Environment-dependent landscapes of coding variant impacts on coproporphyrinogen oxidase

Technical Summary: Environment-dependent landscapes of coding variant impacts on coproporphyrinogen oxidase

Problem Statement
Hereditary coproporphyria (HCP) is an acute hepatic porphyria caused by variants in the CPOX gene, which encodes coproporphyrinogen oxidase (CPOX). While genome sequencing can identify CPOX variants, 74% of clinically reported missense variants are classified as Variants of Uncertain Significance (VUS) due to a lack of functional evidence. A critical complication in interpreting these variants is environmental dependence; for instance, the p.Asn272His variant is classified as benign but triggers HCP-like symptoms (atypical porphyrinogenic response, or APR) specifically in the presence of mercury (Hg²⁺). Existing functional assays are often resource-intensive and unavailable for the vast majority of clinical variants, leaving a gap in understanding how specific amino acid substitutions affect protein function under both baseline and stress conditions.

Methodology
The authors employed a Multiplexed Assay of Variant Effect (MAVE) strategy using a humanized yeast model to systematically measure the functional impact of nearly all possible CPOX amino acid substitutions.

• Model System: A temperature-sensitive yeast strain (Saccharomyces cerevisiae TSA1076) carrying a mutation in the essential HEM13 ortholog was used. Human CPOX cDNA expression was required to rescue growth at the non-permissive temperature.
• Library Construction: Three mutagenic libraries were generated using a modified Precision Oligo-Pool Based Code Alteration (POPCode) approach, targeting different thirds of the CPOX coding sequence. These libraries encompassed nearly all single-nucleotide variants (SNVs) resulting in missense substitutions.
• Experimental Conditions: The libraries were transformed into the yeast strain and grown competitively under two conditions: (1) "baseline" (no mercury) and (2) in the presence of 96 µM mercury.
• Scoring: Functional impact scores were derived from the log-ratio of variant frequencies before and after selection. Scores were rescaled such that 0 represented nonsense variants (loss of function) and 1 represented synonymous variants (neutral).
• Computational Analysis: The authors integrated the experimental data with structural biology and molecular dynamics (MD) simulations. They utilized AlphaFold3 (AF) models and crystal structures (PDB: 2AEX) to analyze secondary structure, solvent accessibility, and global motions. MD simulations (NAMD) and docking studies (AutoDock Vina) were performed on wild-type (WT) and selected mercury-sensitive variants (p.Asn272His, p.Val135Ala, p.Leu155Trp, p.Gly188Gln) to investigate substrate binding preferences (coproporphyrinogen III vs. uroporphyrinogen III) and allosteric effects.

Key Results

• Variant Effect Maps: The study generated high-quality functional maps for >75% of all possible missense variants in both baseline and mercury contexts. The maps recapitulated known sequence-structure-function relationships, such as the intolerance of buried residues and residues at the dimer interface.
• Mercury-Dependent Effects: Mercury exposure reduced the function of the majority (71%) of variants. However, a subset of variants (43% of those tolerated in baseline) showed significantly reduced function specifically under mercury conditions (negative "delta" scores). This subset included the known mercury-sensitive variant p.Asn272His and a newly identified candidate, p.Val135Ala.
• Structural Insights: MD simulations revealed that mercury-sensitive variants (p.Leu155Trp, p.Gly188Gln) act as hinge residues that allosterically alter the conformation of the active site loop and lid. Specifically, these variants induced local unfolding of β-sheet residues near the active site and altered the dynamics of the active site loop.
• Substrate Preference: Docking simulations indicated that while WT CPOX preferentially binds its native substrate (COPRO), the mercury-sensitive variants exhibited an increased preference for uroporphyrinogen III (URO). This shift in substrate preference provides a mechanistic explanation for the accumulation of upstream porphyrins observed in APR.
• Clinical Utility: The baseline map scores effectively distinguished pathogenic from benign variants, outperforming random predictions and performing comparably to top computational predictors (AlphaMissense, VARITY). The maps provided new functional evidence for 73% of CPOX VUSs reported in ClinVar, allowing for reclassification toward pathogenicity or benignity.

Significance and Claims
The authors claim that this study provides the first systematic, environment-dependent atlas of CPOX variant effects. By measuring nearly all possible substitutions, the work offers a comprehensive resource for resolving the "VUS problem" in HCP diagnosis. The paper highlights that variant impacts are not static but can be context-dependent, specifically demonstrating how mercury exposure unmasks pathogenicity in variants that appear benign under standard conditions.

The study illustrates the value of systematic, context-dependent multiplexed assays in bridging the gap between genomic data and clinical understanding. The resulting data allows for:

1. Improved classification of CPOX variants for cascade screening and diagnosis.
2. Identification of new residues critical for mercury-induced APR, expanding the understanding of genotype-phenotype correlations beyond the known p.Asn272His variant.
3. Structural and mechanistic insights into how specific substitutions alter enzyme kinetics and substrate handling via allosteric communication.

The authors note limitations, including the use of a yeast model which may not fully capture human mitochondrial targeting sequence (MTS) defects or splicing impacts, and the inherent noise in high-throughput assays. However, they assert that the empirical calibration of their maps offers immediate clinical value for reclassifying VUSs and guiding vigilance for HCP symptoms in individuals carrying specific variants.