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Understanding the Genetic Basis of Mitochondrial Diseases

Using Drosophila, a group of scientists from Zhejiang University demonstrated how abnormal mitochondrial tRNA metabolism can lead to developmental delay and seizure.

Mitochondrial aminoacyl-tRNA synthetases are nuclear-encoded enzymes that support the conjugation of each of the 20 amino acids to its cognate tRNA molecule. In recent years, scientists have noticed a correlation of mitochondrial disorders with mutations in these enzymes. Mitochondrial diseases are a large, clinically heterogeneous group of disorders that cause an array of health concerns. However, the precise mechanisms by which these mutations affect mitochondrial function and disease development remain to be properly understood.

A team of scientists led by Dr. Ge Wanzhong from the Women’s Hospital, Zhejiang University School of Medicine, and Professor Guan Min-Xin from the Institute of Genetics, Zhejiang University reported the relationship between abnormal mitochondrial tRNA metabolism and developmental delay and seizure.

Based on previous research, it is known that patients with certain mutations in their mitochondrial phenylalanyl-tRNA synthetase (FARS2) developed infantile-onset epileptic mitochondrial encephalopathy with seizure, developmental delay, and truncal hypotonia from birth to age six months. Past works have pinpointed that the primary defects in FARS2 mutations were the deficient aminoacylation of tRNA Phe, where abnormal tRNA Phe metabolism hindered mitochondrial translation and subsequent deficiencies of oxidative phosphorylation. Yet, not much is known about the pathophysiology of FARS2 deficiency due to a lack of animal disease models.

In this study, the team used Drosophila as a model organism to study FARS2 deficiency. Through genome editing with CRISPR/Cas9 system and RNAi approach, they created Drosophila dFARS2 knockout mutants and knockdown models. These models were put through a series of experiments to confirm the validity of the model for studying mitochondrial tRNA synthetase-related diseases. They found that dFARS2 mutations or knockdown of dFARS2 led to a developmental delay and lethality at the second instar larval stages.

To measure seizure susceptibility, the scientists applied mechanical stress and measured the time needed for the flies to right themselves. When mechanical stress was applied, control flies were able to recover quickly while dFARS2 mutants were bang-sensitive and displayed a characteristic seizure pattern. Further examinations of mitochondria in the central brain revealed loosely packed mitochondria with disorganised cristae in FARS2 knockdown flies as compared to densely packed mitochondria with intact cristae in control flies. These morphological defects in FARS2 knockdown flies suggest that seizure phenotypes could be derived from mitochondrial dysfunction.

Looking deeper, the team’s biochemical analysis revealed that FARS2 deficiency led to defects in mitochondrial tRNA Phe metabolism, translation, assembly, and activity of oxidative phosphorylation system (OXPHOS) complexes. More importantly, they found that expressing human disease-causing variants in Drosophila dFARS2 mutants can partially recapitulate some features of the disease. Expressing human wild type FARS2 and FARS2 carrying the p.G309S or p.D142Y variant into the dFARS2 mutants rescued the viability of the mutants.

Altogether, the team’s findings suggest that proper control of mitochondrial tRNA metabolism is crucial for mitochondrial function, unveiling a novel genetic basis for mitochondrial disease. “Our research discoveries will facilitate [a] new understanding of [the] pathogenesis of mitochondrial diseases and new therapies for treating seizures,” said Ge. [APBN]

Source: Fan et al. (2021). FARS2 deficiency in Drosophila reveals the developmental delay and seizure manifested by aberrant mitochondrial tRNA metabolism. Nucleic acids research, 49(22), 13108-13121.