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?

Explanation

## Why heteroplasmy and mitotic segregation is right The pattern marked **A** — maternal transmission only with all offspring of affected mothers affected — is pathognomonic for mitochondrial DNA (mtDNA) inheritance. The variable severity between siblings (son symptomatic, daughter asymptomatic) despite both inheriting the same mtDNA mutation is explained by **heteroplasmy**: each cell contains hundreds of mitochondria with mixed populations of normal and mutant mtDNA. The proportion of mutant mtDNA varies between siblings and even between tissues within the same individual, and this proportion determines whether the **threshold effect** is exceeded in high-energy tissues (CNS, muscle, retina, cochlea). **Mitotic segregation** during cell division can shift the proportion of mutant mtDNA in daughter cells, leading to variable phenotype. This is the hallmark feature that distinguishes mitochondrial inheritance from Mendelian patterns (Nelson Pediatrics 21e Ch 105; Harrison 21e Ch 461). ## Why each distractor is wrong - **X-linked modifier genes**: While X-linked modifiers may explain why LHON predominantly affects males, they do not explain the variable severity between siblings who both inherit the same mtDNA mutation from the mother. Heteroplasmy is the primary mechanism for variable severity in mitochondrial diseases. - **Autosomal dominant nuclear mutations with incomplete penetrance**: Nuclear mutations affecting mitochondrial proteins (e.g., POLG mutations) follow Mendelian inheritance, not maternal-only transmission. The pedigree pattern **A** explicitly shows maternal transmission only, ruling out nuclear inheritance. - **Age-dependent accumulation of somatic mutations in nuclear DNA**: While somatic mutations do accumulate with age, this does not explain why a 3-year-old daughter is asymptomatic while her 5-year-old brother is severely affected. The variable severity is present from early childhood and is driven by heteroplasmy, not age-related nuclear accumulation. **High-Yield:** Maternal transmission only + variable severity among siblings = **heteroplasmy and mitotic segregation of mtDNA**; the proportion of mutant mtDNA in each cell determines phenotype (threshold effect). [cite: Nelson Pediatrics 21e Ch 105; Harrison 21e Ch 461]