Using Pharmacology to Squeeze the Life Out of Childhood Leukemia, and Potential Strategies to Achieve Breakthroughs in Medulloblastoma Treatment.

Eliminating cancer was once thought of as a war. This analogy is still apt today; however, we now realize that cancer is a much more formidable enemy than scientists originally perceived, and in some cases, it harbors a profound ability to thwart our best efforts to defeat it. However, before we were aware of the complexity of cancer, chemotherapy against childhood acute lymphoblastic leukemia (ALL) was successful because it applied the principles of pharmacology. Herein, we provide a historic perspective of the experience at St. Jude Children's Research Hospital. In 1962, when the hospital opened, fewer than 3% of patients experienced durable cure. Through judicious application of pharmacologic principles (e.g., combination therapy with agents using different mechanisms of action) plus appropriate drug scheduling, dosing, and pharmacodynamics, the survival of patients with ALL now exceeds 90%. We contrast this approach to treating ALL with the contemporary approach to treating medulloblastoma, in which genetics and molecular signatures are being used to guide the development of more-efficacious treatment strategies with minimal toxicity. Finally, we highlight the emerging technologies that can sustain and propel the collaborative efforts to squeeze the life out of these cancers. SIGNIFICANCE STATEMENT: Up until the early 1960s, chemotherapy for childhood acute lymphoblastic leukemia was mostly ineffective. This changed with the knowledge and implementation of rational approaches to combination therapy. Although the therapeutics of brain cancers such as medulloblastoma are not as refined (in part because of the blood-brain barrier obstacle), recent extraordinary advances in knowledge of medulloblastoma pathobiology has led to innovations in disease classification accompanied with strategies to improve therapeutic outcomes. Undoubtedly, additional novel approaches, such as immunological therapeutics, will open new avenues to further the goal of taming cancer.

Obstacles to Brain Tumor Therapy: Key ABC Transporters.

The delivery of cancer chemotherapy to treat brain tumors remains a challenge, in part, because of the inherent biological barrier, the blood-brain barrier. While its presence and role as a protector of the normal brain parenchyma has been acknowledged for decades, it is only recently that the important transporter components, expressed in the tightly knit capillary endothelial cells, have been deciphered. These transporters are ATP-binding cassette (ABC) transporters and, so far, the major clinically important ones that functionally contribute to the blood-brain barrier are ABCG2 and ABCB1. A further limitation to cancer therapy of brain tumors or brain metastases is the blood-tumor barrier, where tumors erect a barrier of transporters that further impede drug entry. The expression and regulation of these two transporters at these barriers, as well as tumor derived alteration in expression and/or mutation, are likely obstacles to effective therapy.

Structural basis for the mechanisms of human presequence protease conformational switch and substrate recognition.

Presequence protease (PreP), a 117 kDa mitochondrial M16C metalloprotease vital for mitochondrial proteostasis, degrades presequence peptides cleaved off from nuclear-encoded proteins and other aggregation-prone peptides, such as amyloid ß (Aß). PreP structures have only been determined in a closed conformation; thus, the mechanisms of substrate binding and selectivity remain elusive. Here, we leverage advanced vitrification techniques to overcome the preferential denaturation of one of two ~55 kDa homologous domains of PreP caused by air-water interface adsorption. Thereby, we elucidate cryoEM structures of three apo-PreP open states along with Aß- and citrate synthase presequence-bound PreP at 3.3-4.6 Å resolution. Together with integrative biophysical and pharmacological approaches, these structures reveal the key stages of the PreP catalytic cycle and how the binding of substrates or PreP inhibitor drives a rigid body motion of the protein for substrate binding and catalysis. Together, our studies provide key mechanistic insights into M16C metalloproteases for future therapeutic innovations.

An ABC Transporter Drives Medulloblastoma Pathogenesis by Regulating Sonic Hedgehog Signaling.

Mutations in Sonic hedgehog (SHH) signaling promote aberrant proliferation and tumor growth. SHH-medulloblastoma (MB) is among the most frequent brain tumors in children less than 3 years of age. Although key components of the SHH pathway are well-known, we hypothesized that new disease-modifying targets of SHH-MB might be identified from large-scale bioinformatics and systems biology analyses. Using a data-driven systems biology approach, we built a MB-specific interactome. The ATP-binding cassette transporter ABCC4 was identified as a modulator of SHH-MB. Accordingly, increased ABCC4 expression correlated with poor overall survival in patients with SHH-MB. Knockdown of ABCC4 expression markedly blunted the constitutive activation of the SHH pathway secondary to Ptch1 or Sufu insufficiency. In human tumor cell lines, ABCC4 knockdown and inhibition reduced full-length GLI3 levels. In a clinically relevant murine SHH-MB model, targeted ablation of Abcc4 in primary tumors significantly reduced tumor burden and extended the lifespan of tumor-bearing mice. These studies reveal ABCC4 as a potent SHH pathway regulator and a new candidate to target with the potential to improve SHH-MB therapy. SIGNIFICANCE: These findings identify ABCC4 transporter as a new target in SHH-MB, prompting the development of inhibitors or the repurporsing of existing drugs to target ABCC4.

Osm1 facilitates the transfer of electrons from Erv1 to fumarate in the redox-regulated import pathway in the mitochondrial intermembrane space.

Prokaryotes have aerobic and anaerobic electron acceptors for oxidative folding of periplasmic proteins. The mitochondrial intermembrane space has an analogous pathway with the oxidoreductase Mia40 and sulfhydryl oxidase Erv1, termed the mitochondrial intermembrane space assembly (MIA) pathway. The aerobic electron acceptors include oxygen and cytochrome c, but an acceptor that can function under anaerobic conditions has not been identified. Here we show that the fumarate reductase Osm1, which facilitates electron transfer from fumarate to succinate, fills this gap as a new electron acceptor. In addition to microsomes, Osm1 localizes to the mitochondrial intermembrane space and assembles with Erv1 in a complex. In reconstitution studies with reduced Tim13, Mia40, and Erv1, the addition of Osm1 and fumarate completes the disulfide exchange pathway that results in Tim13 oxidation. From in vitro import assays, mitochondria lacking Osm1 display decreased import of MIA substrates, Cmc1 and Tim10. Comparative reconstitution assays support that the Osm1/fumarate couple accepts electrons with similar efficiency to cytochrome c and that the cell has strategies to coordinate expression of the terminal electron acceptors. Thus Osm1/fumarate is a new electron acceptor couple in the mitochondrial intermembrane space that seems to function in both aerobic and anaerobic conditions.