Targeting Replicative Stress to Treat Hematological Disorders

Background: Combinations of chromosomal translocations, copy-number variations, somatic mutations, and clonal heterogeneity that characterize hematological cancers make every patient unique from a genetic point of view. This variety creates a true challenge for tailored therapy. We have previously described that myeloma (MM) cells present signs of ongoing DNA damage, and activate an ATM/ABL1-dependent DNA damage response (DDR) without overt apoptosis. Here we further characterize the mechanisms of DNA damage and replicative stress in MM, and we extend this knowledge to other hematological malignancies to evaluate a novel and possibly shared approach to synthetic lethality (1).

Results: We studied a panel of MM cell lines together with acute myeloid, lymphoid leukemia and lymphoma cell lines. Several cell lines have demonstrable ongoing DNA damage, activate ATR and CHK1 and also present with signs of replicative stress, such as 53BP1, RPA and RAD51 foci. We next evaluated a gene expression signature specific for increased chromosomal instability and DNA damage in a cohort of MM patients, comparing them with normal plasma cells. Specifically, we identified a subset of patients, representing around 20 percent of individuals with MM that show this signature and also present with an unfavorable prognosis due to a more aggressive disease. Interestingly, in a multivariate analysis, this signature was independent from other poor prognostic criteria, including proliferation and the presence of MMSET/FGFR3 or MAF translocations (2), hence representing a potential novel prognostic signature (3). Gene-set enrichment studies are ongoing in other hematological disorders. However, preliminary data show that subsets of patients with other hematological malignancies also present with intense over expression of genes belonging to the instability signature when compared to normal B cells, in a very similar fashion to that seen with MM. An intact ATR/CHK1 pathway is crucial for the survival of tumor cells in vivo, especially in the presence of activated oncogenes. For instance, Em-myc transgenic mice develop B-cell lymphomas with intense replicative stress that can be blocked by crossing the Em-myc transgenic mice with a hypomorphic Atr mouse strain (Atr-Seckel; AtrS/S). We therefore decided to exploit the concept of replicative stress overload, impeding the capacity of the cells for repairing the excess of damaged DNA. We then silenced ATR using shRNAs, the upstream protein involved in the control of stalled replication origins, in two MM cell lines (H929 and OPM-2) and in the Jurkat cell line. Critically, inhibition of ATR caused a reduction in cellular growth and induction of apoptosis. A similar phenotype was observed using VE-821, a specific ATR inhibitor. Finally, a broad panel of MM cells and leukemia cell lines was used to confirm these growth inhibitory effects.

Conclusion:Replicative stress is present in multiple groups of patients with aggressive types of MM or leukemia. Strategies which couple pre-existing high rates of DNA damage with reduced DNA repair can specifically cause apoptosis of malignant cells and encouragingly spare normal ones, thus providing a strong rationale for potential clinical benefit to those cohorts of patients with otherwise very unfavorable outcomes.

(1)  Cottini F, Hideshima T, Xu C, Sattler M, Dori M, Agnelli L, et al. Rescue of Hippo coactivator YAP1 triggers DNA damage-induced apoptosis in hematological cancers. Nature medicine. 2014;20(6):599-606.

(2)  Zhan F, Huang Y, Colla S, Stewart JP, Hanamura I, Gupta S, et al. The molecular classification of multiple myeloma. Blood. 2006;108(6):2020-8.

(3)  Cottini F, Teru Hideshima, Rikio Suzuki, Yu-Tzu Tai, et al. Synthetic lethal approaches exploiting DNA damage in aggressive myeloma. Cancer Discovery ahead of print.