Mutations of SURF-1 in Leigh disease associated with cytochrome c oxidase deficiency

Loss-of-function mutations of SURF-1 are specifically associated with Leigh syndrome with cytochrome c oxidase deficiency

Mutations of SURF-1, a gene located on chromosome 9q34, have recently been identified in patients affected by Leigh syndrome (LS), associated with deficiency of cytochrome c oxidase (COX), the terminal component of the mitochondrial respiratory chain. To investigate to what extent SURF-1 is responsible for human disorders because of COX deficiency, we undertook sequence analysis of the SURF-1 gene in 46 unrelated patients. We analyzed 24 COX-defective patients classified as having typical Leigh syndrome (LS(COX)), 6 patients classified as Leigh-like (LL(COX)) cases, and 16 patients classified as non-LS(COX) cases. Frameshift, stop, and splice mutations of SURF-1 were detected in 18 of 24 (75%) of the LS(COX) cases. No mutations were found in the LL(COX) and non-LS(COX) group of patients. Rescue of the COX phenotype was observed in transfected cells from patients harboring SURF-1 mutations, but not in transfected cell lines from 2 patients in whom no mutations were detected by sequence analysis. Loss of function of SURF-1 protein is specifically associated with LS(COX), although a proportion of LS(COX) cases must be the result of abnormalities in genes other than SURF-1. SURF-1 is the first nuclear gene to be consistently mutated in a major category of respiratory chain defects. DNA analysis can now be used to accurately diagnose LS(COX), a common subtype of Leigh syndrome.

Revolution in mitochondrial medicine

A revolution in chemical pathology occurred about 40 years ago with the discovery of a patient with mitochondrial dysfunction. The field of mitochondrial medicine has experienced explosive growth during the last decade. More than 50 mtDNA mutations and several nuclear gene mutations have been identified in affected patients. The recent development of animal models will continue the revolution in mitochondrial medicine by facilitating in depth studies of the molecular pathogenesis and development of novel drug and gene therapy strategies for mitochondrial dysfunction. As we enter the next millennium, we can expect mitochondrial medicine to remain a dynamic and rapidly developing field.

Nuclear DNA origin of mitochondrial complex I deficiency in fatal infantile lactic acidosis evidenced by transnuclear complementation of cultured fibroblasts

We have studied complex I (NADH-ubiquinone reductase) defects of the mitochondrial respiratory chain in 2 infants who died in the neonatal period from 2 different neurological forms of severe neonatal lactic acidosis. Specific and marked decrease in complex I activity was documented in muscle, liver, and cultured skin fibroblasts. Biochemical characterization and study of the genetic origin of this defect were performed using cultured fibroblasts. Immunodetection of 6 nuclear DNA-encoded (20, 23, 24, 30, 49, and 51 kDa) and 1 mitochondrial DNA-encoded (ND1) complex I subunits in fibroblast mitochondria revealed 2 distinct patterns. In 1 patient, complex I contained reduced amounts of the 24- and 51-kDa subunits and normal amounts of all the other investigated subunits. In the second patient, amounts of all the investigated subunits were severely decreased. The data suggest partial or extensive impairment of complex I assembly in both patients. Cell fusion experiments between 143B206 rho degrees cells, fully depleted of mitochondrial DNA, and fibroblasts from both patients led to phenotypic complementation of the complex I defects in mitochondria of the resulting cybrid cells. These results indicate that the complex I defects in the 2 reported cases are due to nuclear gene mutations.

Autonomous regulation in mammalian mitochondrial DNA transcription

The regulation of the oxidative phosphorylation system (OXPHOS) biogenesis in eukaryotic cells is unique since it involves the expression of two genomes, the mitochondrial DNA (mtDNA) and the nuclear DNA (nDNA). The considerable effort done in collecting information on the factors that influence the expression of the genes encoded in mtDNA and nDNA has revealed that a multiplicity of regulatory options are available in mammalian cells to perform this task. Thus, at least three archetypal situations can be distinguished: mitochondrial proliferation, mitochondrial differentiation, and mitochondrial local tuning (MLT). Each of them seems to be predominantly under the control of specific strategies of regulation, although the description of the detailed molecular mechanisms involved is still in its beginnings. In the present review, we focus on the evidence supporting the existence of mechanisms for autonomous regulation of mtDNA transcription and its role in the integrated regulation of the OXPHOS system biogenesis.

Clinical mitochondrial genetics

The last decade has been an age of enlightenment as far as mitochondrial pathology is concerned. Well established nuclear genetic diseases, such as Friedreich's ataxia,¹² Wilson disease,³ and autosomal recessive hereditary spastic paraplegia,⁴ have been shown to have a mitochondrial basis, and we are just starting to unravel the complex nuclear genetic disorders which directly cause mitochondrial dysfunction (table 1). However, in addition to the 3 billion base pair nuclear genome, each human cell typically contains thousands of copies of a small, 16.5 kb circular molecule of double stranded DNA (fig 1). Mitochondrial DNA (mtDNA) accounts for only 1% of the total cellular nucleic acid content. It encodes for 13 polypeptides which are essential for aerobic metabolism and defects of the mitochondrial genome are an important cause of human disease.⁹²⁹³ Since the characterisation of the first pathogenic mtDNA defects in 1988,⁵¹³ over 50 point mutations and well over 100 rearrangements of the mitochondrial genome have been associated with human disease⁹⁴⁹⁵ (http://www.gen.emory.edu/mitomap.html). These disorders form the focus of this article.


Keywords: mitochondrial DNA; mitochondrial disease; heteroplasmy; genetic counselling

Complex approach to prenatal diagnosis of cytochrome c oxidase deficiencies

Different severe disorders of cytochrome c oxidase (COX) have been described in children, but only the defects with autosomal inheritance are suitable for prenatal diagnosis. To perform prenatal diagnosis of fatal infantile COX deficiency a complex approach has been used which combined determination of the genetic origin of the defect, and detailed analysis of the function, content and subunit composition of the enzyme in cultured fetal cells. The tissues and cultured fibroblasts of the patient with Leigh's syndrome showed a COX deficiency of systemic character. The decrease of COX activity to 5-11 per cent was accompanied by proportionally decreased content of the assembled COX enzyme. With the help of transmitochondrial cybrids derived from patient fibroblasts it was proven that the COX defect was of nuclear origin. In a successive pregnancy, the function of oxidative phosphorylation (OXPHOS) was analysed in cultured amniocytes by substrate-stimulated ATP production and COX activity was compared with the activity of citrate synthase. The amount and composition of OXPHOS complexes was estimated by two-dimensional (Blue Native/SDS) polyacrylamide gel electrophoresis and was verified immunochemically with specific antibodies. Three independent lines of evidence provided us with reliable data on the function of COX and OXPHOS in fetal cells which were sufficient to rule out the expected enzymatic defect within three weeks after amniocentesis.

Mitochondrial diseases in man and mouse

Over the past 10 years, mitochondrial defects have been implicated in a wide variety of degenerative diseases, aging, and cancer. Studies on patients with these diseases have revealed much about the complexities of mitochondrial genetics, which involves an interplay between mutations in the mitochondrial and nuclear genomes. However, the pathophysiology of mitochondrial diseases has remained perplexing. The essential role of mitochondrial oxidative phosphorylation in cellular energy production, the generation of reactive oxygen species, and the initiation of apoptosis has suggested a number of novel mechanisms for mitochondrial pathology. The importance and interrelationship of these functions are now being studied in mouse models of mitochondrial disease.