological sections per tumour (over 192,000 cells counted).
Cytokine multiplex ELISA
Levels of TNF-α, IFN-γ, IL-1β, IL-6. IL-8, IL-10 and IL-18 were measured in ACP tumour and ACP cystic fluid using a Meso Scale Discovery (MSD) multiplex kit (Meso Scale Diagnostics) as per the manufacturer’s instructions. For solid tumours, protein lysates were extracted from pieces of fresh frozen ACP. Briefly, 6- to 28-mg samples were placed in 250 μl of lysis buffer (150 mM NaCl, 20 mM Tris pH 7.5, 1 mM EDTA, 1 mM EGTA, 1% Triton, with Protease Inhibitor (Roche), 1 mM sodium orthovanadate and 25 mM sodium fluoride). Samples were sonicated on ice and left for 30 min at 4 °C, followed by centrifugation at 20,000g for 10 min and collection of supernatant. Protein concentrations were quantified by Bradford assay. Cytokine levels were normalised against total protein added to the ELISA.
Samples and RNA sequencing data analysis
Computational modular analysis and transcriptomics of laser capture microdissected tumour cells reveal the molecular signatures of specific cellular compartments
Next, we aimed to further dissect the molecular signatures obtained from the RNA-Seq data and establish if these correlated with particular tumour characteristics including diagnosis, percentage of tumour content, CTNNB1 mutation allele frequency and the presence of specific histological features. Weighted gene co-expression network analysis (WGCNA) was used to describe the correlation patterns among genes across samples, resulting in the identification of 12 distinct patterns of gene expression (modules), which were assigned a colour identifier (Fig. 1e). Three major distinct patterns of gene expression across the samples were determined: (1) the brown module, consisting of genes relating to epithelial differentiation and whose expression correlated with percentage tumour content and CTNNB1 mutation allele frequency [Fig. 1f; Suppl. Table 4a, e (Online Resource 6); Suppl. Fig. 2a (Online Resource 3)]; (2) the blue and turquoise modules, containing genes involved in nervous system development, whose expression correlated with the presence of reactive glial tissue [Fig. 1f; Suppl. Table 4a, j, l (Online Resource 6); Suppl. Fig. 3a, b (Online Resource 3)]; (3) the magenta module, including inflammation-related genes whose expression correlated with immune cell markers such as CD14 [Fig. 1f; Suppl. Table 4a, k (online Resource 6); Suppl. Fig. 2d (Online Resource 3)]. Further details of other identified modules are presented in Suppl. Results (Online Resource 1), Suppl. Figs. 2–4 (Online Resource 3) and Suppl. Table 4 (Online Resource 6). WGCNA of the human ACP microarray data published by Gump et al.  also showed moderate preservation of the major modules with our datasets [Suppl. Results (Online Resource 1)], suggesting that the molecular signatures of the main cellular types within ACP samples (i.e. tumour epithelium, reactive glial tissue and inflammatory infiltrate) may be contained in specific modules.
By systematically comparing the molecular signatures obtained from these profiling experiments with the modules previously described, we confirmed that the brown module genes were enriched in the signature of tumour cell compartments (i.e. cluster cells and palisading epithelium) (NES = 2.24, FDR < 0.001) (Fig. 2d). In contrast, both the blue and turquoise module genes were predominantly expressed by reactive glia (blue, NES = − 3.36; turquoise, NES = − 3.06; FDR < 0.001) (Fig. 2d). Supporting the notion that these modules represent specific signatures, genes known to be expressed in tumour epithelium, such as WNT targets (e.g. AXIN2, NOTUM), FGF3 and BMP4 were contained in the brown module [Suppl. Table 4a (Online Resource 6)]. Likewise, genes known to be expressed in the glial reactive tissue, such as GFAP, S100B and NKX2.2 were contained in the blue and turquoise modules [Suppl. Table 4a (Online Resource 6)]. Of interest, the three genes with the strongest brown module membership score (a measure of the degree of co-expression) were TP63, APCDD1L and BCL11B [Suppl. Table 3a (Online Resource 6)]. TP63 has previously been shown to be expressed in the tumour cells in human ACP , but APCDD1L and BCL11B have not been implicated in ACP. Consistently, immunofluorescence of ACP histological sections revealed the expression of BCL11B and TP63 exclusively in tumour cell compartments, including clusters, palisading epithelium and stellate reticulum, but not within surrounding reactive tissue (Fig. 2e). Similarly, immunofluorescence revealed expression of APCDD1L mostly in the cluster cells with no expression in the vast majority of the glial tissue except for cells adjacent to the tumour cells [Suppl. Fig. 5 (Online Resource 3)]. This is in agreement with the differential expression analysis of the laser capture microdissection dataset showing that APCDD1L is highly expressed in the clusters versus glial reactive tissue [40.46-fold, adjusted p value = 0.12; Suppl. Table 2e (Online Resource 4)]. Plotting of normalised expression levels confirmed the relationship of these brown module genes with CTNNB1 mutation allele frequency, further supporting that expression of these genes is enriched within the tumour cells (r = 0.83, 0.95, 0.96 respectively for each gene, p < 1 × 10−5) [Suppl. Fig. 6 (Online Resource 3)]. These datasets represent a useful resource for the identification of novel genes specifically expressed or highly enriched within particular human ACP cell compartments. Together, these studies have revealed the molecular signatures of the tumour epithelium (i.e. cluster cells and palisading epithelium) and glial reactive tissue (astrocytes and immune cells).
Human ACP clusters are molecularly analogous to the enamel knot and activate a transcriptional programme resembling odontogenesis
By comparing the molecular signatures of specific tumour compartments obtained by LCM with curated gene sets derived from distinct cell types present during tooth development , we identified molecular similarities between the ACP β-catenin-accumulating cell clusters and the enamel knot, a critical signalling centre within the developing tooth [28, 55]. Gene set enrichment analysis revealed a significant enrichment of the enamel knot gene signature in human cluster cells when compared with either palisading epithelium (NES = 1.90, FDR = 0.004) or to reactive glial tissue (NES = 1.79, FDR = 0.01) (Fig. 3c). Enamel knot signals act on the inner enamel epithelium at the cap stage of tooth development to control proliferation and tooth morphogenesis, and we identified a strong enrichment between palisading epithelium and cap-stage enamel epithelium , suggesting that palisading and enamel epithelium may be equivalent structures (NES = − 1.69, FDR = 0.008) (Fig. 3c).
We validated these findings further by immunofluorescence against p21/CDKN1A, a marker of the enamel knot, which showed specific signal mostly restricted to human ACP clusters (n = 8 tumour samples; Fig. 3d), consistent with previous reports [10, 17]. In addition, the ectodysplasin receptor (EDAR), whose signalling pathway has been strongly implicated in enamel knot formation but not previously studied in ACP [55, 56], was highly expressed in ACP tumours compared with controls (467.88-fold; adjusted p value = 1.37E−29) and in β-catenin clusters relative to palisading epithelium (10.85-fold; adjusted p value = 0.0001) or glial reactive tissue (357.05-fold; adjusted p value = 9.77E−05) [Suppl. Table 2a, d, e (Online Resource 4)]. Immunofluorescence confirmed the expression of EDAR in human cluster cells and its absence in the glial reactive tissue, further confirming similarities between ACP clusters and the enamel knot (n = 5 human tumours; Fig. 3d).
To test further these findings, we performed RNA-Seq on three Hesx1Cre/+/Ctnnb1lox(ex3)/+ neoplastic and three control pituitaries at postnatal day 1 (P1). As in human ACP, GSEA also confirmed greater expression of inner enamel epithelium (cap stage) (NES = 2.61, FDR < 0.001) and ameloblast (NES = 2.38, FDR < 0.001) genes in Hesx1Cre/+/Ctnnb1lox(ex3)/+ neoplastic pituitaries at P1 compared with wild-type controls, further highlighting the molecular relationship between mouse ACP and tooth development [Fig. 3e; Suppl. Table 2f (Online Resource 4)]. Mouse ACP also contains β-catenin-accumulating cell clusters, and we have previously characterised their expression profiles in both the Hesx1Cre/+/Ctnnb1lox(ex3)/+ embryonic and the Sox2CreERT2/+/Ctnnb1lox(ex3)/+ inducible mouse models of ACP [1, 17]. GSEA revealed a molecular signature of the enamel knot in the clusters from both of these mouse models (embryonic model, NES = 1.90, FDR = 0.003; inducible model NES = 2.44, FDR < 0.001) (Fig. 3f). Expression of p21 has recently been reported to be enriched in the clusters in mouse ACP , but EDAR immunostaining was inconclusive, possibly because the antibody does not recognise mouse EDAR. Together, these results reveal molecular analogies shared between ACP β-catenin-accumulating clusters with the enamel knot and the palisading epithelium with the inner enamel epithelium.
Cell clusters orchestrate paracrine signalling within human and murine ACP
The molecular similarities between human clusters and the enamel knot prompted us to explore further whether the clusters may also act as signalling centres in human ACP tumours. In agreement with this notion, our RNA profiling revealed that human ACP clusters expressed high levels of members of the FGF, BMP and WNT families of secreted factors compared with either palisading epithelium or reactive glia [Suppl. Fig. 7 (Online Resource 3); Suppl. Table 2d, e (Online Resource 4)]. SHH was also highly expressed by the cluster cells (Suppl. Fig. 7a), as previously shown . The activation of the WNT pathway in human ACP has been well documented, and we identified the expression of several WNT ligands (e.g. WNT4, WNT5A, WNT6, WNT7A, WNT10A, WNT10B and WNT16) in the clusters relative to the palisading epithelium or glial reactive tissue [Suppl. Fig. 7b (Online Resource 3); Suppl. Table 2d, e (Online Resource 4); adjusted p value ≤ 0.03]. To reveal the cells responding to FGF, TGFβ and BMP factors, we analysed the expression of the ligands and their receptors in our human RNA-Seq datasets and performed immunofluorescence against downstream signalling effectors indicating pathway activation.
BMP4 and BMP8A were highly expressed by clusters (adjusted p value ≤ 0.001) and BMPR1A and BMPR2 were expressed across all cell types, whist BMPR1B was highly expressed in the glial reactive tissue (adjusted p value = 0.00011) [Suppl. Fig. 7b (Online Resource 3); Suppl. Table 2d, e (Online Resource 4)]. Activation of the pathway, as evidenced by immunofluorescence staining against phosphorylated SMAD1/5/9 was widely present in tumour cells, particularly in proximity to the clusters, but was absent from the palisading epithelium (Fig. 4f). Other members of the TGF superfamily of secreted factors, such as TGFB1-3, as well as their receptors were broadly expressed across all tumour compartments, although TGFB1 appeared non-significantly elevated in clusters relative to both PE (4.9-fold, unadjusted p value = 0.08; adjusted p value not assessed because of inadequate read numbers) and glial tissue (9.5-fold, adjusted p value = 0.37) [Suppl. Fig. 7a, b (Online Resource 3); Suppl. Table 2d, e (Online Resource 4)]. Double immunofluorescence against β-catenin and phospho-SMAD3 revealed the activation of the TGFβ pathway in tumour cells and reactive tissue. Increased signal was apparent within areas of reactive tissue closest to tumour cells (Fig. 4e).
Preclinical studies provide support for an important role of the MAPK/ERK pathway in the pathogenesis of both human and mouse ACP
To further understand the role of the MAPK/ERK pathway in human ACP and to test its potential as a therapeutic target, we targeted this pathway using trametinib, a specific MEK inhibitor that has shown promising results in MAPK/ERK-driven solid tumours [34, 48] and currently being trialled in children and adolescents with a variety of solid tumours (NIH-NCI clinical trial: NCT02124772).
Cytokine profiling of tumour and cystic fluid suggests activation of the inflammasomes in human ACP
In addition, cytokine-encoding genes were highly up-regulated in ACP whole tumours compared with controls [Suppl. Table 6 (Online Resource 8)]. The expression of IL1B, IL18 and IL10 correlated significantly with the immune infiltrate and inflammatory cell markers, particularly CD14 and CD68 (r > 0.73, p < 1 × 10−5), as opposed to CTNNB1 mutation allele frequency [Fig. 8b; Suppl. Table 6 (Online Resource 8)]. This suggests that cytokine expression predominantly derived from immune rather than tumour cells. The presence of cytokines in the ACP tumours was also assessed by multiplex ELISA against IL1B, IL6, IL8 (CXCL8), IL10, IL18, TNF (TNFα) and IFNG (IFNγ), which revealed the expression of all of these but IFNG in protein lysates from eight human ACPs (Fig. 8c). This is in line with recent findings . Immunohistochemistry against IL1B, IL6 and IL8 did not yield reliable results in our hands using different antibodies.
The cystic fluid of ACP has been shown to contain inflammatory modulators [13, 59]. To explore the protein composition in greater detail, comprehensive proteome analysis was completed on cystic fluid from six ACP patients [Suppl. Materials and Methods (Online Resource 1)]. To obtain greater depth in the proteome coverage, samples of fluid were first subjected to a combinatorial peptide bead equalisation [Suppl. Fig. 8a (Online Resource 3)]. In total, 461 proteins were identified in all samples, using an FDR of 1% and requiring at least two unique peptides for each protein [Suppl. Table 7 (Online Resource 9)]. The overall proteome profile of all six cystic fluid samples was similar [Suppl. Fig. 8b (Online Resource 3)], covering an excess of five orders of magnitude. The most abundant proteins included albumin and several inflammation-associated proteins such as apolipoproteins, particularly APOA1 and APOA2, complement system proteins and immunoglobulins [Suppl. Fig. 8c (Online Resource 3]. Ontology analysis revealed an enrichment for terms related to immune/defence response, inflammation and sterol metabolism [Suppl. Fig. 8d (Online Resource 3)]. Cytokines were not detected in the proteome analysis, almost certainly attributable to their very low concentrations. We used multiplex ELISA to demonstrate the presence of IL1B, IL6, IL8, IL10, IL18, TNF and IFNG in the cystic fluid and whole ACP tumour protein lysates (Fig. 8c).
Having shown an expression profile, at both the mRNA and protein levels, supportive of the activation of inflammasomes in human ACP, we sought to explore the similarities of this molecular signature with that caused by other inflammasome activators. Cholesterol and other crystals (e.g. uric acid crystals) are established activators of inflammasomes, which mediate inflammation and are implicated in the pathogenesis of atherosclerosis and arthropathies (e.g. gout) [22, 33]. The ACP inflammatory cytokine profile was similar to that seen in gout, specifically regarding the high levels of IL1B, IL6 and IL18 . Interestingly, we observed strong expression of the inflammasome-induced chemokine CCL2 in association with cholesterol clefts (Fig. 8a), while GSEA revealed a significant enrichment of an atherosclerotic plaque gene signature in ACP tumours relative to control tissues (NES = 2.12, FDR < 0.001) (Fig. 9b). Together, these results suggest an activation of the inflammasomes in human ACP and identify cholesterol as a possible activator.
We provide a rationale that explains the long-standing observation of the histological similarities of ACP tumours with tooth development and odontogenic tumours. We show that ACP β-catenin-accumulating clusters and the enamel knots of developing teeth are molecularly analogous structures, as are ACP palisading epithelium and dental inner enamel epithelium. These pairs share a molecular signature and activate similar genetic programmes. Through the use of mouse models of ACP, we show that the expression of oncogenic β-catenin leading to the activation of the WNT pathway is sufficient to induce enamel epithelial/ameloblast-like gene expression and enamel knot-like cluster formation in Rathke’s pouch derivatives. Importantly, when the same degradation-resistant form of Ctnnb1 (Ctnnb1 lox(ex3) ) is activated in the enamel epithelia under the control of the Krt14 or Sox2 promoters, the result is continuous tooth formation, with the presence of multiple enamel knots and with morphological similarities to ACP [27, 58]. Of note, fully formed teeth including dentin are occasionally seen in human ACP [6, 42, 49].
Analogous to the developing tooth, we highlight a complex system of paracrine signalling occurring between tissue compartments, centred on ACP clusters. We characterise the FGF, MAPK/ERK, TGFβ, BMP and EDAR signalling pathways in ACP. The importance for many of these genes and pathways has been functionally explored in the developing tooth, where their manipulation alters the number of teeth and/or their morphogenesis [20, 28, 38, 55, 57]. As the consequences of gene inactivation have been more extensively studied in the tooth development field, it is plausible to speculate that this knowledge may inform on the genes/pathways that may be more relevant in the pathogenesis of human ACP.
Illustrating this concept, we show that ex vivo MEK inhibition with trametinib results in decreased proliferation increased apoptosis in both mouse and human ACP. Likewise, individuals with Costello syndrome, a condition that results from germline activating mutations in HRAS leading to the over-activation of the MAPK/ERK pathway, show defective enamel mineralisation with increased number, proliferation and irregular orientation of ameloblasts . This phenotype can be rescued by MEK inhibition in murine models of Costello syndrome . We show that MAPK/ERK pathway inhibition ex vivo using trametinib is associated with decreased proliferation and increased apoptosis in both mouse and human ACP, suggesting that other pathways of importance during tooth development may also be relevant in the context of ACP pathogenesis. Future preclinical studies in both genetic and patient-derived xenograft mouse models of ACP will assess the effects of trametinib treatment in tumour development.
Finally, we highlight that inflammasome activation may underlie the inflammation observed in human ACP, and identify that cholesterol crystals may be a potential inflammasome activator (Fig. 10). This finding could have important therapeutic consequences. Prevention of inflammasome activation by the use of IL1R inhibitors (e.g. anakinra) has been remarkably effective in several autoinflammatory diseases, most notably in cryopyrin-associated periodic syndromes (CAPS), gout and atherosclerotic disease [15, 44, 47, 52]. Inhibitors such as anakinra are used in routine clinical practice, including in paediatrics, and are known to cross the blood brain barrier and improve neuroinflammation in patients with severe CAPS [15, 44]. The ability to measure downstream cytokines e.g. IL6 and IL8 within cystic fluid offers an opportunity to develop biomarkers of therapy response to anakinra and other similar drugs. In summary, we reveal a detailed molecular rationale underpinning the resemblance of ACP to tooth development and odontogenic tumours, and additionally, we provide evidence from preclinical testing of the use of novel targeted therapies in ACP, opening the door for biologically driven studies in human patients.
We thank Martyn Cobourne for fruitful discussions on tooth development. We are grateful to Gita Mistry (CCLG Tissue Bank manager) for help identify ACP samples. We are grateful to Dr. Dale Moulding for invaluable help in microscopy. We would like to thank the patients, their families and clinicians who have donated tissues to research. We thank the Childhood Cancer and Leukaemia Group (CCLG) Tissue Bank for access to samples, and contributing CCLG Centres, including members of the ECMC Paediatric network. The CCLG Tissue Bank is funded by Cancer Research UK and CCLG. The human embryonic and fetal material was provided by the Joint MRC/Wellcome Trust (Grant # 099175/Z/12/Z) Human Developmental Biology Resource (http://www.hdbr.org). Tissue samples were also obtained from Plymouth Hospitals NHS Trust as part of the UK Brain Archive Information Network (BRAIN UK), which is funded by the Medical Research Council and brainstrust.
Compliance with ethical standards
Conflict of interest
P.B. has received institutional grants from SOBI, Roche and Novartis; consultancy fees from Roche; and lecturing fees from SOBI and Novartis. The other authors declare that they have no conflict of interest.
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