SDHA mutations causing a multisystem mitochondrial disease: novel mutations and genetic overlap with hereditary tumors.

Defects in complex II of the mitochondrial respiratory chain are a rare cause of mitochondrial disorders. Underlying autosomal-recessive genetic defects are found in most of the 'SDHx' genes encoding complex II (SDHA, SDHB, SDHC, and SDHD) and its assembly factors. Interestingly, SDHx genes also function as tumor suppressor genes in hereditary paragangliomas, pheochromocytomas, and gastrointestinal stromal tumors. In these cases, the affected patients are carrier of a heterozygeous SDHx germline mutation. Until now, mutations in SDHx associated with mitochondrial disease have not been reported in association with hereditary tumors and vice versa. Here, we characterize four patients with isolated complex II deficiency caused by mutations in SDHA presenting with multisystem mitochondrial disease including Leigh syndrome (LS) and/or leukodystrophy. Molecular genetic analysis revealed three novel mutations in SDHA. Two mutations (c.64-2A>G and c.1065-3C>A) affect mRNA splicing and result in loss of protein expression. These are the first mutations described affecting SDHA splicing. For the third new mutation, c.565T>G, we show that it severely affects enzyme activity. Its pathogenicity was confirmed by lentiviral complementation experiments on the fibroblasts of patients carrying this mutation. It is of special interest that one of our LS patients harbored the c.91C>T (p.Arg31*) mutation that was previously only reported in association with paragangliomas and pheochromocytomas, tightening the gap between these two rare disorders. As tumor screening is recommended for SDHx mutation carriers, this should also be considered for patients with mitochondrial disorders and their family members.

A novel mutation in COQ2 leading to fatal infantile multisystem disease.

Coenzyme Q10 (ubiquinone or CoQ10) serves as a redox carrier in the mitochondrial oxidative phosphorylation system. The reduced form of this lipid-soluble antioxidant (ubiquinol) is involved in other metabolic processes as well, such as preventing reactive oxygen species (ROS) induced damage from the mitochondrial membrane. Primary coenzyme Q10 deficiency is a rare, autosomal recessive disorder, often presenting with neurological and/or muscle involvement. Until now, five patients from four families have been described with primary coenzyme Q10 deficiency due to mutations in COQ2 encoding para-hydroxybenzoate polyprenyl transferase. Interestingly, four of these patients showed a distinctive renal involvement (focal segmental glomerular sclerosis, crescentic glomerulonephritis, nephrotic syndrome), which is only very rarely seen in correlation with mitochondrial disorders. The fifth patient deceases due to infantile multi organ failure, also with renal involvement. Here we report a novel homozygous mutation in COQ2 (c.905C>T, p.Ala302Val) in a dizygotic twin from consanguineous Turkish parents. The children were born prematurely and died at the age of five and six months, respectively, after an undulating disease course involving apneas, seizures, feeding problems and generalized edema, alternating with relative stable periods without the need of artificial ventilation. There was no evidence for renal involvement. We would like to raise awareness for this potentially treatable disorder which could be under diagnosed in patients with fatal neonatal or infantile multi-organ disease.