The newly recognized ataxia-ocular apraxia 1 (AOA1; MIM 208920) is the most frequent cause of autosomal recessive ataxia in Japan and is second only to Friedreich ataxia in Portugal. It shares several neurological features with ataxia-telangiectasia, including early onset ataxia, oculomotor apraxia and cerebellar atrophy, but does not share its extraneurological features (immune deficiency, chromosomal instability and hypersensitivity to X-rays). AOA1 is also characterized by axonal motor neuropathy and the later decrease of serum albumin levels and elevation of total cholesterol. We have identified the gene causing AOA1 and the major Portuguese and Japanese mutations. This gene encodes a new, ubiquitously expressed protein that we named aprataxin. This protein is composed of three domains that share distant homology with the amino-terminal domain of polynucleotide kinase 3'- phosphatase (PNKP), with histidine-triad (HIT) proteins and with DNA-binding C2H2 zinc-finger proteins, respectively. PNKP is involved in DNA single-strand break repair (SSBR) following exposure to ionizing radiation and reactive oxygen species. Fragile-HIT proteins (FHIT) cleave diadenosine tetraphosphate, which is potentially produced during activation of the SSBR complex. The results suggest that aprataxin is a nuclear protein with a role in DNA repair reminiscent of the function of the protein defective in ataxia-telangiectasia, but that would cause a phenotype restricted to neurological signs when mutant.
A number of human diseases have been shown to be associated with mutation in the genes encoding leucine-rich-repeat (LRR)-containing proteins. They include 16 different LRR proteins. Mutations of these proteins are associated with 19 human diseases. The mutations occur frequently within the LRR domains as well as their neighboring domains, including cysteine clusters. Here, based on the sequence analysis of the LRR domains and the known structure of LRR proteins, we describe some features of different sequence variants and discuss their adverse effects. The mutations in the cysteine clusters, which preclude the formation of sulfide bridges or lead to a wrong paring of cysteines in extracellular proteins or extracellular domains, occur with high frequency. In contrast, missense mutations at some specific positions in LRRs are very rare or are not observed at all.
We have investigated the myelin P0 gene on chromosome 1 as a candidate gene in two sporadic cases with Dejerine-Sottas disease or hereditary motor and sensory neuropathy (HMSN) type III. We found different mutations, a cysteine substitution for serine 63 in the extracellular domain and an arginine substitution for glycine 167 in the transmembrane domain. The patients were genetically heterozygous for the normal allele and the mutant allele, which was absent in their parents and in one hundred unrelated, healthy controls. The results strongly suggest that a de novo dominant mutation of the P0 gene is responsible for at least some sporadic cases of Dejerine-Sottas disease.
The molecular mechanism resulting in the duplication or deletion of a 1.5 Mb region of 17p11.2-p12, associated, respectively, with Charcot-Marie-Tooth type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP), has been proposed to be an unequal crossing-over during meiosis between the two chromosome 17 homologues generated by misalignment of the proximal and distal CMT1A-REP repeats, two homologous sequences flanking the 1.5 Mb CMT1A/HNPP monomer unit. In a recent study of a large series of de novo cases of CMT1A and HNPP, two distinct sex-dependent mechanisms were identified. Rearrangements of paternal origin, essentially duplications, were indeed generated by unequal meiotic crossing-over between the two chromosome 17 homologues, but duplications and deletions of maternal origin resulted from an intrachromosomal process, either unequal sister chromatid exchange or, in the case of deletion, excision of an intrachromatidal loop. In order to determine how these recombinations occur, 24 de novo crossover breakpoints were localized within the 1.7 kb rearrangement hot spot by comparing the sequences of the parental CMT1A-REPs with the chimeric copy in affected offspring. Nineteen out of 21 paternal crossovers were found in a 741 bp hot spot. All the breakpoints of maternal origin (n = 3), however, were located outside this interval, but in closely flanking sequences, supporting the hypothesis that two distinct sex-dependent mechanisms are involved. Several putative recombination promoting sequences in the hot spot, which are rare or absent in the surrounding 7.8 kb, were identified.
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