VRS contributors confronted numerous trade-offs in developing this specification. As these trade-offs may not be apparent to outside readers, this section highlights the most significant ones and the rationale for our design decisions, including:
Variation Rather than Variant¶
The abstract Variation class is intentionally not labeled “Variant”, despite this being the primary term used in other molecular variation exchange formats (e.g. Variant Call Format, HGVS Sequence Variant Nomenclature). This is because the term “Variant” as used in the Genetics community is intended to describe discrete changes in nucleotide / amino acid sequence. “Variation”, in contrast, captures other classes of molecular variation, including epigenetic alteration and transcript abundance. Capturing these other classes of variation is a future goal of VRS, as there are many annotations that will require these variation classes as the subject.
Allele Rather than Variant¶
The most primitive sequence assertion in VRS is the Allele entity. Colloquially, the words “allele” and “variant” have similar meanings and they are often used interchangeably. However, the VR contributors assert that it is essential to distinguish between the state of a reference sequence from the change from a reference sequence. It is imperative that precise terms are used when modelling data. Therefore, within VRS, “allele” refers to a state of a reference sequence and “variant” refers to a change from a reference sequence.
The word “variant”, which implies change, makes it awkward to refer to the (unchanged) reference allele. Some systems will use an HGVS-like syntax (e.g., NC_000019.10:g.44906586G>G or NC_000019.10:g.44906586=) when referring to an unchanged residue. In some cases, such “variants” are even associated with allele frequencies. Similarly, a predicted consequence is better associated with an allele than with a variant.
Alleles are Fully Justified¶
In order to standardize the representation of sequence variation, Alleles SHOULD be fully justified from the description of the NCBI Variant Overprecision Correction Algorithm (VOCA). Furthermore, normalization rules are identical for all sequence types (DNA, RNA, and protein).
The choice of algorithm was relatively straightforward: VOCA is published, easily understood, easily implemented, and covers a wide range of cases.
The choice to fully justify is a departure from other common variation formats. The HGVS nomenclature recommendations, originally published in 1998, require that alleles be right normalized (3’ rule) on all sequence types. The Variant Call Format (VCF), released as a PDF specification in 2009, made the conflicting choice to write variants left (5’) normalized and anchored to the previous nucleotide.
Fully-justified alleles represent an alternate approach. A fully-justified representation does not make an arbitrary choice of where a variant truly occurs in a low-complexity region, but rather describes the final and unambiguous state of the resultant sequence.
Implementations should normalize Alleles¶
Allele Normalization is the process of comparing a span of reference sequence to a sequence state (often the alternative sequence) and resolving that span to an unambiguous form. The fully-justified Allele normalization in VRS consists of two steps: trimming and shuffling. In the trimming step, common flanking prefix and suffix sequences are removed. For example, a CAG-to-CTG Allele would be trimmed to merely A-to-T, with the position adjusted accordingly. There are four cases of the resulting sequences:
The trimmed sequences are empty: The Allele refers to reference state.
The trimmed sequences are non-empty: The Allele is a substitution (perhaps multi-residue).
The reference sequence is empty: The Allele is a net insertion.
The state sequence is empty: The Allele is a net deletion.
When the Allele refers to a reference state (case 1), trimming would reduce the variant to a null change. However, reduction to a null state would make it impossible to refer to a specific span of reference sequence. In order to permit users to refer to spans of reference sequence, VRS does not require normalizing reference agreement Alleles.
The trimming step applies only when the reference or the state sequences are empty (cases 3 and 4). When these occur in the context of repeating reference sequence that matches the inserted or deleted sequence, the Allele may be shuffled left and right to identify the fully-justified location of the variation. (See Normalization for details.)
In rare cases, data originators might have reason to associate an annotation with a specific repeating unit in the context of repeated sequence. In order to support this case, normalization is not strictly required.
Most users will normalize most Alleles. Normalization should be skipped only when doing so would decrease the intended precision of an Allele.
Sequence ranges use an inter-residue coordinate system. Inter-residue coordinate conventions are used in this terminology because they provide conceptual consistency that is not possible with residue-based systems.
The choice of what to count — residue or inter-residue positions — has significant semantic implications for the interpretation of coordinates. Although inter-residue coordinates and the “0-based” residue coordinates are often numerically identical, we favor “inter-residue” to emphasize the meaning of these coordinates.
When humans refer to a range of residues within a sequence, the most common convention is to use an interval of ordinal residue positions in the sequence. While natural for humans, this convention has several shortcomings when dealing with sequence variation.
For example, interval coordinates are interpreted as exclusive coordinates for insertions, but as inclusive coordinates for substitutions and deletions; in effect, the interpretation of coordinates depends on the variant type, which is an unfortunate coupling of distinct concepts.
The VRS collaborators investigated numerous options for modelling data, generating code, and writing the wire protocol. Required and desired selection criteria included:
language-neutral – or at least C/C++, java, python
high-quality code generation
- supported constructs and data types
lists, maps, and maps of lists/maps
protocol versioning (but not necessarily automatic adaptation)
Initial versions of the VRS logical model were implemented in UML, protobuf, and swagger/OpenAPI, and JSON Schema. We have implemented our schema in JSON Schema. Nonetheless, it is anticipated that some adopters of the VRS logical model may implement the specification in other protocols.
There are many packages and proposals that aspire to a canonical form for json in many languages. Despite this, there are no ratified or de facto winners. Many packages have similar names, which makes it difficult to discern whether they are related or not (often not). Although some packages look like good single-language candidates, none are ready for multi-language use. Many seem abandoned. The need for a canonical json form is evident, and there was at least one proposal for an ECMA standard.
Not using External Chromosome Declarations¶
In ChromosomeLocation, the tuple <species,chromosome name> refers an archetypal chromosome for the species. WikiData and MeSH provide such definitions (e.g., Human Chr 1 at WikiData and MeSH) and were considered, and rejected, for use in VRS. Both ontologies were anticipated to increase complexity that was not justified by the benefit to VRS. In addition, data in WikiData are crowd-sourced and therefore potentially unstable, and the species coverage in MeSH was insufficient for anticipated VRS uses.