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Functional analysis of leukemic CREBBP mutations

Charles G Mullighan

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National Institutes of Health (NIH)
The constellation of mutations and epigenetic alterations (e.g. histone modifications) that promote cancer development and its resistance to treatment are incompletely understood. Significantly, recent genomic sequencing studies to define the mutational spectrum of relapsed acute lymphoblastic leukemia (ALL), B cell non-Hodgkin lymphoma, and transitional cell carcinoma of the bladder have identified lesions in the two genes encoding the KAT3 family of histone acetyltransferases, CREB-binding protein (CBP, CREBBP) and EP300 (p300). ALL is the most common childhood cancer and the focus of this proposal. Relapsed ALL is the leading cause of non-traumatic death in young people, and resistance to glucocorticoids is a hallmark of treatment resistant ALL cells, the basis of which ispoorly understood. CREBBP alterations found in ALL include N-terminal frame-shift and termination mutations that disturb or delete many domains, but also lesions that specifically affect the histone acetyltransferase (HAT) domain and the Nuclear Coactivator Binding Domain (NCBD) that is involved in glucocorticoid receptor function. The long-term goal is to understand how CREBBP mutations contribute to leukemia progression and its resistance to treatment. The objective for this proposal is to determine how CREBBP leukemic mutations affect glucocorticoid-responsive transcription and the response of leukemia cells to glucocorticoid and histone deacetylase inhibitor (HDACi) therapies. Based upon the applicants' published and preliminary data, the central hypothesis states that leukemic cells with null and hypomorphic mutations in CREBBP have altered gene expression that decreases apoptosis in response to glucocorticoids, but such cells have increased susceptibility to drug therapies that increase protein acetylation. The rationale for the proposed research is that ALL-associated mutations found in certain functional domains of the transcriptional "hub" protein CREBBP provide clues to the altered biological pathways that promote leukemia progression and resistance to standard therapies. Once identified, these pathway alterations in CREBBP mutant ALL cells can potentially be exploited by the use of alternative treatments. Three specific aims test the centralhypothesis. Aim 1 is to define how mutant Crebbp affects glucocorticoid-responsive gene expression. Aim 2 is to determine how Crebbp mutations enhance the progression of leukemia in mice. Aim 3 is to establish how CREBBP mutations affect the response of human ALL xenografts to histone deacetylase inhibitors and glucocorticoids. The expected outcome of these studies will provide new mechanistic insight into ALL by determining how ALL-associated CREBBP mutations impact: 1) the transcriptional response to glucocorticoids, 2) the development and progression of leukemia, and 3) the anti-leukemia effects of glucocorticoids and histone deacetylase inhibitors. The translational impact of the results will be to enhance the knowledge-driven therapeutic use of glucocorticoids and HDACi to treat leukemia based on tumor cell genotype.

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