LMO2 is a transcription regulator involved in human T-cell leukemia, including some occurring in X-SCID gene therapy trials, and in B-cell lymphomas and prostate cancer. LMO2 functions in transcription complexes via protein-protein interactions involving two LIM domains and causes a preleukemic T-cell development blockade followed by clonal tumors. Therefore, LMO2 is necessary but not sufficient for overt neoplasias, which must undergo additional mutations before frank malignancy. An open question is the importance of LMO2 in tumor development as opposed to sustaining cancer. We have addressed this using a peptide aptamer that binds to the second LIM domain of the LMO2 protein and disrupts its function. This specificity is mediated by a conserved Cys-Cys motif, which is similar to the zinc-binding LIM domains. The peptide inhibits Lmo2 function in a mouse T-cell tumor transplantation assay by preventing Lmo2-dependent T-cell neoplasia. Lmo2 is, therefore, required for sustained T-cell tumor growth, in addition to its preleukemic effect. Interference with LMO2 complexes is a strategy for controlling LMO2-mediated cancers, and the finger structure of LMO2 is an explicit focus for drug development. [Cancer Res 2009;69(11):4784-90]

LMO2 was discovered via chromosomal translocations in T-cell leukaemia and shown normally to be essential for haematopoiesis. LMO2 is made up of two LIM only domains (thus it is a LIM-only protein) and forms a bridge in a multi-protein complex. We have studied the mechanism of formation of this complex using a single domain antibody fragment that inhibits LMO2 by sequestering it in a non-functional form. The crystal structure of LMO2 with this antibody fragment has been solved revealing a conformational difference in the positioning and angle between the two LIM domains compared with its normal binding. This contortion occurs by bending at a central helical region of LMO2. This is a unique mechanism for inhibiting an intracellular protein function and the structural contusion implies a model in which newly synthesized, intrinsically disordered LMO2 binds to a partner protein nucleating further interactions and suggests approaches for therapeutic targeting of LMO2.

Interfering intracellular antibodies are valuable for biological studies as drug surrogates and as potential macromolecular drugs per se. Their application is still limited because of the difficulty of acquisition of functional intracellular antibodies. We describe the use of the new intracellular antibody capture procedure (IAC 3 ) to facilitate direct isolation of functional single domain antibody fragments using four independent target molecules (LMO2, TP53, CRAF1, and Hoxa9) from a set of diverse libraries. Initially, these have variability in only one of the three antigen-binding CDR regions of VH or VL and first round single domains are affinity matured by iterative randomization of the two other CDRs and reselection. We highlight the approach using a single domain binding to LMO2 protein. Our results show that interfering with LMO2 protein function demonstrates a role specifically in erythroid differentiation, confirm a necessary and sufficient function for LMO2 as a cancer therapy target in T-cell neoplasia and allowed for the first time production of soluble recombinant LMO2 protein by co-expression with intracellular domain antibodies. Cocrystallization of LMO2 and the anti-LMO2 VH protein was successful. These results demonstrate that this third generation IAC 3 offers a robust toolbox for various biomedical applications and consolidates functional features of the LMO2 protein complex, which includes the importance of Lmo2-Ldb1 protein interaction.Antibodies and derivative fragments are molecules that can capture a huge variety of antigens with high specificity and affinity and are indispensable tools and reagents in bioscience and in medicine (1). The utilities of intracellular application of antibody fragments is that, unlike gene knock-out and interfering RNAs or antisense, these molecules interfere with cellular functions directly by binding to target proteins and can therefore target specific properties of proteins such as particular signal transduction via blockading protein-protein interaction (2, 3). This not only provides valuable and robust tools in biological analysis such as functional genomics and proteomics but also serve as drug surrogates to establish the target validity for drug development by ascertaining the biological effects from targeting the protein. To make these options possible on a broad basis, robust screening methods with high quality libraries are needed to allow selection of target-specific intracellular antibodies.Antibody binding sites comprise heavy chain variable domain (VH) 4 and light chain variable domains (VL), each with three complementarity determining regions (CDRs), directly contacting antigen and concerned in binding affinity and specificity. Single domain antibody fragments (DAbs), comprising either VH or VL, encompass a minimal antigen recognition can also be used for intracellular applications, so-called intracellular domain antibodies (iDAbs) (4). Structural analysis of V regions based on a consensus framework sequence have shown that essentially no diff...

Background: Indoleamine 2,3-dioxygenase (IDO), the first step in the kynurenine pathway (KP), is upregulated in some cancers and represents an attractive therapeutic target given its role in tumour immune evasion. However, the recent failure of an IDO inhibitor in a late phase trial raises questions about this strategy. Methods: Matched renal cell carcinoma (RCC) and normal kidney tissues were subject to proteomic profiling. Tissue immunohistochemistry and gene expression data were used to validate findings. Phenotypic effects of loss/gain of expression were examined in vitro. Results: Quinolate phosphoribosyltransferase (QPRT), the final and rate-limiting enzyme in the KP, was identified as being down-regulated in RCC. Loss of QPRT expression led to increased potential for anchorage-independent growth. Gene expression, mass spectrometry (clear cell and chromophobe RCC) and tissue immunohistochemistry (clear cell, papillary and chromophobe), confirmed loss or decreased expression of QPRT, and showed down-regulation of other KP enzymes including kynurenine 3-monoxygenase (KMO) and 3-hydroxyanthranilate-3,4-dioxygenase (HAAO), with a concomitant maintenance or up-regulation of nicotinamide phosphoribosyltransferase (NAMPT), the key enzyme in the NAD+ salvage pathway. Conclusions: Widespread dysregulation of the KP is common in RCC and is likely to contribute to tumour immune evasion, carrying implications for effective therapeutic targeting of this critical pathway

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