We characterized the enhancer landscape of 66 AML patients, identifying 6 novel subgroups and their associated regulatory loci. These subgroups are defined by their super-enhancer (SE) maps, orthogonal to somatic mutations, and are associated with distinct leukemic cell states. Examination of transcriptional drivers for these epigenomic subtypes uncovers a subset of patients with a particularly strong super-enhancer at the retinoic acid receptor alpha (RARA) gene locus. Presence of a RARA SE and concomitant high levels of RARA mRNA predisposes cell lines and ex vivo models to exquisite sensitivity to a selective agonist of RARα, SY-1425 (tamibarotene). Furthermore, only AML patient-derived xenograft (PDX) models with high RARA mRNA were found to respond to SY-1425. Mechanistically, we show that the response to SY-1425 in RARA-high AML cells is similar to that of APL treated with retinoids, characterized by the induction of known retinoic acid response genes, increased differentiation, and loss of proliferation.

The accumulation of misfolded proteins stresses the endoplasmic reticulum (ER) and triggers cell death through activation of the multidomain proapoptotic BCL-2 proteins BAX and BAK at the outer mitochondrial membrane. The signaling events that connect ER stress with the mitochondrial apoptotic machinery remain unclear, despite evidence that deregulation of this pathway contributes to cell loss in many human degenerative diseases. In order to "trap" and identify the apoptotic signals upstream of mitochondrial permeabilization, we challenged Bax ؊/؊ Bak ؊/؊ mouse embryonic fibroblasts with pharmacological inducers of ER stress. We found that ER stress induces proteolytic activation of the BH3-only protein BID as a critical apoptotic switch. Moreover, we identified caspase-2 as the premitochondrial protease that cleaves BID in response to ER stress and showed that resistance to ER stress-induced apoptosis can be conferred by inhibiting caspase-2 activity. Our work defines a novel signaling pathway that couples the ER and mitochondria and establishes a principal apoptotic effector downstream of ER stress.

Myeloid cells are key regulators of the tumor microenvironment, governing local immune responses. Here we report that tumorinfiltrating myeloid cells and circulating monocytes in patients with glioblastoma multiforme (GBM) express ligands for activating the Natural killer group 2, member D (NKG2D) receptor, which cause down-regulation of NKG2D on natural killer (NK) cells. Tumorinfiltrating NK cells isolated from GBM patients fail to lyse NKG2D ligand-expressing tumor cells. We demonstrate that lactate dehydrogenase (LDH) isoform 5 secreted by glioblastoma cells induces NKG2D ligands on monocytes isolated from healthy individuals. Furthermore, sera from GBM patients contain elevated amounts of LDH, which correlate with expression of NKG2D ligands on their autologous circulating monocytes. NKG2D ligands also are present on circulating monocytes isolated from patients with breast, prostate, and hepatitis C virus-induced hepatocellular carcinomas. Together, these findings reveal a previously unidentified immune evasion strategy whereby tumors produce soluble factors that induce NKG2D ligands on myeloid cells, subverting antitumor immune responses.LDH5 | glioma | cancer | tumor evasion

Apoptosis of motor neurons is a well-documented feature in amyotrophic lateral sclerosis (ALS) and related motor neuron diseases (MNDs). However, the role of apoptosis in the pathogenesis of these diseases remains unresolved. One possibility is that the affected motor neurons only succumb to apoptosis once they have exhausted functional capacity. If true, blocking apoptosis should confer no therapeutic benefit. To directly investigate this idea, we tested whether tissue-specific deletion in the mouse CNS of BCL2-associated X protein (BAX) and BCL2-homologous antagonist/killer (BAK), 2 proapoptotic BCL-2 family proteins that together represent an essential gateway to the mitochondrial apoptotic pathway, would protect against motor neuron degeneration. We found that neuronal deletion of Bax and Bak in a mouse model of familial ALS not only halted neuronal loss, but prevented axonal degeneration, symptom onset, weight loss, and paralysis and extended survival. These results show that motor neurons damaged in ALS activate the mitochondrial apoptotic pathway early in the disease process and that apoptotic signaling directly contributes to neuromuscular degeneration and neuronal dysfunction. Hence, inhibiting apoptosis upstream of mitochondrial permeabilization represents a possible therapeutic strategy for preserving functional motor neurons in ALS and other MNDs.

Excessive demands on the protein folding capacity of the endoplasmic reticulum (ER) cause irremediable ER stress and contribute to cell loss in a number of cell degenerative diseases, including type 2 diabetes and neurodegeneration1,2. The signals communicating catastrophic ER damage to the mitochondrial apoptotic machinery remain poorly understood3-6. We used a biochemical approach to purify a cytosolic activity induced by ER stress that causes release of cytochrome c from isolated mitochondria. We discovered that the principal component of the purified pro-apoptotic activity is proto-oncogene CT10-regulated kinase (CRK), an adaptor protein with no known catalytic activity7. Crk-/- cells are strongly resistant to ER stress-induced apoptosis. Moreover, CRK is cleaved in response to ER stress to generate an N-terminal ~14kD fragment with greatly enhanced cytotoxic potential. We identified a putative BCL2 homology-3 (BH3) domain within this N-terminal CRK fragment, which sensitizes isolated mitochondria to cytochrome c release and when mutated significantly reduces CRK's apoptotic activity in vivo. Together these results identify CRK as a pro-apoptotic protein that signals irremediable ER stress to the mitochondrial execution machinery.