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Whole-exome sequencing and autozygosity mapping studies, independently performed in subjects with defective combined mitochondrial OXPHOS-enzyme deficiencies, identified a total of nine disease-segregating FBXL4 mutations in seven unrelated mitochondrial disease families, composed of six singletons and three siblings. All subjects manifested early-onset lactic acidemia, hypotonia, and developmental delay caused by severe encephalomyopathy consistently associated with progressive cerebral atrophy and variable involvement of the white matter, deep gray nuclei, and brainstem structures. A wide range of other multisystem features were variably seen, including dysmorphism, skeletal abnormalities, poor growth, gastrointestinal dysmotility, renal tubular acidosis, seizures, and episodic metabolic failure. Mitochondrial respiratory chain deficiency was present in muscle or fibroblasts of all tested individuals, together with markedly reduced oxygen consumption rate and hyperfragmentation of the mitochondrial network in cultured cells. In muscle and fibroblasts from several subjects, substantially decreased mtDNA content was observed. FBXL4 is a member of the F-box family of proteins, some of which are involved in phosphorylation-dependent ubiquitination and/or G protein receptor coupling. We also demonstrate that FBXL4 is targeted to mitochondria and localizes in the intermembrane space, where it participates in an approximately 400 kDa protein complex. These data strongly support a role for FBXL4 in controlling bioenergetic homeostasis and mtDNA maintenance. FBXL4 mutations are a recurrent cause of mitochondrial encephalomyopathy onset in early infancy.

The authors report a 6 yr old boy with Barth syndrome who presented with cardiomyopathy, neutropenia and hypotonia. Urine gas chromatography showed high level of 3-methylglutaconic acid. The DNA of both the patient and the mother showed a heterozygous 3 bp deletion in exon 8 of the tafazzin gene. This abnormality involves the deletion of the bases TGA starting at cDNA nucleotide 891 (c891_893delTGA), resulting in the absence of glutamic acid at codon 202 from a highly conserved area of the tafazzin protein, consistent with the diagnosis of Barth syndrome. This is the first case report of Barth syndrome in Arab population emphasizing the importance of detailed investigations in cases of hereditary cardiomyopathy.