Mitochondrial disorders in children: toward development of small-molecule treatment strategies

Abstract

This review presents our current understanding of the pathophysiology and potential treatment strategies with respect to mitochondrial disease in children. We focus on pathologies due to mutations in nuclear DNA-encoded structural and assembly factors of the mitochondrial oxidative phosphorylation (OXPHOS) system, with a particular emphasis on isolated mitochondrial complex I deficiency. Following a brief introduction into mitochondrial disease and OXPHOS function, an overview is provided of the diagnostic process in children with mitochondrial disorders. This includes the impact of whole-exome sequencing and relevance of cellular complementation studies. Next, we briefly present how OXPHOS mutations can affect cellular parameters, primarily based on studies in patient-derived fibroblasts, and how this information can be used for the rational design of small-molecule treatment strategies. Finally, we discuss clinical trial design and provide an overview of small molecules that are currently being developed for treatment of mitochondrial disease.

Keywords: children; clinical trial; drug development; mitochondria; outcome measures.

Figure 1. Glycolytic and mitochondrial ATP production, the electron transport chain, and oxidative phosphorylation

(A) Glucose (Glc) and glutamine (Gln) enter the cell via dedicated transporters. In the cytosol, Glc is converted in the glycolysis pathway into pyruvate (Pyr), which is transported to the mitochondrial matrix by the mitochondrial Pyr carrier (MPC). Alternatively, Pyr can be converted into lactate (Lac) by the action of lactate dehydrogenase (LDH). Inside the mitochondrial matrix, Pyr is converted in to acetyl coenzyme A (acetyl‐CoA; not shown) by pyruvate dehydrogenase (PDH) to enter the tricarboxylic acid (TCA) cycle. The latter supplies the oxidative phosphorylation system (OXPHOS) with substrates in the form of reduced nicotinamide adenine dinucleotide (NADH) and reduced flavin adenine dinucleotide (FADH ₂). In addition, Gln can enter the mitochondrial matrix where it is converted by glutaminase into glutamate (not shown), a TCA cycle substrate. Also fatty acids can enter the mitochondrial matrix and enter the TCA cycle following their conversion into acetyl‐CoA (not shown). In healthy cells, the conversion of Glc into Pyr and its further metabolic conversion by the TCA and OXPHOS system constitute the major pathway for ATP generation (marked in red). (B) The mitochondrial electron transport chain (ETC) consists of 4 multisubunit protein complexes (complex I to IV) that are embedded in the mitochondrial inner membrane (MIM). Electrons are donated by NADH (at complex I) and FADH ₂ (at complex II) to coenzyme Q₁₀ (Q), which transports them to complex III. From thereon, electrons are transported to complex IV by cytochrome c (c) where they are donated to molecular oxygen (O₂). Although not discussed here, in addition to the ETC complexes also other proteins can provide coenzyme Q₁₀ and cytochrome c with electrons (red boxes) in a tissue‐dependent manner. During electron transport, energy is liberated and used expel protons (H⁺) from the mitochondrial matrix intro the inter‐membrane space (IMS) between the MIM and mitochondrial outer membrane (MOM). As a consequence, the mitochondrial matrix displays an increased pH and the MIM has a highly negative‐inside membrane potential (Δψ). (C) Together, the pH (ΔpH) and potential difference (Δψ) across the MIM determine the magnitude of the proton‐motive force (PMF), which is used by the F_oF₁‐ATPase (complex V) to drive mitochondrial ATP production from inorganic phosphate (P_i) and ADP. In addition to ATP generation, virtually all other mitochondrial processes including ion exchange and pre‐protein import require a proper ΔpH and/or Δψ. The magnitude of the PMF not only depends on the combined action of the ETC and complex V (i.e., the oxidative phosphorylation system; OXPHOS) but also is affected by other electrogenic systems. These include uncoupling proteins (UCPs), the P_i transporter (P_iT), and the adenine nucleotide translocator (ANT). This figure was compiled based on (Koopman et al, 2010; Valsecchi et al, 2010; Liemburg‐Apers et al, 2011; Koopman et al, 2012, 2013 and Liemburg‐Apers et al, 2015).

Figure 2. Currently identified mutations in nDNA‐encoded OXPHOS subunits and assembly factors causing mitochondrial disease in children

OXPHOS structural subunits in which a pathological mutation was reported are depicted in red for each complex. Assembly factors are indicated (in blue). The data in this figure were compiled from (Koopman et al, 2012, 2013; Mayr et al, 2015; Ostergaard et al, 2015; van Rahden et al, 2015).