unexpected tissue-specificity and complex genetics [2]. Somatic
mutations in mtDNA also accumulate in tissues over time and have
been implicated in aging and age-related pathology [3]. Finally,
depletion of mtDNA is also pathogenic due to global disruption of
mitochondrial gene expression and can result from inherited mutations
in nuclear genes or represent a secondary consequence of other
disease conditions and treatments [4–7].
Recent estimates indicate that the mammalian mitochondrial
proteome comprises 1130–1500 proteins [8,9]. Since mtDNA encodes
only thirteen of these, the remaining majority are nuclear gene
products that are translated in the cytoplasm and imported into the
organelle. Consequently, mitochondrial dynamics is controlled by
gene expression programs in the nucleus [10] and signaling pathways
that relay information back and forth between these two organelles
[11]. These two processes are essential to maintain cellular homeostasis
and alter mitochondrial biogenesis and function in response to
changing cellular needs or environmental conditions.
Mutations in nuclear genes that encode mitochondrial proteins
cause mitochondrial diseases that are inherited in a Mendelian
fashion. A subset of these involves factors that function directly in
mtDNA expression and maintenance (e.g. mtDNA polymerase, Pol γ,
and Twinkle helicase) [12]. In addition, nuclear mutations that affect
factors that do not reside in mitochondria, but nonetheless perturb
mitochondrial homeostasis or inter-organelle signaling pathways are