PALB2 Mutational Status in Haematopoeitic Malignancies - a Potential Therapeutic Target?

PALB2 protein is one of the principal components of Homologous Recombination Repair (HRR) that facilitates recruitment of other HRR proteins such as BRCA1, BRCA2 and Rad51. Biallelic PALB2 inactivation is associated with a severe form of Fanconi Anemia (FA-N), however, biallelic PALB2 mutations also occur in patients with a FA-N syndrome variant characterised by a predisposition to B-cell lymphoma development. The association of PALB2 with FA-N and FA-N variant suggests that PALB2 may play a pathogenic role in a wider range of sporadic haematopoietic malignancies. To address this possibility, the mutational status of the PALB2 gene was assessed in 30 myelodysplastic syndrome (MDS) samples, 23 acute lymphoblastic leukaemias (ALL), 171 chronic lymphocytic leukaemias (CLL) and 24 paediatric non-Hodgkin lymphomas (NHL) of T- or B-cell origin.

Overall, pathogenic (truncating or splicing errors)orlikely pathogenic sequence changes (missense alterations not previously reported but predicted to alter protein function) were detected in 17 (6.7%) of these patients. PALB2 was most frequently altered in NHL (25%), followed by MDS (6%), CLL (5%) and ALL (4%). The overall incidence of changes observed for B-cell NHL was higher than the frequency detected in T-cell NHL. PALB2 sequence changes were distributed across the whole gene including the DNA binding, BRCA1-interacting and BRCA2-interacting domains. Of note, a single truncating mutation; in the BRCA1-interacting domain, c.886delA; was detected in three cases, two with progressive CLL and one with MDS and interestingly only became detectable in one of these CLL patients following ibrutinib treatment. Furthermore, a polymorphic variant (c.2993G>A), previously associated with an increased breast cancer risk, was present at higher frequency than in the general population in cohorts of both CLL and ALL patients.

HRR defective tumour cells can be targeted by pharmacological inhibition of co-operating DNA repair pathways using the principal of synthetic lethality. To explore whether this concept can be utilised in the context of PALB2 deficiency, we evaluated the effectiveness of inhibition of two pathways; PARP and ATR; that co-operate with PALB2. The cytotoxicity induced by either an ATR inhibitor or a PARP inhibitor was assessed in lymphoblastoid cell lines derived from a paediatric B-cell NHL patient carrying the c.1676_1677delAAinsG and c.2586+1G>A mutations. As predicted, exposure of lymphoid cells with PALB2 mutations to either ATR or PARP inhibitor alone exerted a cytotoxic effect which was enhanced when both inhibitors were applied in combination.

In conclusion, our results suggest that PALB2 is altered in a subset of patients with different haematopoietic malignancies. This potentially provides another avenue for targeted therapies utilising the concept of synthetic lethality via application of PARP and ATR inhibitors that are currently being tested in clinical trials.