Melanoma stem cells

Tumor-initiating cells have been identified in a variety of cancers. Frank and colleagues identify a subpopulation enriched for malignant melanoma–initiating cells (MMICs) that express the chemoresistance mediator ATP-binding cassette subfamily B member 5 (ABCB5). Expression of ABCB5 in primary and metastatic melanomas is higher than in benign melanocytic lesions and correlates with the clinical progression of malignant melanoma. Using primary patient-derived tumor cells in human-to-NOD/SCID (non-obese diabetic/severe combined immunodeficient) mouse xenotransplantation experiments, the authors show that injection of 106 ABCB5-positive, unsegregated, and ABCB5-negative cells results in 100%, 64% and 9% primary tumor formation, respectively, demonstrating that the MMIC frequency is greater in the ABCB5-positive melanoma population These ABCB5-positive cells not only self-renew but also reestablish tumor heterogeneity. Systemic administration of an anti-ABCB5 monoclonal antibody induces antibody-dependent cellular cytotoxicity in ABCB5-positive cells and inhibits tumor growth, thereby supporting the hypothesis that MMICs are required for tumorigenicity. Because ABCB5 expression most likely contributes to chemoresistance of melanoma, it will be interesting to determine whether MMIC-targeted therapeutics can break this resistance. (Nature 451, 345–349, 2008) JWT

shRNA cancer genome screens

Two papers demonstrate the mettle of genome-wide RNA interference (RNAi) screens for discovering mutations that affect cancer proliferation. In a recent issue of Cell, Green and colleagues focus on genes influencing the proto-oncogene BRAF, a serine-threonine kinase that functions downstream of RAS pathways. BRAF mutations, in particular V600E, are often present in human cancers and in melanoma occur at a frequency of 50–70%. Because expression of BRAF-V600E induces senescence in primary human melanocytes, at least one additional mutation in a gene is likely to be required to inactivate BRAF-V600E–mediated senescence for cancer development. Using an RNAi screen, Green's group identifies 17 potential tumor-suppressor genes required for BRAF-V600E to trigger senescence in human primary foreskin fibroblasts and melanocytes. One, the secreted protein insulin growth factor binding protein 7 (IGFBP7), inhibits signaling downstream of BRAF and triggers both senescence and apoptosis in primary cells. What's more, recombinant IGFBP7 induces apoptosis in BRAF-V600E–positive human melanoma cell lines and completely inhibits growth of BRAF-V600E–positive, but not BRAF-V600E–negative, xenografts in mice. Loss of IGFBP7 expression in BRAF-V600E–positive melanomas and cell lines is triggered by promoter hypermethylation. Future experiments are required to determine whether IGFBP7 can be developed into a melanoma therapeutic. In Science, Elledge, Hannon and colleagues describe a high-throughput short hairpin RNA (shRNA) screening system in which shRNAs that affect cancer proliferation are identified by using half-hairpin barcodes for microarray decoding. (Cell 132, 363–374, 2008; Science 319, 620–624, 2008) JWT

New HIV receptor

Depletion of the CD4+ T-cells in the gut by HIV is a hallmark of AIDS pathogenesis, underlying the irreversible damage to the immune system caused by HIV infection. How this occurs has not been known. Now researchers at the NIAID, led by Anthony Fauci, have solved the mystery. Looking first at natural killer (NK) cells, which are rendered powerless by HIV envelope protein, the researchers showed that the HIV envelope protein gp120 binds integrin α4β7. Although other integrin-binding proteins had been identified, this was the first such interaction detected between an HIV protein and an integrin. The gp120 protein contains a tripeptide identical to a region shared by other integrin-binding proteins, which is believed to be involved with the interaction. Through competition experiments and mutagenesis studies, the researchers showed that precisely that region was involved with gp120-integrin interactions. This region of gp120 is highly conserved, although it resides in a variable portion of the molecule. Finally, the authors show that this interaction is involved in viral synapse formation, which facilitates the cell-to-cell spread of virus; in the presence of integrin-gp120 complexes, LFA-1, the integrin associated with synapses, is activated. Existing treatments for other indications (such as multiple sclerosis monoclonal and natalizumab) already target the α4 integrin subunit, opening up the possibility of testing such drugs against HIV. (Nat. Immunol. 9, 301–309, 2008) LD

MAStering crop biofortification

Plant varieties identified by marker-assisted selection (MAS) are gaining prominence as a publicly acceptable alternative to transgenic crops, such as Golden Rice. Harjes et al. dissect the diversity in provitamin A levels among almost 300 inbred maize lines to reveal that variation at the lycopene epsilon cyclase locus, which encodes an enzyme that converts lycopene to α-carotene, substantially affects β-carotene and β-cryptoxanthin abundances in kernels. Increased flux through the β-branch of carotenoid biosynthesis associated with the most favorable allele they recover results from reduced transcription from this locus. Although β-carotene levels can vary as much as 30-fold between strains, visual selection is ineffective for selecting the best provitamin A–accumulating lines. Now, relatively inexpensive MAS—involving PCR-mediated scoring of variation at this locus—can substitute for expensive HPLC-based screening of locally adapted strains in resource-poor environments. This should enable easy selection of maize stocks within breeding programs aimed at addressing vitamin A deficiency in sub-Saharan Africa and Latin America without the need for transgenesis. (Science 319, 330–333, 2008) PH

Scrutinizing reprogramming

Several groups have now shown that adult fibroblasts and epithelial cells can be reprogrammed to a pluripotent, embryonic-like state by retroviral delivery of the four transcription factors Oct4, Sox2, Klf4 and c-Myc, or a similar set of factors. Although this process is known to involve epigenetic changes and the activation of pluripotency-associated genes, it will be important to work out a detailed picture of the molecular events that mediate reprogramming. To this end, Jaenisch and colleagues expressed the four reprogramming factors in mouse embryonic fibroblasts from doxycyline-inducible lentiviral vectors, allowing factor expression to be turned on and off. Using fluorescence-activated cell sorting to monitor the appearance of several pluripotency-associated genes during reprogramming, they found that alkaline phosphatase was expressed first, followed by SSEA1, and finally Nanog and endogenous Oct4. The authors also showed that the four factors must be expressed for at least 12–16 days for reprogramming to occur. (Cell Stem Cell 2, 151–159, 2008) KA

Written by Kathy Aschheim, Laura DeFrancesco, Peter Hare & Jan-Willem Theunissen