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The locus for Friedreich ataxia (FRDA), a severe neurodegenerative disease, is tightly linked to markers D9S5 and D9S15, and analysis of rare recombination events has suggested the order cen–FRDA–D9S5–D9S15–qter. We report here the construction of a YAC contig extending 800 kb centromeric to D9S5 and the isolation of five new microsatellite markers from this region. In order to map these markers with respect to the FRDA locus, all within a 1-cM confidence interval, we sought to increase the genetic information of available FRDA families by considering homozygosity by descent and association with founder haplotypes in isolated populations. This approach allowed us to identify one phase-known recombination and one probable historic recombination on haplotypes from Réunion Island patients, both of which place three of the five markers proximal to FRDA. This represents the first identification of close FRDA flanking markers on the centromeric side. The two other markers allowed us to narrow the breakpoint of a previously identified distal recombination that is >180 kb from D9S5 (26P). Taken together, the results place the FRDA locus in a 450-kb interval, which is small enough for direct search of candidate genes. A detailed rare cutter restriction map and a cosmid contig covering this interval were constructed and should facilitate the search of genes in this region.

About 60% of both Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) is due to deletions of the dystrophin gene. For cases with a deletion mutation, the “reading frame” hypothesis predicts that BMD patients produce a semifunctional, internally deleted dystrophin protein, whereas DMD patients produce a severely truncated protein that would be unstable. To test the validity of this theory, we analyzed 258 independent deletions at the DMD/BMD locus. The correlation between phenotype and type of deletion mutation is in agreement with the “reading frame” theory in 92% of cases and is of diagnostic and prognostic significance. The distribution and frequency of deletions spanning the entire locus suggests that many “in-frame” deletions of the dystrophin gene are not detected because the individuals bearing them are either asymptomatic or exhibit non-DMD/non-BMD clinical features.

We have performed Southern blot analysis on a large, four-generation kindred with Duchenne muscular dystrophy (DMD). Probes 754 (DXS 84), pERT87-1, pERT87-8, pERT87-15 (DXS164), and pXJ-1.1 did not hybridize to digested genomic DNA of affected males. Obligate-carrier mothers and unaffected brothers showed signals of a single X-chromosome copy intensity, and suspected noncarrier sisters demonstrated either a single band of two-copy intensity or informative polymorphisms. Uniform hybridization was seen with probes C7 (DXS28) and D2 (DXS43), which map distal to the DMD locus, and with OTC, which maps proximally. This deletion was present in six affected individuals and has been transmitted through 3 generations to date. On high-resolution chromosome analysis, a deletion within band Xp21 was consistently observed in one affected male studied and in one of the two X chromosomes in obligate carriers. This large molecular and cytogenetically visible deletion in affected DMD individuals without glycerol kinase deficiency, chronic granulomatous disease, retinitis pigmentosa (RP), or ornithine transcarbamylase deficiency is a very rare finding and should prove useful in specifically cloning additional probes within and flanking the DMD locus.

We report the results of our investigations of a large, inbred, aboriginal Canadian kindred with nine muscular dystrophy patients. The ancestry of all but two of the carrier parents could be traced to a founder couple, seven generations back. Seven patients presented with proximal myopathy consistent with limb girdle-type muscular dystrophy (LGMD), whereas two patients manifested predominantly distal wasting and weakness consistent with Miyoshi myopathy (distal autosomal recessive muscular dystrophy) (MM). Age at onset of symptoms, degree of creatine kinase elevation, and muscle histology were similar in both phenotypes. Segregation of LGMD/MM is consistent with autosomal recessive inheritance, and the putative locus is significantly linked (LOD scores >3.0) to six marker loci that span the region of the LGMD2B locus on chromosome 2p. Our initial hypothesis that the affected patients would all be homozygous by descent for microsatellite markers surrounding the disease locus was rejected. Rather, two different core haplotypes, encompassing a 4-cM region spanned by D2S291-D2S145-D2S286, segregated with the disease, indicating that there are two mutant alleles of independent origin in this kindred. There was no association, however, between the two different haplotypes and clinical variability; they do not distinguish between the LGMD and MM phenotypes. Thus, we conclude that LGMD and MM in our population are caused by the same mutation in LGMD2B and that additional factors, both genetic and nongenetic, must contribute to the clinical phenotype.