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The Friedreich's ataxia locus (FRDA) is tightly linked to markers D9S5 and D9S15 located in 9q13-q21. Cumulated maximum lod scores between FRDA and D9S5 and between FRDA and D9S15 are above 36 and 61, respectively, at a recombination fraction of 0, indicating that recombination events needed to orient the search of the gene are very difficult to identify and ascertain. We have established a 1 Megabase PFGE map around D9S5 and D9S15 and isolated a corresponding 530 kb YAC contig. We found that the two markers are 260 kb apart. This result was surprising, since D9S5 and D9S15 were independently isolated, but in agreement with the strong linkage between the two loci (lod score > 35 at a recombination fraction of 0). Seven clusters of rare cutter enzyme sites (CpG islands), which are potential indicators of genes, were identified in the 1 Megabase region by PFGE analysis and YAC mapping. The search for genes around the CpG islands is in progress. To map the Friedreich ataxia locus in the absence of clearly identified recombination events, we chose an alternative approach based on haplotype analysis of patients from small populations with precise geographic and historical origins, such as the Louisiana-Acadians, deported from Nova-Scotia about 150 years ago and who remained isolated for historical and cultural reasons. In this population, a single mutation, associated with a specific haplotype may account for the majority of Friedreich ataxia cases. Haplotypes different from the major haplotype at one or the other extremity can indicate ancient recombinations.(ABSTRACT TRUNCATED AT 250 WORDS)

Friedreich's ataxia (FA) is an autosomal recessive neurodegenerative disorder, the disease locus (FRDA) of which has been assigned to 9q13-q21.1 by genetic linkage analysis in affected families. We report two large inbred Tunisian families with FA manifestations that did not show the expected linkage. The disease locus could be excluded from a large (12 cMo) region around FRDA. This is the first report providing evidence for nonallelic genetic heterogeneity for the FA clinical phenotype. On subsequent analysis, all patients had very low levels of serum vitamin E whereas the parents and healthy sibs had normal vitamin E levels. This establishes that the selective vitamin E deficiency with normal fat absorption is an autosomal recessive trait, which is associated in the two families reported here with the FA phenotype.

We investigated linkage disequilibrium between Friedreich's ataxia (FRDA) and four tightly linked multi-allele markers in 140 families from France and Italy. These markers include three microsatellites (D9S111, D9S15 and D9S110) and one RFLP (D9S5). Their chromosomal order, D9S111-D9S15-D9S110-D9S5, had previously been established by physical mapping. Linkage disequilibrium was evaluated between each marker and FRDA and between markers. Extended haplotypes were obtained and their frequencies on FRDA and normal chromosomes were evaluated. We obtained evidence of strong allelic association of FRDA with D9S5 only. Analysis of linkage disequilibrium between markers revealed a significant decrease between D9S110 and D9S5, suggesting the presence of a recombination hot spot in the interval between these markers. Probably for this reason, no major FRDA-associated extended haplotype could be identified. Our data suggest the presence of a few common disease-causing mutations in the examined population, and indicate a putative localization for the FRDA gene. Transcribed sequences have been found in this candidate region.

Friedreich ataxia (FRDA) is an autosomal recessive degenerative disorder that affects the cerebellum, spinal cord, and peripheral nerves. The FRDA gene was localized in 9q13-q21 within 0.7 centimorgan of the D9S5 and D9S15 loci. One recently reported recombination event and haplotype analysis in a population with a founder effect suggested that the FRDA locus is on the D9S5 side. Using a conserved probe from the D9S5 locus, we have now identified an approximately 7-kilobase (kb) transcript and report cloning of its cDNA. The corresponding gene, X11, extends at least 80 kb in a direction opposite D9S15. The gene is expressed in the brain, including the cerebellum, but is not detectable in several nonneuronal tissues and cell lines. In situ hybridization of adult mouse brain sections showed prominant expression in the granular layer of the cerebellum. Expression was also found in the spinal cord. The cDNA contains an open reading frame encoding a 708-amino acid sequence that shows no significant similarity to other known proteins but contains a unique, 24-residue-long, putative transmembrane segment. On the basis of its genomic localization and its neuronal site of expression, particularly in the cerebellum, this "pioneer" gene represents a candidate for FRDA. Direct evidence of its involvement in FRDA will require a search for causative point mutations in patients.

The absence of recombination between the mutation causing Friedreich ataxia and the two loci which originally assigned the disease locus to chromosome 9 has slowed attempts to isolate and characterize the genetic defect underlying this neurodegenerative disorder. A proximity of less than 1 cM to the linkage group has been proved by the generation of high maximal lod score (Z) to each of the two tightly linked markers D9S15 (Z = 96.69; recombination fraction [theta] = .01) and D9S5 (Z = 98.22; theta = .01). We report here recombination events which indicate that the FRDA locus is located centromeric to the D9S15/D9S5 linkage group, with the most probable order being cen-FRDA-D9S5-D9S15-qter. However, orientation of the markers with respect to the centromere, critical to the positional cloning strategy, remains to be resolved definitively.

Friedreich ataxia is a neurodegenerative disorder with autosomal recessive inheritance. Precise linkage mapping of the Friedreich ataxia locus (FRDA) in 9q13-q21 should lead to the isolation of the defective gene by positional cloning. The two closest DNA markers, D9S5 and D9S15, show very tight linkage to FRDA, making difficult the ordering of the three loci. We present a linkage study of three large Friedreich ataxia families of Tunisian origin, with several multiallelic markers around D9S5 and D9S15. Haplotype data were used to investigate genetic homogeneity of the disease in these geographically related families. A meiotic recombination was found in a nonaffected individual, which excludes a 150-kb segment, including D9S15, as a possible location for the Friedreich ataxia gene and which should orient the search in the D9S5 region.

Friedreich ataxia (FA) is a severe autosomal recessive neurodegenerative disease. The defective gene has been previously assigned to chromosome 9q13-q21 by demonstration of tight linkage to the two independent loci D9S15 and D9S5. Linkage data indicate that FRDA is at less than 1 cM from both markers. Previous physical mapping has shown that probes defining D9S15 (MCT112) and D9S5 (26P) are less than 260 kb apart and are surrounded by at least six CpG clusters within 450 kb, which might indicate the presence of "candidate" genes for FA. We isolated and characterized a 530 kb YAC (yeast artificial chromosome) contig that contains five of the CpG clusters. The YACs were used to search for new polymorphic markers needed to map FRDA precisely with respect to the cloned segment. In particular, we found a (CA)n microsatellite polymorphism, GS4, that detects 13 alleles with a PIC value of 0.83 and allows the definition of haplotypes extending over 310 kb when used in combination with polymorphic markers at D9S5 and D9S15.

Eleven Acadian families with Friedreich ataxia (FA) who were from southwest Louisiana were studied with a series of polymorphic markers spanning 310 kb in the D9S5-D9S15 region previously shown to be tightly linked to the disease locus. In particular, three very informative microsatellites were tested. Evidence for a strong founder effect was found, since a specific extended haplotype spanning 230 kb from 26P (D9S5) to MCT112 (D9S15) was present on 70% of independent FA chromosomes and only once (6%) on the normal ones. There was no evident correlation between haplotypes and clinical expression. The typing of an additional microsatellite (GS4) located 80 kb from MCT112 created a divergence of the main FA-linked haplotype, generating four minor and one major haplotype. A similar split was observed with GS4 in a patient homozygous for a rare 26P-to-MCT112 haplotype. These results suggest that GS4 is flanking marker for the disease locus, although other interpretations are possible.

Friedreich ataxia (FA) is an autosomal recessive degenerative disease of the nervous system of unknown biochemical cause. The FA gene has been shown to be in close linkage with the two chromosome 9 markers D9S5 and D9S15, and linkage disequilibrium between FA and D9S15 has been detected in French families by Hanauer et al. We used new highly informative markers at the above loci to analyze Italian FA families for linkage and linkage disequilibrium. The new markers were a three-allele BstXI RFLP at D9S5 (PIC = .55) and a six-allele microsatellite, typed by polymerase chain reaction, at D9S15 (PIC = .75). We obtained maximum lod scores of 8.25 between FA and D9S5, 10.55 between FA and D9S15, and 9.52 between D9S5 and D9S15, all at zero recombination. Our results, combined with those reported by other authors, reduce maxlod-1 (maximum lod score minus 1) confidence limits to less than 1.1 cM between FA and D9S5, 1.2 cM between FA and D9S15, and 1.4 cM between D9S5 and D9S15. Linkage disequilibrium with FA was found only for D9S15 when all families were evaluated but was also found for a D9S5/D9S15 haplotype in a subgroup of southern Italian families. We conclude that FA, D9S5, and D9S15 are tightly clustered and that studies of geographically restricted groups may reveal a limited number of mutations responsible for the disease in the Italian population. We present preliminary evidence from pulsed-field gel electrophoresis that D9S5 and D9S15 may be less than 450 kb apart. Linkage disequilibrium between FA and D9S15 suggests that the disease gene may be at an even shorter distance from this marker locus, which therefore represents a very good starting point for cloning attempts.

The gene for Friedreich ataxia (FA), a severe recessive neurodegenerative disease, has previously been shown to be tightly linked to the polymorphic markers D9S15 and D9S5 on human chromosome 9. In addition, the observation of linkage disequilibrium suggested that D9S15 is within 1 centimorgan (cM) of the disease locus, FRDA. Although D9S5 did not show recombination with FRDA, its localization was less precise (0-5 cM) due to its lower informativeness. We have now identified additional polymorphisms at both marker loci. Two cosmids spanning 50 kilobases around D9S5 were isolated, and a probe derived from one of them detects an informative three-allele polymorphism. We have found a highly polymorphic microsatellite sequence at D9S15 which is rapidly typed by the DNA polymerase chain reaction. The polymorphism information contents at the D9S5 and D9S15 loci have been increased from 0.14 to 0.60 and from 0.33 to 0.74, respectively. With the additional polymorphisms the lod (log10 odds ratio) score for the D9S15-FRDA linkage is now 48.10 at recombination fraction theta = 0.005 and for D9S5-FRDA, the lod score is 27.87 at theta = 0.00. We have identified a recombinant between D9S15 and FRDA. However, due to the family structure, it will be of limited usefulness for more precise localization of FRDA. The linkage disequilibrium previously observed between D9S15 and FRDA is strengthened by analysis of the haplotypes using the microsatellite polymorphism, while weaker but significant disequilibrium is found between D9S5 and FRDA. Extended haplotypes that encompass D9S5 and D9S15 show a strikingly different distribution between chromosomes that carry the FA mutation and normal chromosomes. This suggests that both marker loci are less than 1 cM from the FRDA gene and that a small number of mutations account for the majority of FA cases in the French population studied. D9S5 and D9S15 are thus excellent start points to isolate the disease gene.