p53 Pathway Alterations Influence Survival and Treatment Response In Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is a heterogeneous malignancy, with survival rates that range from months to decades. Numerous genetic markers have been identifying that predict for poor survival outcomes. One of the most significant prognostic risk factors is loss of integrity of the p53 pathway. It is well known that loss of the short arm of chromosome 17 (17p-), containing the p53 locus, is a major predictor poor outcome in CLL, however, there is a lack of clarity how specific p53 mutations influence cancer progression. It is our contention that understanding the impact of p53 mutations in CLL progression will facilitate the development of better treatments options. We are using a mouse model of B-CLL (Eµ-TCL1) that also carry a p53 hot-spot mutation p53R172H, corresponding to R175H in humans, to study the impact of p53 mutation on treatment response and survival. Cohorts of Eµ-TCL1 and Eµ-TCL1;p53R172H/+ were treated with fludarabine at 35mg/kg per day for 5 days each month. We have observed significant changes in survival and selective pressure on the p53 locus following fludarabine treatment, leading to a refractory population of clones. Mice in the Eu-TCL1 cohort received a median of 1 treatment cycle (Range; 1-3), while the mice in the Eµ-TCL1;p53R172H/+ cohort received a median of 3 treatment cycles (Range;1-4), suggesting that the mutant cohort are likely to benefit initially with treatment only to succumb to a refractory clone of CLL. Mice in the Eµ-TCL1 cohort had an overall survival of 373 days (range; 341-464 days) in comparison to 322 days (range; 270-382days) in the Eµ-TCL1;p53R172H/+ cohort, indicating a trend toward inferior survival in the Eµ-TCL1;p53R172H/+ cohort. Interestingly, mice in the Eµ-TCL1 cohort had significant splenomegaly (40%) as compared to the Eµ-TCL1;p53R172H/+ cohort (0%). However, we observed that all Eµ-TCL1;p53R172H/+ had extensive extranodal disease with mice having abdominal and/or cervical mass, which is suggestive of Richter’s transformation. Together, these data indicate that fludarabine based therapies may have a profound negative transformational impact on CLL progression when used as a treatment strategy in the context of p53 mutations. These results would suggest that less cytotoxic therapies may benefit patients harboring p53 mutations. To this end, we are evaluating the effect of Ibrutinib therapy in the context of p53 mutations. We are currently treating cohorts of Eµ-TCL1 and Eµ-TCL1;p53R172H/+ mice with Ibrutinib 25mg/kg/day. Our preliminary results suggest Ibrutinib’s innovative mechanism may lessen some of these selective pressures on the p53 pathway and may indicate Ibrutinib is a superior treatment option compared to fludarabine when p53 mutations are present. In conjunction with these mouse studies, we are also determining the impact of p53 mutation in human patients by examining changes in CLL patient samples and cell lines following fludaribine and Ibrutinib treatment. This dual mouse/human strategy represents an ideal system in which to study genetic anomalies in the p53 pathway and examine the dynamic molecular pressures exerted by both fludarabine and Ibrutinib in CLL.