Complete loss of expression of the ANT1 gene causes a clinical syndrome mainly characterised by cardiomyopathy and myopathy. This report expands the clinical spectrum of ANT1-related human diseases, and emphasises the crucial role of the mitochondrial ADP/ATP carriers in muscle function and pathophysiology of human myopathies.

DGUOK [dG (deoxyguanosine) kinase] is one of the two mitochondrial deoxynucleoside salvage pathway enzymes involved in precursor synthesis for mtDNA (mitochondrial DNA) replication. DGUOK is responsible for the initial rate-limiting phosphorylation of the purine deoxynucleosides, using a nucleoside triphosphate as phosphate donor. Mutations in the DGUOK gene are associated with the hepato-specific and hepatocerebral forms of MDS (mtDNA depletion syndrome). We identified two missense mutations (N46S and L266R) in the DGUOK gene of a previously reported child, now 10 years old, who presented with an unusual revertant phenotype of liver MDS. The kinetic properties of normal and mutant DGUOK were studied in mitochondrial preparations from cultured skin fibroblasts, using an optimized methodology. The N46S/L266R DGUOK showed 14 and 10% residual activity as compared with controls with dG and deoxyadenosine as phosphate acceptors respectively. Similar apparent negative co-operativity in the binding of the phosphate acceptors to the wild-type enzyme was found for the mutant. In contrast, abnormal bimodal kinetics were shown with ATP as the phosphate donor, suggesting an impairment of the ATP binding mode at the phosphate donor site. No kinetic behaviours were found for two other patients with splicing defects or premature stop codon. The present study represents the first characterization of the enzymatic kinetic properties of normal and mutant DGUOK in organello and our optimized protocol allowed us to demonstrate a residual activity in skin fibroblast mitochondria from a patient with a revertant phenotype of MDS. The residual DGUOK activity may play a crucial role in the phenotype reversal.

Objective: To investigate whether mutations in the SURF1 gene are a cause of Charcot-MarieTooth (CMT) disease. Methods:We describe 2 patients from a consanguineous family with demyelinating autosomal recessive CMT disease (CMT4) associated with the homozygous splice site mutation c.107-2A.G in the SURF1 gene, encoding an assembly factor of the mitochondrial respiratory chain complex IV. This observation led us to hypothesize that mutations in SURF1 might be an unrecognized cause of CMT4, and we investigated SURF1 in a total of 40 unrelated patients with CMT4 after exclusion of mutations in known CMT4 genes. The functional impact of c.107-2A.G on splicing, amount of SURF1 protein, and on complex IV activity and assembly was analyzed.Results: Another patient with CMT4 was found to harbor 2 additional SURF1 mutations. All 3 patients with SURF1-associated CMT4 presented with severe childhood-onset neuropathy, motor nerve conduction velocities ,25 m/s, and lactic acidosis. Two patients had brain MRI abnormalities, including putaminal and periaqueductal lesions, and developed cerebellar ataxia years after polyneuropathy. The c.107-2A.G mutation produced no normally spliced transcript, leading to SURF1 absence. However, complex IV remained partially functional in muscle and fibroblasts. Conclusions:We found SURF1 mutations in 5% of families (2/41) presenting with CMT4. SURF1 should be systematically screened in patients with childhood-onset severe demyelinating neuropathy and additional features such as lactic acidosis, brain MRI abnormalities, and cerebellar ataxia developing years after polyneuropathy. Peripheral neuropathies are a well-known complication of mitochondrial DNA and nuclearencoded mitochondrial gene mutations. For instance, patients with mutations in the nuclearencoded mitochondrial genes MFN2 and GDAP1, which encode outer mitochondrial membrane proteins, usually present with axonal and demyelinating forms of Charcot-Marie-Tooth (CMT) disease, respectively.1 Moreover, patients with mutations in the mitochondrial DNA gene MTATP6, which encodes the ATP6 subunit of the mitochondrial respiratory chain (MRC) complex V, may present with axonal CMT (CMT2). 2The determination of the genetic cause is a major challenge in rare neuromuscular diseases such as autosomal recessive demyelinating CMT (CMT4). We investigated a consanguineous family in which 2 patients with CMT4 harbored a homozygous splice site mutation in SURF1, encoding an assembly factor of the MRC complex IV (cytochrome c oxidase [COX]). Despite this defect, we detected some residual assembly and function of COX in fibroblasts and muscle of both patients. We then screened for SURF1 mutations in a cohort of 40 unrelated patients