The discovery that expansion of unstable repeats can cause a variety of neurological disorders has changed the landscape of disease-oriented research for several forms of mental retardation, Huntington disease, inherited ataxias, and muscular dystrophy. The dynamic nature of these mutations provided an explanation for the variable phenotype expressivity within a family. Beyond diagnosis and genetic counseling, the benefits from studying these disorders have been noted in both neurobiology and cell biology. Examples include insight about the role of translational control in synaptic plasticity, the role of RNA processing in the integrity of muscle and neuronal function, the importance of Fe-S-containing enzymes for cellular energy, and the dramatic effects of altering protein conformations on neuronal function and survival. It is exciting that within a span of 15 years, pathogenesis studies of this class of disorders are beginning to reveal pathways that are potential therapeutic targets.

Trinucleotide Repeat Disorders

Mutations in the proteolytic enzyme calpain 3 cause limb-girdle muscular dystrophy type 2A

The gene responsible for Friedreich's ataxia, a disease characterized by neurodegeneration and cardiomyopathy, has recently been cloned and its product designated frataxin. A gene in Saccharomyces cerevisiae was characterized whose predicted protein product has high sequence similarity to the human frataxin protein. The yeast gene (yeast frataxin homolog, YFH1) encodes a mitochondrial protein involved in iron homeostasis and respiratory function. Human frataxin also was shown to be a mitochondrial protein. Characterizing the mechanism by which YFH1 regulates iron homeostasis in yeast may help to define the pathologic process leading to cell damage in Friedreich's ataxia.

https://science.sciencemag.org/content/sci/276/5319/1709.full.pdf

Regulation of Mitochondrial Iron Accumulation by Yfh1p, a Putative Homolog of Frataxin

Friedreich ataxia is a progressive neurodegenerative disorder caused by loss of function mutations in the frataxin gene. In order to unravel frataxin function we developed monoclonal antibodies raised against different regions of the protein. These antibodies detect a processed 18 kDa protein in various human and mouse tissues and cell lines that is severely reduced in Friedreich ataxia patients. By immunocytofluorescence and immunocytoelectron microscopy we show that frataxin is located in mitochondria, associated with the mitochondrial membranes and crests. Analysis of cellular localization of various truncated forms of frataxin expressed in cultured cells and evidence of removal of an N-terminal epitope during protein maturation demonstrated that the mitochondrial targetting sequence is encoded by the first 20 amino acids. Given the shared clinical features between Friedreich ataxia, vitamin E deficiency and some mitochondriopathies, our data suggest that a reduction in frataxin results in oxidative damage.

/pdf/frataxin-is-reduced-in-friedreich-ataxia-patients-and-is-4jwcjo405e.pdf

Frataxin is Reduced in Friedreich Ataxia Patients and is Associated with Mitochondrial Membranes

The roles of iron in health and disease

Background Friedreich's ataxia, the most common inherited ataxia, is associated with a mutation that consists of an unstable expansion of GAA repeats in the first intron of the frataxin gene on chromosome 9, which encodes a protein of unknown function. Methods We studied 187 patients with autosomal recessive ataxia, determined the size of the GAA expansions, and analyzed the clinical manifestations in relation to the number of GAA repeats and the duration of disease. Results One hundred forty of the 187 patients, with ages at onset ranging from 2 to 51 years, were homozygous for a GAA expansion that had 120 to 1700 repeats of the trinucleotides. About one quarter of the patients, despite being homozygous, had atypical Friedreich's ataxia; they were older at presentation and had intact tendon reflexes. Larger GAA expansions correlated with earlier age at onset and shorter times to loss of ambulation. The size of the GAA expansions (and particularly that of the smaller of each pair) was associated with the ...

Clinical and Genetic Abnormalities in Patients with Friedreich's Ataxia

Summary Thelocus forFriedreich ataxia (FRDA), a severeneurodegenerative disease, istightly linked tomarkers D9S5 andD9S15, andanalysis ofrarerecombination eventshassuggested theorder cen-FRDA-D9S5-D9S15-qter. Wereportheretheconstruction ofa YACcontig extending 800kbcentromeric toD9S5andtheisolation of five new microsatellite markers fromthis region. Inorder tomap these markers with respect totheFRDAlocus, all within a1-cMconfidence interval, we sought toincrease thegenetic information ofavailable FRDAfamilies byconsidering homozygosity bydescent andassociation withfounder haplotypes inisolated populations. This approach allowed ustoidentify one phase-known recombination andoneprobable historic recombination on haplotypes fromReunion Island patients, bothofwhichplace three ofthefive markers proxima toFRDA. This represents thefirst identification ofclose FRDAflanking markers on thecentromeric side. Thetwoother markers allowed ustonarrowthebreakpoint ofapreviously identified distal recombination that is>180kb fromD9S5(26P). Takentogether, theresults place theFRDAlocus ina450-kb interval, which issmall enough fordirect search ofcandidate genes.Adetailed rarecutterrestriction map andacosmid contig covering this interval wereconstructed andshould facilitate thesearch ofgenesinthis region.

Recombinations inIndividuals HomozygousbyDescent Localize theFriedreich Ataxia Locusina Cloned 450-kb Interval

Next-generation sequencing (NGS) has an established diagnostic value for inherited ataxia. However, the need of a rigorous process of analysis and validation remains challenging. Moreover, copy number variations (CNV) or dynamic expansions of repeated sequence are classically considered not adequately detected by exome sequencing technique. We applied a strategy of mini-exome coupled to read-depth based CNV analysis to a series of 33 patients with probable inherited ataxia and onset <50 years. The mini-exome consisted of the capture of 4,813 genes having associated clinical phenotypes. Pathogenic variants were found in 42% and variants of uncertain significance in 24% of the patients. These results are comparable to those from whole exome sequencing and better than previous targeted NGS studies. CNV and dynamic expansions of repeated CAG sequence were identified in three patients. We identified both atypical presentation of known ataxia genes (ATM, NPC1) and mutations in genes very rarely associated with ataxia (ERCC4, HSD17B4). We show that mini-exome bioinformatics data analysis allows the identification of CNV and dynamic expansions of repeated sequence. Our study confirms the diagnostic value of the proposed genetic analysis strategy. We also provide an algorithm for the multidisciplinary process of analysis, interpretation, and validation of NGS data.

Claude Mignard

Papers

Clinical and Genetic Abnormalities in Patients with Friedreich's Ataxia

Recombinations inIndividuals HomozygousbyDescent Localize theFriedreich Ataxia Locusina Cloned 450-kb Interval

Mini‐Exome Coupled to Read‐Depth Based Copy Number Variation Analysis in Patients with Inherited Ataxias