Clinical and molecular advances in autosomal dominant cerebellar ataxias: from genotype to phenotype and physiopathology

Genetic ataxia

Advances in molecular genetics have led to identification of an increasing number of genes responsible for inherited ataxic disorders. Consequently, DNA testing has become a powerful method to unambiguously establish the diagnosis in some of these disorders; however, there are limitations in this approach. Furthermore, the ethical, social, legal and psychological implications of the genetic test results are complex, necessitating appropriate counseling. This article intends to help the practicing neurologist clinically differentiate these disorders, choose appropriate genetic tests, and recognize the importance of counseling.

Instability of the human minisatellite CEB1 in rad27Delta and dna2-1 replication-deficient yeast cells

Convergent studies in human and yeast model systems have shown that some minisatellite loci are relatively stable in somatic cells but not in the germline, and little is known about the mechanism(s) that can destabilize them. Unlike microsatellite sequences, mini satellites are not destabilized by mismatch repair mutations. We report here that the absence of Rad27 and Dna2 functions but not RNase H(35) or Exo1, which play an essential role in the processing of Okazaki fragments during replication, destabilize the human minisatellite CEB1 in mitotically growing Saccharomyces cerevisiae cells, up to 14% per generation in rad27Delta cells. Analysis using minisatellite variant repeat mapping by polymerase chain reaction of the internal structure of 17 variants reveals that the majority of rearrangements in rad27Delta cells are extremely complex contraction events that contain deletions, often accompanied by duplications of motif unit. Altogether, these results suggest that the improperly processed 5' flap structures that accumulate when replication is impaired can act as a potent stimulator of minisatellite destabilization and can provoke an unexpectedly broad range of mutagenic events. This replication-dependent phenomenon differs from the recombination-induced instability in yeast meiotic cells.

Spinocerebellar ataxia type 7 (SCA7) shows a cone-rod dystrophy phenotype

Autosomal dominant spinocerebellar ataxia 7 is associated with retinal degeneration. SCA7, the causative gene, encodes ataxin-7, a ubiquitous 892 amino acid protein of variable sub-cellular localization, and the disease is due to expansion of an unstable CAG repeat in the coding region of the gene. Recent increases in understanding of the mechanisms ofSCA7 -related retinopathy from in vitro and murine model studies prompted us to perform a detailed study of the retinal phenotype of affected members of a family with SCA7 mutation (45-47 CAG repeats). There was a spectrum of severity from mild to severe dysfunction. Early functional abnormalities were at both photoreceptor and post-receptoral levels. When cone and rod photoreceptor dysfunction was present, it was approximately equal. Regional retinal dysfunction was evident: there was more dysfunction centrally than peripherally with least effect in the midperiphery. In vivo cross-sectional retinal images with optical coherence tomography showed an early disease stage of altered foveal lamination (abnormal area of low reflectivity splitting the outer retina-choroidal complex) accompanied in the parafovea by reduced retinal thickness. Later disease stages showed foveal and parafoveal retinal thinning. The phenotype in this family with SCA7 is that of a cone-rod dystrophy. These observations increase interest in a recent hypothesis that ataxin-7 may interfere with the function of CRX (cone-rod homeobox), a transcription factor regulating photoreceptor genes and a cause of a cone-rod dystrophy phenotype in man.

Diagnosis of five spinocerebellar ataxia disorders by multiplex amplification and capillary electrophoresis

The autosomal-dominant spinocerebellar ataxias (ADCA) are a heterogeneous group of neurodegenerative disorders with variable expression and phenotypic overlap. An accurate diagnosis relies on detection of a mutation in a specific causative gene, which is typically an abnormal number of CAG trinucleotide repeats. To streamline testing in a clinical setting, we converted our current panel of tests for the spinocerebellar ataxias (SCA) types SCA1, SCA2, SCA3, SCA6, and SCA7 from five independent amplification reactions analyzed by polyacrylamide gel electrophoresis (PAGE) to a single multiplex amplification reaction analyzed by capillary electrophoresis (CE). Multiplex amplification was facilitated by the use of chimeric primers; different lengths and fluorochromes distinguished the amplicons. During CE with commercially available molecular weight standards, the SCA amplicons migrated faster than predicted, thereby underestimating their length compared to that determined previously by PAGE. This was observed to varying degrees for each of the five loci, with the greatest size differential occurring in amplicons with greater (CAG)(n). To determine accurate amplicon length, and therefore an accurate number of CAG repeats, a size correction formula was calculated for each locus. This multiplex semi-automated assay has been reliable during 1 year of use in a clinical setting during which 57 samples were tested and five positive samples were detected.

Association of CAG repeat loci on chromosome 22 with schizophrenia and bipolar disorder

Chromosome 22 has been implicated in schizophrenia and bipolar disorder in a number of linkage, association and cytogenetic studies. Recent evidence has also implicated CAG repeat tract expansion in these diseases. In order to explore the involvement of CAG repeats on chromosome 22 in these diseases, we have created an integrated map of all CAG repeats > or =5 on this chromosome together with microsatellite markers associated with these diseases using the recently completed nucleotide sequence of chromosome 22. Of the 52 CAG repeat loci identified in this manner, four of the longest repeat stretches in regions previously implicated by linkage analyses were chosen for further study. Three of the four repeat containing loci, were found in the coding region with the CAG repeats coding for glutamine and were expressed in the brain. All the loci studied showed varying degrees of polymorphism with one of the loci exhibiting two alleles of 7 and 8 CAG repeats. The 8-repeat allele at this locus was significantly overrepresented in both schizophrenia and bipolar patient groups when compared to ethnically matched controls, while alleles at the other three loci did not show any such difference. The repeat lies within a gene which shows homology to an androgen receptor related apoptosis protein in rat. We have also identified other candidate genes in the vicinity of this locus. Our results suggest that the repeats within this gene or other genes in the vicinity of this locus are likely to be implicated in bipolar disorder and schizophrenia.

Clinical and genetic analysis of hereditary and sporadic ataxia in central Italy

We have clinically and genetically evaluated 24 affected patients belonging to 22 Italian Friedreich ataxia (FA) families, 52 patients from 32 kindreds with proven autosomal dominant cerebellar ataxia (ADCA), 9 patients belonging to 5 families with autosomal recessive hereditary ataxia (ARCA) and 103 sporadic cases, 89 of which affected by idiopathic late onset cerebellar ataxia (ILOCA). Genotype-phenotype correlation analyses in FA patients have evidenced an inverse relationship between GAA repeat expansion length and age of onset, disease duration, and presence of cardiomyopathy. Among autosomal dominant types, spinocerebellar ataxia 2 (SCA2) genotype has been found in 31% of our ADCA families, resulting the most frequent form of ataxia. Phenotypic analysis of the various SCA subtypes evidenced a marked heterogeneity of symptoms with a substantial overlap between different syndromes.

Neuropathic arthropathy of the elbow. A report of five cases

BACKGROUND

Neuropathic arthropathy of the elbow is rare and characterized by a painless but unstable articulation. The functional capacity of patients with this condition has not been reviewed in detail.

METHODS

Five male patients, with an average age of fifty-one years, were treated for neuropathic arthropathy of the elbow. The underlying conditions associated with the arthropathy included syringomyelia, insulin-dependent diabetes mellitus, end-stage renal failure, and two cases of polyneuropathy of unknown cause. Four patients sought medical attention after a specific traumatic event. Peripheral sensory and motor dysfunction was present in each patient. Radiographs of the elbow revealed dislocation, fracture fragmentation, and heterotopic ossification. Our management of the neuropathic elbows centered on maintenance of a functional arc of motion through physical therapy aimed at regaining muscle strength and the use of orthoses for support. Operative treatment was performed for an associated ulnar or radial nerve compression syndrome in three patients, and an open reduction and internal fixation of an unstable proximal ulnar nonunion associated with loose implants was performed in one.

RESULTS

The patients were followed for an average of nineteen months, with a range of twelve to thirty-six months. All patients had a pain-free elbow with a functional range of motion at the most recent follow-up examination, and none wished to have further treatment. The operatively treated ulnar nonunion united successfully. All three patients treated surgically for an associated nerve compression syndrome had recovery of nerve function.

CONCLUSION

In the face of instability and gross distortion of the joint, the patients in this series demonstrated remarkably good function.

Clinical and genetic aspects of spinocerebellar degeneration

After decades of confusion as a result of the marked clinical variability of spinocerebellar degeneration, molecular analyses have permitted the identification of loci and genes, which constitute the basis of a new classification. However, even greater genetic heterogeneity is suspected and several phenotypes, such as complex forms of spastic paraplegia and autosomal recessive ataxias, have not yet been thoroughly explored. Unexpectedly, the genes responsible for Friedreich's ataxia and a form of autosomal recessive spastic paraplegia place these diseases in the category of mitochondrial disorders. The unstable mutations caused by trinucleotide repeat expansions are responsible for a growing number of inherited cerebellar ataxias.

Mapping of spinocerebellar ataxia 13 to chromosome 19q13.3-q13.4 in a family with autosomal dominant cerebellar ataxia and mental retardation

We examined a large French family with autosomal dominant cerebellar ataxia (ADCA) that was excluded from all previously identified spinocerebellar ataxia genes and loci. The patients-seven women and a 4-year-old boy-exhibited slowly progressive childhood-onset cerebellar gait ataxia associated with cerebellar dysarthria, moderate mental retardation (IQ 62-76), and mild developmental delays in motor acquisition. Nystagmus and pyramidal signs were also observed in some cases. This unique association of clinical features clearly distinguishes this new entity from other previously described ADCA. Cerebral magnetic-resonance imaging showed moderate cerebellar and pontine atrophy in two patients. We performed a genomewide search and found significant evidence for linkage to chromosome 19q13.3-q13.4, in an approximately 8-cM interval between markers D19S219 and D19S553.