Leucine→Proline Is a Particularly Pathogenic-Enriched Substitution Pair in ClinVar Missense Variants: 66.2% Pathogenic Fraction (Wilson 95% CI [64.7, 67.7]) Across 3,909 Records — A Hydrophobic-Helix-to-Proline-Helix-Disruptor Pair Affecting α-Helical Geometry in Folded Domains

Abstract

We analyze the Leucine → Proline (L → P) substitution pair in ClinVar missense single-nucleotide variants — one of the largest single-pair Pathogenic-fraction effects we observe 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). Result: across 3,909 L → P missense records (2,589 Pathogenic + 1,320 Benign), the per-pair Pathogenic fraction is 66.2% (Wilson 95% CI [64.7, 67.7]) — substantially above the corpus-baseline ~28% Pathogenic fraction. Mechanism: Leucine is the most-frequent amino acid in α-helical regions of human proteins (~14% of helix residues; Pace & Scholtz 1998), while Proline is a known α-helix breaker (MacArthur & Thornton 1991) due to the φ-angle constraint imposed by its cyclic side chain. The L → P substitution therefore disrupts α-helix geometry at typically helix-forming Leu positions, with high pathogenic consequence. We provide the full per-target-AA distribution for Leucine-reference substitutions for context: L→P 66.2% [64.7, 67.7]; L→R 65.8% [63.1, 68.4]; L→Q 56.6% [51.8, 61.3]; L→H 53.7% [47.8, 59.6]; L→W 52.5% [45.2, 59.6]; L→S 36.9% [33.5, 40.5]; L→F 24.4% [22.8, 26.1]; L→V 20.1% [18.5, 21.8]; L→M 15.6% [12.6, 19.2]; L→I 12.1% [9.6, 15.0]. The 5.5× per-target-AA range (66.2 / 12.1) within Leucine-reference substitutions reflects the broad chemistry-class spread among Leu's substitution-accessible neighbors. The Pathogenic-skew of L → P (and similarly L → R at 65.8% — charge introduction at hydrophobic core position) defines the high-Pathogenic regime. The Benign-skew of L → I (12.1%) and L → V (20.1%) defines the low-Pathogenic regime — both branched-chain hydrophobic conservative substitutions. For variant-prioritization pipelines: an observed L → P substitution carries a 66% Pathogenic prior, vs L → I at only 12% — a 5.5× per-prior difference within the same reference AA.

1. Background

Leucine (Leu, L) is a hydrophobic branched-chain amino acid with side chain (-CH₂-CH(CH₃)-CH₃). Leu has 6 codons (the most of any amino acid: TTA, TTG, CTT, CTC, CTA, CTG), reflecting its high abundance (~10% of human proteome residues). Functional roles:

• α-helix-forming preference: Leu is the most-frequent residue in α-helices (~14% of helix residues; Pace & Scholtz 1998).
• Hydrophobic core packing: Leu is buried in protein cores at high frequency.
• Membrane-helix anchoring: Leu is enriched in transmembrane α-helices.
• Leucine-zipper coiled-coil motif: heptad-repeat Leu residues at "d" positions in coiled coils.

Proline (Pro, P) is the only proteogenic amino acid with a cyclic side chain (the side chain ring back-bonds to the α-N). Pro has unique φ-angle restrictions that make it an α-helix breaker (MacArthur & Thornton 1991): Pro at internal helix positions destabilizes the helix.

The L → P substitution at typically helix-forming Leu positions therefore introduces a maximally-disruptive residue. This paper measures the per-pair Pathogenic fraction of L → P across ClinVar, with Wilson 95% confidence intervals (Wilson 1927).

2. Method

ClinVar missense (alt ≠ X) variants from MyVariant.info / dbNSFP v4. We focus on ref = L; group by alt AA; require ≥100 total per pair for stable per-pair Pathogenic-fraction estimates with Wilson 95% CI (Wilson 1927; Brown et al. 2001).

3. Results

3.1 The L → P headline finding

The L → P pair has 3,909 total records — among the largest single-pair samples in our cache. The Pathogenic-fraction Wilson 95% CI is tight ([64.7, 67.7]) and substantially above the corpus-baseline ~28% Pathogenic.

3.2 Full per-target-AA Pathogenic fraction (sorted descending)

The 10 Leu-derived pairs span a 5.5× range (66.2 / 12.1) in Pathogenic fraction.

3.3 The L → P proline-helix-breaker mechanism

L → P at 66.2% Pathogenic is the most Pathogenic Leucine-reference substitution. Mechanism:

1. Leucine is α-helix preferred: Leu has the highest helical-propensity P_α index of any amino acid (Pace & Scholtz 1998). Most Leu residues in folded human proteins are in α-helices.
2. Proline is α-helix-breaker: Pro's pyrrolidine ring fixes the φ angle to ~−65°, incompatible with the canonical α-helix geometry (φ ≈ −57°). Pro at internal helix positions destabilizes the helix.
3. L → P substitutions therefore disrupt α-helix geometry at typically helix-forming positions, with high pathogenic consequence.

The 3,909 records is among the largest single-pair samples; the 66.2% Pathogenic fraction with tight CI [64.7, 67.7] is robust.

3.4 The L → R Pathogenic-enrichment (charge in hydrophobic core)

L → R at 65.8% Pathogenic is nearly identical to L → P. Mechanism: Arg introduces a positive charge at typically-buried hydrophobic Leu positions, requiring desolvation of the charged side chain in a hydrophobic environment — energetically unfavorable. The ~66% Pathogenic fraction reflects this maximum-electrostatic disruption.

3.5 The L → I conservative-class minimum (12.1%)

L → I at 12.1% Pathogenic is the most Benign-skewed Leucine-reference substitution. Mechanism:

• Both Leu (-CH₂-CH(CH₃)-CH₃) and Ile (-CH(CH₃)-CH₂-CH₃) are branched-chain hydrophobic amino acids.
• Both share the same chemical formula (C₆H₁₃NO₂); they are structural isomers differing only in side-chain branching geometry.
• Both prefer α-helical or β-strand secondary structure.
• For most hydrophobic-core-packing positions, L and I are functionally interchangeable.

The high Benign count (503 vs 69 Pathogenic) reflects population-genome variation: L → I is a common population variant.

3.6 The L → V near-conservative substitution (20.1%)

L → V at 20.1% Pathogenic is the second-least-Pathogenic Leu substitution. Val is a smaller branched-chain hydrophobic residue. The 20.1% Pathogenic fraction reflects the subset of Leu positions where the precise side-chain volume matters.

3.7 The chemistry-class continuum

The Leu-derived Pathogenic fractions cluster into 3 tiers:

• Tier 1 — Severely Pathogenic (P-fraction > 50%): L → P (helix-breaker), L → R/Q/H (charge or polar in core), L → W (large aromatic).
• Tier 2 — Mid-range (P-fraction 20–37%): L → S (hydroxyl), L → F (aromatic), L → V (smaller branched).
• Tier 3 — Conservative (P-fraction 12–16%): L → M (sulfur-containing hydrophobic), L → I (isomer).

4. Confound analysis

4.1 Stop-gain explicitly excluded

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

4.2 ClinVar curatorial bias

Leu Pathogenic variants are over-reported in disease genes with critical α-helical Leu residues (membrane channels, transcription factors with leucine-zipper domains, structural-protein helical bundles). The L → P 66.2% Pathogenic fraction partly reflects curation focus on these gene families.

4.3 Codon-mutability not normalized

Leu has 6 codons. The per-target-AA mutational rates differ across alt AAs. L → P (CTN → CCN), L → R (CTN → CGN, plus AGR), L → I (CTN → ATN, plus ATA), L → V (CTN → GTN), L → M (CTG → ATG), L → S (TTR/CTN → TCN/AGY), L → F (TTR → TTY), L → Q (CTN → CAR), L → H (CTN → CAY), L → W (TTG → TGG) are accessible by single transitions or transversions.

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. Leu-derived substitutions with < 100 records (L → A, L → G, L → T, L → N, L → K, L → C, L → Y, L → D, L → E) are not analyzed.

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. L → P is among the most Pathogenic single substitution pairs in ClinVar at 66.2% (Wilson CI [64.7, 67.7]) — driven by proline's α-helix-breaking property at typically-helical Leu positions.
2. L → R at 65.8% is nearly identical — driven by charge introduction at hydrophobic-core Leu positions.
3. L → I at 12.1% is the most Benign Leucine substitution — branched-chain isomer chemistry-conservative.
4. The 5.5× per-target-AA range within Leucine spans from helix-disrupting (P, R) to chemistry-conservative (I).
5. For variant-prioritization pipelines: per-target-AA priors within Leu should be applied; L → P/R ~66%, L → I ~12%.

6. Limitations

1. Stop-gain excluded (§4.1).
2. ClinVar curatorial bias (§4.2) toward α-helical disease-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.
• Verification mode: 6 machine-checkable assertions: (a) all P-fractions in [0, 1]; (b) Wilson CIs contain the point estimate; (c) all 10 reported pairs have N ≥ 100; (d) L→P P-fraction > 0.6; (e) L→I P-fraction < 0.15; (f) sample sizes match input file contents.

node analyze.js
node analyze.js --verify

8. References

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9. Cooper, D. N., & Krawczak, M. (1990). The mutational spectrum of single base-pair substitutions causing human genetic disease. Hum. Genet. 85, 55–74.
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