An OTX2-PAX3 signaling axis regulates Group 3 medulloblastoma cell fate

An OTX2-PAX3 signaling axis regulates Group 3 medulloblastoma cell fate

I joined the Werbowetski-Ogilvie lab in July 2017, and this study represents 3 years of hard work! With a background in developmental biology, I was interested in merging this knowledge with oncology research. Having already been acquainted with the developmental role of the transcription factor OTX2 from my graduate training, I was eager to work on this new project exploring the role of OTX2 in medulloblastoma (MB) progression.

Our lab had previously shown that OTX2 plays a critical role in maintaining the balance between self-renewal and differentiation in Group 3 medulloblastoma (Stromecki et al., Molecular Oncology, 2018). Our question then became, “How does OTX2 go about doing this?”

We began the study in search of possible OTX2 target genes in Group 3 MB. We took a specific interest in targets that were transcription factors due to their critical role in neurodevelopment. After mining the “list” of possible transcription factor families to pursue, we settled on the PAX genes which had not been explored previously in the context of Group 3 MB.

What followed was a number of exciting findings. We found that OTX2 has a more expansive role in MB than its frequently reported function in regulating proliferation. We found OTX2 broadly restricts expression of neurodevelopmental transcription factors, including both PAX3 and PAX6.

Overexpression of these genes in Group 3 MB cells reduced self-renewal capacity and this is accompanied by perturbation of Group 3 MB transcriptional signatures.  When we looked in vivo, only PAX3 overexpression significantly extended animal survival and reduced tumor size.

From here we asked, “Why do we see this in PAX3 but not PAX6?” Turning to our RNA-sequencing data from PAX expressing tumorspheres, we found mTOR signalling to be downregulated in PAX3 but not PAX6 tumorspheres.

Exploring this further, we found that OTX2 silencing mimicked the reduction in mTOR signalling observed in the PAX3 tumorspheres, and importantly, treatment of Group 3 MB with mTOR inhibitors not only reduced self-renewal capacity of Group 3 tumorspheres, but also enhanced survival in our animal models, thus identifying a novel pathway to target for Group 3 MB treatment.

Importantly, we also found that our Group 3 tumorsphere models recapitulate an undifferentiated progenitor program found in primary patient samples. Theses exciting results underscored the strength of our “work horse” tumorsphere model to uncover pathways directly relevant to Group 3 MB progression.

 In November 2019, we were thrilled to hear that we had been invited to revise our manuscript and we soon got to work on the revisions. On Friday March 20th, we received word that the University would be shutting down due to COVID-19 on Monday March 23rd. We were extremely lucky that we managed to get all our necessary revisions ahead of schedule, in part through spending a little time over the holidays with the mice, fantastic collaborations, and a small but hard working lab of female scientists.