Defining the ‘molecular signature’ of PARP inhibitor sensitivity in blood cancers

Despite significant advances in the understanding of the biology of blood cancers, identification of the most effective and safe form of treatment continues to present a formidable challenge to clinicians. Treatment of myeloid leukaemia, in particular, invariably involves intensive chemotherapy and/or bone marrow transplantation. In younger patients, the approach to therapy aims to achieve complete remission. In contrast, patients over 60 years of age obtain a gentler course of therapy with the emphasis on controlling disease and improving quality of life. Current chemotherapeutics such as the use of alkylating agents are also associated with increased morbidity and drug resistance stimulating the search for alternative treatment strategies.

A major feature of blood cancers such as acute myeloid leukaemia (AML) is chromosomal instability such as deletions, translocations and chromosome losses. Chemotherapeutic agents and -irradiation cause significant numbers of DNA double-strand breaks (DSB) and other DNA damage and leukaemia associated with gross chromosomal rearrangements. More recent research has proposed that defects in the double strand DNA repair pathway cause chromosomal instability in haematological disorders. In this regard, chromosome instability disorders such as Blooms syndrome (BS) and Fanconi’s Anaemia (FA) have an increased propensity to transform to AML. Mutations in other signaling components associated with DSB in DNA, such as NBS1, and DNA ligase IV have been identified in leukaemia.

Importantly, I have previously shown that the major pathway for the repair of DNA DSB, the non-homologous end joining pathway (NHEJ) is up-regulated in Bloom Syndrome cells, associated with grossly inaccurate repair (1). I subsequently reported that acute and chronic myeloid leukaemic patient cells have similar NHEJ abnormalities (2)

If the breakdown in DNA damage repair signalling is the basis of chromosomal instability, then the particular factors responsible for giving survival advantages to the leukaemia clone are in fact targets for therapy. Defects in the pathways of double-strand DNA repair, involving homologous recombination, non-homologous end-joining and other DNA damage response pathways would render tumour cells sensitive to DNA repair inhibition such as inhibition of Poly ADP Ribose polymerase (PARP) activity. I have showed that chromosomal instability cell lines such as BLM and FA are highly sensitive to PARP inhibitors (3). In addition, in a pilot study of AML cell lines and primary AML cells were determined that 15% cell lines and AML patients respectively were hypersensitive to the PARP inhibition (4).

There are currently over 60 clinical trials in progress in which PARP inhibitors are being tested in a very wide range of malignancies, including breast and ovarian cancers. Importantly, however, leukaemia is not prominent in these trials. The central problem in the use of PARP inhibitors in clinical medicine is to identify the patients who would benefit from this novel treatment. In this regard, I have previously identified a subset of leukaemia patients with defects in mismatch DNA repair that confer PARP inhibitor sensitivity (5).

This project seeks to delineate the molecular response ‘signature’ that confers and modulates lethal synergy to PARP inhibitors in leukaemia.

1. Gene expression/Transcriptome profiling/RNA silencing screens will be used to characterise a subset of genes that predict sensitivity to PARP inhibitors.

2. Once identified, specific RNA silencing/in vitro mutagenesis will be used to down regulate expression of those genes that confer PARP inhibitor sensitivity. DNA repair studies would be used to determine the effect of disrupted gene expression loss on pathways of DNA damage signalling.

3. Cytotoxicity studies of PARP inhibitors with other chemotherapeutics would be tested to identify combination treatments in RNA silenced cells.