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Common causes of hypoglycemia include hyperinsulinism, hormonal deficiencies, fatty acid oxidation disorders, and glycogen storage diseases; however, rare causes should also be considered for the condition. Mitochondrial complex III deficiency shows an autosomal recessive or a mitochondrial inheritance pattern. To date, mitochondrial complex III deficiency, nuclear type 3 attributable to a pathogenic variant of the UQCRB gene (MIM 615158) has been identified in only 2 pediatric patients; both presented with hypoglycemia and lactic acidosis. In this paper, we present a patient with mitochondrial complex III deficiency, nuclear type 3, UQCRB variant associated with acute hypoglycemia and lactic acidosis episodes. The male patient was admitted on the first day of life with tachypnea, metabolic acidosis, and hypoglycemia. Up to 10 years of age, he was admitted 7 times with abdominal pain, vomiting, and fever. His blood tests revealed hypoglycemia, metabolic acidosis, and hyperlactatemia. At 10 years of age, a whole-exome sequencing (WES) analysis was performed identifying a homozygous c.309_313delAGAAA (p.Glu104ArgfsTer10) pathogenic variant of the UQCRB gene. Once the common causes of hypoglycemia are excluded, it is essential to perform a WES analysis for other rare causes. Thus, rare disorders such as mitochondrial complex III deficiency can be diagnosed.

Isolated complex III (CIII) deficiencies are among the least frequently diagnosed mitochondrial disorders. Clinical symptoms range from isolated myopathy to severe multi-systemic disorders with early death and disability. To date, we know of pathogenic variants in genes encoding five out of 10 subunits and five out of 13 assembly factors of CIII. Here we describe rare bi-allelic variants in the gene of a catalytic subunit of CIII, UQCRFS1, which encodes the Rieske iron-sulfur protein, in two unrelated individuals. Affected children presented with low CIII activity in fibroblasts, lactic acidosis, fetal bradycardia, hypertrophic cardiomyopathy, and alopecia totalis. Studies in proband-derived fibroblasts showed a deleterious effect of the variants on UQCRFS1 protein abundance, mitochondrial import, CIII assembly, and cellular respiration. Complementation studies via lentiviral transduction and overexpression of wild-type UQCRFS1 restored mitochondrial function and rescued the cellular phenotype, confirming UQCRFS1 variants as causative for CIII deficiency. We demonstrate that mutations in UQCRFS1 can cause mitochondrial disease, and our results thereby expand the clinical and mutational spectrum of CIII deficiencies.

Mitochondrial complex III deficiency nuclear type 2 is an autosomal-recessive disorder caused by mutations in TTC19 gene. TTC19 is involved in the preservation of mitochondrial complex III, which is responsible for transfer of electrons from reduced coenzyme Q to cytochrome C and thus, contributes to the formation of electrochemical potential and subsequent ATP generation. Mutations in TTC19 have been found to be associated with a wide range of neurological and psychological manifestations. Herein, we report on a 15-year-old boy born from first-degree cousin parents, who initially presented with psychiatric symptoms. He subsequently developed progressive ataxia, spastic paraparesis with involvement of caudate bodies and lentiform nuclei with cerebellar atrophy. Eventually, the patient developed gastrointestinal involvement. Using whole-exome sequencing (WES), we identified a novel homozygous frameshift mutation in the TTC19 gene in the patient (NM_017775.3, c.581delG: p.Arg194Asnfs*16). Advanced genetic sequencing technologies developed in recent years have not only facilitated identification of novel disease genes, but also allowed revelations about novel phenotypes associated with mutations in the genes already linked with other clinical features. Our findings expanded the clinical features of TTC19 mutation to potentially include gastrointestinal involvement. Further functional studies are needed to elucidate the underlying pathophysiological mechanisms.