Association of tumour mutational burden with outcomes in patients with advanced solid tumours treated with pembrolizumab: prospective biomarker analysis of the multicohort, open-label, phase 2 KEYNOTE-158 study

BACKGROUND

Tumour mutational burden (TMB) has been retrospectively correlated with response to immune checkpoint blockade. We prospectively explored the association of high tissue TMB (tTMB-high) with outcomes in ten tumour-type-specific cohorts from the phase 2 KEYNOTE-158 study, which assessed the anti-PD-1 monoclonal antibody pembrolizumab in patients with selected, previously treated, advanced solid tumours.

METHODS

In the multi-cohort, open-label, non-randomised, phase 2 KEYNOTE-158 study, patients were enrolled from 81 academic facilities and community-based institutions across 21 countries in Africa, the Americas, Asia, and Europe. Eligible patients were aged 18 years or older, had a histologically or cytologically confirmed advanced (ie, unresectable or metastatic, or both) incurable solid tumour (eligible tumour types were anal, biliary, cervical, endometrial, mesothelioma, neuroendocrine, salivary, small-cell lung, thyroid, and vulvar), progression on or intolerance to one or more lines of standard therapy, had measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST; version 1.1) assessed by independent central radiological review, Eastern Cooperative Oncology Group performance status of 0 or 1, life expectancy of at least 3 months, adequate organ function, and a tumour sample for biomarker analysis. Participants were given pembrolizumab 200 mg intravenously every 3 weeks for up to 35 cycles. Tissue TMB (tTMB) was assessed in formalin-fixed paraffin-embedded tumour samples using the FoundationOne CDx assay (Foundation Medicine, Cambridge, MA, USA). The prespecified definition of tTMB-high status was at least 10 mutations per megabase. The primary endpoint was the proportion of patients with an objective response (complete or partial response) as per Response Evaluation Criteria in Solid Tumours (version 1.1) by independent central review. This prespecified analysis assessed the association between antitumour activity and tTMB in treated patients with evaluable tTMB data. Efficacy was assessed in all participants who received at least one dose of pembrolizumab, had evaluable tTMB data, and were enrolled at least 26 weeks before data cutoff (June 27, 2019), and safety was assessed in all participants who received at least one dose of pembrolizumab and had tTMB-high status. KEYNOTE-158 is registered at ClinicalTrials.gov, NCT02628067, and is ongoing.

FINDINGS

Between Jan 15, 2016, and June 25, 2019, 1073 patients were enrolled. 1066 participants were treated as of data cutoff (June 27, 2019), of whom 805 (76%) were evaluable for TMB, and 105 (13%) of 805 had tTMB-high status and were assessed for safety. 1050 (98%) of 1066 patients enrolled by at least 26 weeks before data cutoff, of whom 790 (75%) were evaluable for TMB and included in efficacy analyses. 102 (13%) of these 790 patients had tTMB-high status (≥10 mutations per megabase), and 688 (87%) patients had non-tTMB-high status (<10 mutations per megabase). Median study follow-up was 37·1 months (IQR 35·0-38·3). Objective responses were observed in 30 (29%; 95% CI 21-39) of 102 patients in the tTMB-high group and 43 (6%; 5-8) of 688 in the non-tTMB-high group. 11 (10%) of 105 patients had treatment-related serious adverse events. 16 (15%) participants had a grade 3-5 treatment-related adverse event, of which colitis was the only such adverse event that occurred in more than one patient (n=2). One patient had fatal pneumonia that was assessed by the investigator to be treatment related.

INTERPRETATION

tTMB-high status identifies a subgroup of patients who could have a robust tumour response to pembrolizumab monotherapy. tTMB could be a novel and useful predictive biomarker for response to pembrolizumab monotherapy in patients with previously treated recurrent or metastatic advanced solid tumours.

FUNDING

Merck Sharp & Dohme Corp, a subsidiary of Merck & Co, Inc.

Abstract 5671: Alignment of TMB measured on clinical samples: Phase IIB of the Friends of Cancer Research TMB Harmonization Project

Introduction:

Tumor mutational burden (TMB) is the number of somatic mutations per megabase in a tumor's genome and has shown promise as a predictive biomarker of response to immune checkpoint inhibitors across several cancers. TMB is typically measured by whole exome sequencing (WES TMB) or by targeted next-generation sequencing gene panels (panel TMB). As more assays are developed to estimate TMB, harmonization is emerging as an unmet need and is a key goal of the Friends of Cancer Research (Friends) TMB Harmonization Project. Phase I of the Harmonization Project demonstrated correlation between panel TMB and WES TMB using TCGA data and defined theoretical sources of variability across panels. In phase IIA, sustainable TMB reference standard materials generated from human derived cell lines were used to characterize variability in TMB measurements across panels and assessed for utility in TMB alignment. Phase IIB aims to characterize variability in TMB measurements in clinical samples and to establish best practices for estimating and aligning TMB in order to improve consistency across panels.

Methods:

Fifteen laboratories (16 targeted gene panels) at different stages of development participated in phase IIB. Thirty formalin-fixed paraffin-embedded (FFPE) samples with &gt;30% tumor content were acquired; tumor DNA was isolated by a single reference lab. TMB values were calculated for DNA extracted from lung (N=10), bladder (N=10), and gastric tumors (N=10) using WES and a uniform bioinformatics pipeline agreed upon by all Consortium members. DNA samples were also sent to all laboratories, and each used their own sequencing and bioinformatics pipelines to estimate TMB from the genes represented in their respective panels. For each tumor sample, a median across panel TMB estimates was calculated; individual panel TMB estimates were translated to fold-changes relative to the sample median to quantify variability. Association between WES TMB (reference) and panel TMB will be assessed by regression analysis; dependence of association on cancer type was investigated.

Results:

A subset of tumor samples (9 bladder, 7 lung, and 5 gastric) was analyzed using 11 panels at the time of abstract submission. Median panel TMB values ranged 0.60 - 40.26 across samples, with median of median values of 5.35. Fold-change from sample-level medians ranged 0x - 6.67x. Assessment of these clinical samples by WES and all 16 gene panels, as well as regression analysis results, are forthcoming.

Conclusions:

The Friends TMB Harmonization Project has made substantial progress in characterization of TMB measurement variability and association between WES TMB and panel TMB. These are important steps toward alignment of TMB estimates generated by different gene panels which may improve the interpretation of findings within clinical development programs and ultimately enhance the usefulness of this predictive biomarker in clinical decision making.

Citation Format: Diana M. MERINO, Laura M. Yee, Lisa M. McShane, P. Mickey Williams, Tomas Vilimas, Rajesh Patidar, J. Carl Barrett, Shu-Jen Chen, Jen-Hao Cheng, Jeffrey M. Conroy, Dinesh Cyanam, Kenneth R. Eyring, David A. Fabrizio, Vincent Funari, Elizabeth P. Garcia, Sean T. Glenn, Christopher D. Gocke, Vikas Gupta, Lisa M. Haley, Matthew D. Hellmann, Laurel Keefer, Lauryn R. Keeler, Brett Kennedy, Alexander J. Lazar, Laura E. MacConaill, Kristen L. Meier, Arnaud Papin, Naiyer A. Rizvi, Ethan Sokol, Phillip Stafford, John F. Thompson, Warren Tom, Victor J. Weigman, Mingchao Xie, Chen Zhao, Mark D. Stewart, Jeff Allen. Alignment of TMB measured on clinical samples: Phase IIB of the Friends of Cancer Research TMB Harmonization Project [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5671.

A Novel Next-Generation Sequencing Approach to Detecting Microsatellite Instability and Pan-Tumor Characterization of 1000 Microsatellite Instability-High Cases in 67,000 Patient Samples

Microsatellite instability (MSI) is an important biomarker for predicting response to immune checkpoint inhibitor therapy, as emphasized by the recent checkpoint inhibitor approval for MSI-high (MSI-H) solid tumors. Herein, we describe and validate a novel method for determining MSI status from a next-generation sequencing comprehensive genomic profiling assay using formalin-fixed, paraffin-embedded samples. This method is 97% (65/67) concordant with current standards, PCR and immunohistochemistry. We further apply this method to >67,000 patient tumor samples to identify genes and pathways that are enriched in MSI-stable or MSI-H tumor groups. Data show that although rare in tumors other than colorectal and endometrial carcinomas, MSI-H samples are present in many tumor types. Furthermore, the large sample set revealed that MSI-H tumors selectively share alterations in genes across multiple common pathways, including WNT, phosphatidylinositol 3-kinase, and NOTCH. Last, MSI is sufficient, but not necessary, for a tumor to have elevated tumor mutation burden. Therefore, MSI can be determined from comprehensive genomic profiling with high accuracy, allowing for efficient MSI-H detection across all tumor types, especially those in which routine use of immunohistochemistry or PCR-based assays would be impractical because of a rare incidence of MSI. MSI-H tumors are enriched in alterations in specific signaling pathways, providing a rationale for investigating directed immune checkpoint inhibitor therapies in combination with pathway-targeted therapies.

Abstract CT194: Exploratory subgroup analysis of atezolizumab (atezo) clinical characteristics in patients (pts) with low circulating tumor DNA (ctDNA) in B-F1RST—a Phase II trial evaluating blood-based tumor mutational burden (bTMB) in NSCLC

Background

B-F1RST (ITT, n = 152) was the first prospective trial to evaluate bTMB as a predictive biomarker in 1L NSCLC. In the biomarker evaluable population (BEP, ctDNA with max somatic allele frequency [MSAF] ≥ 1%), bTMB high (≥ 16) predicted a better overall response rate (ORR) to atezo vs bTMB low (&lt; 16; 28.6 vs 4.4%). In exploratory analyses, numerically higher ORR was also seen in the biomarker non-evaluable population (ORR, 34.5%), although these pts were not evaluable for bTMB due to reduced assay sensitivity at very low ctDNA levels (MSAF &lt; 1%). Here we further evaluate the ≥ 1% MSAF and &lt; 1% MSAF subgroups.

Methods

Baseline characteristics were compared across MSAF &lt; 1% (n = 29) and MSAF ≥ 1% (n = 119) subgroups. Factors with a notable difference between groups (P &lt; 0.15) were included in an inverse probability weighting (IPW) method to adjust for baseline imbalances. Clinical outcomes from unadjusted and adjusted models were compared.

Results

Baseline factors with imbalances between MSAF groups (P &lt; 0.15) were age, smoking status, PD-L1 status, no. of target lesions and SLD (Table). In the unadjusted analysis, ORR for MSAF &lt; 1% vs ≥ 1% was 34.5 vs 10.1% (odds ratio [OR], 4.69; P = 0.002). After IPW adjustment, ORR was 19.9 vs 11.1% (OR, 1.99; P = 0.43). Unadjusted mPFS for MSAF &lt; 1% vs ≥ 1% was 6.8 vs 4.0 mo (HR, 0.63; P = 0.065), and adjusted mPFS was 2.8 vs 4.0 mo (HR, 0.88; P = 0.72).

Conclusions

Consistent with previous findings regarding low levels of ctDNA, pts with MSAF &lt; 1% had better baseline prognostic factors than those with MSAF ≥ 1%, likely accounting for their better outcomes. After adjusting for baseline imbalances, ORR and mPFS did not differ significantly between subgroups. These results do not alter interpretation of high bTMB results in the BEP (MSAF ≥ 1%). Clinical validation of the bTMB assay continues in B-F1RST and BFAST.

Table.Unadjusted and Adjusted Baseline Characteristics Included in the IPW ModelUnadjustedAdjustedMSAF ≥ 1%MSAF &lt; 1%PMSAF ≥ 1%MSAF &lt; 1%Pn11929119.027.5Age &lt; 65 y, %29.448.30.0933.030.20.80Never smoker, %5.013.80.146.66.90.52PD-L1+, %a37.851.70.1340.949.20.65Number of target lesions, mean (SD)2.38 (1.23)1.79 (0.86)0.022.27 (1.20)2.26 (1.13)0.98SLD, mmmedian (range)70.0 (12.7, 257.0)42.4 (13.0, 200.0)0.00162.0 (12.7, 257.0)48.2 (13.0, 200.0)0.77HR, hazard ratio; SD, standard deviation; SLD, sum of longest diameters.a PD-L1+ defined as ≥ 1% PD-L1 on tumor cells by any commercially available assay.

Citation Format: Mark A. Socinski, Sarah M. Paul, Cindy Yun, Sylvia Hu, Vincent Shen, Vamsidar Velcheti, Tony S. Mok, David R. Gandara, Young Kwang Chae, Erica Schleifman, David A. Fabrizio, David S. Shames, See Phan, Edward S. Kim. Exploratory subgroup analysis of atezolizumab (atezo) clinical characteristics in patients (pts) with low circulating tumor DNA (ctDNA) in B-F1RST—a Phase II trial evaluating blood-based tumor mutational burden (bTMB) in NSCLC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr CT194.

Beyond microsatellite testing: assessment of tumor mutational burden identifies subsets of colorectal cancer who may respond to immune checkpoint inhibition

Background

The clinical application of PD1/PD-L1 targeting checkpoint inhibitors in colorectal cancer (CRC) has largely focused on a subset of microsatellite instable (MSI-high) patients. However, the proposed genotype that sensitizes these patients to immunotherapy is not captured by MSI status alone. Estimation of tumor mutational burden (TMB) from comprehensive genomic profiling is validated against whole exome sequencing and linked to checkpoint response in metastatic melanoma, urothelial bladder cancer and non-small cell lung carcinoma. We sought to explore the subset of microsatellite stable (MSS) CRC patients with high TMB, and identify the specific genomic signatures associated with this phenotype. Furthermore, we explore the ability to quantify TMB as a potential predictive biomarker of PD1/PD-L1 therapy in CRC.

Methods

Formalin-fixed, paraffin embedded tissue sections from 6,004 cases of CRC were sequenced with a CLIA-approved CGP assay. MSI and TMB statuses were computationally determined using validated methods. The cutoff for TMB-high was defined according to the lower bound value that satisfied the 90% probability interval based on the TMB distribution across all MSI-High patients.

Results

MSS tumors were observed in 5,702 of 6,004 (95.0%) cases and MSI-H tumors were observed in 302 (5.0%) cases. All but one (99.7%) MSI-H cases were TMB-high (range, 6.3-746.9 mut/Mb) and 5,538 of 5,702 (97.0%) MSS cases were TMB-low (range, 0.0-10.8 mut/Mb). Consequently, 164 of 5,702 (2.9%) MSS cases were confirmed as TMB-high (range, 11.7-707.2 mut/Mb), representing an increase in the target population that may respond to checkpoint inhibitor therapy by 54% (466 vs. 302, respectively). Response to PD-1 inhibitor is demonstrated in MSS/TMB-high cases.

Conclusions

Concurrent TMB assessment accurately classifies MSI tumors as TMB-high and simultaneously identifies nearly 3% or CRC as MSS/TMB-high. This subgroup may expand the population of CRC who may benefit from immune checkpoint inhibitor based therapeutic approaches.

Abstract B16: Validation and clinical feasibility of a Foundation Medicine assay to identify immunotherapy response potential through tumor mutational burden (TMB)

Background: The ability of tumors to evade immune surveillance by overexpressing immune checkpoint proteins has been exploited for therapeutic intervention through antibodies designed to interrupt their signaling. A number of patients across a range of disease types, including melanoma, lung, renal and bladder cancer, have demonstrated robust and durable responses using checkpoint inhibitor therapies (CPITs). Still, identifying the most likely responders remains an urgent need for proper clinical management. Tumor mutational burden (TMB) measures the overall number of somatic protein coding mutations per area of sequence counted occurring in a tumor specimen. This measure has been associated with both response and survival for multiple CPITs across an array of indications. It is hypothesized that immunotherapies are more effective for tumors with high TMB because these cells are more likely to express immune-reactive neoantigens. In this study we describe Foundation Medicine's (FMI) work to develop and validate a TMB result as part of the current FoundationOne (F1) and FoundationOne Heme (F1H) comprehensive genomic profiling assays.

Methods: We developed an analysis method to determine TMB based on data from both the F1 and F1H comprehensive genomic profiling assays. TMB is calculated by counting all synonymous and non-synonymous somatic variants across 315 or 405 genes. Germline alterations and known and likely driver alterations are excluded to avoid sample bias, as both F1 and F1H specifically target genes with cancer associations. The resulting mutation count is normalized by expressing the number as a mutation density with units of mutations per megabase (mut/Mb) of coding target territory. Analytic validation of TMB focused on accuracy, precision and sensitivity, while initial clinical feasibility was assessed in a cohort of 65 metastatic melanoma patients receiving immunotherapy. To determine accuracy, we compared the TMB values generated from F1 against a CLIA validated whole-exome sequencing (WES) method on 29 patients with TMB values ranging from &lt;1 mut/Mb up to 600 mut/Mb. Precision was defined as the reliability of the TMB metric when determined from 10 clinical samples replicated 4-6 times. Sensitivity was evaluated by determining the lower limit of sample tumor purity at which a TMB value could be reliably assessed through a dilution series of tumor/normal pairs ranging from 80% to 5% tumor. We also assessed the clinical feasibility of the F1 TMB result by examining its ability to predict clinical response to anti-PD1 or PD-L1 immunotherapy in a cohort of 65 metastatic melanoma patients. The patients were evaluated for best response per RECIST criteria, progression free survival (PFS) and overall survival (OS).

Results: Foundation Medicine's TMB measure provides accurate and precise results across a range of tumor mutational burden values on samples with as little as 20% tumor purity. In a cohort of 65 metastatic melanoma patients, the median TMB value was 37.9 mut/Mb in the responder group and 6.6 mut/Mb in the non-responder group (p&lt;0.0001, Mann-Whitney test). Additionally, TMB-high (≥20 mut/Mb) patients demonstrated superior PFS and OS compared to TMB non-high patients (median PFS and OS not reached for TMB-High through 66 months vs. medians of 3 months PFS and 12 months OS for TMB non-high, p-value &lt;0.001).

Conclusions: We have developed and validated a TMB result as part of the FoundationOne and FoundationOne Heme platforms. Initial clinical feasibility results demonstrate that the FoundationOne TMB value can be used to predict the likely response of metastatic melanoma patients to anti-PD1/PD-L1 checkpoint inhibitors, while feasibility in NSCLC and bladder cancer have been presented elsewhere.

Citation Format: Daniel S. Lieber, Mark R. Kennedy, Douglas B. Johnson, Joel R. Greenbowe, Garrett M. Frampton, Alexa B. Schrock, Jeffrey S. Ross, Phillip J. Stephens, Siraj M. Ali, Vincent A. Miller, David A. Fabrizio. Validation and clinical feasibility of a Foundation Medicine assay to identify immunotherapy response potential through tumor mutational burden (TMB). [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2016 Oct 20-23; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2017;5(3 Suppl):Abstract nr B16.

Abstract 643: Identification of novel pancreatic cancer-specific antibodies and their target antigens through a next generation immune sequencing platform

One of the major challenges in developing new therapies for pancreatic cancer is the lack of known specific and sensitive targets to the cancer cells. In this study, we present a powerful way to identify novel specific targets to pancreatic cancer by leveraging the antibody responses of tumor infiltrating lymphocytes (TILs) using a novel next-generation immune sequencing approach. We first deeply characterize the antibody repertoire produced by TILs, followed by antibody expression, screening and functional characterization to identify their biological targets. We aim to manipulate the antibody cancer targeting specificity into potential therapeutics by utilizing antibody-drug conjugation (ADC) technology.

We obtained tumor resections and matched normal adjacent tissue from patients diagnosed with pancreatic ductal adenocarcinoma or acinar cell carcinoma, and characterized each sample's TIL immune repertoire using AbVitro's proprietary Next-Generation Immune Sequencing platform. Interestingly, results indicated the presence of millions of B and T cells in both tumor and normal tissues. Strikingly, tumor tissues across all patients were characterized by an abundance of expanded B-cell lineages expressing IgG, whereas normal adjacent tissue and healthy control samples contained almost exclusively IgA expressing cells. Antibody candidates were selected from the sequencing data and heavy and light chains were synthesized and expressed in a mammalian cell-based expression system. The antibodies were then screened for cell surface binding, tissue specificity and cell killing potential against pancreatic cancer cell lines and FFPE human cancer tissues. Selected antibodies from these screens were then chosen as candidates for antigen identification.

Our study uncovered multiple antibodies that specifically bind to pancreatic cancer tissues but not to healthy tissues. Interestingly, some of the antibodies also showed strong binding to other types of cancer such as lung squamous cell carcinoma. Initial target identification efforts for these antibodies yielded a short list of antigens which are known to be expressed at high levels in the pancreas or pancreatic cancer cell lines. We are currently performing studies to further understand the antigen specificity and sensitivity of these antibodies and evaluate their cell killing potential as ADCs, as well as investigating the diagnostic value of the novel antigen targets. In summary, our study demonstrates the potential of next-generation sequencing in TIL analysis for the discovery of novel cancer-specific targets of potential therapeutic value.

Citation Format: David A. Fabrizio, Sonia Timberlake, Brian Belmont, Stephen J. Goldfless, Adrian W. Briggs, Teresa J. Broering, Francois Vigneault. Identification of novel pancreatic cancer-specific antibodies and their target antigens through a next generation immune sequencing platform. [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 643. doi:10.1158/1538-7445.AM2015-643

DRAGON, a Bone Morphogenetic Protein Co-receptor

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor (TGF)beta superfamily of ligands that regulate many crucial aspects of embryonic development and organogenesis. Unlike other TGFbeta ligands, co-receptors for BMP ligands have not been described. Here we show that DRAGON, a glycosylphosphatidylinositol-anchored member of the repulsive guidance molecule family, which is expressed early in the developing nervous system, enhances BMP but not TGFbeta signaling. DRAGON binds directly to BMP2 and BMP4 but not to BMP7 or other TGFbeta ligands. The enhancing action of DRAGON on BMP signaling is also reduced by administration of Noggin, a soluble BMP antagonist, indicating that the action of DRAGON is ligand-dependent. DRAGON associates directly with BMP type I (ALK2, ALK3, and ALK6) and type II (ActRII and ActRIIB) receptors, and its signaling is reduced by dominant negative Smad1 and ALK3 or -6 receptors. In the Xenopus embryo, DRAGON both reduces the threshold of the ability of Smad1 to induce mesodermal and endodermal markers and alters neuronal and neural crest patterning. The direct interaction of DRAGON with BMP ligands and receptors indicates that it is a BMP co-receptor that potentiates BMP signaling.

Reconstitution and analysis of soluble inhibin and activin receptor complexes in a cell-free system

Activins and inhibins compose a heterogeneous subfamily within the transforming growth factor-beta (TGF-beta) superfamily of growth and differentiation factors with critical biological activities in embryos and adults. They signal through a heteromeric complex of type II, type I, and for inhibin, type III receptors. To characterize the affinity, specificity, and activity of these receptors (alone and in combination) for the inhibin/activin subfamily, we developed a cell-free assay system using soluble receptor-Fc fusion proteins. The soluble activin type II receptor (sActRII)-Fc fusion protein had a 7-fold higher affinity for activin A compared with sActRIIB-Fc, whereas both receptors had a marked preference for activin A over activin B. Although inhibin A and B binding was 20-fold lower compared with activin binding to either type II receptor alone, the mixture of either type II receptor with soluble TGF-beta type III receptor (TbetaRIII; betaglycan)-Fc reconstituted a soluble high affinity inhibin receptor. In contrast, mixing either soluble activin type II receptor with soluble activin type I receptors did not substantially enhance activin binding. Our results support a cooperative model of binding for the inhibin receptor (ActRII.sTbetaRIII complex) but not for activin receptors (type II + type I) and demonstrate that a complex composed of activin type II receptors and TbetaRIII is both necessary and sufficient for high affinity inhibin binding. This study also illustrates the utility of this cell-free system for investigating hypotheses of receptor complex mechanisms resulting from crystal structure analyses.

DRAGON: a member of the repulsive guidance molecule-related family of neuronal- and muscle-expressed membrane proteins is regulated by DRG11 and has neuronal adhesive properties

DRG11, a transcription factor expressed in embryonic dorsal root ganglion (DRG) and dorsal horn neurons, has a role in the development of sensory circuits. We have used a genomic binding strategy to screen for the promoter region of genes regulated by DRG11. One gene with a promoter region binding to the DNA binding domain of DRG11 encodes a novel membrane-associated [glycosyl-phosphatidylinositol (GPI)-anchored] protein that we call DRAGON. DRAGON expression is transcriptionally regulated by DRG11, and it is coexpressed with DRG11 in embryonic DRG and spinal cord. DRAGON expression in these areas is reduced in DRG11 null mutants. DRAGON is expressed, however, in the neural tube before DRG11, and unlike DRG11 it is expressed in the brain and therefore must be regulated by other transcriptional regulatory elements. DRAGON shares high sequence homology with two other GPI-anchored membrane proteins: the mouse ortholog of chick repulsive guidance molecule (mRGM), which is expressed in the mouse nervous system in areas complementary to DRAGON, and DRAGON-like muscle (DL-M), the expression of which is restricted to skeletal and cardiac muscle. A comparative genomic analysis indicates that the family of RGM-related genes--mRGM, DRAGON, and DL-M--are highly conserved among mammals, zebrafish, chick, and Caenorhabditis elegans but not Drosophila. DRAGON, RGM, and DL-M mRNA expression in the zebrafish embryo is similar to that in the mouse. Neuronal cell adhesion assays indicate that DRAGON promotes and mRGM reduces adhesion of mouse DRG neurons. We show that DRAGON interacts with itself homophilically. The dynamic expression, ordered spatial localization, and adhesive properties of the RGM-related family of membrane-associated proteins are compatible with specific roles in development.