Mutation analysis of the SDHAF1 gene revealed mutations in five patients with SDH-defective infantile leukoencephalopathy. The missense mutation detected in patients #1 and #2 was reported previously in a large multiconsanguineous kindred of Turkish origin with several affected children [2, 3]. Family history of the siblings described here (patients #1 and #2) indicates common ancestry with those patients. The homozygous nonsense mutation demonstrated in patient #3 and the homozygous missense mutation detected in patients #4 and #5 were not reported before.
Succinate dehydrogenase participates in the electron transfer in the respiratory chain and in succinate catabolism in the Krebs cycle and consists of four subunits, all encoded by the nuclear genome . Isolated complex II deficiency is a relatively rare cause of mitochondrial disease compared to other respiratory chain defects, but is associated with a wide range of clinical features . Mutations in the four genes, SDH-A, -B, -C, -D, have been reported, with remarkably diverse phenotypes. Mutations in the SDHA gene were found to be associated with Leigh syndrome , late onset neurodegenerative disease  and dilated cardiomyopathy . Heterozygous germline mutations in SDHA, SDHB, SDHC, and SDHD cause hereditary paragangliomas and pheochromocytomas , and germline mutations in SDHB and SDHC were found to be associated with gastrointestinal stromal tumors . Recently, a mitochondrial encephalopathy was reported to be caused by SDHD mutations .
Whereas an increasing number of assembly factors have been identified for complex I, III, and cytochrome oxidase, little was known concerning the assembly of complex II. Recently two genes involved in this process were detected in humans, and the first, termed SDHAF1, was found by linkage analysis in two families with SDH-defective infantile leukoencephalopathy . Yeast experimentation indicated that the protein encoded by this gene is required for the stable assembly and full function of the SDH complex. The protein was thus termed SDH assembly factor 1 (SDHAF1) .
In the same year, mutations in the SDHAF2 (SDH5) gene encoding a protein necessary for the flavination of the subunit SDHA were detected in patients with paraganglioma . The cause of such diverse phenotypes associated with defective assembly factors of complex II remains enigmatic to date.
Our results confirm the pathogenicity of SDHAF1 mutations in infantile leukoencephalopathy due to defective succinate dehydrogenase. Our patients with complex II deficiency not associated with leukoencephalopathy but with other, diverse clinical phenotypes including myopathy with exercise intolerance, acute liver failure, psychomotor delay, muscle weakness, and hearing impairment did not carry a SDHAF1 mutation. Further studies will clarify whether infantile leukoencephalopathy with accumulation of succinate, readily detectable by in vivo proton MR spectroscopy of the brain, is pathognomonic for SDHAF1 deficiency.
To date, clinical features comprising motor regression with spasticity and neuroradiological features including bilateral leukoencephalopathy with elevated succinate on cerebral proton MRS are the suggestive findings pointing to a SDHAF1 mutation.
Treatment with riboflavin was found to be effective in selected mitochondrial disorders, including SDH deficiency . In our cohort, riboflavin treatment was applied in patients #2, #4, and #5 (SDHAF1 mutations) as well as #6 (SDH-defective myopathy with exercise intolerance). Riboflavin treatment resulted in no discernable effect in the 3 patients with SDHAF1 mutations. Patient #6 had clear benefit from this treatment with markedly prolonged motor endurance.
The clinical course in our five patients with SDHAF1 mutations is strikingly diverse. Patients #1, #2, and #4, all carrying missense mutations, died at age 18 months, 11 years, and 5 years, respectively, with histopathological features of Leigh syndrome in one of them. In contrast, patient #3, who carries a stop mutation, follows a milder course with spastic paraparesis as the main clinical feature 14 years after onset and stable white matter changes on MRI over many years. We hypothesize that an atypical starting codon could be present resulting in synthesis of a partially functional protein. Further studies are needed to elucidate the pathomechanism of this stop mutation. This variation points to influential further genetic or epigenetic factors shaping the phenotype of defective complex II assembly due to mutated SDHAF1.