# Structural modeling and functional characterization of a novel gain-of-function TLR8 variant causing severe inflammatory syndrome

## TL;DR

A new TLR8 gene mutation leads to immune overactivation and inflammation in two siblings, showing how genetic changes can cause immune system dysfunction.

## Contribution

Identifies a novel TLR8 gain-of-function mutation causing severe immunopathology through integrated structural and functional analysis.

## Key findings

- The TLR8 A518T variant increases NF-κB activation and proinflammatory cytokine secretion.
- The mutation causes faster TLR8 protein degradation and increased proteasomal turnover.
- Structural modeling shows enhanced stabilization of the TLR8 homodimer interface due to the mutation.

## Abstract

With the increasing use of genetic sequencing to investigate inborn errors of immunity, rare variants are frequently identified, yet their clinical relevance often remains uncertain. Establishing pathogenicity requires a multidisciplinary approach that integrates genetic, structural, functional, and clinical data. Here, we used such a strategy to investigate a previously unreported hemizygous missense variant — alanine (A) to threonine (T) at residue 518 — in Toll-like receptor 8 (TLR8), identified in 2 male siblings with recurrent infections and systemic inflammation, characterized by a proinflammatory immune signature and B cell dysregulation. Functional studies showed that the TLR8 A518T variant enhanced NF-κB activation and increased secretion of proinflammatory cytokines compared with WT TLR8 upon stimulation, consistent with a gain-of-function effect. Protein degradation and turnover assays revealed reduced abundance of the mutant TLR8 protein due to faster turnover and increased proteasomal degradation. Computational modeling predicted enhanced structural stabilization of the active TLR8 homodimer interface via additional water-mediated hydrogen bonds introduced by the A518T substitution. Together, these findings integrating structural modeling with functional assays identify a novel TLR8 ligand-specific gain-of-function mutation resulting in complex immunopathology in 2 siblings.

A newly discovered TLR8 gene mutation causes immune overactivation and inflammation in two siblings, linking genetic change to immune system dysfunction and disease.