I have been seeing a patient for several years who carried a clinical diagnosis of a mitochondrial disorder based on chronic lactic acidosis, brain abnormalities and other organ involvement. However, a molecular diagnosis was elusive despite mtDNA sequencing and various gene panels. I therefore recently sent whole exome sequencing on this patient, and he was found to have compound heterozygous deletions in the FARS2 gene, a mitochondrial phenylalanine tRNA synthetase.
Fewer than 5 cases have been described with mutations in this gene, and all were far more severe in their presentation than my patient, who is in his teenage years, and mildly to moderately developmentally delayed. This case lends further evidence to the idea that with the increasing clinical utilization of whole exome sequencing will widen the clinical spectrum of Mendelian disorders.
Hilary Vernon MD PhD