The Most Prevalent Freeman-Sheldon Syndrome Mutations in the Embryonic Myosin Motor Share Functional Defects

Area of Science:

• Biochemistry
• Molecular Biology
• Genetics

Background:

• Embryonic myosin is crucial for fetal development but downregulated postnatally.
• Freeman-Sheldon syndrome (FSS), a severe distal arthrogryposis, results from mutations in the embryonic myosin motor domain.
• Investigating FSS mutations is hindered by limited access to human embryonic muscle tissue.

Purpose of the Study:

• To characterize the functional impact of FSS-associated mutations on human embryonic myosin motor function.
• To compare the kinetics of wild-type (WT) embryonic myosin with common FSS mutants.

Main Methods:

• Utilized a recombinant expression system to produce homogeneous samples of WT and FSS mutant human myosin motors.
• Performed kinetic analyses to determine parameters such as ADP release rate, ADP affinity, and ATPase Vmax.

Main Results:

• The WT embryonic myosin motor exhibits contractile speeds comparable to slow type I/β cardiac myosin.
• All three studied FSS mutations (R672H, R672C, T178I) significantly altered kinetic properties.
• Mutations led to a 5-9-fold slower ATP hydrolysis step and a 2-35-fold reduced Vmax, indicating impaired myosin cycling.

Conclusions:

• FSS mutations drastically disrupt embryonic myosin motor function by slowing key kinetic steps.
• The observed kinetic impairments provide a molecular basis for the severe muscle overcontraction seen in Freeman-Sheldon syndrome.
• Recombinant expression systems are valuable tools for studying the functional consequences of disease-associated mutations when native tissue is scarce.