Project Summary
Glioblastoma (GBM) is the most common and aggressive primary malignant brain cancer in the adult population. Current
standard of care: tumor resection, ionizing radiation (IR) and (TMZ) produce minimum clinical benefits for most patients
due to innate or acquired resistance to treatment. Thus, optimization therapeutic approaches targeting oncogenic pathways
in glioblastoma is an urgent need. Genomic alterations lead to dysregulation of Cyclin Dependent Kinase 4/6-Rb pathway
in 84% of the 230 GBM cases profiled by The Cancer Genome Atlas. This results in the decoupling E2F from regulatory
mechanisms to permit unchecked transcription of genes required for cell division as well as DNA-damage response. This
points to CDK4/6 pharmacological inhibitors (CDK4/6i) as a promising strategy to treat GBM. Accordingly, three CDK4/6i
(Abemaciclib, Ribociclib and Palbociclib) approved by the FDA for treatment of breast cancer are now in clinical trials for
GBM patients. A priori genomics based selection of GBM patients for treatment with CDK4/6i are Rb-competent tumors
(wildtype RB1), and CDK4/6 amplification or deletion of CDKN2A, which codes for P16INK4a, an endogenous inhibitor
of CDK4/6 enzymatic activity. Studies point to CDK4/6-Rb dysregulation as an early evolutionary event which requires
additional alterations to progress to malignancy, and there is insufficient evidence utilizing the incorporation of additional
molecular biomarkers in the prediction of durable patient response. This deficiency is exemplified in preclinical studies
showing adaptive resistance in patients previously predicted to respond to CDK4/6i.This is corroborated by my preliminary
data showing a variability in Abemaciclib treatment response amongst GBM cancer stem cells (CSCs) that cannot be
explained by CDK4/6-Rb alterations alone. My hypothesis is that the identification of molecular features driving the
malignant phenotype, and co-occurring with CDK4/6-Rb pathway dysregulation, such as alterations in the other oncogenic
pathways in p53, receptor tyrosine kinase receptor signaling and MYC, as well as epigenetic and transcriptional adaptation
will strengthen our predictive capabilities in determining durable response versus resistance in GBM patients. I will test
my hypothesis by measuring the sensitivity of a representative panel of 20 genomically heterogeneous GBM patient-derived
CSCs to Abemaciclib and Ribociclib against control treatments. I will incorporate this response with their various somatic
genomic alterations co-occurring with CDK4/6-RB axis dysregulation. This will enable the identification of baseline
molecular correlates of GBM response with CDK4/6i. I will then interrogate the alterations in CDK4/6 copy number
amplifications, present in extrachromosomal DNA elements, global transcriptome and targeted proteomics in response to
CDK4/6 inhibition to identify potential escape mechanisms that lead to resistance in GBMs. Finally, to evaluate the
translational impact of our studies, I will determine if the phenotypes observed in vitro are reproduced in orthotopic patient-
derived xenografts (PDX). I will also determine if CDK4/6i can potentiate the genotoxic effects of TMZ. The results from
this research will provide evidence towards interpreting upcoming clinical trials results and identifying GBM patient cohorts
that will benefit from treatment with CDK4/6 inhibitors. Thus, the proposed project has significant translational potential,
aligning with the mission of the National Cancer Institute to develop evidence-based treatment strategies for cancer patients.