Stefano DiDonato

Pathways to motor incoordination: the inherited ataxias

Two groups of hereditary ataxias are most relevant to humans — the autosomal recessive ataxias and the autosomal dominant spinocerebellar ataxias. Recessive ataxias are multisystem disorders that are characterized by inactivating mutations that result in loss of protein function. By contrast, cell death associated with dominant spinocerebellar ataxias is mostly restricted to the CNS, and cellular control of protein folding and processing is affected. The purpose of this review is to provide an integrated view of the field, encompassing the similarities — which are few — and the differences — which are many — between pathological processes that cause ataxia. In reviewing the current knowledge of ataxias, we discuss recent insights into the pathogenic mechanisms that lead to specific neuronal dysfunction and neurodegeneration.

A MERRF/MELAS Overlap Syndrome Associated with a New Point Mutation in the Mitochondrial DNA tRNA^Lys Gene

Several members of a three-generation kindred from Sardinia were affected by a maternally inherited syndrome characterized by features of both myoclonus epilepsy with ragged-red fibers (MERRF) and mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS). Clinically, symptoms such as myoclonus epilepsy, neural deafness and ataxia were variably associated with stroke-like episodes and/or migrainous attacks. Morphologically, numerous ME-LAS-associated SDH-stained vessels were observed in muscle biopsies, either alone or in combination with ragged-red fibers, the morphological hallmark of MERRF. Sequence analysis of the mtDNA tRNA genes revealed the presence of a single, heteroplasmic T → C transition at nt 8356, in the region of the tRNALys gene corrsponding to the T-Ψ-C stem. The T → C(8356) transition was exclusively found in the maternal lineage of our family, and the relative amount of the mutant mtDNA species in muscle was correlated with the severity of the clinical presentation. Therefore, we propose that the T → C(8356) transition is responsible for the mitochondrial encephalomyopathy found in our family, and must be added to the expanding list of the pathogenetically relevant mutations of human mtDNA.

Cloning of human and rat cDNAs encoding the mitochondrial single-stranded DNA-binding protein (SSB)

We have retro-transcribed and amplified by PCR the full-length cDNAs specifying the rat and human precursors of the single-stranded mitochondrial DNA (mtDNA)-binding protein (mtSSB). Each deduced sequence is composed of a 16-amino-acid (aa) N-terminal basic pre-sequence and a mature protein (132 aa in humans and 135 aa in the rat). The mature proteins are highly conserved among themselves and with the mtSSB from Xenopus laevis (Xl). Moreover, three regions of the protein are similar to corresponding domains of the SSB of Escherichia coli and to the E. coli F-sex factor SSB, indicating the existence of a broad class of DNA-binding proteins with structural and functional similarities both in prokaryotes and in prokaryote-derived organelles of higher organisms.

Cholesterol Defect Is Marked across Multiple Rodent Models of Huntington's Disease and Is Manifest in Astrocytes

Brain cholesterol, which is synthesized locally, is a major component of myelin and cell membranes and participates in neuronal functions, such as membrane trafficking, signal transduction, neurotransmitter release, and synaptogenesis. Here we show that brain cholesterol biosynthesis is reduced in multiple transgenic and knock-in Huntingtons disease (HD) rodent models, arguably dependent on deficits in mutant astrocytes. Mice carrying a progressively increased number of CAG repeats show a more evident reduction in cholesterol biosynthesis. In postnatal life, the cholesterol-dependent activities of neurons mainly rely on the transport of cholesterol from astrocytes on ApoE-containing particles. Our data show that mRNA levels of cholesterol biosynthesis and efflux genes are severely reduced in primary HD astrocytes, along with impaired cellular production and secretion of ApoE. Consistently, in CSF of HD mice, ApoE is mostly associated with smaller lipoproteins, indicating reduced cholesterol transport on ApoE-containing lipoproteins circulating in the HD brain. These findings indicate that cholesterol defect is robustly marked in HD animals, implying that strategies aimed at selectively modulating brain cholesterol metabolism might be of therapeutic significance.

Genotype to phenotype correlations in mitochondrial encephalomyopathies associated with the A3243G mutation of mitochondrial DNA

We studied 22 subjects carrying the A3243G point mutation of human mitochondrial DNA (mtDNA). In 14 cases the clinical phenotype was characterized by mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS), while 8 patients had chronic progressive external ophthalmoplegia (CPEO). The proportion of A3243G heteroplasmy in muscle was determined by two methods: densitometry on a diagnostic restriction-fragment length polymorphism and solid-phase mini-sequencing. We found a highly significant inverse correlation between the percentage of A3243G mutation and the specific activity of complex 1, the respiratory complex with the highest number of mtDNA-encoded subunits, suggesting a direct effect of the mutation on mtDNA translation. No correlation was observed between the percentage of mutated mtDNA and the presence or absence of specific clinical features, such as stroke, ophthalmoplegia and diabetes mellitus. However, in the MELAS group the percentage of mutated mtDNA molecules was strongly correlated with the age of onset, while no such correlation was found in the CPEO group, suggesting a different time-dependent evolution of the mutation in the two groups. Finally, in contrast with other mtDNA mutations associated with ragged-red fibres (RRF), in both MELAS3243 and CPE03243 we observed a high proportion of RRF that were positive to the histochemical reaction to cytochromec oxidase, a morphological feature that seems to be specific for the neuromuscular phenotypes associated with mutations affecting the tRNALeu(UUR) gene.

Respiratory chain and mitochondrial DNA in muscle and brain in Parkinson's disease patients

There are several reports of a defect of complex I in the substantia nigra (SN) of Parkinsons disease (PD) patients. To evaluate whether this is specific to dopaminergic neurons or the phenotypically relevant consequence of a widespread failure of the mitochondrial oxidative phosphorylation (OXPHOS) system, we measured respiratory enzyme activities in muscle homogenates from 16 PD patients and eight age-matched controls, and in muscle isolated mitochondria of six PD patients and six age-matched controls. We found no difference between the PD and control groups. In addition, we detected, by polymerase chain reaction, the mitochondrial DNA (mtDNA) “common deletion” (CD) in muscle specimens of 14 of 17 PD patients, but we obtained similar results in age-matched controls. In both groups, the amount of CD-specific deleted (Δ) mtDNA ranged from 0.0% to 0.1%. Our data suggest that PD cannot be attributed to a multisystem decline of mitochondrial OXPHOS, and that lesions of muscle mtDNA in PD are likely due to normal aging. However, there was a remarkable accumulation of ΔmtDNA in the SN of a PD patient and an age-matched control, suggesting that the SN is exquisitely sensitive to age-dependent damage of the mitochondrial genome.

Fumarase deficiency is an autosomal recessive encephalopathy affecting both the mitochondrial and the cytosolic enzymes

A 7-month-old boy died in a demented state after a clinical history characterized by generalized seizures, psychomotor deterioration, and fumaric aciduria. We found a marked deficiency of both mitochondrial and cytosolic fumarases in skeletal muscle, brain, cerebellum, heart, kidney, liver, and cultured fibroblasts. Fumarase activities were 30 to 50% compared with controls in both mitochondria and cytosol from cultured fibroblasts of the parents. Antifumarase cross-reacting material was present in negligible amounts in the patients tissues. Our data indicate that this disease is an autosomal recessive encephalopathy, due to a single mutation affecting the gene encoding both forms of the enzyme.

Chromosomal localization of mitochondrial transcription factor A (TCF6), single-stranded DNA-binding protein (SSBP), and Endonuclease G (ENDOG), three human housekeeping genes involved in mitochondrial biogenesis

By using a PCR-based screening of a somatic cell hybrid panel and FISH, we have assigned the loci of mitochondrial single-stranded DNA-binding protein (SSBP), mitochondrial transcription factor A (TCF6), and mitochondrial endonuclease G (ENDOG) genes to human chromosomes 7q34, 10q21, and 9q34.1, respectively. The products of these three genes are involved in fundamental aspects of mitochondrial biogenesis, such as replication and transcription of the mitochondrial genome. The chromosomal localization of these genes is important to testing whether the corresponding proteins may play a role in the etiopathogenesis of human disorders associated with qualitative or quantitative abnormalities of mitochondrial DNA.

Late‐onset riboflavin‐responsive myopathy with combined multiple acyl coenzyme A dehydrogenase and respiratory chain deficiency

We studied the effect of riboflavin treatment on the clinical status and on the activities of β-oxidation and respiratory chain enzymes in a 69-year-old patient with late-onset myopathy. Before treatment, she was very weak and wasted in the limbs and trunk muscles; also, she could not walk or attend to daily activities. Marked lipid storage was present in the muscle biopsy. The activities of short-chain acyl coenzyme A (acyl-CoA) dehydrogenase (SCAD), medium-chain acyl-CoA dehydrogenase (MCAD), and long-chain acyl-CoA dehydrogenase (LCAD) in isolated muscle mitochondria were reduced to less than 10% of control values. This defect in fatty acid oxidation was associated with a marked deficiency of two flavin-dependent respiratory chain complexes: complex I activity was 20% and complex II activity was 25% of control values. By contrast, the activities of the nonflavin-dependent complex III and complex IV were normal. Western blot analysis of the patients muscle mitochondrial extracts with antibodies raised against purified SCAD, MCAD, and the α-and β-subunits of the electron transfer flavoprotein (ETF) showed absence of SCAD cross-reacting material (CRM), markedly decreased MCAD-CRM, and normal amounts of both α- and β-ETF-CRM. After riboflavin treatment, the patients clinical status dramatically improved and morphologic changes in muscle disappeared. SCAD activity increased to 55% of control values, whereas MCAD, LCAD, and complex I and complex II activities normalized. SCAD and MCAD immunoreactivity was restored to normal. On the basis of our experience and the data in the literature, we concluded that some lipid storage myopathies can show dramatic response to riboflavin.

Defective respiratory capacity and mitochondrial protein synthesis in transformant cybrids harboring the tRNA(Leu(UUR)) mutation associated with maternally inherited myopathy and cardiomyopathy.

We studied the physiometabolic effects of a mitochondrial DNA (mtDNA) heteroplasmic point mutation, the A-->G3260 transition associated with maternally inherited myopathy and cardiomyopathy. To eliminate the possible influence of the autochthonous nuclear gene set, we fused myoblast-derived cytoplasts of a patient with a human tumoral cell line deprived of mtDNA (Rho degrees). The presence and amount of the mutant G3260 vs the wild-type A3260 were measured by solid phase minisequencing. We observed a marked reduction of the percentage of mutant mtDNA in the culture system compared with that measured in the donors muscle biopsy, suggesting the presence of negative selection against the mutation. Furthermore, stable mitotic segregation of the two mtDNA populations was observed in 18 of 19 transformant clones, suggesting the presence of intraorganelle and possibly intracellular homoplasmy in the precursor cells of the donor. Several indexes of mtDNA-related respiratory capacity, including oxygen consumption, complex I- and complex IV-specific activities, and lactate production, were markedly abnormal in the clones containing a high proportion of mutant mtDNA, as compared with those containing homoplasmic wild-type mtDNA, possibly because of impaired mitochondrial protein synthesis. We conclude that (a) the A-->G3260 transition is indeed responsible for the mitochondrial disorder identified in the donor patient, and (b) transformant cybrid system gives direct evidence of the mitochondrial origin of a genetic disorder and should be adopted for the evaluation of the pathogenic potential of the mtDNA mutations.