Dual and opposing roles of primary cilia in medulloblastoma development

Abstract

Recent work has shown that primary cilia are essential for Hedgehog (Hh) signaling during mammalian development. It is also known that aberrant Hh signaling can lead to cancer, but the role of primary cilia in oncogenesis is not known. Cerebellar granule neuron precursors (GNPs) can give rise to medulloblastomas, the most common malignant brain tumor in children. The primary cilium and Hh signaling are required for GNP proliferation. We asked whether primary cilia in GNPs have a role in medulloblastoma growth in mice. Genetic ablation of primary cilia blocked medulloblastoma formation when this tumor was driven by a constitutively active Smoothened protein (Smo), an upstream activator of Hh signaling. In contrast, removal of cilia was required for medulloblastoma growth by a constitutively active glioma-associated oncogene family zinc finger-2 (GLI2), a downstream transcription factor. Thus, primary cilia are either required for or inhibit medulloblastoma formation, depending on the initiating oncogenic event. Remarkably, the presence or absence of cilia was associated with specific variants of human medulloblastomas; primary cilia were found in medulloblastomas with activation in HH or WNT signaling but not in most medulloblastomas in other distinct molecular subgroups. Primary cilia could serve as a diagnostic tool and provide new insights into the mechanism of tumorigenesis.

Fig. 1. Kif3a is required for SmoM2-driven medulloblastoma formation

(a) Hematoxylin stained sagittal sections of control and mutant cerebella. Expression of SmoM2 in GNPs using hGFAP::Cre induces medulloblastoma by P10. Removal of Kif3a, an essential gene for ciliogenesis, in SmoM2 expressing cells completely blocks tumorigenesis resulting in atrophic cerebella similar to those of hGFAP::Cre;Kif3a^fl/fl mice. (b) SmoM2-YFP is highly enriched in primary cilia (green, arrow) associated with the basal body (anti-γ-tubulin staining, shown in red, arrowhead). In hGFAP::Cre; SmoM2^fl/+; Kif3a^fl/fl mice only the basal body is present (arrowhead). (c, d) BrdU incorporation (1h survival) at E16. GNPs proliferation in hGFAP::Cre; SmoM2^fl/+; Kif3a^fl/fl is similar to that observed in hGFAP::Cre; Kif3a^fl/fl or wild type mice. In contrast, already by E16, hGFAP::Cre; SmoM2^fl/+ mice show an expanded EGL containing significantly more proliferating cells. *: P < 0.05. Scale bar = 0.5 mm (a), 5 µm (b) and 100 µm (c).

Fig. 2. Kif3a suppresses Gli2ΔN-driven medulloblastoma formation

(a–c) Hematoxylin and eosin (H&E) stained sagittal sections of control and mutant cerebella at P23. (a) hGFAP::Cre; CLEG2^fl/+ mice have ectopic clusters of cells (arrows) without tumor formation. (b) All hGFAP::Cre; CLEG2^fl/+; Kif3a^fl/fl mice develop medulloblastomas between P11 and P30. A green dotted line in b demarcates domains of the tumors containing type 1 and type 2 tumor cells (see text). (c) High magnification of red box in b, showing the two domains with different cell types. (d) Western blot analysis of Gli3 proteins isolated from P23 cerebella shows the decrease of Gli3 repressor (Gli3R) in hGFAP::Cre; CLEG2^fl/+; Kif3a^fl/fl mice compared with wild-type and hGFAP::Cre; CLEG2^fl/+ mice. Asterisk indicates a nonspecific band, which serves as a loading control. This banding pattern is similar to a previous report, which showed that this Gli3 antibody detects specific Gli3 bands that are absent in Gli3 mutants and non-specific bands. (e,f) BrdU-labeling (1h survival) reveals that the ectopic clusters in hGFAP::Cre; CLEG2^fl/+ mice are not proliferating (arrows), while tumors in hGFAP::Cre; CLEG2^fl/+; Kif3a^fl/fl mice contained many proliferating cells. (g) hGFAP::Cre; CLEG2^fl/+; Kif3a^fl/fl mice (P < 0.001) and hGFAP::Cre; CLEG2^fl/+; Gli3^Xt/+ mice (P < 0.02) die significantly earlier than hGFAP::Cre; CLEG2^fl/+ mice. (h) Hh-responsive genes are up-regulated in both SmoM2- and Gli2ΔN-driven tumors (see text). *: P < 0.05. Scale bar = 0.5 mm (a,b,e,f), 10 µm (c).

Fig. 3. Primary cilia are present in a subset of human medulloblastomas

(a) Nine of twenty-three classic and five of six desmoplastic medulloblastomas have primary cilia in most cells. Eight of nine anaplastic medulloblastomas have few or no ciliated cells (shown is an example of non-ciliated anaplastic medulloblastoma). Cilia and basal bodies are stained by anti-acetylated tubulin (green, arrows) and anti-γ-tubulin (red, arrowheads), respectively. Scale bar = 5 µm. (b) Model summarizing results of dual roles of primary cilia in SmoM2- and Gli2ΔN-driven tumorigenesis. SmoM2 is insensitive to inhibition by Ptch1 and constitutively localizes to the cilia, where it activates Gli2 and inhibits Gli3 repressor (Gli3R) formation, leading to medulloblastoma (top left). Without the cilia, SmoM2 cannot activate downstream pathways; cerebellum remains small as in ciliary mutants and no tumors develop (bottom left). Gli2ΔN is constitutively active and independent of primary cilia, yet these mice do not develop medulloblastoma possibly due to thepresence of Gli3 repressor; however, these mice develop type 2 tumors later in life (top right). In the absence of cilia, Gli2ΔN induces medulloblastoma and type 2 tumors earlier in life. This may be due to the elimination of Gli3 repressor when cilia are removed (bottom right).