Investigation Of The Underlying Molecular Mechanism Of Targeting The MYC-PARP1 Axis In Small Cell Lung Cancer By Combined Inhibition Of PARP And BET

Small cell lung cancer(SCLC)is the most aggressive subtype of lung cancer,accounting for 15%of all lung cancers.In the past four decades,combination of chemotherapy with or without radiation is the standard treatment option for SCLC.Patients with SCLC are initially high responsive to standard treatment.Unfortunately,almost all SCLC patients relapse within one year,leading to five-year survival rate under 7%.MYC paralogs and PARP1 play important roles in SCLC development.MYC paralogs are exclusively amplified or overexpressed in about 20%SCLC patients.PARP1,a DNA damage repair protein involved in DNA single-strand break repair(SSBR)and double-strand break repair(DSBR),is highly expressed in SCLC.However,the correlation between MYC paralogs and PARP1 in SCLC is still unclear.Thus,the aim of this study is to identify the correlation between MYC paralogs and PARP1 in SCLC,and to test whether targeting the MYC paralog-PARP1 axis is effective in the treatment of SCLC.Furthermore,this study aims to provide a rational treatment strategy for SCLC patients especially in a MYC paralog-dependent setting.Methods:(1)Integrative analysis of multiple SCLC RNA-seq datasets,including the RNA-seq dataset of SCLC cell lines downloaded from CCLE,the SCLC RNA-seq dataset from 81 human primary SCLC tumors obtained from George et al,2015 and RNA-seq dataset from 14 murine SCLC tumors downloaded from GSE89660.(2)In vitro investigations:A panel of MYC paralog-dependent and independent SCLC cell lines were chosen for test the synergy between PARP inhibitor BMN673 and BET inhibitor JQ1.(3)In vivo studies:SCLC cells were subcutaneously injected into the flank sites of 4-5 weeks old immunocompromised female mice.Once the tumors reached about 100mm3,mice were randomly divided into four groups:vehicle,BMN673 or JQ1 single agent group,and combination group.Animals were treated by oral gavage with JQ1(40mg/kg)every 6 days per week.BMN673 was resuspended for intraperitoneal administration at a dose of 0.33mg/kg every 6 days per week.The tumor volume curve was plotted after 3-weeks treatment.Moreover,the fresh SCLC patient tumor was collected and transplanted into the subcutaneous sites of NOD/SCID mice to establish SCLC patient derived xenograft(PDX)models,after about two month's growth,the tumor was dissected for ex vivo culture.Following 24 hours recovery,tumors were treated with indicated drugs for another 48 hours and then collected for histopathological analysis.Results:(1)Patients with high PARP1 expression had a significantly longer overall survival(OS)and progression free survival(PFS)than that with low PARP1 expression.(2)MYC paralog expression was positively correlated with the expression of PARP1 and other DDR-related genes.(3)MYC paralogs transcriptionally regulated the expression of PARP1 and other DDR-related genes.(4)Combination of PARP inhibitor BMN673 and BET inhibitor JQ1 yielded strongly synergistic anti-tumor effects in MYC paralog-dependent SCLC,but not in MYC paralog-independent SCLC.(5)BET inhibition resulted in decreased DNA damage repair activity,sensitizing SCLC cells to PARP inhibitor BMN673.Conclusions:(1)PARP1 expression le vel could be used as a prognostic biomarker for predicting the clinical outcomes of SCLC patients.(2)The expression of MYC paralogs is positively associated with the expression of DDR-related proteins in SCLC,MYC paralog-activated SCLC may highly depend on DDR pathway for unchecked cell proliferation.(3)Combined inhibition of BET and PARP effectively treats MYC paralog-dependent SCLC in vitro and in vivo and thus provides a plausible strategy for clinical trials.(4)MYC paralog status might be used as a biomarker for predicting the effectiveness of BMN673-JQ1 combination in SCLC.