Peter D. Aplan, M.D.
Dr. Aplan uses the tools of molecular biology and molecular genetics to understand the causes of hematopoietic malignancy, including myelodysplastic syndrome (MDS), acute myelogenous leukemia (AML), T cell acute lymphoblastic leukemia (ALL), and B cell precursor ALL. The general approach taken is to identify mutations, such as chromosomal translocations and single nucleotide variants, in leukemic cells, and study these mutations using a combination of in vitro and in vivo models. Recent accomplishments include generation of the first genetically engineered model for MDS (the NUP98-HOXD13 mouse), a genetically engineered model for progenitor B1 cell ALL, and the discovery of templated sequence insertion polymorphisms (TSIPs) in humans.
A large number of hematologic malignancies are associated with non-random, recurrent chromosomal translocations. These translocations typically either activate latent proto-oncogenes or create novel fusion genes. Our approach to studying leukemia in general is as follows:
- Identify and clone the genes located at translocation breakpoints.
- Test the oncogenic potential of these genes in animal models.
- If the animals develop leukemia, or other hematologic malignancy, study the process of malignant transformation over time (i.e., can occult, pre-malignant lesions be identified in clinically healthy animals).
- Identify collaborating events (i.e., genes and pathways) that, when abrogated, can collaborate with the gene of interest.
- Use the animal model for pre-clinical testing of cancer treatment modalities.
1. We developed a transgenic mouse model of T-cell leukemia by overexpressing SCL and LMO1 in the thymus. We identified mutations of the Notch1 gene in 70% of these mice, demonstrating that Notch1 activation was a progression event that collaborated with SCL/LMO1. The Notch1 mutations are important in maintaining the malignant phenotype of these cells, as T-cell lines established from SCL/LMO1 mice with Notch1 mutations are sensitive to gamma-secretase inhibitors. We used cre-lox technology to generate mice in which SCL can be activated conditionally. These mice show impaired thymocyte differentiation, with a dramatic increase in immature CD4-/CD8- thymocytes.
2. We have cloned several chromosomal translocations that generate NUP98 fusion proteins. We generated transgenic mice that express NUP98HOXD13 (hereafter NHD13) or NUP98TOP1 in the hematopoietic compartment. The NHD13 mice develop a highly penetrant myelodysplastic syndrome (MDS) that resembles the human disease in terms of peripheral blood cytopenias, dysplasia, apoptosis, and progression to frank leukemia. We used retroviral tagging experiments to identify genes that collaborate with NHD13 in the process of leukemic transformation, and identified a number of expected (Meis1) collaborating genes, as well as several novel and unexpected collaborators (MN1, Erg, Mir29a). We have shown for the first time that MDS is transplantable, by transplanting bone marrow from NHD13 mice with MDS. We generated "knock-in" ES cells that express NHD13 under control of endogenous NUP98 regulatory elements, and have derived immortal, IL-3 dependent hematopoietic precursors from these ES cells.
3. Evaluation of CALM-AF10 as an oncogenic fusion. The CALM-AF10 fusion is caused by a t(10;11) chromosomal translocation seen in a wide spectrum of acute leukemia, but most commonly in T-ALL. We have generated transgenic mice that express the CALM-AF10 fusion in the hematopoietic compartment, 50-70% of these mice develop acute leukemia, primarily AML, but often with lymphoid features.. These mice also show increased expression of HOXA cluster genes and impaired thymocyte differentiation, suggesting that the cause of leukemia is, at least in part, impaired blood cell differentiation caused by overexpression of HOXA cluster genes.
Gross chromosomal rearrangements (GCR), including chromosomal translocations, deletions, inversions, and amplifications are present in the majority of cancer cells. These GCRs can lead to production of oncogenic fusion genes, amplification of proto-oncogenes, and deletion of tumor suppressor genes. However, the molecular mechanisms which lead to these GCR are largely unknown. Therefore, our specific goals are:
- To establish one or more model systems in which we can reproducibly generate GCR.
- To determine if certain genomic regions or sequence motifs are likely to be involved in these GCRs.
- To investigate what gene(s) might be important in preventing these GCRs.
An unsolved, fundamental question concerning chromosomal translocations can be phrased as follows: "Do recurrent, non-random translocations occur between 'recombinogenic' regions of the genome that are extraordinarily susceptible to breakage/religation events, or are the regions involved not particularly recombinogenic, but simply regions that lead to the production of oncogenic fusion proteins which confer a growth advantage to the cell?" To help address this question, we developed an in vitro system in which we can produce chromosomal aberrations, through the use of the Isce-I restriction endonuclease, that do not provide a growth advantage to the cell. Using this system, we have learned that gross chromosomal rearrangements are a rare result of improper DNA repair of a double strand break, with a frequency roughly 1% that of small interstitial deletions.
We are using this system to determine if gross chromosomal rearrangements will be more common if additional breaks are introduced, via genotoxic chemotherapy such as etoposide or bleomycin and/or VDJ recombination. Our preliminary results suggest that the use of etoposide and I-SceI does not lead to increased numbers of GCRs. However, the use of RAG1/RAG2 on a substrate that contains a VDJ recombination signal sequence has led to larger deletions, on the scale of 100s of kb. In addition, we are using this system to test whether purine/pyrimidine repeat regions are recombinogenic.
Selected Key Publications
- Proc. Natl. Acad. Sci. U.S.A. 105: 14088-93, 2008. [ Journal Article ]
- Blood. 118: 6247-57, 2011. [ Journal Article ]
Enforced expression of Lin28b leads to impaired T-cell development, release of inflammatory cytokines, and peripheral T-cell lymphoma..Blood. 120: 1048-59, 2012. [ Journal Article ]
NUP98-PHF23 is a chromatin-modifying oncoprotein that causes a wide array of leukemias sensitive to inhibition of PHD histone reader function.Cancer Discov. 4: 564-77, 2014. [ Journal Article ]
Repair of DNA double-strand breaks by templated nucleotide sequence insertions derived from distant regions of the genome.Proc. Natl. Acad. Sci. U.S.A. 111: 7729-34, 2014. [ Journal Article ]
Dr. Aplan received a B.S. in biophysics (1979) and an M.D. (1983) from Pennsylvania State University in 1983. From 1983 to 1987, he trained in pediatrics and was chief resident at the Children's Hospital of Buffalo. He received fellowship training in pediatric hematology/oncology at the NCI from 1987 to 1992 and served as assistant, then associate, professor of pediatrics and microbiology and immunology at Roswell Park Cancer Institute from 1992 to 1999. He moved to the NCI in 1999 where he is an investigator in the Genetics Branch.
|Yang-Jo Chung Ph.D.||Staff Scientist|
|Joseph Dufraine Ph.D.||Postdoctoral Fellow (CRTA)|
|Ghanwa Khawaja||Research Biologist (Contr)|
|Subhadip Kundu Ph.D.||Postdoctoral Fellow (CRTA)|
|Vijay Negi Ph.D.||Postdoctoral Fellow (Visiting)|
|Masahiro Onozawa M.D., Ph.D.||Guest Researcher|
|Mianmian Yin Ph.D.||Postdoctoral Fellow (CRTA)|