4.2.3. Mutations in Complex III Assembly Factors

Together with cytochrome *b*, most Complex III pathologies are derived from mutations in the nuclear *BCS1L* gene. Its yeast homolog *BCS1* is an AAA protease that was shown to be required for the expression/maturation of Rip1 [60,61]. Later studies indicated that Bcs1 promotes one of the steps in the translocation of Rip1 necessary for incorporation of the iron-sulfur center [80]. Bcs1 exists as a heptamer with a contractile central cavity that participates in the translocation of the folded Rip1. Additionally, Bcs1 is associated with the complex III assembly module and its dissociation ends the maturation process [81].

Mutations in *BCS1L* comprise a wide spectrum of pathologies, including: GRACILE syndrome (growth retardation, aminoaciduria, cholestasis, iron overload, lactic acidosis, early death) [82–88]; Björstand syndrome, characterized by hearing loss and *pili torti* [89–94]; encephalopathy [95–98]; lactic acidosis, liver dysfunction and tubulopathy [99–102]; muscle weakness, focal motor seizures and optic atrophy [103], among others. Besides low steady state levels of Complex III, cells from patients with *BSC1L* mutations have impaired mitochondrial import of the protein as evidenced by its accumulation in the cytosol [91]. An adult harboring a R69C missense mutation was diagnosed with aminoaciduria, seizures, bilateral sensorineural deafness, and learning difficulties. Yeast complementation studies corroborate that the R69C mutation impairs the respiratory capacity of the cell [104].

Assembly of Rip1 is thought to be the last step in the biogenesis of Complex III [81]. In addition to Bcs1, this assembly step was found to require the product of the yeast nuclear *MZM1* gene [105]. The first case of Complex III deficiency caused by a mutation in *LYRM7*, the human homolog of *MZM1*, was reported in 2013 [106]. The equivalent mutation in yeast resulted in decreased oxygen consumption as a result of reduced steady state levels of Rieske protein and Complex III. Since then, Complex III deficiency caused by *LYRM7* has been identified in patients presenting with leukoencephalopathy [107–109] and liver dysfunction [110].

Mutations have also been reported in recent years in the human *UQCC2* and *UQCC3* genes that code for protein homologs of yeast complex III assembly factors Cbp6 and Cbp4. Complex III deficiency was found in a patient with a homozygous mutation in *UQCC2* [111]. This study demonstrated that the biochemical phenotype produced by the *UQCC2* mutation is similar to that reported in yeast [57], as cytochrome *b* synthesis and stability was decreased in the patient's fibroblasts [111]. A homozygous mutation in *UQCC2* leading to a Complex III deficiency was also reported in a consanguineous baby presenting neonatal encephalomyopathy. This mutation resulted in a secondary deficiency of Complex I [112]. The authors proposed that assembled Complex III is required for the stability or assembly of complexes I and IV, which may be related to supercomplex formation. Interestingly, a recent study [113] showed that the ND1 subunit of Complex I co-immunoprecipitated with newly synthesized UQCRFS1 of Complex III in mammalian mitochondria, indicating a possible coordination of the assembly of the two complexes.


**Table 3.** Pathologies resulting from mutations in genes encoding Complex III subunits and assembly factors, and their yeast homologs.


**Table 3.** *Cont.*

<sup>1</sup> Review describing many mutations. <sup>2</sup> In different patients. <sup>3</sup> Mutation causing a splicing defect. <sup>4</sup> In the same patient.

During assembly of yeast Complex III, Cbp4 is recruited by the Cbp3–Cbp6-cytochrome *b* ternary complex following release of the latter from the mitoribosome [57]. Wanschers et al. [114] described a homozygous mutation in *UQCC3*, the human homolog of *CBP4*, in a patient diagnosed with isolated Complex III deficiency. Cultured fibroblasts from the patient were partially deficient in cytochrome *b* and had no detectable UQCC3 protein. The authors concluded that UQCC3 functions in Complex III assembly downstream of UQCC1 and UQCC2, as the absence of UQCC3 did not affect the levels of UQCC1 and UQCC2 [114]. These observations are consistent with the above mentioned sequential interaction of Cbp4 with the Cbp3-Cbp6-cytochrome *b* complex during synthesis and assembly of cytochrome *b* [57].
