P2-02-09: TP53 Mutation Patterns in Breast Cancer Subgroups

Tumor protein 53 (TP53) is the most commonly altered gene in human cancers. In breast cancers, TP53 is mutated in approximately 30% of all cases, but this frequency fluctuates widely within the different molecular subclasses. Different types of mutations may be observed, such as substitutions (replacement of a nucleotide pair by another one), or complex mutations (deletions or insertions of one or more nucleotides). Mutation types may reflect mechanism of DNA lesion or DNA repair deficiencies. Furthermore, mutations can give rise to different effects such as truncating mutations leading to loss of function, or missense mutations often leading to dominant negative activity. Those mutation effects can be advantageous in tumorigenesis and thus can be subject to selective pressure.

Here we classified 572 breast tumors in three groups, according to microarray data: luminal, basal and molecular apocrine. TP53 status was assessed by a yeast-based functional assay (FASAY) and cDNA sequencing. We then assessed whether any feature of TP53 mutations would be preferentially associated to a specific subtype of breast cancer.

- In term of TP53 mutation frequencies, as expected, lowest frequency was observed in luminal subgroup (26%) and highest in basal (90%) and molecular apocrine (70%). Notably, much higher rate of TP53 mutations occurred in luminal B subgroup (41%) than in luminal A one (17%), suggesting that TP53 may be an important feature in progression from luminal A to B.

- In term of mutations types, luminal tumors showed high frequency of substitutions, while molecular apocrine and basal presented increased rate of deletions and insertions, reflecting probably increased rate of DNA breaks. This suggests that same mutational events may occur in basal and molecular apocrine tumors.

- In term of mutation effects, we found high frequency of missense mutations in luminal tumors (notably AT to GC) and much higher rate of truncating mutations in basal tumors. These observations point to an existence of different selection pressure in each of them, such as a strong pressure for P53 mutations with potential dominant negative inhibition of P73/P63 (recently shown to favor invasion), in luminal tumors. Collectively, these results point not only to different mechanisms of P53 gene inactivation, but also different functional consequences among the different breast cancer subclasses.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-02-09.

Multiple molecular mechanisms contribute to radiation sensitivity in mantle cell lymphoma

Mantle cell lymphomas (MCL) are characterized by their aggressive behavior and poor response to chemotherapy regimens. We report here evidence of increased in vitro radiation sensitivity in two cell lines that we have generated from two MCL patients (UPN1 and UPN2). However, despite their increased radiation sensitivity, UPN2 cells were totally resistant to apoptotic cell death, whereas UPN1 cells underwent massive apoptosis 6 h after irradiation. The frequency of induced chromosomal abnormalities was higher in UPN1 as compared to UPN2. Distinct mechanisms have been found to contribute to this phenotype: a major telomere shortening (UPN1 and UPN2), deletion of one ATM allele and a point mutation in the remaining allele in UPN2, mutation of p53 gene (UPN1 and UPN2) with absence of functional p53 as revealed by functional yeast assays. After irradiation, Ku70 levels in UPN1 increased and decreased in UPN2, whereas in the same conditions, DNA-PKcs protein levels decreased in UPN1 and remained unchanged in UPN2. Thus, irradiation-induced apoptotic cell death can occur despite the nonfunctional status of p53 (UPN1), suggesting activation of a unique pathway in MCL cells for the induction of this event. Overall, our study demonstrates that MCL cells show increased radiation sensitivity, which can be the result of distinct molecular events. These findings could clinically be exploited to increase the dismal response rates of MCL patients to the current chemotherapy regimens.

Efficacy of fludarabine, intermittent sequential high-dose cytosine arabinoside, and mitoxantrone (FIS-HAM) salvage therapy in highly resistant acute leukemias

Patients with refractory acute leukemias after intensive induction and salvage attempts have a particularly poor prognosis and therapeutic options are limited. In the current study, the pharmacologically based FIS-HAM regimen was applied, which included fludarabine 15 mg/m2 q 12 h (days 1, 2, 8, and 9), cytosine arabinoside as a 45-min infusion every 3 h at 750 mg/m2 per single application (days 1, 2, 8, and 9), and mitoxantrone 10 mg/m2 (days 3, 4, 10, 11). Twenty-six intensively pretreated patients [median age: 38 years; range: 22-65; 16 cases of acute myeloid leukemia (AML) and 10 of acute lymphoblastic leukemia (ALL)] were included. Of 16 patients with AML, 5 achieved a complete remission (CR, 31%), 1 a partial remission (PR, 6%), 2 were nonresponders (13%), and 8 succumbed to early death (ED, 50%). Of 10 patients with ALL, 5 achieved a CR, 1 a PR, 1 was a nonresponder, and 3 died early. Overall, the CR rate was 38%. The median disease-free survival time was 50 days and median survival 90 days. Two patients underwent allogeneic bone marrow transplantation and are alive after 27 and 28 months. Neutropenia amounted to a median of 46 days. Toxicity WHO III/IV included infection (61%), diarrhea (48%), nausea/vomiting (43%), impairment of heart function (30%), and mucositis (26%). The current data indicate a significant activity of FIS-HAM chemotherapy in advanced acute leukemias. However, due to its pronounced toxicity, this regimen should be restricted to third-line therapy for patients expecting a suitable donor for allogeneic transplantation, and supportive treatment should be optimized.