Localization and genomic structure of human deoxyhypusine synthase gene on chromosome 19p13.2-distal 19p13.1.

The amino acid hypusine is formed post-translationally in a single cellular protein, the eukaryotic translation initiation factor 5A, by two enzymes, namely deoxyhypusine synthase and deoxyhypusine hydroxylase. Hypusine is found in all eukaryotes and in some archaebacteria, but not in eubacteria. The deoxyhypusine synthase cDNA was cloned and mapped by fluorescence in situ hybridization on chromosome 19p13.11-p13.12. Rare cDNAs containing internal deletions were also found. We localized the deoxyhypusine synthase gene on a high resolution cosmid/BAC contig map of chromosome 19 to a region in 19p13.2-distal 19p13.1 between MANB and JUNB. Analysis of the genomic exon/intron structure of the gene coding region showed that it consists of nine exons and spans a length of 6.6kb. From observation of the genomic structure, it seems likely that the internally deleted forms of mature RNA are the result of alternative splicing, rather than of artifacts.

A fine physical map of the CACNA1A gene region on 19p13.1-p13.2 chromosome.

The P/Q-type Ca(2+) channel alpha(1A) subunit gene (CACNA1A) was cloned on the short arm of chromosome 19 between the markers D19S221 and D19S179 and found to be responsible for Episodic Ataxia type 2, Familial Hemiplegic Migraine and Spinocerebellar Ataxia type 6. This region was physically mapped by 11 cosmid contigs spanning about 1. 4Mb, corresponding to less than 70% of the whole region. The cosmid contig used to characterize the CACNA1A gene accounted only for the coding region of the gene lacking, therefore, the promoter and possible regulation regions. The present study improves the physical map around and within the CACNA1A by giving a complete cosmid or BAC contig coverage of the D19S221-D19S179 interval. A number of new STSs, whether polymorphic or not, were characterized and physically mapped within this region. Four ESTs were also assigned to cosmids belonging to specific contigs.

The AXH module: an independently folded domain common to ataxin-1 and HBP1.

Ataxin-1 (ATX1), a human protein responsible for spinocerebellar ataxia type 1 in humans, shares a region of homology, named AXH module, with the apparently unrelated transcription factor HBP1. Here, we describe the first characterisation of the AXH module in terms of its structural properties and stability. By producing protein constructs spanning the AXH modules of ATX1 and HBP1 and by comparing their properties, we have identified the minimal region sufficient for forming independently folded units (domains). Knowledge of the AXH domain boundaries allows us to map many of the interactions of ATX1 with other molecules onto the AXH module. We further show that the AXH of ATX1 is a dimerisation domain and is able to recognise RNA with the same nucleotide preference previously described for the full-length protein. AXH is therefore a novel protein-protein and RNA binding motif.

Spinocerebellar ataxia type 6 and episodic ataxia type 2: differences and similarities between two allelic disorders.

Spinocerebellar ataxia type 6 (SCA6) is one of three allelic disorders caused by mutations of CACNA1A gene, coding for the pore-forming subunit of calcium channel type P/Q. SCA6 is associated with small expansions of a CAG repeat at the 3' end of the gene, while point mutations are responsible for its two allelic disorders (Episodic Ataxia type 2 and Familial Hemiplegic Migraine). Genetic, clinical, pathological and pathophysiological data of SCA6 patients are reviewed and compared to those of other SCAs with expanded CAG repeats as well as to those of its allelic channelopathies, with particular reference to Episodic Ataxia type 2. Overall SCA6 appears to share features with both types of disorders, and the question as to whether it belongs to polyglutamine disorders or to channelopathies remains unanswered at present.

Phenotypic effects of expanded ataxin-1 polyglutamines with interruptions in vitro.

Spinocerebellar ataxia type 1 is a neurodegenerative disease caused by expansion of an uninterrupted glutamine repeat in ataxin-1 protein. Protein aggregation and immunoreactivity to 1C2 monoclonal antibody are two distinct pathognomonic features of expanded ataxin-1, as well as of other polyglutamine disorders. Rare cases of non-affected elderly subjects carrying expanded ataxin-1 alleles were found in random population. However, in these alleles the glutamine stretch was interrupted by histidines. Due to lack of phenotype, these alleles should be considered "normal". Most importantly, occurrence of these unusual alleles provides a unique opportunity to investigate which molecular properties of expanded ataxin-1 are not coupled to polyglutamine pathogenesis. Towards this goal, we compared in vitro the immunoreactivity to 1C2 antibody and the ability to form aggregates of interrupted and uninterrupted alleles. Immunoblotting showed that expanded-interrupted ataxin-1 had an affinity to 1C2 resembling that of normal ataxin-1. On the contrary, filter assay showed that aggregation rate of expanded-interrupted ataxin-1 resembles that of expanded-uninterrupted ataxin-1. These observations indicate that affinity for 1C2 does not directly correlate with self-aggregation of ataxin-1. Moreover, self-aggregation is not directly affected by histidine interruptions. In conclusion, these results support the hypothesis that mechanisms underlying neuronal degeneration are triggered by protein misfolding rather than by protein aggregation.

CAG repeat instability, cryptic sequence variation and pathogeneticity: evidence from different loci.

Different aspects of expanded polyglutamine tracts and of their pathogenetic role are taken into consideration here. (i) The (CAG)n length of wild-type alleles of the Huntington disease gene was analysed in instability-prone tumour tissue from colon cancer patients to test whether the process leading to the elongation of alleles towards the expansion range involves single-unit stepwise mutations or larger jumps. The analysis showed that length changes of a single unit had a relatively low frequency. (ii) The observation of an expanded spinocerebellar ataxia (SCA)1 allele with an unusual pattern of multiple CAT interruptions showed that cryptic sequence variations are critical not only for sequence length stability but also for the expression of the disease phenotype. (iii) Small expansions of the (CAG)n sequence at the CACNA1A gene have been reported as causing SCA6. The analysis of families with SCA6 and episodic ataxia type 2 showed that these phenotypes are, in fact, expressions of the same disorder caused either by point mutations or by small (CAG)n expansions. A gain of function has been hypothesized for all proteins containing an expanded polyglutamine stretch, including the alpha 1A subunit of the voltage-gated calcium channel type P/Q coded by the CACNA1A gene. Because point mutations at the same gene with similar phenotypic consequences are highly unlikely to have this effect, an alternative common pathogenetic mechanism for all these mutations, including small expansions, can be hypothesized.

Allele and haplotype frequency distribution of the EcoRI, RsaI, and MspI COL1A2 RFLPs among various human populations.

The EcoRI, RsaI, and MspI RFLPs (restriction fragment length polymorphisms) of the COL1A2 gene, one of the two genes that encode for the polypeptides of type I collagen, have been studied in four West African and two Asian populations to evaluate their potential effectiveness as anthropological markers. All three RFLPs were in Hardy-Weinberg equilibrium. The comparisons between present data on two of the major human groups and those on Europeans and Amerindians show a considerable heterogeneity for each of the three RFLPs under study. EcoRI, in particular, appears to be highly effective in distinguishing Africans, Europeans, and Asians from each other. As expected, the analysis at the haplotype level considerably improves the discriminating efficiency of these three markers by creating a clear-cut distinction between Tharus and Indonesians, the two Asian populations of the present survey. In fact, even though these two populations exhibit the same frequencies for the RsaI and MspI alleles, the frequency of the MspI(-) allele among the RsaI(-) chromosomes is 0.5 +/- 0.14 in the Indonesian sample and 0 + 0.04 in the Tharu sample.

Population variation analysis at nine loci containing expressed trinucleotide repeats.

The polymorphisms of nine loci containing reiterated CAG repeats were examined in four populations from three continents. Their normal variation was analysed across populations or in subsets of loci grouped according to either the presence/absence of disease-associated expansions or CAG interruptions. A unifying feature of the allele distributions of all loci in all populations was the marked non-normality. Significantly larger numbers of alleles, average lengths, length ranges and variances in repeat number were observed in loci with vs. without known expansions. Significantly longer alleles were found at loci with vs. without interruption of the (CAG)n motif. The nine loci detected levels of inter-population variability comparable to other loci. Altogether the data are at odds with a model assuming that autosomal expressed trinucleotides accumulate variation exclusively by insertion/deletion of a single unit.

Disequilibrium of multiple DNA markers on the human Y chromosome.

We characterized four DNA polymorphisms on the Y chromosomes of 123 males from five Caucasian populations. Three markers on the male specific portion of the chromosome varied appreciably in frequency among the populations. When combined, these markers define a limited number of haplotypes compared with the maximum expected on the basis of random association. The associations found in the five groups are qualitatively similar and are thus considered to be relatively stable on an evolutionary time-scale and possibly to predate the divergence of Caucasian populations. However, the haplotype frequencies varied markedly among populations, even between weakly isolated areas such as northern vs. southern Sardinia. This may indicate rapid progression towards fixation of alternative types of Y chromosomes. We also report data suggesting that the same associations no longer hold when examining a marker as close as 275 bp from the boundary of the pseudoautosomal region on the Y chromosome.

The trinucleotide repeat expansion on chromosome 6p (SCA1) in autosomal dominant cerebellar ataxias.

Affected members of 73 families with a variety of autosomal dominant late onset cerebellar ataxias (ADCAs) were investigated for the trinucleotide (CAG) repeat expansion which is found in pedigrees exhibiting linkage to the SCA1 locus on chromosome 6. Most of the families were too small for linkage analysis. The mutation was only found in ADCA type I, in 19 out of 38 such kindreds investigated (50%). It was slightly more common in Italian (59%) than British (50%) families, and was also found in Malaysian, Bangladeshi and Jamaican kindreds. Overall, ADCA type I patients with the expansion had a lower incidence of hyporeflexia and facial fasciculation than those without. The trinucleotide expansion was not found in eight families with ADCA and maculopathy or 24 kindreds with a pure type of ADCA, confirming that these syndromes are genetically distinct. It was also not detected in 12 patients with sporadic degenerative ataxias. DNA analysis for the SCA1 mutation is useful diagnostically in single patients or small families, and can be used for presymptomatic testing where appropriate.

Acetazolamide-responsive episodic ataxia in an Italian family refines gene mapping on chromosome 19p13.

Episodic ataxia type 2 is an autosomal dominant disorder with attacks of vertigo and ataxia which respond to acetazolamide treatment. The gene, distinct from the KCNA1 responsible for episodic ataxia type 1, has been mapped on chromosome 19p13 in a 11-12 cM region. A large Italian kindred affected with acetazolamide-responsive episodic ataxia is reported, with onset in adulthood, a strong vestibular component during attacks and a high frequency of cerebellar vermis degeneration. The genetic analysis (i) showed strong linkage between the disease and the 19p13 microsatellite markers in a region which widely overlaps that previously reported and (ii) set a new distal boundary of the gene-containing region. Combining present and previous mapping data, the gene of episodic etaxia type 2 is most probably located in an interval approximately 1.5 Mb between markers D19S221 and D19S226.

Characterization of a small family (CAIII) of microsatellite-containing sequences with X-Y homology.

Four X-linked loci showing homology with a previously described Y-linked polymorphic locus (DYS413) were identified and characterized. By fluorescent in situ hybridization (FISH), somatic cell hybrids, and YAC screening, the X-linked members of this small family of sequences (CAIII) all map in Xp22, while the Y members map in Yq11. These loci contribute to the overall similarity of the two genomic regions. All of the CAIII loci contain an internal microsatellite of the (CA)n type. The microsatellites display extensive length polymorphism in two of the X-linked members as well as in the Y members. In addition, common sequence variants are found in the portions flanking the microsatellites in two of the X-linked members. Our results indicate that, during the evolution of this family, length variation on the Y chromosome was accumulated at a rate not slower than that on the X chromosome. Finally, these sequences represent a model system with which to analyze human populations for similar X- and Y-linked polymorphisms.

Autosomal dominant pure cerebellar ataxia. Neurological and genetic study.

A family with late-onset autosomal dominant pure cerebellar ataxia was studied both neurologically and genetically. Neuroimaging and electrophysiological results were in agreement with the clinical evidence showing involvement of the cerebellar system only, even many years after onset. No atrophy of inferior olives was observed by magnetic resonance imaging, while cerebellar atrophy was extremely marked. A very slow disease progression was observed in all patients. The disease can be differentiated from autosomal dominant olivo-ponto-cerebellar atrophies, and in particular from spinocerebellar ataxia type 1 mapping on chromosome 6p, which shows an early multisystemic involvement and a more rapid progression toward inability. A genetic study of the family with the 6p DNA marker D6S89 closely linked to the spinocerebellar ataxia type 1 locus was performed. Results showed significant exclusion of a linkage between the disease and the marker within a distance of 8.5% recombination, indicating that genetic heterogeneity underlies phenotypic differences.

HLA-linked spinocerebellar ataxia: a clinical and genetic study of large Italian kindreds.

Five families with late onset autosomal dominant spinocerebellar ataxia, were studied. Linkage between the disease and HLA loci on the short arm of chromosome 6 was shown in the two largest pedigrees. Clinical study of 26 patients and neuropathological study in one are reported. The disease was characterized by cerebellar and pyramidal involvement variably associated with cranial nerve and peripheral nervous system disorders. A remarkable concordance of the main clinical features was observed in patients with similar disease duration. Comparison with previous reports of HLA-linked spinocerebellar ataxia kindreds showed differences in clinical phenotypes. Although these might be due to genetic variation, the hypothesis is suggested that the phenotype might appear more homogeneous if disease duration is taken into account.

The gene for spinal cerebellar ataxia 1 (SCA1) is flanked by two closely linked highly polymorphic microsatellite loci.

The gene for one form of autosomal dominant spinal cerebellar ataxia (SCA1), is mapped by linkage to chromosome 6p, very close to the microsatellite locus D6S89. Eight large Italian kindreds segregating SCA1, as defined by very close linkage to D6S89, were genotyped with five microsatellite markers linked closely to D6S89, all mapping within a 6 cM interval on 6p. Multipoint linkage analysis and haplotypes from recombinants map SCA1 between two of these markers, D6S274 and D6S259, 5-6 cM apart. A single rare four marker haplotype within this interval shows linkage disequilibrium with the disease locus in southern Italy and is transmitted with SCA1 in five kindreds originating from this area.