Abstract 3698: The effect of epigenetic silencing and p53 mutation on DLL4 expression in human cancer stem cell disorder: The Li-Fraumeni syndrome

Abstract

Abstract Tumorigenesis results from an accumulation of mutational and epigenetic changes that alter normal cell growth and survival pathways. Li-Fraumeni syndrome (LFS) is a clinically and genetically heterogeneous inherited cancer syndrome that can provide powerful insights into our understanding of the somatic mutations present in sporadic cancers. LFS is currently linked to heterozygous mutation in the tumor suppressor gene p53 (TP53), which encodes a transcription factor that responds to diverse cellular stresses through the regulation of target genes that induce apoptosis, cell cycle arrest, DNA repair, and senescence. We previously showed a balanced reciprocal chromosomal translocation t(11;15)(q23;q15) in the non-cancerous skin fibroblasts of a bilateral breast cancer patient in LFS family. This prompted us to investigate the breakpoint region of the translocation, which uncovered a gene that encodes DLL4, (Delta-like ligand 4; locus at 15q15.1), a Notch ligand that is a key target in tumor vasculature. DLL4 regulates T-cell lineage thymopoiesis and further affects cancer immunosurveillance. In the breakpoint region of chromosome 15q15, we examined THBS1, DNAJC17, Rad51, DLL4, CHAC1 and INO80 gene expression levels. We found that specifically DLL4 is abrogated in all the LFS cell lines as well as MDA231 breast cancer cell line, and drastically down-regulated in MCF7 breast and IMR32 brain cancer cell lines and several human organ-specific tumor samples. Furthermore, DNA methylation studies revealed that DLL4 promoter is silenced only in MCF7 and MDA231 but not in LFS cell lines. ChIP and siRNA knockdown assays demonstrated that p53 binds to DLL4 promoter through its association with CTCF, a chromosomal networking protein CCCTC binding factor. These results imply a possible karyotype-phenotype correlation with respect to DLL4 in LFS and breast cancer initiation and progression. The drastic reduction or absence in the expression of DLL4 in normal skin fibroblasts of LFS patients as well as breast and brain cancer cells is significant and supports the concept that this ligand may also play a role in cancer immune-surveillance; and its resuscitation in the tumor microenvironment may stimulate T-cell immunity and suppress tumor growth. Citation Format: Zhixing Yao, Zaki A. Sherif. The effect of epigenetic silencing and p53 mutation on DLL4 expression in human cancer stem cell disorder: The Li-Fraumeni syndrome. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3698.