Ribonucleotide Reductase Subunit Switching in Hepatoblastoma Drug Response and Relapse

Hepatoblastoma is the most common primary liver cancer in infants and young children. Despite being a very rare cancer that accounts for only 0.5-2% if all childhood cancer cases, HB has the largest increase in incidence among childhood cancers in the United States and worldwide. The five-year survival rate of children with the aggressive forms of HB, including those that have developed metastatic or recurrent diseases, is less than 40% due to the lack of effective treatment. We aim to identify targetable mechanisms underlying the progression and drug resistance of high-risk HB. Our recent work on HB mouse and organoid models, patient-derived xenografts (PDX) and primary patient samples revealed a significant upregulation of ribonucleotide reductase (RNR) subunit M2 (RRM2) in high-risk HB. RNR is the sole enzymatic complex in mammal cells that converts ribonucleotides to deoxyribonucleotides and plays a critical role in regulating cell division and DNA repair. We found standard chemotherapy agents as well as two RRM2 inhibitors, triapine and MK1775, werecapable of reducing RRM2 expression effectively in vitro. However, we found a significant induction of another RNR subunit M2B (RRM2B) in treated cells in corresponding to RRM2 reduction. While no changes in drug response were noticed in RRM2B knockout (KO) HB cells. RRM2B levels in HB cells showed a strong impact on cells’ ability to recover after chemotherapy. RRM2BOE HB cells showed a significant increase in their colony formation potential after chemotherapy where RRM2BKO cells formed much fewer colonies after treatment compared to the control cells. Interestingly, we noticed a reversed subunit switch from RRM2B to RRM2 during the recovery period when cell proliferation was restored. RRM2, indeed, had a much higher enzymatic activity in converting ribonucleotides to deoxyribonucleotides than RRM2B and promoted cell growth much more efficiently than RRM2B when both were overexpressed in HB cells. Finally, combining the RRM2 inhibitor MK1775 with standard chemotherapy in HB PDX models, although showing no additional benefit in reducing tumor size, significantly delayed tumor relapse after drug withdrawal. In this study, we demonstrated an intriguing switching between two RNR subunits, RRM2 and RRM2B in HB cells undergoing drug treatment and during their recovery afterwards. Our data suggest that RRM2 supports HB growth while its switching to RRM2B is critical to tumor cell survival under drug treatment. When tumor relapses, there is a reversed subunit switch from RRM2B to RRM2 to supports the recurrent growth of the tumor, which can serve as a potential therapeutic target in preventing HB relapse