Ndufa9, or NADH dehydrogenase [ubiquinone] 1 alpha subcomplex subunit 9, is closely associated with the activity and function of mitochondrial respiratory chain complex I [1]. Mitochondrial respiratory chain complex I consists of 44 different subunits and 3 functional modules, with Ndufa9 being a Q-module subunit required for complex I assembly or stability [2]. The gene plays a crucial role in cellular energy production via oxidative phosphorylation and is essential for normal mitochondrial function.

In mice, Ndufa9 promotes mitochondrial respiration, thermogenesis, and the browning of white adipose tissue. When Ndufa9 was studied in white fat browning model mice, it was found to enhance mitochondrial function, mitochondrial complex I activity, ATP synthesis, and mitochondrial respiration, all of which contribute to the browning process [1]. In addition, gene knockout using transcription activator-like effector nucleases (TALENs) in HEK293T cells showed that loss of Ndufa9 led to impaired assembly of complex I, with cells unable to grow in galactose medium. Re-expression of Ndufa9 restored the defects in complex I assembly, indicating its role in stabilizing the junction between membrane and matrix arms of complex I, a late and critical step for complex I biogenesis and activity [3].

Mutations in Ndufa9 have also been linked to Leigh syndrome. For example, via exome sequencing, a novel homozygous Ndufa9 missense variant was identified in a patient with childhood-onset progressive generalized dystonia and axonal peripheral neuropathy. Complex I assembly analysis in patient fibroblasts showed reduced complex I abundance and an accumulation of Q-module subassemblies, with the severity of the clinical phenotype correlating with the severity of the effects of the Ndufa9 variant on complex I assembly [2]. A pathogenic mutation in Ndufa9 was also found in a Kurdish patient with neonatally fatal Leigh syndrome and complex I deficiency, and lentiviral transduction with wild-type Ndufa9 restored complex I activity in the patient's fibroblasts [4].

In conclusion, Ndufa9 is essential for mitochondrial respiratory chain complex I assembly and function. Studies using mouse models and gene knockout experiments have revealed its role in processes like white adipose tissue browning and its association with Leigh syndrome. These findings provide important insights into the mechanisms of energy metabolism-related biological processes and mitochondrial-associated diseases, potentially guiding future research and treatment strategies in these areas.