Systematically testing human HMBS missense variants to reveal mechanism and pathogenic variation

Area of Science:

• Biochemistry
• Genetics
• Molecular Biology

Background:

• Defects in hydroxymethylbilane synthase (HMBS) are linked to Acute Intermittent Porphyria (AIP), a neurological disorder.
• A significant portion of HMBS variants are missense, often classified as variants of uncertain significance (VUS), complicating diagnosis.

Approach:

• Utilized saturation mutagenesis, high-throughput selection, and sequencing to create comprehensive variant effect maps for both HMBS isoforms.
• Achieved functional impact scores for over 84% of all possible amino-acid substitutions.
• Integrated molecular dynamics simulations to explore active site dynamics and residue roles.

Key Points:

• Variant effect maps largely align with biochemical predictions but reveal unexpected tolerance at the dimerization interface.
• Identified novel insights into residue roles influencing active site dynamics.
• Developed a powerful tool for classifying the pathogenicity of HMBS missense variants.

Conclusions:

• The generated HMBS variant effect maps provide crucial evidence to differentiate pathogenic from benign variants.
• This approach offers proactive diagnostic support for both known and previously unobserved clinical missense variants.
• Enhances diagnostic accuracy and understanding of HMBS-related porphyrias.