Loss of PTEN in Pediatric AML Confers Sensitivity to PARP Inhibition
Authors: Kamens Jennifer Lauren , Cotton Anitria , Lam Jeannie W , Dang Jinjun , Seth Aman , Hansen Baranda S , Pruett-Miller Shondra M. , Shelat Anang , Gruber Tanja Andrea 
Published: November 2021

Abstract
Pediatric Acute Myeloid Leukemia (AML) is a rare, but deadly cancer. Outcomes over the last 20 years have remained stagnant with an overall 5-year survival rate &lt; 70% and relapse rates around 50%. Further, few new therapies have been successfully introduced to improve these outcomes. Here we report that exploiting deficiencies in DNA damage repair (DDR) is a potential therapeutic strategy for AML. Poly-ADP Ribose Polymerase (PARP) inhibitors were initially developed to target deficient homologous recombination (HR) in BRCA1/2 mutated cancers by blocking single stranded base repair following DNA damage, leading to an accumulation of double stranded DNA breaks, thereby inducing apoptosis.
To evaluate the activity of PARP inhibition in pediatric AML, talazoparib was tested as a single agent and in combination with standard chemotherapeutic agents in human AML cell lines representing low (Kasumi-1 and ME-1), intermediate (AML193), and high-risk (CTS, CMS, MOLM-13, and CHRF288-11) disease based on their genomic mutations. Talazoparib showed the highest efficacy as a single agent in all four cell lines with genomic lesions found in high-risk AML subtypes.
After combination drug screens, topotecan (synergistic) and gemcitabine (additive) were chosen to move forward to in vivo testing. Our investigational combination was tested in vivo in four murine models representing pediatric AML subtypes harboring AML1-ETO9a (low risk), MLL-AF6 (high risk), CBAF2T3-GLIS2/JAK2 V617F (high risk) and NUP98-KDM5A (high risk) oncogenes. Mice received a backbone of either current standard of care chemotherapy (SOC; anthracycline plus cytarabine) or topotecan plus gemcitabine. NUP98-KDM5A and MLL-AF6 positive mice receiving single agent talazoparib were found to have prolonged survival compared to vehicle alone (p=0.019 and p&lt;0.0001, respectively) which was further enhanced by the addition of chemotherapy irrespective of backbone (p &lt;0.0001). Conversely, mice with AML1-ETOa positive leukemia had no response to single agent PARP inhibitor. While a few mice benefitted from the addition of talazoparib to SOC, this result was not statistically significant (p= 0.42). Early response by bioluminescent imaging confirmed that mice with MLL-AF6 and NUP98-KDM5A driven leukemias who received talazoparib in combination with chemotherapy had the lowest leukemia burdens while the AML1-ETOa cohort did not benefit from the addition of this targeted agent. Interestingly, mice harboring CBAF2T3-GLIS2/JAK2 V617F were not responsive to PARP inhibitors, which was inconsistent with the CMS cell line that has same oncogenic fusion gene but lacks the JAK2 V617F mutation.
Synergy experiments with ATM inhibitor AZD0156 demonstrated tremendous synergy with talazoparib in sensitive cell lines with almost no synergy in those that were resistant, suggesting that sensitive cell lines are unable to efficiently activate the HR pathway to repair double stranded breaks induced by PARP inhibition whereas resistant cells can overcome inhibition. To determine the HR response to DNA damage in our cell lines, we exposed them to 1uM topotecan for 2 hours and then measured γH2AX response at 0, 4 and 24 hours. γH2AX is a sensor of DNA damage and therefore increases with DNA damage and decreases with repair. PARP inhibitor sensitive cell lines had persistence of gamma H2AX at 24hrs while resistant cell lines had at least partial resolution of damage, confirming that PARP inhibitor sensitive cell lines have aberrant DNA damage response through HR.
RNA sequencing of our cell lines revealed a correlation between Phosphatase and tensin homolog (PTEN) transcript levels and PARP sensitivity. Western blotting confirmed that PTEN was downregulated or absent in both cell lines and murine leukemias that were sensitive to PARP inhibitors. In contrast to the CMS cell line that carries the CBFA2T3-GLIS2 fusion, murine leukemias with CBAF2T3-GLIS2/JAK2 V617F had high levels of PTEN, supporting the hypothesis that sensitivity to PARP inhibitors is due to loss of PTEN.
In conclusion, we report that a subset of pediatric AML with high- risk features are sensitive to PARP inhibition due to deficient DDR through HR. Downregulation of PTEN is a candidate biomarker of response to PARP inhibitors in these patients. This data illuminates a promising therapeutic vulnerability in a patient population where new targeted treatments are vital to improve outcomes.

Disclosures
No relevant conflicts of interest to declare.