Hypertrophic Cardiomyopathy with Asymmetric Septal Hypertrophy

A 22-year-old college athlete presents with near-syncope during sprinting. His paternal uncle died suddenly at age 35. On examination, a harsh systolic ejection murmur is heard at the left sternal border that increases with Valsalva and standing, but decreases with squatting and hand-grip. Echocardiography demonstrates the structure marked **A** (asymmetric septal hypertrophy with septal thickness 24 mm versus posterior wall 11 mm), systolic anterior motion of the anterior mitral leaflet, and a resting left ventricular outflow tract gradient of 80 mmHg. Which of the following genetic mutations is MOST COMMONLY responsible for this morphologic phenotype of hypertrophic cardiomyopathy?