MLL3 regulates the CDKN2A tumor suppressor locus in liver cancer

Mutations in genes encoding components of chromatin modifying and remodeling complexes are among the most frequently observed somatic events in human cancers. For example, missense and nonsense mutations targeting the mixed lineage leukemia family member 3 (MLL3, encoded by KMT2C) histone methyltransferase occur in a range of solid tumors, and heterozygous deletions encompassing KMT2C occur in a subset of aggressive leukemias. Although MLL3 loss can promote tumorigenesis in mice, the molecular targets and biological processes by which MLL3 suppresses tumorigenesis remain poorly characterized. Here we combined genetic, epigenomic, and animal modeling approaches to demonstrate that one of the mechanisms by which MLL3 links chromatin remodeling to tumor suppression is by co-activating the Cdkn2a tumor suppressor locus. Disruption of Kmt2c cooperates with Myc overexpression in the development of murine hepatocellular carcinoma (HCC), in which MLL3 binding to the Cdkn2a locus is blunted, resulting in reduced H3K4 methylation and low expression levels of the locus-encoded tumor suppressors p16/Ink4a and p19/Arf. Conversely, elevated KMT2C expression increases its binding to the CDKN2A locus and co-activates gene transcription. Endogenous Kmt2c restoration reverses these chromatin and transcriptional effects and triggers Ink4a/Arf-dependent apoptosis. Underscoring the human relevance of this epistasis, we found that genomic alterations in KMT2C and CDKN2A were associated with similar transcriptional profiles in human HCC samples. These results collectively point to a new mechanism for disrupting CDKN2A activity during cancer development and, in doing so, link MLL3 to an established tumor suppressor network.

sAPP Enhances the Transdifferentiation of Adult Bone Marrow Progenitor Cells to Neuronal Phenotypes

The remediation of neurodegeneration and cognitive decline in Alzheimer's Disease (AD) remains a challenge to basic scientists and clinicians. It has been suggested that adult bone marrow stem cells can transdifferentiate into different neuronal phenotypes. Here we demonstrate that the alpha-secretase-cleaved fragment of the amyloid precursor protein (sAPPalpha), a potent neurotrophic factor, potentiates the nerve growth factor (NGF)/retinoic acid (RA) induced transdifferentiation of bone marrow-derived adult progenitor cells (MAPCs) into neural progenitor cells and, more specifically, enhances their terminal differentiation into a cholinergic-like neuronal phenotype. The addition of sAPPalpha to NGF/RA-stimulated MAPCs resulted in their conversion to neuronal-like cells as evidenced by the extension of neurites and the appearance of immature synaptic complexes. MAPCs differentiated in the presence of sAPPalpha and NGF/RA exhibited a 40% to as much as 75% increase in neuronal proteins including NeuN, beta-tubulin III, NFM, and synaptophysin, compared to MAPCs differentiated by NGF/RA alone. This process was accompanied by an increase in the levels of choline acetyltransferase, a marker of cholinergic neurons, compared to those of GABAergic and dopaminergic neuronal subtypes. MAPCs immunopositive for sAPPalpha were identified within the septohippocampal system of transgenic PS/APP mice injected intravenously with sAPPalpha-transfected MAPCs and found in close proximity to the cerebral vasculature. Given that in AD cholinergic neurons are severely vulnerable to neurodegeneration and that the levels of sAPPalpha are significantly reduced, these findings suggest the combined use of sAPPalpha and MAPCs offers a new and potentially powerful therapeutic strategy for AD treatment.