Jervell Lange-Nielsen LQTS + SNHL

A 7-year-old boy is brought to the pediatric cardiology clinic after experiencing a syncopal episode while swimming at school. His parents report that he failed newborn hearing screening bilaterally and has required hearing aids since infancy. On examination, he is found to have bilateral profound sensorineural hearing loss. An ECG shows a QTc interval of 560 ms with broad-based T waves. The audiogram and cardiac findings are consistent with the pattern marked **B** in the diagram. Which of the following genetic mutations is most likely responsible for this clinical presentation?