Abstract P4-01-02: Low ATM Expression Levels Predict Resistance to Anthracyclins in TP53 and CHK2 Wild-Type Breast Tumors
Authors: Knappskog S , Chrisanthar R , Løkkevik E , Anker G , østenstad B , Lundgren S , Risberg T , Mjaaland I , Staalesen V , Miletic H , Leirvaag B , Lillehaug JR , Lønning PE. 
Published: April 2011

Background: Mutations in TP53 and its upstream p53-activator Chk2, are known to be associated with resistance to anthracycline and mitomycin treatment in locally advanced breast cancer. However, this association is not fully predictive, as some tumours are therapy resistant despite harbouring wild-type TP53 and CHK2. ATM is an upstream activator of both p53 and Chk2. Here, we explored ATM mutational status and expression levels with respect to anthracycline/mitomycine resistance in locally advanced breast cancers having primary chemotherapy with doxorubicin or 5FU-mitomycin and previously analyzed for TP53 and CHK2 status.

Methods: In order to investigate ATM's potential predictive role, we performed MLPA-analysis, qPCR on ATM mRNA, ATM promoter methylation analyses and complete sequencing of the entire ATM coding region in patient biopsies from two prospective studies on resistance to doxorubicin and 5-FU/mitomycin in locally advance breast cancer (n=36 and n=38, respectively).

Results: We performed complete sequencing of the ATM coding exons in 74 patients including 17 patients revealing primary resistance to chemotherapy. ATM mutations were detected in tumours from 5 patients. No mutation was detected among tumors resistant to chemotherapy. In addition, we observed 9 polymorphic variants present in more than one tumour, affecting or not affecting the amino acid sequence of the ATM protein (n = 34 patients in total). No correlation was observed between polymorphism status and response to therapy. No methylation of the ATM promoter, large deletions or duplication/amplifications of the ATM gene was observed. ATM expression levels varied substantially, with a ratio of 49 between the highest versus lowest level recorded. Among patients with no mutations in TP53 or CHK2, low ATM expression levels were significantly correlated to lack of response to doxorubicin or 5-FU/mitomycin (p=0.023; Mann-Whitney test). In contrast, ATM-levels were not suppressed in non-responding tumors harbouring TP53 or CHK2 mutations (p=0.642; Mann-Whitney test). Defining ATM mRNA levels in the lower 50%, 33% or 25% of the patient cohort to be reduced, tumors with either reduced levels of ATM or mutations affecting TP53 or CHK2 revealed a high risk for therapy resistance (p=0.004, p=0.001 and p=0.006, using the different ATM-level cut-offs; Fischer exact test). Interestingly, in a similar prospective study with a smaller number of patients displaying progressive disease upon treatment with epirubicin (n = 10), we find 2 out 5 TP53 and CHK2-wild-type non-responding tumors to express very low levels of ATM (within the lowest 5% of the cohort).

Conclusions: Our data suggests that low expression of ATM may substitute for TP53 and CHK2 mutations causing resistance to anthracycline-and mitomycin therapy in breast cancer.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-01-02.