Back in the year 2009, we presented in this journal a neonate with an unusual combination of congenital lactic acidosis and bilateral calcifications in the adrenal medulla1. After prenatal ultrasound examination at 21 weeks had revealed intrauterine growth retardation and pericardial effusions, labor was induced at 38 weeks of pregnancy. The patient was hypotonic at birth, had respiratory insufficiency necessitating intubation and artificial ventilation, hepatomegaly and severe cardiomegaly. Intensive care was withdrawn in accordance with parents’ wishes on day 38. At the time, the patient’s causal genetic defect had remained unknown, yet clinical and biochemical evidence ticked all the boxes of an underlying mitochondrial defect. Postmortem spectrophotometric evaluation then revealed decreased oxidative phosphorylation activities in heart muscle (complex IV) and liver (complexes I, III, IV and V), and normal activities in skeletal muscle and cultured skin fibroblasts. Blue-native polyacrylamide gel electrophoresis of liver extracts confirmed decreased activities of complexes III and IV and showed the presence of subcomplexes of complex V. Thanks to the great genetic leap forward taken by medical research ever since, we were now able to thoroughly investigate the patient’s whole exome extracted from preserved cultured skin fibroblasts. These investigations picked up compound heterozygous missense variants in the gene encoding the mitochondrial leucyl-tRNA synthetase (LARS2): c.302 A > G (p.His101Arg) and c.516 G > T (p.Arg172Ser). Nuclear-encoded mitochondrion-specific aminoacyl-tRNA synthetases exist for 17 of the 20 proteogenic amino acids, and pathogenic mutations have now been reported for each one of these2. At the time we published our casus, LARS2 defects had not been reported, yet between the year 2013 and now, a total of 23 patients have been described that carry disease-causing LARS2 variants (Table 1)1,3,4,5,6,7,8,9,10,11,12,13. Their diverse associated disease phenotypes, which include Perrault syndrome, multisystem disease, leukodystrophy and intellectual disability, do not appear to have an obvious link with genotype. With the casus we presented in this journal and now have characterized at the molecular level, more than twenty patients with 18 unique combinations of disease-causing LARS2 variants have been reported so far, only one was homozygous the others concerned compound heterozygous variants.

Table 1 Published patients carrying disease-causing LARS2 variants.
Full size table

Mitochondrial aminoacyl-tRNA synthetase defects unavoidably lead to defective intramitochondrial translation, disturbing the synthesis of mitochondrion-encoded subunits and the integrity of the oxidative phosphorylation complexes. LARS2 protein loads leucine onto its destined tRNA to form an aminoacylated tRNA, subsequently allowing addition of the amino acid to a growing polypeptide chain. To establish this function, LARS2 protein contains a catalytic domain, an anticodon binding domain, and a dimerization domain. Both variants reported in this patient lie within the catalytic domain, the protein domain that binds ATP, leucine and the 3’ end of tRNALeu, allowing the release of leucyl-tRNA. The c.516 G > T variant has been described before alongside the c.1028 C > T variant in compound heterozygous form, in a patient with congenital sensorineural hearing loss9. The variant is present at low frequency in the general population and is categorized as possibly damaging with a score of 0.623 on PolyPhen-2 v2.2.2r398. In addition, the patient carries the not previously described heterozygous LARS2 variant c.302 A > G (p.His101Arg), a missense alteration categorized as probably damaging with a score of 1.000 on PolyPhen-2 v2.2.2r398. Both variants are predicted to be disease causing using Mutation Taster14, and the affected positions are more than 80% conserved.

The majority of reported LARS2 variants are missense mutations, as are the ones we report in this patient. This is consistent with the essential activity of the gene reflected by a probability of loss of function intolerance of 0 on gnomAD (https://gnomad.broadinstitute.org), as complete loss of function most probably would lead to fetal death. Many patients suffer from a combination of gonadal dysgenesis and deafness termed Perrault syndrome. The disease phenotype of this previously reported patient is, however, a more severe form of LARS2 deficiency. In fact, this is only the second reported case of a patient carrying LARS2 variants who dies within the first 2 months of life. Both reported patients suffered from congenital lactic acidosis and had impairment of heart and lung function, but our patient is the only one in which adrenal abnormalities were noted. It has been established that seemingly comparable mutations in different mitochondrial aminoacyl tRNA synthetases lead to diseases with extremely different age of onset and may affect tissues differentially. Such phenotypic heterogeneity could be due to modifying genetic factors to which we now remain largely ignorant. We are hopeful that such insight will appear after persevering research in mitochondrial defects for the decades to come.