RESUMO
Friedreich’s ataxia is a commonly inherited neurodegenerative disease with an autosomal recessive pattern of inheritance, and was described by Nikolaus Friedreich first in 1863. Friedreich’s ataxia is caused due to hyperexpansion of the intronic GAA trinucleotide repeats or mutations in the FXN gene on chromosome 9q13. This gene codes for a mitochondrial protein, frataxin, which is highly conserved in many species and has functions in iron-sulfur cluster biosynthesis. Friedreich’s ataxia mainly results from a deficiency of the frataxin protein, due to mutations in the FXN gene. Formation of sticky DNA, formation of DNA-RNA hybrid and epigenetic changes, including methylation of DNA and histone modifications, are the proposed mechanisms for disruption of FXN gene expression. Most cases of Friedreich’s ataxia are homozygous and caused due to expansion of the GAA trinucleotide repeat in the first intron of the FXN gene, however, some cases can be heterozygous, with GAA expansion in one allele and point mutation or deletion in the FXN gene on the other allele. Therefore, diagnosis of the disease based on only the clinical symptoms becomes difficult. Molecular diagnosis is, therefore, important, in order to detect GAA repeat expansions as well as mutations in the FXN gene. This review represents an overview of the molecular diagnostic studies in Friedreich’s ataxia, including an overview of the disease, as well as the gene and protein involved in the disease and techniques that can be useful in diagnosis of the Friedreich’s ataxia. The described methods include tools that are based on analysis of DNA as well as analysis of mRNA and protein levels. A brief description of mutations found in compound heterozygous Friedreich’s ataxia patients, is also provided.
RESUMO
It is now established that a small fraction of genomic DNA does adopt the non-canonical B-DNA structure or ‘unusual’ DNA structure. The unusual DNA structures like DNA-hairpin, cruciform, Z-DNA, triplex and tetraplex are represented as hotspots of chromosomal breaks, homologous recombination and gross chromosomal rearrangements since they are prone to the structural alterations. Friedreich’s ataxia (FRDA), the autosomal recessive degenerative disorder of nervous and muscles tissue, is caused by the massive expansion of (GAA) repeats that occur in the first intron of Frataxin gene X25 on chromosome 9q13-q21.1. The purine strand of the DNA in the expanded (GAA) repeat region folds back to form the (R∙R*Y) type of triplex, which further inhibits the frataxin gene expression, and this clearly suggests that the shape of DNA is the determining factor in the cellular function. FRDA is the only disease known so far to be associated with DNA triplex. Structural characterization of GAA-containing DNA triplexes using some simple biophysical methods like UV melting, UV absorption, circular dichroic spectroscopy and electrophoretic mobility shift assay are discussed. Further, the clinical aspects and genetic analysis of FRDA patients who carry (GAA) repeat expansions are presented. The potential of some small molecules that do not favour the DNA triplex formation as therapeutics for FRDA are also briefly discussed.
RESUMO
Introducción: La ataxia de Friedreich (FRDA) es una enfermedad autosómica recesiva debida a una mutación en el gen X25. Dicho gen está localizado en el cromosoma 9 y codifica para la proteína frataxina. La enfermedad es causada por la repetición del trinucleótido GAA. En individuos normales la secuencia GAA se encuentra repetida entre siete y veintidós veces, mientras que, en pacientes con ataxia de Friedreich GAA puede estar repetida cientos o miles de veces. Objetivos: Evaluar si existe correlación entre el tamaño de la expansión, la edad de inicio de FRDA y su severidad en la muestra seleccionada. Métodos: - Se estudiaron once pacientes con fenotipo típico de ataxia de Friedreich. El análisis molecular por PCR determinó la expansión del trinucleótido GAA. Se analizó la correlación entre la edad de inicio de FRDA y su progresión con el número de repeticiones GAA. Resultados y conclusiones: - El análisis molecular por PCR mostró ocho pacientes homocigotos para la expansión, y tres negativos. El promedio del tamaño de las expansiones en los alelos es 622±5 con un promedio correspondiente de la edad inicio de FRDA 13±8. Para el tamaño de la muestra no se observó una correlación estadística significativa entre la edad de inicio de la enfermedad y el número de repeticiones, pero sí una tendencia a correlacionarse de forma inversa (p<0.11). El diagnóstico molecular de FRDA sumado a la comprensión de su fisiología y a la utilización de los criterios de inclusión de Harding, constituye un paso importante en el logro de un tratamiento óptimo de la enfermedad.
Introduction: - Friedreich's ataxia is an autosomal recessive disease due to a mutation in gene X25. This gene codes for frataxin and it is located on chromosome 9. The disease is caused by a triplet particular sequence of bases (GAA). Normally, the GAA sequence is repeated 7 to 22 times, but in people with Friedreich's ataxia, it can be repeated hundreds or even over thousand times. Objectives: To determine if there is a correlation between clinical and molecular findings in our FRDA patients. Methods: Eleven patients with the typical Friedreich´s ataxia phenotype were studied by PCR we determined the size of the GAA expansions, and analyzed the correlation of age at onset and rate of disease progression with the number of GAA repetitions. Results and conclusions: Molecular analysis by PCR showed eight homozygous patients for the expansion and three negative. The average of the size of the expansions in the allele was of 622±5 with an average in the age of beginning of 13±8. For the sample size, there was no significant statistical correlation between the age of beginning of the disease and the number of repetitions, although there was like an inverse correlation. Besides understanding of FRDA physiology and the Harding clinical inclusion criteria, molecular diagnosis is an important step in the achievement of an optimal therapeutic treatment.