The role of ATMIN transcript variants in pancreatic cancer and DNA repair

Pancreatic cancer (PC) has a dismal prognosis with a five-year survival rate of 7%. Treatment of inoperable PC involves non-specific chemotherapy that has limited efficacy and considerable side effects. Identification of novel biomarkers or pathways provides an alternative treatment strategy that targets only the PC cells. A cohort of PC patients have mutations in the genes involved in DNA damage response (DDR), e.g., Breast Cancer 1 and 2 (BRCA1/2) and Ataxia Telangiectasia Mutated (ATM).Daily, thousands of DNA damage events occur that are repaired by specific DNA repair pathways that regulates the integrity and stability of the genome and prevents cancer. ATM-interactor (ATMIN) protein, an essential regulator of DYNLL1 expression and ATM signalling, is required for the activation of DDR pathways upon changes in chromatin structure and replication stress. The aim of this study is to investigate the role of ATMIN transcript variants in PC and DNA repair.

Expression of ATMIN transcript variants (ATMIN_001, 003 and 004) were examined using qRT-PCR. There was a~19-fold increase in ATMIN_001 in pancreatic tumour biopsies compared to the normal pancreas tissue and the expression of ATMIN_003 and 004 variants were only upregulated in pancreatic tumour biopsies. ATMIN_001, 003 and 004 variants were also detected in AsPC-1 and Hpaf-1 pancreatic cell lines with the absence of ATMIN_004 in Capan-1 cells. CRISPR/Cas9-based approaches were used to stably knockout ATMIN in the PC cell lines, producing heterozygous and biallelic mutations. Western blotting was used to establish complete and partial loss of ATMIN expression in the mutant cell lines. ATMIN transcript variants 001 and 003 were then overexpressed in all ATMIN-mutant null cell lines. ATMIN-mutant null cell lines showed a significant reduction in proliferation and viability that could be alleviated with ATMIN_001 overexpression, but not with ATMIN_003 overexpression. Similarly, homologous recombination (HR), non-homologous end joining (NHEJ), and base excision repair (BER) DDR pathways also showed significantly deregulated activity in ATMIN-mutant null cells that were rescued with ATMIN_001 overexpression but failed to be recovered with ATMIN_003 overexpression. The effect of olaparib and gemcitabine were then assessed, and results showed that ATMIN-mutated null and ATMIN_003 overexpressed cell lines were significantly sensitive to the drugs especially as a combination chemotherapy.

In summary, for the first time the effect of ATMIN variants on DNA repair and characterising an interaction between ATMIN and chemotherapeutic agents in PC cells have been assessed. The results provide a basis of potential new drug targets for PC. This could involve targeting the ATM/ATMIN interaction using small molecule drugs, or potentially developing variant specific therapy.