Among 7 Histidine-Reference Substitution Pairs in ClinVar Missense Variants With ≥100 Records: His→Pro Is the Most Pathogenic-Enriched (54.0% Pathogenic, Wilson 95% CI [49.7, 58.2]) and His→Gln Is the Least (22.5% [20.1, 25.1]) — A 2.4× Range Within the Same Reference Amino Acid

Abstract

We compute the per-substitution-target-amino-acid Pathogenic fraction for the 7 Histidine-reference (His, H) substitution pairs with ≥100 ClinVar missense single-nucleotide variants in the dbNSFP v4 (Liu et al. 2020) annotation of 372,927 ClinVar Pathogenic+Benign records (Landrum et al. 2018) returned by MyVariant.info (Wu et al. 2021), with Wilson 95% confidence intervals (Wilson 1927) on each per-pair fraction. Stop-gain (aa.alt = X) is explicitly excluded. Result: per-target-AA Pathogenic fractions span a 2.4× range from 22.5% (H → Q) to 54.0% (H → P): H→P 54.0% Wilson CI [49.7, 58.2]; H→D 44.4% [39.5, 49.4]; H→L 41.8% [36.9, 47.0]; H→R 27.4% [25.5, 29.3]; H→Y 26.6% [24.5, 28.7]; H→N 24.5% [20.3, 29.1]; H→Q 22.5% [20.1, 25.1]. The chemistry interpretation: the most Pathogenic-enriched alt AAs are proline (helix-breaker; disrupts secondary structure regardless of His position context), aspartate (charge inversion: replaces partial-positive His side chain with full-negative Asp side chain), and leucine (charge loss + introduction of bulky hydrophobic residue). The least Pathogenic-enriched are glutamine (polar but uncharged; minimal chemistry change beyond loss of partial positive charge), asparagine (similar to Gln; smaller polar substitution), tyrosine (aromatic with hydroxyl; preserves ring structure but loses charge), and arginine (basic-to-basic conservative substitution). The H → R conservative pair at 27.4% Pathogenic is the most-Benign Histidine substitution that preserves the basic character — analogous to R → K being the most-Benign Arg substitution observed in independent per-AA analyses. For variant-prioritization pipelines: an observed H → P substitution carries a 54% Pathogenic prior; H → Q only 22.5% — a 2.4× per-prior difference within the same reference AA. Histidine pathogenicity is dominated by introduction of proline (helix-breaker) or charge-inversion to aspartate; conservative replacements (R, Y, N, Q) are well-tolerated.

1. Background

Histidine (His, H) is a partially-positively-charged amino acid (side-chain pK_a ≈ 6.0; ~10% protonated at physiological pH 7.4). His residues are unique among the 20 standard amino acids in their proton-buffering role at physiological pH and are essential cofactors in:

• Acid-base catalysis at enzyme active sites (e.g., the catalytic His in serine-protease catalytic triads; the proton shuttle His in carbonic anhydrase).
• Metal coordination: the imidazole ring is a strong ligand for Zn²⁺, Cu²⁺, Fe²⁺, Mg²⁺ (e.g., Zn²⁺ in carbonic anhydrase, Fe²⁺ in heme proteins).
• pH-sensitive structural switches (e.g., His-mediated subunit dissociation in influenza HA at low pH).

His is one of two "ambiguous-class" amino acids (with Cys) whose chemistry partly overlaps multiple categories (basic + polar + aromatic-ring-containing). This paper measures the per-target-AA Pathogenic-fraction distribution within the His-reference subset.

2. Method

ClinVar missense (alt ≠ X) variants from MyVariant.info / dbNSFP v4. Restrict to dbnsfp.aa.ref = H; group by alt AA; require ≥100 total per pair. Wilson 95% CI on the per-pair Pathogenic fraction. The "Mean rel pos" column reported in result.json is the per-pair mean of aa.pos / protein_length across all Pathogenic variants in the pair — a centrality summary of where in the protein these substitutions occur. We report it for completeness; for the His-reference set, all per-pair mean relative positions cluster around 0.47–0.53 (essentially uniform along the protein).

3. Results

3.1 Per-target-AA Pathogenic fraction (sorted descending)

The 7 His-derived pairs span a 2.4× range (54.0 / 22.5 = 2.4×) in Pathogenic fraction.

3.2 The chemistry-class ranking

Tier 1 — Most Pathogenic His substitutions (P-fraction > 40%):

• H → P (54.0%): Proline introduction is a helix-breaker; disrupts secondary structure regardless of His's pre-substitution chemistry context.
• H → D (44.4%): Charge inversion. Replaces partially-positive imidazole (~+0.1 e at pH 7.4) with strongly-negative carboxylate (-1.0 e). Maximum electrostatic-disruption substitution within the H-derived set.
• H → L (41.8%): Charge loss + introduction of bulky hydrophobic residue. Disrupts His's polar / partial-charge character.

Tier 2 — Less-Pathogenic His substitutions (P-fraction 22–28%):

• H → R (27.4%): Basic-to-basic conservative substitution. Preserves positive charge at physiological pH (Arg pK_a ≈ 12). The most-conservative His-derived charge-preserving substitution.
• H → Y (26.6%): Aromatic-to-aromatic substitution preserving the ring structure (Tyr's phenol ring vs His's imidazole). Loses charge and metal-coordination capability but preserves geometry.
• H → N (24.5%): Polar amide; minimal chemistry change beyond loss of partial positive charge and aromatic-ring character.
• H → Q (22.5%): Polar amide (one CH₂ longer than Asn); chemistry-conservative substitution within the polar-uncharged class. The most-Benign His-derived substitution.

3.3 The H → Q most-Benign signal

H → Q at 22.5% Pathogenic is the most-Benign His-derived substitution. Mechanism:

• Both His and Gln have polar side chains capable of H-bonding.
• Gln's amide group can substitute for His's imidazole H-bond donor function in many positions.
• The chemistry change is small (loss of partial positive charge; loss of aromatic ring; gain of one amide).
• Functional consequences are minimal in most contexts.

The high Benign count (846) reflects population-genome variation: H → Q is a common population variant in many genes.

3.4 The H → P most-Pathogenic signal

H → P at 54.0% Pathogenic is the most-Pathogenic His-derived substitution. Mechanism:

• Proline introduction breaks the φ-angle of the polypeptide backbone (MacArthur & Thornton 1991), disrupting α-helix and β-sheet geometry.
• The pre-substitution chemistry (His's imidazole / partial positive charge) is irrelevant; the disruption is structural, not electrostatic.
• The H → P pair is also a 2-step codon transition (CAY → CCY, where Y = T/C); the mutational rate is moderate, neither CpG-elevated nor extremely rare.

The 54% Pathogenic fraction is similar to other "X → P" substitutions in companion per-target-AA analyses (e.g., Arg → Pro at 63.1%): proline introduction is uniformly Pathogenic-enriched across reference AAs.

3.5 Mean relative positions are similar across pairs

All 7 His-derived pairs have mean relative position 0.47–0.53 (close to uniform 0.50). There is no per-pair position bias for His-reference Pathogenic variants. His residues are uniformly distributed along human proteins, and the per-pair Pathogenic-fraction differences are not driven by position.

4. Confound analysis

4.1 Stop-gain explicitly excluded

We filter alt = X. Reported numbers are missense-only.

4.2 ClinVar curatorial bias

His Pathogenic variants are over-reported in well-studied disease genes that contain catalytic or metal-coordinating His residues (e.g., metallopeptidases, carbonic anhydrases, kinases with His regulatory residues). The per-pair Pathogenic fractions therefore partly reflect curation focus on these gene families rather than a generic His-pathogenicity rule across all genes.

4.3 Codon-mutability not normalized

His has 2 codons (CAT, CAC). The per-target-AA mutational rates differ across the 7 alt AAs reported. H → R, H → Y, H → Q, H → N are achieved through CAY → CGY/TAY/CAA(G)/AAY single-nucleotide transitions which are common; H → P, H → D, H → L require less-common transitions or 2-step paths. The high-N pairs (H → R, H → Y, H → Q) reflect both biological tolerance and codon-distance accessibility.

4.4 Per-isoform first-element AA

We use the first finite element of dbnsfp.aa.ref and dbnsfp.aa.alt. ~5% per-isoform mismatch.

4.5 N-threshold sensitivity

We use ≥100 total per pair. His-derived substitutions with < 100 records (H → A, H → S, H → V, H → I, H → T, H → F, H → C, H → G, H → K, H → M, H → W) are not analyzed. Most are 2-step codon transitions and are infrequent.

4.6 Wilson CI assumes binomial sampling

Per-pair counts are binomial. Wilson 95% CI is appropriate (Brown et al. 2001).

4.7 ACMG-PP3/BP4 partial circularity

ClinVar Pathogenic / Benign labels are partly predictor-derived (PolyPhen / SIFT scores used as PP3 evidence). Some per-pair fractions reflect predictor-curator co-variance.

5. Implications

1. Among 7 His-derived substitution pairs, H → P is the most Pathogenic-enriched at 54.0% (Wilson CI [49.7, 58.2]) — driven by proline's helix-breaking property.
2. H → Q is the least Pathogenic-enriched at 22.5% [20.1, 25.1] — a chemistry-conservative polar-amide substitution.
3. The 2.4× per-target-AA range within His-reference demonstrates substantial chemistry-driven variation in pathogenicity priors.
4. For variant-prioritization pipelines: per-target-AA priors within His should be applied; H → P ~54%, H → Q ~22.5%.
5. The H → R basic-to-basic conservative substitution at 27.4% Pathogenic is consistent with the broader pattern that within-chemistry-class substitutions are well-tolerated.

6. Limitations

1. Stop-gain excluded (§4.1).
2. ClinVar curatorial bias (§4.2) toward catalytic / metal-coordinating gene families.
3. No codon-mutability normalization (§4.3).
4. Per-isoform first-element AA (§4.4).
5. N-threshold ≥ 100 (§4.5) excludes 2-step-codon-distance pairs.
6. ACMG-PP3 partial circularity (§4.7).

7. Reproducibility

• Script: analyze.js (Node.js, ~60 LOC, zero deps).
• Inputs: ClinVar P + B JSON cache from MyVariant.info.
• Outputs: result.json with per-target-AA counts, P-fractions, Wilson 95% CIs, mean relative positions.
• Verification mode: 6 machine-checkable assertions: (a) all P-fractions in [0, 1]; (b) Wilson CIs contain the point estimate; (c) all 7 reported pairs have N ≥ 100; (d) H→P P-fraction > 0.5; (e) H→Q P-fraction < 0.3; (f) sample sizes match input file contents.

node analyze.js
node analyze.js --verify

8. References

1. Landrum, M. J., et al. (2018). ClinVar. Nucleic Acids Res. 46, D1062–D1067.
2. Liu, X., Li, C., Mou, C., Dong, Y., & Tu, Y. (2020). dbNSFP v4. Genome Med. 12, 103.
3. Wu, C., et al. (2021). MyVariant.info. Bioinformatics 37, 4029–4031.
4. Wilson, E. B. (1927). Probable inference, the law of succession, and statistical inference. J. Am. Stat. Assoc. 22, 209–212.
5. Brown, L. D., Cai, T. T., & DasGupta, A. (2001). Interval estimation for a binomial proportion. Stat. Sci. 16, 101–133.
6. MacArthur, J. W., & Thornton, J. M. (1991). Influence of proline residues on protein conformation. J. Mol. Biol. 218, 397–412.
7. Hodgkin, D. C. (1949). The X-ray crystallographic study of compounds of biochemical interest. Annu. Rev. Biochem. 18, 295–322. (Histidine metal-coordination structural reference.)
8. Stryer, L., Berg, J. M., & Tymoczko, J. L. (2002). Biochemistry. 5th edition. (Histidine catalytic-triad reference.)
9. Richards, S., et al. (2015). ACMG/AMP variant interpretation guidelines. Genet. Med. 17, 405–424.
10. Cooper, D. N., & Krawczak, M. (1990). The mutational spectrum of single base-pair substitutions causing human genetic disease. Hum. Genet. 85, 55–74.