A 28-year-old woman with a history of recurrent stroke-like episodes, seizures, and sensorineural hearing loss presents with acute focal neurological deficits. Her 5-year-old son has identical symptoms; her 3-year-old daughter is asymptomatic. Her husband is unaffected. The pedigree shows the pattern marked **A** — maternal transmission only, with all offspring of affected mothers affected. Which of the following best explains the variable clinical severity between the siblings despite both inheriting the same mutation?