Craniofacial morphology in children of mothers with the m.3243A>G mutation in mitochondrial DNA

Transcriptomic analysis reveals mitochondrial dysfunction in the pathogenesis of Nager syndrome in sf3b4-depleted zebrafish

Nager syndrome (NS) is a rare acrofacial dysostosis caused by heterozygous loss-of-function variants in the splicing factor 3B subunit 4 (SF3B4). The main clinical features of patients with NS are characterized by facial-mandibular and preaxial limb malformations. The migration and specification of neural crest cells are crucial for craniofacial development, and mitochondrial fitness appears to play a role in such processes. Here, by analyzing our previously published transcriptome dataset, we aim to investigate the potential involvement of mitochondrial components in the pathogenesis of craniofacial malformations, especially in sf3b4 mutant zebrafish. We identified that oxidative phosphorylation (OXPHOS) defects and overproduction of reactive oxygen species (ROS) due to decreased antioxidants defense activity, which leads to oxidative damage and mitochondrial dysfunction. Furthermore, our results highlight that fish lacking sf3b4 gene, primarily display defects in mitochondrial complex I. Altogether, our findings suggest that mitochondrial dysfunction may contribute to the development of the craniofacial anomalies observed in sf3b4-depleted zebrafish.

Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease

OBJECTIVES

Homoplasmic maternally inherited, m.14674T>C or m. 14674T>G mt-tRNA(Glu) mutations have recently been identified in reversible infantile cytochrome c oxidase deficiency (or 'benign COX deficiency'). This study sought other genetic defects that may give rise to similar presentations.

PATIENTS

Eight patients from seven families with clinicopathological features of infantile reversible cytochrome c oxidase deficiency were investigated.

METHODS

The study reviewed the diagnostic features and performed molecular genetic analyses of mitochondrial DNA and nuclear encoded candidate genes.

RESULTS

Patients presented with subacute onset of profound hypotonia, feeding difficulties and lactic acidosis within the first months of life. Although recovery was remarkable, a mild myopathy persisted into adulthood. Histopathological findings in muscle included increased lipid and/or glycogen content, ragged-red and COX negative fibres. Biochemical studies suggested more generalised abnormalities than pure COX deficiency. Clinical improvement was reflected by normalisation of lactic acidosis and histopathological abnormalities. The m.14674T>C mt-tRNA(Glu) mutation was identified in four families, but none had the m. 14674T>G mutation. Furthermore, in two families pathogenic mutations were also found in the nuclear TRMU gene which has not previously been associated with this phenotype. In one family, the genetic aetiology still remains unknown.

CONCLUSIONS

Benign COX deficiency is better described as 'reversible infantile respiratory chain deficiency'. It is genetically heterogeneous, and patients not carrying the m.14674T>C or T>G mt-tRNA(Glu) mutations may have mutations in the TRMU gene. Diagnosing this disorder at the molecular level is a significant advance for paediatric neurologists and intensive care paediatricians, enabling them to select children with an excellent prognosis for continuing respiratory support from those with severe mitochondrial presentation in infancy.