Rational design of a SOCS1-edited tumor-infiltrating lymphocyte therapy using CRISPR/Cas9 screens

Cell therapies such as tumor-infiltrating lymphocyte (TIL) therapy have shown promise in the treatment of patients with refractory solid tumors, with improvement in response rates and durability of responses nevertheless sought. To identify targets capable of enhancing the antitumor activity of T cell therapies, large-scale in vitro and in vivo clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 screens were performed, with the SOCS1 gene identified as a top T cell-enhancing target. In murine CD8+ T cell-therapy models, SOCS1 served as a critical checkpoint in restraining the accumulation of central memory T cells in lymphoid organs as well as intermediate (Texint) and effector (Texeff) exhausted T cell subsets derived from progenitor exhausted T cells (Texprog) in tumors. A comprehensive CRISPR tiling screen of the SOCS1-coding region identified sgRNAs targeting the SH2 domain of SOCS1 as the most potent, with an sgRNA with minimal off-target cut sites used to manufacture KSQ-001, an engineered TIL therapy with SOCS1 inactivated by CRISPR/Cas9. KSQ-001 possessed increased responsiveness to cytokine signals and enhanced in vivo antitumor function in mouse models. These data demonstrate the use of CRISPR/Cas9 screens in the rational design of T cell therapies.

Tumor-derived IFN triggers chronic pathway agonism and sensitivity to ADAR loss

Interferons (IFNs) are cytokines that play a critical role in limiting infectious and malignant diseases ^1-4 . Emerging data suggest that the strength and duration of IFN signaling can differentially impact cancer therapies, including immune checkpoint blockade ^5-7 . Here, we characterize the output of IFN signaling, specifically IFN-stimulated gene (ISG) signatures, in primary tumors from The Cancer Genome Atlas. While immune infiltration correlates with the ISG signature in some primary tumors, the existence of ISG signature-positive tumors without evident infiltration of IFN-producing immune cells suggests that cancer cells per se can be a source of IFN production. Consistent with this hypothesis, analysis of patient-derived tumor xenografts propagated in immune-deficient mice shows evidence of ISG-positive tumors that correlates with expression of human type I and III IFNs derived from the cancer cells. Mechanistic studies using cell line models from the Cancer Cell Line Encyclopedia that harbor ISG signatures demonstrate that this is a by-product of a STING-dependent pathway resulting in chronic tumor-derived IFN production. This imposes a transcriptional state on the tumor, poising it to respond to the aberrant accumulation of double-stranded RNA (dsRNA) due to increased sensor levels (MDA5, RIG-I and PKR). By interrogating our functional short-hairpin RNA screen dataset across 398 cancer cell lines, we show that this ISG transcriptional state creates a novel genetic vulnerability. ISG signature-positive cancer cells are sensitive to the loss of ADAR, a dsRNA-editing enzyme that is also an ISG. A genome-wide CRISPR genetic suppressor screen reveals that the entire type I IFN pathway and the dsRNA-activated kinase, PKR, are required for the lethality induced by ADAR depletion. Therefore, tumor-derived IFN resulting in chronic signaling creates a cellular state primed to respond to dsRNA accumulation, rendering ISG-positive tumors susceptible to ADAR loss.

Genetic interrogation of replicative senescence uncovers a dual role for USP28 in coordinating the p53 and GATA4 branches of the senescence program

In this study, Mazzucco et al. describe a panel of genetic screens to identify genes required for replicative senescence and uncover a role in senescence for the potent tumor suppressor and ATM substrate USP28. Their findings suggest a role for ubiquitination in senescence and imply a common node downstream from ATM that links the TP53 and GATA4 branches of the senescence response.

Senescence is a terminal differentiation program that halts the growth of damaged cells and must be circumvented for cancer to arise. Here we describe a panel of genetic screens to identify genes required for replicative senescence. We uncover a role in senescence for the potent tumor suppressor and ATM substrate USP28. USP28 controls activation of both the TP53 branch and the GATA4/NFkB branch that controls the senescence-associated secretory phenotype (SASP). These results suggest a role for ubiquitination in senescence and imply a common node downstream from ATM that links the TP53 and GATA4 branches of the senescence response.

Project DRIVE: A Compendium of Cancer Dependencies and Synthetic Lethal Relationships Uncovered by Large-Scale, Deep RNAi Screening

Elucidation of the mutational landscape of human cancer has progressed rapidly and been accompanied by the development of therapeutics targeting mutant oncogenes. However, a comprehensive mapping of cancer dependencies has lagged behind and the discovery of therapeutic targets for counteracting tumor suppressor gene loss is needed. To identify vulnerabilities relevant to specific cancer subtypes, we conducted a large-scale RNAi screen in which viability effects of mRNA knockdown were assessed for 7,837 genes using an average of 20 shRNAs per gene in 398 cancer cell lines. We describe findings of this screen, outlining the classes of cancer dependency genes and their relationships to genetic, expression, and lineage features. In addition, we describe robust gene-interaction networks recapitulating both protein complexes and functional cooperation among complexes and pathways. This dataset along with a web portal is provided to the community to assist in the discovery and translation of new therapeutic approaches for cancer.

KEAP1 loss modulates sensitivity to kinase targeted therapy in lung cancer

Inhibitors that target the receptor tyrosine kinase (RTK)/Ras/mitogen-activated protein kinase (MAPK) pathway have led to clinical responses in lung and other cancers, but some patients fail to respond and in those that do resistance inevitably occurs (Balak et al., 2006; Kosaka et al., 2006; Rudin et al., 2013; Wagle et al., 2011). To understand intrinsic and acquired resistance to inhibition of MAPK signaling, we performed CRISPR-Cas9 gene deletion screens in the setting of BRAF, MEK, EGFR, and ALK inhibition. Loss of KEAP1, a negative regulator of NFE2L2/NRF2, modulated the response to BRAF, MEK, EGFR, and ALK inhibition in BRAF-, NRAS-, KRAS-, EGFR-, and ALK-mutant lung cancer cells. Treatment with inhibitors targeting the RTK/MAPK pathway increased reactive oxygen species (ROS) in cells with intact KEAP1, and loss of KEAP1 abrogated this increase. In addition, loss of KEAP1 altered cell metabolism to allow cells to proliferate in the absence of MAPK signaling. These observations suggest that alterations in the KEAP1/NRF2 pathway may promote survival in the presence of multiple inhibitors targeting the RTK/Ras/MAPK pathway.

DOI:http://dx.doi.org/10.7554/eLife.18970.001

CRISPR Screens Provide a Comprehensive Assessment of Cancer Vulnerabilities but Generate False-Positive Hits for Highly Amplified Genomic Regions

CRISPR/Cas9 has emerged as a powerful new tool to systematically probe gene function. We compared the performance of CRISPR to RNAi-based loss-of-function screens for the identification of cancer dependencies across multiple cancer cell lines. CRISPR dropout screens consistently identified more lethal genes than RNAi, implying that the identification of many cellular dependencies may require full gene inactivation. However, in two aneuploid cancer models, we found that all genes within highly amplified regions, including nonexpressed genes, scored as lethal by CRISPR, revealing an unanticipated class of false-positive hits. In addition, using a CRISPR tiling screen, we found that sgRNAs targeting essential domains generate the strongest lethality phenotypes and thus provide a strategy to rapidly define the protein domains required for cancer dependence. Collectively, these findings not only demonstrate the utility of CRISPR screens in the identification of cancer-essential genes, but also reveal the need to carefully control for false-positive results in chromosomally unstable cancer lines.

SIGNIFICANCE

We show in this study that CRISPR-based screens have a significantly lower false-negative rate compared with RNAi-based screens, but have specific liabilities particularly in the interrogation of regions of genome amplification. Therefore, this study provides critical insights for applying CRISPR-based screens toward the systematic identification of new cancer targets. Cancer Discov; 6(8); 900-13. ©2016 AACR.See related commentary by Sheel and Xue, p. 824See related article by Aguirre et al., p. 914This article is highlighted in the In This Issue feature, p. 803.

Abstract LB-017: Disordered methionine metabolism in MTAP/CDKN2A-deleted cancers leads to marked dependence on PRMT5

Metabolic genes are increasingly recognized as targets of somatic genetic alteration in human cancer often leading to profound changes in intracellular metabolite concentrations. 5-Methylthioadenosine Phosphorylase (MTAP) is a key enzyme in the methionine salvage pathway that metabolizes methylthioadenosine (MTA) to adenine and methionine. Its chromosomal position proximal to CDKN2A results in frequent collateral homozygous deletion in a wide range of human cancers. By interrogating data from a large scale deep-coverage pooled shRNA screen across 390 cancer cell line models we found that the viability of MTAP null cancer cells is strongly impaired upon shRNA-mediated depletion of the protein arginine methyltransferase PRMT5. In MTAP deleted cells there is marked accumulation of the substrate MTA and surprisingly, we find that MTA is a specific inhibitor of the catalytic activity of PRMT5. In keeping with these data, knockout of MTAP in an MTAP-proficient cell line led to increased MTA levels and rendered them sensitive to PRMT5 depletion. Moreover, reconstitution of MTAP in an MTAP-deficient cell line fully rescued PRMT5 dependence. Collectively, these findings indicate that the collateral loss of MTAP in CDNK2A deleted cancers leads to accumulation of MTA that thereby creates a hypomorphic PRMT5 state that is selectively sensitized towards further PRMT5 inhibition.

Citation Format: Konstantinos Mavrakis, E Robert McDonald III, Michael R. Schlabach, Eric Billy, Gregory R. Hoffman, Antoine deWeck, David A. Ruddy, Kavitha Venkatesan, Greg McAllister, Rosalie deBeaumont, Samuel Ho, Yue Liu, Yan Yan-Neale, Guizhi Yang, Fallon Lin, Hong Yin, Hui Gao, David Randal Kipp, Songping Zhao, Joshua T. McNamara, Elizabeth R. Sprague, Young Shin Cho, Justin Gu, Ken Crawford, Vladimir Capka, Kristen Hurov, Jeffrey A. Porter, John Tallarico, Craig Mickanin, Emma Lees, Raymond Pagliarini, Nicholas Keen, Tobias Schmelzle, Francesco Hofmann, Frank Stegmeier, William R. Sellers. Disordered methionine metabolism in MTAP/CDKN2A-deleted cancers leads to marked dependence on PRMT5. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-017.

Disordered methionine metabolism in MTAP/CDKN2A-deleted cancers leads to dependence on PRMT5

5-Methylthioadenosine phosphorylase (MTAP) is a key enzyme in the methionine salvage pathway. The MTAP gene is frequently deleted in human cancers because of its chromosomal proximity to the tumor suppressor gene CDKN2A. By interrogating data from a large-scale short hairpin RNA-mediated screen across 390 cancer cell line models, we found that the viability of MTAP-deficient cancer cells is impaired by depletion of the protein arginine methyltransferase PRMT5. MTAP-deleted cells accumulate the metabolite methylthioadenosine (MTA), which we found to inhibit PRMT5 methyltransferase activity. Deletion of MTAP in MTAP-proficient cells rendered them sensitive to PRMT5 depletion. Conversely, reconstitution of MTAP in an MTAP-deficient cell line rescued PRMT5 dependence. Thus, MTA accumulation in MTAP-deleted cancers creates a hypomorphic PRMT5 state that is selectively sensitized toward further PRMT5 inhibition. Inhibitors of PRMT5 that leverage this dysregulated metabolic state merit further investigation as a potential therapy for MTAP/CDKN2A-deleted tumors.

Abstract 2847: High complexity barcoding to study clonal dynamics in response to cancer therapy

Targeted therapies, such as erlotinib and imatinib, lead to dramatic clinical responses, but the emergence of resistance presents a significant challenge. Recent studies have revealed intratumoral heterogeneity as a potential source for the emergence of therapeutic resistance. However, it is still unclear if relapse/resistance is driven predominantly by pre-existing or de novo acquired alterations. To address this question, we developed a high-complexity barcode library, ClonTracer, which contains over 27 million unique DNA barcodes and thus enables the high resolution tracking of cancer cells under drug treatment. Using this library in two clinically relevant resistance models, we demonstrate that the majority of resistant clones pre-exist as rare subpopulations that become selected in response to therapeutic challenge. Furthermore, our data provide direct evidence that both genetic and non-genetic resistance mechanisms pre-exist in cancer cell populations. The ClonTracer barcoding strategy, together with mathematical modeling, enabled us to quantitatively dissect the frequency of drug-resistant subpopulations and evaluate the impact of combination treatments on the clonal complexity of these cancer models. Hence, monitoring of clonal diversity in drug-resistant cell populations by the ClonTracer barcoding strategy described here may provide a valuable tool to optimize therapeutic regimens towards the goal of curative cancer therapies.

Citation Format: Hyo-eun C. Bhang, David A. Ruddy, Viveksagar Krishnamurthy Radhakrishna, Rui Zhao, Iris Kao, Daniel Rakiec, Pamela Shaw, Marissa Balak, Justina X. Caushi, Elizabeth Ackley, Nicholas Keen, Michael R. Schlabach, Michael Palmer, William R. Sellers, Franziska Michor, Vesselina G. Cooke, Joshua M. Korn, Frank Stegmeier. High complexity barcoding to study clonal dynamics in response to cancer therapy. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2847. doi:10.1158/1538-7445.AM2015-2847

A chemical genetics approach for the functional assessment of novel cancer genes

Assessing the functional significance of novel putative oncogenes remains a significant challenge given the limitations of current loss-of-function tools. Here, we describe a method that employs TALEN or CRISPR/Cas9-mediated knock-in of inducible degron tags (Degron-KI) that provides a versatile approach for the functional characterization of novel cancer genes and addresses many of the shortcomings of current tools. The Degron-KI system allows for highly specific, inducible, and allele-targeted inhibition of endogenous protein function, and the ability to titrate protein depletion with this system is able to better mimic pharmacologic inhibition compared with RNAi or genetic knockout approaches. The Degron-KI system was able to faithfully recapitulate the effects of pharmacologic EZH2 and PI3Kα inhibitors in cancer cell lines. The application of this system to the study of a poorly understood putative oncogene, SF3B1, provided the first causal link between SF3B1 hotspot mutations and splicing alterations. Surprisingly, we found that SF3B1-mutant cells are not dependent upon the mutated allele for in vitro growth, but instead depend upon the function of the remaining wild-type alleles. Collectively, these results demonstrate the broad utility of the Degron-KI system for the functional characterization of cancer genes.

Abstract IA28: Haploinsufficiency in cancer

Breast cancer is a collection of diseases with distinct clinical behaviors and underlying genetic causes. We have searched genetically for genes that have cancer relevant phenotypes including genes that alter cellular proliferation, cellular transformation, cell survival, cellular senescence, and genes that are essential for the proliferation of cancer cells. We have approached this by the generation of libraries of shRNAs for loss of function experiments and libraries of ORFs for gain of function experiments. We will discuss these technologies and how they can be applied to the functional dissection of genes important for breast cancer. Using these new technologies we have identified new oncogenes and tumor suppressors. We find that tumor cells selectively delete negative growth regulators and suggest that the deletion of clusters of these genes may drive tumorigenesis by haploinsufficiency. These recurrent deletions also avoid deletion of one copy of many essential genes through haploinsufficiency. We put forward the Cancer Gene Island model that postulates that tumors select for hemizygous loss of islands of gene enriched in negative regulators of proliferation and depleted in essential genes to promote tumor cell proliferation through cumulative haploinsufficiency.

Citation Format: Teresa Davoli, Nicole L. Solimini, Natalya N. Pavlova, Qikai Xu, Kristen Mengwasser, Laura M. Sack, Anthony C. Liang, Michael R. Schlabach, Ji Luo, Anna E. Burrows, Anthony N. Anselmo, Mamie Z. Li, Stephen J. Elledge. Haploinsufficiency in cancer. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr IA28.

Recurrent hemizygous deletions in cancers may optimize proliferative potential

Tumors exhibit numerous recurrent hemizygous focal deletions that contain no known tumor suppressors and are poorly understood. To investigate whether these regions contribute to tumorigenesis, we searched genetically for genes with cancer-relevant properties within these hemizygous deletions. We identified STOP and GO genes, which negatively and positively regulate proliferation, respectively. STOP genes include many known tumor suppressors, whereas GO genes are enriched for essential genes. Analysis of their chromosomal distribution revealed that recurring deletions preferentially overrepresent STOP genes and underrepresent GO genes. We propose a hypothesis called the cancer gene island model, whereby gene islands encompassing high densities of STOP genes and low densities of GO genes are hemizygously deleted to maximize proliferative fitness through cumulative haploinsufficiencies. Because hundreds to thousands of genes are hemizygously deleted per tumor, this mechanism may help to drive tumorigenesis across many cancer types.

A SUMOylation-dependent transcriptional subprogram is required for Myc-driven tumorigenesis

Myc is an oncogenic transcription factor frequently dysregulated in human cancer. To identify pathways supporting the Myc oncogenic program, we used a genome-wide RNA interference screen to search for Myc-synthetic lethal genes and uncovered a role for the SUMO-activating enzyme (SAE1/2). Loss of SAE1/2 enzymatic activity drives synthetic lethality with Myc. Inactivation of SAE2 leads to mitotic catastrophe and cell death upon Myc hyperactivation. Mechanistically, SAE2 inhibition switches a transcriptional subprogram of Myc from activated to repressed. A subset of these SUMOylation-dependent Myc switchers (SMS genes) is required for mitotic spindle function and to support the Myc oncogenic program. SAE2 is required for growth of Myc-dependent tumors in mice, and gene expression analyses of Myc-high human breast cancers suggest that low SAE1 and SAE2 abundance in the tumors correlates with longer metastasis-free survival of the patients. Thus, inhibition of SUMOylation may merit investigation as a possible therapy for Myc-driven human cancers.

A genome-wide RNAi screen identifies multiple synthetic lethal interactions with the Ras oncogene

Oncogenic mutations in the small GTPase Ras are highly prevalent in cancer, but an understanding of the vulnerabilities of these cancers is lacking. We undertook a genome-wide RNAi screen to identify synthetic lethal interactions with the KRAS oncogene. We discovered a diverse set of proteins whose depletion selectively impaired the viability of Ras mutant cells. Among these we observed a strong enrichment for genes with mitotic functions. We describe a pathway involving the mitotic kinase PLK1, the anaphase-promoting complex/cyclosome, and the proteasome that, when inhibited, results in prometaphase accumulation and the subsequent death of Ras mutant cells. Gene expression analysis indicates that reduced expression of genes in this pathway correlates with increased survival of patients bearing tumors with a Ras transcriptional signature. Our results suggest a previously underappreciated role for Ras in mitotic progression and demonstrate a pharmacologically tractable pathway for the potential treatment of cancers harboring Ras mutations.

Profiling essential genes in human mammary cells by multiplex RNAi screening

By virtue of their accumulated genetic alterations, tumor cells may acquire vulnerabilities that create opportunities for therapeutic intervention. We have devised a massively parallel strategy for screening short hairpin RNA (shRNA) collections for stable loss-of-function phenotypes. We assayed from 6000 to 20,000 shRNAs simultaneously to identify genes important for the proliferation and survival of five cell lines derived from human mammary tissue. Lethal shRNAs common to these cell lines targeted many known cell-cycle regulatory networks. Cell line-specific sensitivities to suppression of protein complexes and biological pathways also emerged, and these could be validated by RNA interference (RNAi) and pharmacologically. These studies establish a practical platform for genome-scale screening of complex phenotypes in mammalian cells and demonstrate that RNAi can be used to expose genotype-specific sensitivities.

Cancer proliferation gene discovery through functional genomics

Retroviral short hairpin RNA (shRNA)-mediated genetic screens in mammalian cells are powerful tools for discovering loss-of-function phenotypes. We describe a highly parallel multiplex methodology for screening large pools of shRNAs using half-hairpin barcodes for microarray deconvolution. We carried out dropout screens for shRNAs that affect cell proliferation and viability in cancer cells and normal cells. We identified many shRNAs to be antiproliferative that target core cellular processes, such as the cell cycle and protein translation, in all cells examined. Moreover, we identified genes that are selectively required for proliferation and survival in different cell lines. Our platform enables rapid and cost-effective genome-wide screens to identify cancer proliferation and survival genes for target discovery. Such efforts are complementary to the Cancer Genome Atlas and provide an alternative functional view of cancer cells.

Second-generation shRNA libraries covering the mouse and human genomes

Loss-of-function phenotypes often hold the key to understanding the connections and biological functions of biochemical pathways. We and others previously constructed libraries of short hairpin RNAs that allow systematic analysis of RNA interference-induced phenotypes in mammalian cells. Here we report the construction and validation of second-generation short hairpin RNA expression libraries designed using an increased knowledge of RNA interference biochemistry. These constructs include silencing triggers designed to mimic a natural microRNA primary transcript, and each target sequence was selected on the basis of thermodynamic criteria for optimal small RNA performance. Biochemical and phenotypic assays indicate that the new libraries are substantially improved over first-generation reagents. We generated large-scale-arrayed, sequence-verified libraries comprising more than 140,000 second-generation short hairpin RNA expression plasmids, covering a substantial fraction of all predicted genes in the human and mouse genomes. These libraries are available to the scientific community.

A genetic screen for candidate tumor suppressors identifies REST

Tumorigenesis is a multistep process characterized by a myriad of genetic and epigenetic alterations. Identifying the causal perturbations that confer malignant transformation is a central goal in cancer biology. Here we report an RNAi-based genetic screen for genes that suppress transformation of human mammary epithelial cells. We identified genes previously implicated in proliferative control and epithelial cell function including two established tumor suppressors, TGFBR2 and PTEN. In addition, we uncovered a previously unrecognized tumor suppressor role for REST/NRSF, a transcriptional repressor of neuronal gene expression. Array-CGH analysis identified REST as a frequent target of deletion in colorectal cancer. Furthermore, we detect a frameshift mutation of the REST gene in colorectal cancer cells that encodes a dominantly acting truncation capable of transforming epithelial cells. Cells lacking REST exhibit increased PI(3)K signaling and are dependent upon this pathway for their transformed phenotype. These results implicate REST as a human tumor suppressor and provide a novel approach to identifying candidate genes that suppress the development of human cancer.

The nonspecific binding of Fe3+ to transferrin in the absence of synergistic anions

An obligatory role for barbonate (or other synergistic anions) in the specific binding of Fe3+ by transferrin has been a point of controversy for two decades. There are an equal number of confirmatory and negative reports of specific Fe3+-transferrin binary complexes. A criticism of previous studies is the use of only one synthetic route, and limited product testing. This study reports the development of several preparative routes aimed at the formation of a specific Fe3+-transferrin complex, and the characterization of the products by spectrophotometry and chemical reactivity. The preparative routes described include: (a) displacement of carbonate from Fe3+-transferrin-CO32- at low pH followed by removal of CO2 by several techniques; (b) addition of FeCl3 to apotransferrin under CO2-free conditions; (c) oxidation of Fe2+ in the presence of apotransferrin under CO2-free conditions; (d) reaction of apotransferrin with nonsubstituting Fe3+ complexes in the absence of CO2; and (e) attempts to displace anions from weak Fe3+-transferrin-anion complexes. The product were examined with regard to their visible spectra, and their examined with regard to their visible spectra, and their reactivity with: (a) NaHCO3, (b) Fe3+-nitrilotriacetic acid in NaHCO3, and (c) citrate. The results are compared with the characteristics of Fe3+-transferrin-anion complexes and nonspecific Fe3+, transferrin mixtures. The data indicate that in the absence of synergistic anions the affinity of the specific metal binding sites of transfe-rin for Fe3+ is so low as to not compete favorably with hydrolytic polymerization and nonspecific binding effects.

The synergistic binding of anions and Fe3+ by transferrin. Implications for the interlocking sites hypothesis

The finding that transferrin does not bind Fe3+ at the specific metal binding sites in the absence of carbonate and synergistic anions emphasizes the fundamental importance of the anion binding site to the chemistry of Fe3+-transferrin-CO32-. An important question regards the chemical and structural requirements for carbonate substitution. This has been, however, an area of some dispute in the literature. We have utilized four synthetic routes for the preparation of Fe3+-transferrin-anion complexes. The products have been examined with regard to spectral properties, and reaction with: (a) NaHCO3, (b) Fe3+-nitrilotriacetic acid in NaHCO3, and (c) sodium citrate under CO2-free conditions. The results provide information as to which anions are synergistic, and the basic properties of the Fe3+-transferrin-anion complexes that are formed. The 6 inorganic anions that were tested were all found to be nonsynergistic. Dihydroxyacetone and glyceraldehyde were also nonsynergistic. Dicarboxylic acids were found to form stable Fe3+-transferrin-anion complexes which were only slowly displaced by carbonate. Several monocarboxylic acids with proximal aldehyde, ketone, alcohol, amino, or thiol functional groups proved to be synergistic. CPK molecular model studies suggested the functional group and the carboxylic acid must be able to fit within a site between 6.3 and 7.0 A in maximal length. One large substituent could be accommodated by the site, however, two methylgroups on the alpha carbon to a carboxylate group could not be accommodated. Chloroacetate and monocarboxylic acids were nonsynergistic. The results are interpreted in terms of an interlocking sites hypothesis which envisions the synergistic anion as interacting with the protein via its its carboxyl group and bonding with the Fe3+ via its proximal functional group.