Molecular genetics of the hereditary ataxias

In children with Friedreich ataxia, muscle and ataxia parameters are associated

AIM

In children with Friedreich ataxia (FRDA), ataxia is assessed using the surrogate marker the International Cooperative Ataxia Rating Scale (ICARS). We aimed to determine whether ICARS scores in children with FRDA are confounded by muscle weakness.

METHOD

In 12 children with FRDA (10 males, two females; mean age 13 y 6 mo, SD 2 y 6 mo) and 12 age-matched children without FRDA (nine males; three females), we determined the association between muscle and ataxia parameters (i.e. muscle ultrasound density (MUD), muscle force, sensory evoked potentials, and ICARS scores). Children with FRDA were included on the basis of FXN gene analysis. Children in the comparison group were included on basis of uneventful pregnancy and normal cognitive and neurological development.

RESULTS

In children with FRDA, muscle ultrasound density was homogeneously increased in the biceps, quadriceps, and tibialis anterior muscles (median 4SD). FRDA muscle weakness was significantly more pronounced in proximal than in distal muscles (-2SD vs -0.5SD respectively; p=0.004), with a stronger impairment of leg muscles than of arm muscles (-2SD vs -0. SD respectively; p=0.001). Comparing MUD between children with FRDA and an age-matched comparison group revealed a relatively strong increase in MUD in the proximal leg muscles in the FRDA group. Under the condition of persistently absent sensory evoked potentials, leg ICARS subscores in the FRDA group appeared to be positively associated with leg muscle force until a maximal plateau level of ICARS subscores was reached.

INTERPRETATION

In children with FRDA, ataxia scales based on ICARS are confounded by muscle weakness. Longitudinal ICARS evaluations in children with FRDA do not necessarily indicate altered ataxia.

Pregnancy with Friedreich ataxia: a retrospective review of medical risks and psychosocial implications

OBJECTIVE

Friedreich ataxia (FRDA) is an autosomal recessive, neurodegenerative disease. Recent medical advances have improved the average life expectancy, and as such, many female patients are contemplating pregnancy. However, little research exists exploring the medical or psychosocial complications that arise from pregnancy with this disease.

STUDY DESIGN

In this study, we retrospectively examined 31 women with FRDA who had 65 pregnancies, resulting in 56 live offspring.

RESULTS

FRDA did not appear to increase the risk of spontaneous abortion, preeclampsia, or preterm birth. Despite the sensory and proprioceptive loss that occurs in FRDA, nearly four fifths of births were vaginal. Of babies, 94.4% were discharged home with their mothers. Equal numbers of women reported that pregnancy made their disease symptoms worse, better, or unchanged.

CONCLUSION

This study demonstrates that women with FRDA can have uncomplicated pregnancies that do not uniformly complicate disease symptomatology.

Absence of aprataxin gene mutations in a Greek cohort with sporadic early onset ataxia and normal GAA triplets in frataxin gene

Phenotype of patients with the aprataxin gene mutation varies and according to previous studies, screening of aprataxin gene could be useful, once frataxin gene mutation is excluded in patients with normal GAA expansion in frataxin gene. In the present study, we sought to determine possible causative mutations in aprataxin gene (all exons and flanking intronic sequences) in 14 Greek patients with sporadic cerebellar ataxia all but one without GAA expansion in frataxin gene (1 patient was heterozygous). No detectable point mutation or deletion was found in the aprataxin gene of all the patients. Our results do not confirm the previous studies. This difference may be attributed to the different populations studied and possible different genetic background. It is still questionable whether the screening for aprataxin mutation in Greek patients' Friedreich ataxia phenotype is of clinical importance; larger, multicenter studies are necessary to clarify this issue.

Measuring the rate of progression in Friedreich ataxia: implications for clinical trial design

Friedreich ataxia is an autosomal recessive neurodegenerative disorder characterized by ataxia of all four limbs, dysarthria, and arreflexia. A variety of measures are currently used to quantify disease progression, including the Friedreich Ataxia Rating Scale, examiner-rated functional disability scales, self-reported activities of daily living and performance measures such as the timed 25-foot walk, 9-hole pegboard test, PATA speech test, and low-contrast letter acuity vision charts. This study examines the rate of disease progression over one and two years in a cohort of 236 Friedreich ataxia patients using these scales and performance measure composites. The Friedreich Ataxia Rating Scale and performance-measure composites captured disease progression, with a greater sensitivity to change over 2 years than over 1 year. The measures differed in their sensitivity to change and in possible bias. These results help to establish norms for progression in FRDA that can be useful in measuring the long-term success of therapeutic agents and defining sample-size calculations for double-blind clinical trials.

Idebenone in Friedreich's ataxia

BACKGROUND

Friedreich's ataxia is an autosomal recessive neurodegenerative disease where impaired mitochondrial function and excessive production of free radicals play a central pathogenetic role. Idebenone, a synthetic analogue of coenzyme Q, is a powerful antioxidant that was first administrated to Friedreich's ataxia patients less than 10 years ago.

OBJECTIVE

The aim of this study was to evaluate the efficacy of idebenone administration and define the optimal dosage.

METHODS

A critical evaluation of all open and double-blinded idebenone trials in Friedreich's ataxia patients was undertaken.

RESULTS/CONCLUSIONS

Idebenone is well tolerated in paediatric and adult patients. Most trials demonstrated a positive effect on cardiac hypertrophy. The neurological function is in general not modified in adult patients, but a dose-dependent effect was demonstrated in young Friedreich's ataxia patients. Further double-blinded high-dose trials should evaluate idebenone in Friedreich's ataxia early in the disease course.

Efficacy and safety of idebenone in the treatment of Friedreich ataxia: a review of early results and future prospects

Friedreich ataxia (FRDA) is an autosomal-recessive neurodegenerative disorder characterized by difficulty walking, dysarthria and absent reflexes. At present, no treatment or cure exists, but the pathophysiology of FRDA suggests that antioxidants and related compounds may be efficacious. Research in the field has focused on idebenone, a lipid-soluble synthetic short-chain analogue of the quinone coenzyme Q10. Idebenone protects mitochondrial membranes from oxidative damage in vitro and may facilitate electron transport within the mitochondrial electron transport chain. In FRDA, idebenone decreases cardiac hypertrophy, but the clinical significance of this finding remains unknown. Recent Phase II trials using higher doses of idebenone have suggested a dose-dependent improvement in neurologic symptoms. To confirm these results, a Phase III trial has begun in the USA with the goal of assessing the efficacy of idebenone for the treatment of neurologic dysfunction in FRDA.

Identification and sizing of GAA trinucleotide repeat expansion, investigation for D-loop variations and mitochondrial deletions in Iranian patients with Friedreich's ataxia

Friedreich's Ataxia (FA) is the commonest genetic cause of ataxia and is associated with the expansion of a GAA repeat in intron 1 of the frataxin gene. Iron accumulation in the mitochondria of patients with FA would result in hypersensitivity to oxidative stress. Mitochondrial DNA (mtDNA) could be considered a candidate modifier factor for FA disease, since mitochondrial oxidative stress is thought to be involved in the pathogenesis of this disease. We studied 25 Iranian patients (16 females and 9 males) from 12 unrelated families. DNA from each patient was extracted and frequency and length of (GAA)(n) repeat was analyzed using a long-range polymerase chain reaction (PCR) test. Also we investigated impact of GAA size on neurological findings, age of onset and disease development. In order to identify polymorphic sites and genetic background, the sequence of two hypervariable regions (HVR-I and HVR-II) of mtDNA was obtained from FA patients harbouring GAA trinucletide expansions. Alignment was made with the revised cambridge reference sequence (rCRS) and any differences recorded as single base substitution (SBS), insertions and deletions. Homozygous GAA expansion was found in 21 (84%) of all cases. In four cases (16%), no expansion was observed, ruling out the diagnosis of Friedreich's ataxia. In cases with GAA expansions, ataxia, scoliosis and pes cavus, cardiac abnormalities and some neurological findings occurred more frequently than in our patients without GAA expansion. Molecular analysis was imperative for diagnosis of Friedreich's ataxia, not only for typical cases, but also for atypical ones. Diagnosis bases only on clinical findings is limited, however, it aids in better screening for suspected cases that should be tested. Our results showed that the rate of D-loop variations was higher in FA patients than control (P<0.05). mtDNA deletions were present in 76% of our patients representing mtDNA damage, which may be due to iron accumulation in mitochondria.

Expression of the human ferritin light chain in a frataxin mutant yeast affects ageing and cell death

Ferritin is one of the major eukaryotic proteins involved in regulating iron metabolism and maintaining iron homeostasis. However, Saccaromyces cerevisiae is an exception, possessing no ferritin and using other means to store excess iron. The only potential iron storage protein identified in yeast so far is the homologue of human frataxin (YFH1p). In this study, we found that dysfunction of yeast frataxin shortens mean lifespan by 49% compared to the WT control. Interestingly, the human ferritin L gene can, at least partially, complement the function of yeast frataxin, extending lifespan and protecting cells from death induced by oxidative stress or excess iron. Our findings indicate that ferritin L can perform functions in yeast that are similar to its functions in mammals, and suggest that common mechanisms may exist for preventing iron and oxidative damage in single- and multi-cellular eukaryotic organisms. Clearly, elucidation of the function of human ferritin in yeast would help in gaining a better understanding the molecular basis of iron storage diseases.

Friedreich's ataxia

Friedreich's ataxia, the most common hereditary ataxia, is caused by expansion of a GAA triplet located within the first intron of the frataxin gene on chromosome 9q13. There is a clear correlation between size of the expanded repeat and severity of the phenotype. Frataxin is a mitochondrial protein that plays a role in iron homeostasis. Deficiency of frataxin results in mitochondrial iron accumulation, defects in specific mitochondrial enzymes, enhanced sensitivity to oxidative stress, and eventually free-radical mediated cell death. Friedreich's ataxia is considered a nuclear encoded mitochondrial disease. This review discusses the major and rapid progress made in Friedreich's ataxia from gene mapping and identification of the gene to pathogenesis and encouraging therapeutic implications.

Sticky DNA, a long GAA.GAA.TTC triplex that is formed intramolecularly, in the sequence of intron 1 of the frataxin gene

Friedreich's ataxia is caused by the massive expansion of GAA.TTC repeats in intron 1 of the frataxin (X25) gene. Our prior investigations showed that long GAA.TTC repeats formed very stable triplex structures which caused two repeat tracts to adhere to each other (sticky DNA). This process was dependent on negative supercoiling and the presence of divalent metal ions. Herein, we have investigated the formation of sticky DNA from plasmid monomers and dimers; sticky DNA is formed only when two tracts of sufficiently long (GAA.TTC)(n) (n = 59-270) are present in a single plasmid DNA and are in the direct repeat orientation. If the inserts are in the indirect (inverted) repeat orientation, no sticky DNA was observed. Furthermore, kinetic studies support the intramolecular nature of sticky DNA formation. Electron microscopy investigations also provide strong data for sticky DNA as a single long triplex. Hence, these results give new insights into our understanding of the capacity of sticky DNA to inhibit transcription and thereby reduce the level of frataxin protein as related to the etiology of Friedreich's ataxia.

Friedreich's ataxia: clinical and molecular study of 25 Brazilian cases

INTRODUCTION

Friedreich's ataxia is a neurodegenerative disorder whose clinical diagnostic criteria for typical cases basically include: a) early age of onset (< 20 or 25 years), b) autosomal recessive inheritance, c) progressive ataxia of limbs and gait, and d) absence of lower limb tendon reflexes.

METHODS

We studied the frequency and the size of expanded GAA and their influence on neurologic findings, age at onset, and disease progression in 25 Brazilian patients with clinical diagnosis of Friedreich's ataxia - 19 typical and 6 atypical - using a long-range PCR test.

RESULTS

Abnormalities in cerebellar signs, in electrocardiography, and pes cavus occurred more frequently in typical cases; however, plantar response and speech were more frequently normal in this group when the both typical and atypical cases were compared. Homozygous GAA expansion repeats were detected in 17 cases (68%) - all typical cases. In 8 patients (32%) (6 atypical and 2 typical), no expansion was observed, ruling out the diagnosis of Friedreich's ataxia. In cases with GAA expansions, foot deformity, cardiac abnormalities, and some neurologic findings occurred more frequently; however, abnormalities in cranial nerves and in tomographic findings were detected less frequently than in patients without GAA expansions.

DISCUSSION

Molecular analysis was imperative for the diagnosis of Friedreich's ataxia, not only for typical cases but also for atypical ones. There was no genotype-phenotype correlation. Diagnosis based only on clinical findings is limited; however, it aids in better screening for suspected cases that should be tested. Evaluation for vitamin E deficiency is recommended, especially in cases without GAA expansion.

The alpha-ketoglutarate dehydrogenase complex

The alpha-ketoglutarate dehydrogenase complex (KGDHC) is an important mitochondrial constituent, and deficiency of KGDHC is associated with a number of neurological disorders. KGDHC is composed of three proteins, each encoded on a different and well-characterized gene. The sequences of the human proteins are known. The organization of the proteins into a large, ordered multienzyme complex (a "metabolon") has been well studied in prokaryotic and eukaryotic species. KGDHC catalyzes a critical step in the Krebs tricarboxylic acid cycle, which is also a step in the metabolism of the potentially excitotoxic neurotransmitter glutamate. A number of metabolites modify the activity of KGDHC, including inactivation by 4-hydroxynonenal and other reactive oxygen species (ROS). In human brain, the activity of KGDHC is lower than that of any other enzyme of energy metabolism, including phosphofructokinase, aconitase, and the electron transport complexes. Deficiencies of KGDHC are likely to impair brain energy metabolism and therefore brain function, and lead to manifestations of brain disease. In general, the clinical manifestations of KGDHC deficiency relate to the severity of the deficiency. Several such disorders have been recognized: infantile lactic acidosis, psychomotor retardation in childhood, intermittent neuropsychiatric disease with ataxia and other motor manifestations, Friedreich's and other spinocerebellar ataxias, Parkinson's disease, and Alzheimer's disease (AD). A KGDHC gene has been associated with the first two and last two of these disorders. KGDHC is not uniformly distributed in human brain, and the neurons that appear selectively vulnerable in human temporal cortex in AD are enriched in KGDHC. We hypothesize that variations in KGDHC that are not deleterious during reproductive life become deleterious with aging, perhaps by predisposing this mitochondrial metabolon to oxidative damage.