The aggressive and unpredictable behavior of relapsed T-cell acute lymphoblastic leukemia presents a major clinical challenge, as relapsed disease continues to have a dismal prognosis in both children and adults, despite recent advances in the treatment. A major hurdle in the development of new chemotherapies is identifying targets that are specific for malignant T-cells, while leaving normal cells untouched. Detailed analysis of mutations gained in T-ALL cells as they developed increased relapse potential revealed that PRL3, a protein tyrosine phosphatase, is highly expressed in a majority of T-ALL, but is not expressed in normal hematopoietic cells. Additionally, PRL3 over-expression significantly enhanced T-ALL development in vivo, and PRL3 inhibition rapidly and specifically kills T-ALL cells in vitro.
The goal of this proposal will be to extend these studies by testing several lead compounds on a large panel of T-ALL cell lines and to examine the efficacy of PRL3 inhibition in targeting T-ALL cells in an in vivo xenograft model. Additionally, the function of PRL3 in T-ALL malignancy will be defined. The long-term goal of this project is to provide a novel therapeutic approach to relapsed T-ALL.
Jessica Blackburn answered questions about her research (September 2014):
What were you initially studying with your grant funded by ALSF?
I am interested in finding new therapeutics to treat relapsed leukemia. I initially used a zebrafish model of T-cell acute lymphoblastic leukemia (T-ALL) and serial transplant into over 6,000 recipient zebrafish to identify genetic changes that occurred in single T-ALL cells as they acquired increased ability to form relapse. I found that ~60% of zebrafish T-ALL had acquired amplification or high expression of Protein Tyrosine Phosphatase 4A3 (PTP4A3, also known as PRL3) as they developed increased relapse potential. PRL3 is also highly expressed in human T-ALL cell lines and patient samples. Additionally, zebrafish engineered to express high levels of PRL3 in their T cells develop T-ALL significantly faster than control animals, indicating that PRL3 enhances T-ALL progression.
What have you found?
I have found that knock-down of PRL3 in human T-ALL cell lines induced apoptosis in T-ALL cells in vitro, and slowed in vivo growth in a mouse xenograft model, suggesting that PRL3 may play an important role in T-ALL survival, and represents a potential therapeutic target for this disease. Additionally, a commercially available PRL3 inhibitor selectively killed T-ALL cell lines that expressed PRL3 in vitro.
What are your next steps?
While the PRL3 inhibitor has a good effect in killing T-ALL cells, it is a research compound and not currently suitable for clinical use. However, there are 12 FDA approved general phosphatase inhibitors that may block PRL3 phosphatase activity and are currently used clinically, although not as anti-cancer drugs. The next steps of this project are to test these inhibitors for effectiveness against PRL3 in vitro using human T-ALL cell lines, and to move the most promising hits to pre-clinical trials using xenografts of T-ALL cell lines and primary patient samples in mice. Because these compounds are already FDA approved, drugs showing promise against T-ALL can be fast tracked to clinical trials in leukemia patients. A second direction of this project is to identify the pathways affected by PRL3 phosphatase activity that play a role in cell survival, in the hopes of identifying additional druggable targets for the treatment of T-ALL.
What does this mean for children with cancer and their families?
Targeted therapeutics have had good success against a variety of cancers and will be the way forward in treating patients. Phosphatases are generally considered tumor suppressors, and have been largely ignored as therapeutic targets. My work showing the direct contribution of the phosphatase PRL3 to T-ALL progression and survival may open up a new line of research into the oncogenic role of phosphatases in leukemia, and could provide impetus for the development of phosphatase inhibitors for use in cancer patients.
Has this research been published?
The results from the initial screen in zebrafish has been published in Cancer Cell, 2014; 25(3), pages 366-78.
What has this grant from ALSF allowed you to do that you wouldn’t have been able to do otherwise?
My postdoctoral advisor’s laboratory exclusively used zebrafish as an in vivo T-ALL model. Funding from ALSF has allowed me to extend my findings in zebrafish T-ALL to using human cells lines in mouse xenograft, and my future plans include pre-clinical trials using xenograft of primary patient samples. These studies would have been cost prohibitive without the ALSF grant. Additionally, I have used the data generated during this first year of ALSF funding as preliminary data for a successful NIH K99/R00 application. This award will provide critical funding as I start my own laboratory focused on pediatric leukemia research.
Why did you choose to work in this field/on this topic?
Relapsed leukemia is a major clinical challenge in large part due to ineffective and highly toxic chemotherapies. Targeted therapies and personalized medicine, where drugs are given based on a patient’s cancer’s unique spectrum of mutations and gene expression, are the way forward in cancer medicine, will provide less toxic and more effective treatment options for children with cancer. The major focus of my work is to understand the molecular underpinnings of leukemia progression and relapse to aid in the development of these types of targeted therapies.