Resistance Mechanisms: Preclinical Insights

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Different mechanisms of resistance may be applicable to different types of cancer, likely depending on the germline or other mutation profile, or other factors, such as origin of the disease or prior treatment. This is a complex and rapidly evolving area, and one that is still in its relative infancy, but this section will outline some insights into mechanisms of resistance from preclinical studies.

1. Restoration of HRR is one mechanism leading to PARP inhibitor resistance, and can occur via several routes:

• BRCA reversion mutations can restore protein function and lead to PARP inhibitor and platinum-based chemotherapy resistance [1].
  • There are documented cases of BRCA1 reversion mutations exhibiting a MMEJ signature, suggesting that POLQ (required for MMEJ) could be a driver of resistance [2].
• Restoration of BRCA1 via other mechanisms than restoration mutations (e.g. alternative splicing or alternative translation initiation) [3-5].
• Mutations that compromise regulation of DNA end-resection via loss of 53BP1, MAD2L2/Rev7 or the Shieldin complex and enable HRR in the absence of BRCA1 [6-10].
  • The two main repair pathways for double-strand break (DSB) repair are HRR and NHEJ. While HRR allows for precise repair, NHEJ is more error-prone. DNA end resection of DSB is an important step in HRR repair, as it produces 3’ overhangs that prevent NHEJ and allow proteins involved in HRR repair to be recruited [11].

2. POLQ inhibitors may suppress acquired PARP inhibitor resistance, whilst also conferring Synthetic Lethality in HRR and NHEJ deficient cancers [2].

3. Loss of PARP1 activity caused by mutations in the PARP1 DNA binding domain can cause PARP inhibitor resistance and impair PARP1 trapping [12].

4. Stabilisation of stalled replication forks has recently emerged as another novel PARP inhibitor resistance mechanism. It has been shown that loss of PTIP (the MLL3/4 complex protein) protected BRCA2-deficient cells from DNA damage by inhibiting the recruitment of the MRE11 nuclease, and subsequent DNA degradation of stalled replication forks, which prevented PARP inhibitor-induced lethality [13].

5. PARP-inhibitor resistant, BRCA1-deficient cells, are known to become dependent on ATR for survival [14].

6. Activation of trans-lesion DNA synthesis through loss of CHD4 promotes resistance by allowing less efficient DNA repair to occur [15].

7. Upregulation of the PgP transporter for drug efflux, resulting in reduced availability of PARP inhibitor [16]. The next generation PARP inhibitor AZD2461, which has poor PgP affinity, may prove to overcome this mechanism of resistance [17].

8. PARG mutations can lead to PARP resistance via a mechanism that doesn’t restore HRR. Loss of PARG results in PAR chains not being degraded, so PARP activity has a bigger manifestation than it otherwise would and is not blocked by normal doses of PARP inhibitors [18]. 

References

1. Lin KK, Harrell MI, Oza AM et al. BRCA Reversion Mutations in Circulating Tumor DNA Predict Primary and Acquired Resistance to the PARP Inhibitor Rucaparib in High-Grade Ovarian Carcinoma. Cancer Discov 2019; 9: 210-219.
2. Ceccaldi R, Liu JC, Amunugama R et al. Homologous-recombination-deficient tumours are dependent on Poltheta-mediated repair. Nature 2015; 518: 258-262.
3. Wang Y, Bernhardy AJ, Cruz C et al. The BRCA1-Delta11q Alternative Splice Isoform Bypasses Germline Mutations and Promotes Therapeutic Resistance to PARP Inhibition and Cisplatin. Cancer Res 2016; 76: 2778-2790.
4. Drost R, Dhillon KK, van der Gulden H et al. BRCA1185delAG tumors may acquire therapy resistance through expression of RING-less BRCA1. J Clin Invest 2016; 126: 2903-2918.
5. Cruz C, Castroviejo-Bermejo M, Gutierrez-Enriquez S et al. RAD51 foci as a functional biomarker of homologous recombination repair and PARP inhibitor resistance in germline BRCA-mutated breast cancer. Ann Oncol 2018; 29: 1203-1210.
6. Bunting SF, Callen E, Wong N et al. 53BP1 inhibits homologous recombination in Brca1-deficient cells by blocking resection of DNA breaks. Cell 2010; 141: 243-254.
7. Gupta R, Somyajit K, Narita T et al. DNA Repair Network Analysis Reveals Shieldin as a Key Regulator of NHEJ and PARP Inhibitor Sensitivity. Cell 2018; 173: 972-988 e923.
8. Dev H, Chiang TW, Lescale C et al. Shieldin complex promotes DNA end-joining and counters homologous recombination in BRCA1-null cells. Nat Cell Biol 2018; 20: 954-965.
9. Mirman Z, Lottersberger F, Takai H et al. 53BP1–RIF1–shieldin counteracts DSB resection through CST- and Polα-dependent fill-in. Nature 2018; 560: 112-116.
10. Xu G, Chapman JR, Brandsma I et al. REV7 counteracts DNA double-strand break resection and affects PARP inhibition. Nature 2015; 521: 541-544.
11. Liu T, Huang J. DNA End Resection: Facts and Mechanisms. Genomics Proteomics Bioinformatics 2016; 14: 126-130.
12. Pettitt SJ, Krastev DB, Brandsma I et al. Genome-wide and high-density CRISPR-Cas9 screens identify point mutations in PARP1 causing PARP inhibitor resistance. Nat Commun 2018; 9: 1849.
13. Ray Chaudhuri A, Callen E, Ding X et al. Replication fork stability confers chemoresistance in BRCA-deficient cells. Nature 2016; 535: 382-387.
14. Yazinski SA, Comaills V, Buisson R et al. ATR inhibition disrupts rewired homologous recombination and fork protection pathways in PARP inhibitor-resistant BRCA-deficient cancer cells. Genes Dev 2017; 31: 318-332.
15. Guillemette S, Serra RW, Peng M et al. Resistance to therapy in BRCA2 mutant cells due to loss of the nucleosome remodeling factor CHD4. Genes Dev 2015; 29: 489-494.
16. Rottenberg S, Jaspers JE, Kersbergen A et al. High sensitivity of BRCA1-deficient mammary tumors to the PARP inhibitor AZD2281 alone and in combination with platinum drugs. Proc Natl Acad Sci U S A 2008; 105: 17079-17084.
17. Oplustil O'Connor L, Rulten SL, Cranston AN et al. The PARP Inhibitor AZD2461 Provides Insights into the Role of PARP3 Inhibition for Both Synthetic Lethality and Tolerability with Chemotherapy in Preclinical Models. Cancer Res 2016; 76: 6084-6094.
18. Gogola E, Duarte AA, de Ruiter JR et al. Selective Loss of PARG Restores PARylation and Counteracts PARP Inhibitor-Mediated Synthetic Lethality. Cancer Cell 2018; 33: 1078-1093 e1012.