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GNOme – Glycan Naming and Subsumption Ontology

Authors Nathan J. Edwards Wenjin Zhang

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GNOme – Glycan Naming and Subsumption Ontology
Wenjin Zhang 1 and Nathan J. Edwards 1
     Clinical and Translational Glycoscience Research Center, Georgetown University, Washington, D.C., USA

                                  The Glycan Naming and Subsumption Ontology (GNOme) is an OBOFoundry ontology that
                                  organizes the stable glycan accessions of GlyTouCan glycan sequence registry for reasoning
                                  and browsing by subsumption. The ontology enables the fast, intuitive, and interactive
                                  exploration of GlyTouCan’s glycan structure accessions for glycan sequences; facilitates text-
                                  based lookup for common synonyms for structures and their GlyTouCan accessions; provides
                                  a framework for automated reasoning about glycan subsumption relationships; and annotates
                                  glycans with well-defined characterization categories. As part of the OBOFoundry, GNOme
                                  can be readily used by other ontology and standards initiatives to refer to glycans at varying
                                  degrees of characterization and is currently integrated with the GlyGen glycoinformatics
                                  resource to help users find “related glycans'' and to propagate species and glycan classification
                                  annotations by subsumption.

                                  Keywords 1
                                  Glycans, ontology, subsumption

1. Introduction

    Glycan sequence formats, especially GlycoCT [1] and WURCS [2], in common use by the
glycobiology and glycoinformatics community for describing glycan molecules, explicitly specify, or
indicate the absence of knowledge about, every detail of a glycan’s structure. Experimental techniques
for characterizing glycans are often unable to fully characterize glycans, and these sequence formats
precisely record which details of a glycan structure are known or not known. However, the resulting
long, complex, and cryptic sequences cannot be readily communicated or shared. The GlyTouCan [3]
glycan (sequence) registry provides stable accessions for glycan sequences to facilitate communication
and knowledge sharing. Unfortunately, the number of accessions and their glycan sequences in
GlyTouCan have grown beyond 100,000, and it has become very difficult to find the corresponding
GlyTouCan accessions for experimentally characterized glycans since they are not organized with
respect to the degree of glycan characterization of each sequence, that is, by subsumption.
    The Glycan Naming and Subsumption Ontology (GNOme) is an OBOFoundry [4] ontology that
organizes the stable glycan sequence accessions of GlyTouCan for reasoning and display by
subsumption. The ontology enables the fast, intuitive, and interactive exploration of GlyTouCan glycan
structure accessions for experimental glycans characterized to a specific extent; facilitates text-based
lookup for common synonyms for structure and their GlyTouCan accessions; provides a framework for
automated reasoning about glycan subsumption relationships; and annotates glycans with well-defined
characterization categories. As part of the OBOFoundry, GNOme can be readily used by other ontology
and standards initiatives to refer to glycans at varying degrees of characterization, and is currently
integrated with the GlyGen glycoinformatics resource [5] help users find “related glycans” and to
propagate species and glycan classification annotations by subsumption.

International Conference on Biomedical Ontologies 2021, September 16–18, 2021, Bozen-Bolzano, Italy
EMAIL: (A. 2)
ORCID: 0000-0001-5168-3196 (A. 2)
                               © 2021 Copyright for this paper by its authors.
                               Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).

                               CEUR Workshop Proceedings (
2. Methods

    The GNOme ontology is computationally determined from the glycan sequences and accessions of
GlyTouCan. The glycans, are first grouped by molecular weight, then all structures in a group aligned
with each other, establishing their mutual subsumption relationships. Redundant subsumption
relationships, which can be established by transitivity of the subsumption partial order, are removed.
The subsumption-based characterization categories, in most characterized to least characterized order:
Saccharide, Topology, Composition, Base Composition, and Molecular Weight are derived from the
ROCS ontology [6] developed by the GlyTouCan project. Each subsumption category indicates the
partial presence or complete absence of specific information in the structure description sequence.
While the molecular weight category is not part of ROCS, it can be readily computed from the structure
description, is often the result of mass-spectrometry based glycan characterization, is invariant with
respect to our subsumption definition, and represents the grouping that drives our computational
alignment strategy. The removal of specific classes of structural information can transform a structure
to another with a subsuming characterization category, though not all such transformations will be
reflected by a registered sequence in GlyTouCan. Table 1 shows the categories and the specific
information whose absence defines membership in each category.

Table 1
GNOme subsumption categories and missing structure information. X indicates the complete absence
of the indicated structure information.

       Subsumption      Superclass    Stereochemistry     Linkage & Ring         Anomers &
         Category                                           Information     Carbon Bond Positions

     Molecular Weight        ✘                ✘                 ✘                      ✘

     Base Composition                         ✘                 ✘                      ✘

     Composition                                                ✘                      ✘

     Topology                                                                          ✘


    We automatically categorize and partition the glycans based on the presence or absence of these
specific pieces of information and determine whether the glycan sequences in a molecular weight group
are related by the removel transformations described above.
    The GNOme ontology uses GlyTouCan accessions to define its primary class terms, with the
required OBO Foundry structure (e.g. GNO_G00912UN). When a glycan’s sequence describes a glycan
composition or base composition that is consistent with a so-called composition string that succinctly
describes the number of each type of monosaccharide, we associate the composition string, in various
formats, with the GNOme term, as a synonym.
    When GlyTouCan accessions representing structure sequences that have been published as GNOme
terms in a release are subsequently archived or replaced, this is reflected in the ontology by marking it
as obsolete and indicating the replacement accession if available.
    In order to support glycoinformatics resources that capture a subset of GlyTouCan’s structures,
GNOme also releases ontology restrictions to subsets of GlyTouCan accessions, in which only the
relevant GNOme terms are retained and subsumption relationships inferred by transitivity from the
primary ontology. GNOme supports ontology restrictions for GlyGen, BSCDB [7], and GlyCosmos

3. Results and Discussion
3.1. Construction of OBO Foundry Ontology
    The GNOme ontology has been constructed as an OBO Foundry ontology with prefix GNO and
released as an OWL-format ontology with automatically generated OBO and JSON derived formats
using the robot tool [9]. In addition to GNOme classes for each supported GlyTouCan accession,
GNOme creates molecular weight class terms, to represent the molecular weight grouping of
subsumption relationships (to two decimal places), and a root Glycan class that subsumes each of the
molecular weight terms. The subclass predicate is used to represent the subsumption relationship. The
subclass relationships define a DAG, not a tree, since a given structure may have multiple immediately
subsuming structures.
    GNOme provides a variety of annotation properties that provide more information to each class term
representing a GlyTouCan accession and its sequence:
    •    has_glytoucan_id, has_glytoucan_link: GlyTouCan accession and deep linking URL to the
    corresponding GlyTouCan webpage.
    •    has_subsumption_category: GNOme URI of the subsumption category.
    •    has_basecomposition; has_composition; has_topology: GNOme URI of the structure with the
    appropriate information removed, if it exists.
    •    has_structure_browser_link, has_composition_browser_link: URL of deep link to interactive
    GNOme browser web-applications for structures and compositions, where appropriate.
    •    has_Byonic_name, hasExactSynonym: Synonyms for the GNOme term, including specific
    predicate for composition strings as formatted by the Byonic glycopeptide identification search
    engine. hasExactSynonym is defined in the oboInOwl ontology.
    In addition, GNOme uses the definition property from the Information Artifact Ontology, predicates
from the oboInOwl ontology (hasExactSynonym in particular), and typical Web Ontology Language
terms, such as label. The GNOme ontology strives to meet and abide by the OBO Foundry principles,
adopting annotation properties consistent with other OBO Foundry ontologies, and as such is well-
integrated with ontology support services such as OntoBee [10] and OLS [11], and can be readily
referenced by other OBO Foundry ontologies.
    In all, GNOme (version 1.7.2, June 16, 2021) defines 11 annotation properties, 5 named instances
(subsumption categories), and 111,696 classes. The classes are made up of 1 root glycan class, 1
subsumption category class, 13,698 molecular weight classes, and 97,996 classes representing
GlyTouCan accessions, of which 7,775 represent GlyTouCan obsoleted accessions.

3.2.    Interactive Exploration of Glycans by Subsumption

    The glycan structures represented by GlyTouCan accessions and sequences are not easily understood
as human readable labels or other text that is the basis for the widely available ontology browsing and
interrogation tools. Glycobiologists, GNOme’s primary target audience, typically use so-called
cartoons or images to communicate and describe a glycan structure. While these images do not
necessarily reveal all details of a structure’s characterization or sequence, they represent the most
accessible method for interactive navigation and browsing of glycan structures. We have constructed
built-for-purpose, visual, image-based web-applications for browsing and exploring glycan structures
by subsumption. The web-applications rely on users to visually select from available glycan topologies
and then navigate up and down the subsumption relationships by choosing visual representations of
glycans that match their conceptual glycan of interest.
    The Structure Browser web-application has two panes - the Topology Selector and Subsumption
Navigator. The Topology Selector provides buttons to select the monosaccharide composition of the
glycan of interest, displaying representative topology images in real-time as the monosaccharide
composition changes. The interactive update of topologies provides feedback to the user on the types
of glycan structures consistent with the current state of the monosaccharide composition buttons.
Selecting a topology switches focus to the Subsumption Navigator pane, where the immediate parents
(subsuming) and children (subsumed by) of the current structure are shown. A double click refocuses
the Subsumption Navigator on a structure higher or lower in the subsumption hierarchy. The
Composition Browser web-application works similarly but is focused on glycans with subsumption
category Composition or Base Composition. A pop-up menu provides the opportunity to jump to
GlyTouCan or GlyGen, place descendants or ancestor accessions on the clipboard, or switch between
Structure and Composition Browsers.
    Importantly, the Structure Browser and Composition Browser web-applications permit deep linking
by GlyTouCan accession, synonym, or composition (button state), and can be embedded in other web-
sites. Interactive users of the Browsers can use the Find or Align tools to explore subsumption near
structures identified by accession, synonym, or GlycoCT/WURCS sequence. The GNOme Structure
Browser, restricted to the GlyGen glycan set, is integrated with the GlyGen glycoscience
knowledgebase as a “Related Glycans” button on each Glycan page; and GlyCosmos integrates the
Structure Browser as one of its many glycan search strategies.

3.3.    Automated Subsumption Reasoning

    In addition to the interactive use of the GNOme Structure Browser to find related glycans, the
GlyGen project also makes use of the subsumption relationships computed and provided by GNOme
for annotation propagation. A long-standing issue with glycan annotation is the scattering of annotations
for a single conceptual glycan structure across many accessions related by subsumption. Mass-
spectrometry based inference typically annotates composition or base composition category structures,
while more detailed characterization techniques might determine the location of fucosylation on
complex monosaccharides and carbon bond positions of glycosidic linkage. In this case, propagation of
important annotations, such as the species observed with a glycan, via the subsumption relationships of
GNOme, allow information known for more well characterized structure to be associated with less well
characterized structures that subsume them. In addition to species, GlyGen also propagates glycan type
and subtype annotations via subsumption relationships.

3.4.    Use by Other Ontology Projects

   GNOme, as an OBO Foundry ontology, has been adopted by a number of other ontology projects,
including ChEBI [12] and the Protein Ontology (PRO) [13], by the HUPO Protein Standards Initiative
post-translational modification ontology (PSIMOD) [14], and the HUPO Protein Standards Initiative
effect to support glycopeptide identifications in mzIdentML [15].

4. Conclusion

   We hope GNOme helps bridge the semantic gap between glycoinformatics data-resources and the
glycobiology community they serve.

5. References

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