MC1R carries an unusually high VUS burden — 64% of its 317 amino acid positions have at least one variant of uncertain significance. CodeXome identifies 48 of these MC1R VUS (23.5%) as likely benign on the basis of primate evolutionary recurrence alone, distributed across all structural regions of the seven-pass transmembrane receptor. The Deep Time Ancestry Score extends this further, providing functional evidence for variants where gnomAD population data is sparse.
MC1R encodes the melanocortin-1 receptor, a seven-pass transmembrane G-protein-coupled receptor that binds melanocyte-stimulating hormone (MSH) and controls melanogenesis. The gene is a major determinant of human pigmentation and a well-established genetic risk factor for both melanoma and non-melanoma skin cancer. Over 30 variant alleles correlate with skin and hair color phenotypes, making MC1R one of the most variable pigmentation genes in the human population.
This same variability creates an interpretation problem. MC1R is intronless and small — a single coding region of 954 nucleotides encoding 317 amino acids — but 204 of those residues (64%) carry variants of uncertain significance in ClinVar. The gene's compact size and structural complexity make these VUS particularly difficult to resolve through traditional approaches, which limits MC1R's tractability for research into skin cancer risk and treatment response.
This study evaluates whether deep-time evolutionary recurrence across primates can identify a meaningful fraction of MC1R VUS as likely benign, and whether that resolution holds across all structural regions of the receptor — extracellular, transmembrane, and cytoplasmic. The analysis also examines whether the Deep Time Ancestry Score extends evolutionary evidence to variants where gnomAD population frequency data is sparse or absent.
Using CodeXome's Gene Profile module, every ClinVar VUS in MC1R was mapped to its orthologous site across 55 primate genera. Each VUS amino acid substitution was checked for natural recurrence in the primate alignment using the "Changes relative to human" filter. Variants found to recur in one or more primate lineages were classified as likely benign on evolutionary evidence. Variants absent from the primate record were retained for second-stage analysis using the Deep Time Ancestry Score (DTAS), CodeXome's machine learning model trained on the proprietary primate dataset.
Structural location of each variant — extracellular loop, transmembrane helix, or cytoplasmic loop — was annotated from MC1R's known seven-pass topology to test whether evolutionary resolution was confined to specific protein regions.
Finding 1 — A high VUS burden in a small gene. Of MC1R's 317 amino acid residues, 204 (64%) are associated with VUS in ClinVar. This is among the highest VUS densities of any well-characterized GPCR, reflecting both the gene's compact size and its extensive natural variation in human populations.
Finding 2 — Evolutionary recurrence resolves 23.5% of MC1R VUS. CodeXome identifies 48 MC1R VUS as likely benign based on primate evolutionary recurrence — 23.5% of the gene's total ClinVar VUS pool. This rate sits within the 10–40% per-gene range observed across the broader 14,000-gene VUS survey, confirming that MC1R behaves as a typical case rather than an outlier.
Finding 3 — Resolution is structurally independent. The 48 resolved VUS are distributed across all three structural compartments of the receptor: extracellular loops, transmembrane helices, and cytoplasmic regions. Evolutionary evidence does not preferentially resolve VUS in any single compartment, indicating that the signal is driven by residue-specific tolerance rather than region-level constraint.
Finding 4 — A representative VUS shared across 15 primate genera. A valine-to-methionine substitution at a representative MC1R position is currently a ClinVar VUS with limited gnomAD support. CodeXome shows this exact substitution recurring across 15 primate genera and persisting for over 30 million years of evolutionary time, with independent recurrence in human populations. The depth and breadth of this evolutionary signal provides strong functional evidence that the substitution is tolerated — and therefore likely benign — despite its current uncertain classification.
Finding 5 — DTAS extends evolutionary evidence beyond direct recurrence. For MC1R VUS that are not directly observed in the primate record, the Deep Time Ancestry Score provides a predictive functional signal trained on the primate dataset. A representative example at position 219 demonstrates that DTAS can produce a confident likely-benign or likely-pathogenic call in cases where gnomAD population frequency data is too sparse to support classification through standard ACMG criteria.
MC1R demonstrates how evolutionary evidence performs on a gene with a difficult interpretation profile: small, structurally complex, heavily varied in human populations, and clinically consequential. The 23.5% resolution rate is meaningful in absolute terms — 48 fewer VUS for researchers to triage — and the structural independence of that resolution suggests the underlying biology is robust. Evolutionary tolerance at a residue tracks the residue's tolerance, not the structural compartment it sits in.
The valine-to-methionine example illustrates the strength of the empirical signal. A substitution shared across 15 primate genera over 30 million years has been tested by natural selection in dozens of independent lineages. That depth of biological replication is not available from any human population dataset, regardless of cohort size, because human populations represent a single evolutionary moment. Evolutionary recurrence provides a fundamentally different kind of evidence — biological, longitudinal, and population-agnostic.
For variants without direct primate recurrence, DTAS extends the evidence layer further. The position 219 example shows that even in the absence of sufficient gnomAD coverage, evolutionary patterns from the primate dataset can support functional inference. This is particularly relevant for MC1R variants that appear primarily in non-European populations, where gnomAD coverage remains historically thinner.
MC1R serves as a worked example of the broader CodeXome approach applied to a single, well-characterized gene:
For research programs working on pigmentation, melanoma risk stratification, or population-specific skin cancer susceptibility, CodeXome reduces the noise floor in MC1R variant lists and provides a biological evidence layer that complements existing predictors and population frequency data.
Distribution of CodeXome-resolved VUS across MC1R's seven transmembrane domains, extracellular loops, and cytoplasmic regions. Resolved variants (purple) are not confined to any single structural compartment, demonstrating that evolutionary evidence operates independently of protein region or domain class.
The CodeXome platform lets you browse residue-level evolutionary evidence across 55 primate genera, live in your browser. No signup for the Gene Previewer.