The Friedreich ataxia region: characterization of two novel genes and reduction of the critical region to 300 kb.

Friedreich ataxia is a severe neurodegenerative autosomal recessive disorder of unknown biochemical defect. The Friedreich ataxia locus (FRDA) is tightly linked to the centromeric side of the D9S5 locus. We have used 'exon-trapping' to identify two new genes, approximately 100 and 200 kb centromeric to D9S5, respectively. One gene appears ubiquitously expressed while the other is prominently expressed in muscle. The ubiquitous transcript codes for a protein containing a 20 aa repeat reminiscent of simple repeats found in several ribonucleoproteins. Using the single-strand conformation polymorphism (SSCP) procedure, we searched for mutations in affected patients in the coding sequence of the two genes, as well as in a gene that we had previously identified in the same region. Eight polymorphic DNA changes but no causative mutations were found, suggesting that the genes are not candidates for Friedreich ataxia. The discovery of a simple sequence repeat polymorphism in the most centromeric gene allowed the localization within that gene of the breakpoint of a previously described recombination in a Friedreich ataxia family, therefore excluding the two distal genes from the FRDA region. The lack of causative mutations in the three genes and the position of the recombination further delineate the FRDA locus to a 300 kb interval.