Phenylalanine→Serine Is the Most Pathogenic-Enriched Phenylalanine-Reference Substitution Pair in ClinVar Missense Variants: 57.4% Pathogenic Fraction (Wilson 95% CI [54.3, 60.5]) Across 958 Records — Plus Per-Target-AA Distribution Across the 6 Phenylalanine-Reference Substitution Pairs

Abstract

We analyze the per-substitution-target-amino-acid Pathogenic fraction for the 6 Phenylalanine-reference (Phe, F) 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). Stop-gain (aa.alt = X) explicitly excluded. Result: per-target-AA Pathogenic fractions span a 2.21× range from 26.0% (F → Y) to 57.4% (F → S) within Phenylalanine-reference substitutions: F→S 57.4% Wilson CI [54.3, 60.5]; F→C 57.0% [52.2, 61.7]; F→I 52.4% [46.5, 58.2]; F→V 51.5% [46.4, 56.5]; F→L 34.3% [32.4, 36.3]; F→Y 26.0% [20.5, 32.3]. The chemistry interpretation: the most Pathogenic-enriched alt AAs are serine (aromatic-to-polar-hydroxyl), cysteine (aromatic-to-thiol), isoleucine and valine (aromatic-to-branched-chain-hydrophobic). The least Pathogenic-enriched is tyrosine — the chemistry-conservative aromatic-to-aromatic substitution preserving the ring structure (Tyr is Phe with one para-hydroxyl). The next-least is leucine (aromatic-to-branched-chain-hydrophobic-acyclic; preserves bulk hydrophobicity). The F → Y conservative substitution at 26.0% Pathogenic reflects that aromatic-to-aromatic substitution is well-tolerated in most contexts (the para-hydroxyl on Tyr can substitute for the H on Phe in many positions). Phenylalanine residues participate in aromatic-aromatic stacking, hydrophobic-core packing, and π-cation interactions; substitutions disrupting the aromatic ring (F → S, C, I, V) destroy these functional roles; substitutions preserving the aromatic ring (F → Y) or hydrophobic bulk (F → L) preserve most function. For variant-prioritization pipelines: the per-target-AA chemistry within Phenylalanine spans a 2.21× range; F → S/C ~57%, F → Y ~26%.

1. Background

Phenylalanine (Phe, F) is an aromatic hydrophobic amino acid with side chain (-CH₂-C₆H₅; benzyl group). Phe is one of three aromatic amino acids (with Tyr and Trp); the three are biochemically related and often interchangeable in aromatic-stacking positions. Functional roles include:

• Aromatic-aromatic stacking interactions in protein cores and at protein-protein interfaces.
• Hydrophobic core packing in folded proteins.
• π-cation interactions with lysine and arginine side chains.
• Substrate-binding pockets that exploit the planar aromatic ring (e.g., heme-binding, NAD-binding, chromophore-binding).

This paper measures the per-target-AA Pathogenic-fraction distribution within the Phe-reference subset.

2. Method

ClinVar missense (alt ≠ X) variants from MyVariant.info / dbNSFP v4. Restrict to ref = F; group by alt AA; require ≥100 total per pair. Wilson 95% CI on the per-pair Pathogenic fraction (Wilson 1927; Brown et al. 2001).

3. Results

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

The 6 Phe-derived pairs span a 2.21× range (57.4 / 26.0) in Pathogenic fraction.

3.2 The chemistry-class ranking

Tier 1 — Most Pathogenic Phe substitutions (P-fraction > 50%):

• F → S (57.4%): Aromatic-to-polar-hydroxyl. Maximum chemistry disruption: removes aromatic ring; introduces small polar side chain.
• F → C (57.0%): Aromatic-to-thiol. Removes aromatic ring; introduces reactive sulfhydryl group that can form aberrant disulfides.
• F → I (52.4%): Aromatic-to-branched-chain-hydrophobic. Preserves hydrophobic character but disrupts aromatic-stacking and π-interactions.
• F → V (51.5%): Aromatic-to-branched-chain-hydrophobic (smaller). Same mechanism as F → I.

Tier 2 — Mid-range Phe substitution (P-fraction ~34%):

• F → L (34.3%): Aromatic-to-branched-chain-hydrophobic (Leu has one extra CH₂ vs Val/Ile). Preserves hydrophobic bulk.

Tier 3 — Most Benign Phe substitution (P-fraction < 30%):

• F → Y (26.0%): Aromatic-to-aromatic. The chemistry-conservative substitution preserving the ring structure (Tyr is Phe with one para-hydroxyl). Most chemistry-conservative F-derived substitution.

3.3 The F → Y conservative aromatic-class minimum

F → Y at 26.0% Pathogenic is the least Pathogenic Phenylalanine-reference substitution. Mechanism:

• Both Phe (-CH₂-C₆H₅) and Tyr (-CH₂-C₆H₄-OH) carry an aromatic benzene-ring side chain.
• Tyr is essentially Phe with one additional hydroxyl (para position on the ring).
• Both can participate in aromatic-aromatic stacking (Phe-Phe, Phe-Tyr, Phe-Trp).
• For most aromatic-stacking positions, F and Y are functionally interchangeable; the additional Tyr hydroxyl can also H-bond, providing additional functional capability.

The 26% Pathogenic fraction reflects the subset of Phe positions where the absence of the para-hydroxyl matters (e.g., specific binding pockets, chromophore-binding residues, oxidoreductase active-site residues).

The relatively low Benign count (154) reflects that F → Y is not as common a population variant as some other conservative pairs (e.g., I → V).

3.4 The F → S Pathogenic-enriched signal

F → S at 57.4% Pathogenic is the most Pathogenic Phenylalanine-reference substitution. Mechanism:

• Phe is typically buried in hydrophobic cores or at aromatic-stacking interfaces.
• Ser is a small polar residue with a hydroxyl side chain.
• F → S removes the aromatic ring, removes the bulk, and introduces polarity at typically-hydrophobic positions.
• The hydrophobic-core position is destabilized; aromatic-stacking interactions are abolished.

The 57.4% Pathogenic fraction reflects strong selection against this substitution.

3.5 The F → C alternative-aromatic-disruption (57.0%)

F → C is essentially identical in Pathogenic fraction to F → S (57.0% vs 57.4%; Wilson CIs overlap heavily). The mechanism is similar: aromatic ring removed, replaced with a smaller polar/reactive side chain (Cys -SH).

3.6 The F → L midrange (34.3%)

F → L at 34.3% Pathogenic is intermediate. Mechanism: Leu preserves the hydrophobic bulk but lacks the aromatic ring. For positions where hydrophobic packing is the dominant role, F → L is tolerable; for positions where aromatic-stacking is essential, F → L is disruptive.

The 34.3% reflects this mixed mechanism: ~1/3 of Phe positions in ClinVar Pathogenic genes are aromatic-stacking-dependent, ~2/3 are hydrophobic-packing-only.

4. Confound analysis

4.1 Stop-gain explicitly excluded

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

4.2 ClinVar curatorial bias

Phe Pathogenic variants are over-reported in disease genes with critical aromatic-stacking or hydrophobic-core Phe residues (membrane proteins, nuclear receptors, kinases with aromatic substrate-binding pockets, chromophore-binding rhodopsin-family GPCRs).

4.3 Codon-mutability not normalized

Phe has 2 codons (TTT, TTC). The per-target-AA mutational rates differ across the 6 alt AAs reported. F → L (TTY → CTY / TTR), F → Y (TTY → TAY), F → C (TTY → TGY), F → S (TTY → TCY), F → I (TTY → ATY), F → V (TTY → GTY) 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. Phe-derived substitutions with < 100 records (F → A, F → G, F → T, F → N, F → Q, F → K, F → R, F → H, F → D, F → E, F → M, F → W, F → P) 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. Among 6 Phe-derived substitution pairs, F → S is the most Pathogenic-enriched at 57.4% (Wilson CI [54.3, 60.5]) — driven by aromatic-ring removal + polarity introduction.
2. F → Y is the least Pathogenic-enriched at 26.0% [20.5, 32.3] — a conservative aromatic-to-aromatic substitution.
3. F → C at 57.0% is nearly tied with F → S — similar aromatic-ring-removal mechanism.
4. For variant-prioritization pipelines: per-target-AA priors within Phe should be applied; F → S/C ~57%, F → Y ~26%.
5. The F-derived substitutions split into aromatic-disrupting (P-fraction 51–57%) vs aromatic-preserving (F → Y at 26%) vs hydrophobic-bulk-preserving (F → L at 34%) — three chemistry tiers.

6. Limitations

1. Stop-gain excluded (§4.1).
2. ClinVar curatorial bias (§4.2) toward aromatic-stacking 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 6 reported pairs have N ≥ 100; (d) F→S P-fraction > 0.5; (e) F→Y P-fraction < 0.30; (f) sample sizes match input file contents.

node analyze.js
node analyze.js --verify

8. References

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