From genetic variants to pathophysiological mechanisms: implication of α2δ-4 in exertional heat stroke

On Friday November the 6th, Clara Charpentier will defend her PhD "From genetic variants to pathophysiological mechanisms: implication of α2δ-4 in exertional heat stroke".

This PhD was carried out under the supervision of Julien Fauré of the team "Myologie cellulaire et Pathologies" of the GIN and Nathalie Roux Buisson.

Jury :

- Sylvie Ducreux, Université Lyon 1, rapporteure
- Olivier Biondi, Université Evry Paris Saclay, rapporteur
- Christine Berthier, Université Lyon 1, examinatrice
- Mireille Albrieux, Université Grenoble Alpes, examinatrice
- Nathalie Roux Buisson, CHU - Grenoble Alpes, co-directrice de thèse
- Julien Fauré, CHU - Grenoble Alpes, directeur de thèse

Abstract :

Exertional Heat Stroke (EHS) is a life-threatening medical emergency characterized by a core temperature exceeding 40°C associated with acute central neurological impairment. This condition primarily affects young, healthy individuals during strenuous physical exercise, a scenario frequently encountered among military personnel and endurance athletes. Although EHS has traditionally been considered an environmentally induced disorder, a growing body of evidence suggests an underlying individual genetic susceptibility, drawing a close pathophysiological parallel with Malignant Hyperthermia (MH).

MH is a rare, life-threatening pharmacogenetic disorder triggered by exposure to certain volatile halogenated anesthetics. At the molecular level, it mainly results from mutations in the RYR1 gene, encoding the ryanodine receptor (RyR1), which leads to dysregulated calcium handling in skeletal muscle and a massive, uncontrolled release of calcium (Ca²⁺) from the sarcoplasmic reticulum into the myoplasmic cytosol.

The functional proximity between EHS and MH is clinically established: approximately 50% of individuals who have suffered a severe EHS episode display a positive In Vitro Contracture Test (IVCT) to caffeine and halothane, reflecting an increased sensitivity of myocytic calcium release. Nevertheless, pathogenic variants identified in canonical MH genes (RYR1, CACNA1S) account for only a minority of EHS cases. This diagnostic impasse points to the involvement of other genetic factors in EHS susceptibility.

To explore this predisposition, Whole Exome Sequencing (WES) analysis identified variations in the CACNA2D4 gene in four unrelated military patients who experienced an EHS crisis. The CACNA2D4 gene encodes the ∝₂d4 isoforms of the regulatory subunit of voltage-gated calcium channels. Our work demonstrates that a CACNA2D4 transcript is expressed in mouse skeletal muscle.

To evaluate the in vivo impact of these alterations, viable and fertile mouse models (Knock-In S306^+/- and Knock-Out CACNA2D4) were developed. When subjected to an intense exercise protocol, KI-S306^+/- animals exhibited a significant increase in blood creatine phosphokinase (CPK) levels compared to wild-type (WT) littermates, reflecting enhanced rhabdomyolysis.

Altogether, these data point to the CACNA2D4 gene as a major candidate gene predisposing to Exertional Heat Stroke. The aim of my PhD thesis is to perform an in-depth functional and molecular characterization of this protein in skeletal muscle tissue.