Friedreich's ataxia phenotype not linked to chromosome 9 and associated with selective autosomal recessive vitamin E deficiency in two inbred Tunisian families

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.

Abnormal messenger RNA expression and a missense mutation in patients with X-linked adrenoleukodystrophy

A candidate gene for X-linked adrenoleukodystrophy (ALD) has been identified via positional cloning strategies. We now report messenger RNA expression in fibroblasts from 6 unrelated ALD patients. Four patients lacked the normal 4.2 kb transcript, three of them having deletions of the ALD gene. A fifth patient with a deletion of 1.6 kb had a smaller 4.0 kb transcript. The last patient had a normal sized transcript and a missense mutation at base 1258 leading to Glu-291-Lys substitution in a region of the candidate gene protein which is conserved in the 70 kD peroxisomal membrane protein. These results provide further evidence that this candidate gene is indeed the ALD gene.

The FMR-1 protein is cytoplasmic, most abundant in neurons and appears normal in carriers of a fragile X premutation

Fragile X mental retardation syndrome is caused by the unstable expansion of a CGG repeat in the FMR-1 gene. In patients with a full mutation, abnormal methylation results in suppression of FMR-1 transcription. FMR-1 is expressed in many tissues but its function is unknown. We have raised monoclonal antibodies specific for the FMR-1 protein. They detect 4-5 protein bands which appear identical in cells of normal males and of males carrying a premutation, but are absent in affected males with a full mutation. Immunohistochemistry shows a cytoplasmic localization of FMR-1. The highest levels were observed in neurons, while glial cells contain very low levels. In epithelial tissues, levels of FMR-1 were higher in dividing layers. In adult testis, FMR-1 was detected only in spermatogonia. FMR-1 was not detected in dermis and cardiac muscle except under pathological conditions.

Origin of the expansion mutation in myotonic dystrophy

Myotonic dystrophy (DM) is caused by the expansion of a CTG trinucleotide repeat. The mutation is in complete linkage disequilibrium with a nearly two-allele insertion/deletion polymorphism, suggesting a single origin for the mutation or predisposing mutation. To trace this-ancestral event, we have studied the association of CTG repeat alleles in a normal population to alleles of the insertion/deletion polymorphism and of a (CA)n repeat marker 90 kilobases from the DM mutation. The results strongly suggest that the initial predisposing event(s) consisted of a transition from a (CTG)5 allele to an allele with 19 to 30 repeats. The heterogeneous class of (CTG)19-30 alleles which has an overall frequency of about 10%, may constitute a reservoir for recurrent DM mutations.

Putative X-linked adrenoleukodystrophy gene shares unexpected homology with ABC transporters

Adrenoleukodystrophy (ALD) is an X-linked disease affecting 1/20,000 males either as cerebral ALD in childhood or as adrenomyeloneuropathy (AMN) in adults. Childhood ALD is the more severe form, with onset of neurological symptoms between 5-12 years of age. Central nervous system demyelination progresses rapidly and death occurs within a few years. AMN is a milder form of the disease with onset at 15-30 years of age and a more progressive course. Adrenal insufficiency (Addison's disease) may remain the only clinical manifestation of ALD. The principal biochemical abnormality of ALD is the accumulation of very-long-chain fatty acids (VLCFA) because of impaired beta-oxidation in peroxisomes. The normal oxidation of VLCFA-CoA in patients' fibroblasts suggested that the gene coding for the VLCFA-CoA synthetase could be a candidate gene for ALD. Here we use positional cloning to identify a gene partially deleted in 6 of 85 independent patients with ALD. In familial cases, the deletions segregated with the disease. An identical deletion was detected in two brothers presenting with different clinical ALD phenotypes. Candidate exons were identified by computer analysis of genomic sequences and used to isolate complementary DNAs by exon connection and screening of cDNA libraries. The deduced protein sequence shows significant sequence identity to a peroxisomal membrane protein of M(r) 70K that is involved in peroxisome biogenesis and belongs to the 'ATP-binding cassette' superfamily of transporters.

Striking founder effect for the fragile X syndrome in Finland

The fragile X mental retardation syndrome is caused by the expansion of an unstable CGG repeat in a 5' exon of the FMR1 gene. Significant linkage disequilibrium between this mutation and flanking microsatellite markers has been observed previously in Caucasian populations, a very unusual finding for an X-linked disease which severely impairs reproduction fitness in affected males. This reflects the multistep process at the origin of the full mutation. We have analyzed the FRAXAC2 and DXS548 microsatellites in 26 fragile X families originating from various parts of Finland, and report a striking founder effect much stronger than the linkage disequilibrium observed previously in other more heterogeneous populations. One DXS548 allele was present on 92% of fragile X chromosomes and on 17% of normal chromosomes. A single haplotype accounted for 73% of fragile X chromosomes, and was found only once in 34 normal chromosomes, corresponding to a relative risk of about 90 compared to its absence. The broad geographic origin of the high-risk haplotype and its expected frequency suggest that it was present in initial settlers of Finland, and could thus have been carried silently through 100 generations.

Adrenoleukodystrophy gene: unexpected homology to a protein involved in peroxisome biogenesis

Adrenoleukodystrophy (ALD) is an X-linked peroxisomal disorder characterized by a progressive demyelination of the central nervous system and adrenal insufficiency. Clinical phenotypes of different severity are frequently observed within the same kindred. ALD is characterized biochemically by the accumulation of very-long-chain fatty acids (VLCFA) due to an impairment in the beta-oxidation of these fatty acids in peroxisome. From the observation that oxidation of VLCFA-CoA is normal in fibroblasts from patients with ALD, it was concluded that the gene coding for VLCFA-CoA synthetase was a candidate gene for ALD. Using positional cloning strategies, we have identified a gene which was found partially deleted in 7% of 85 independent patients with ALD. The predicted protein (ALDP) sequence shows significant homology to the 70-kDa peroxisomal membrane protein which is involved in peroxisome biogenesis and belongs to the 'ATP binding' superfamily of transporters. ALDP thus encodes a putative peroxisomal transporter molecule which may be involved in the import or anchoring of VLCFA-CoA synthetase.

Gene in the region of the Friedreich ataxia locus encodes a putative transmembrane protein expressed in the nervous system

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.

Study of large inbred Friedreich ataxia families reveals a recombination between D9S15 and the disease locus

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.

Inheritance of the fragile X syndrome: size of the fragile X premutation is a major determinant of the transition to full mutation

The fragile X mental retardation syndrome is caused by unstable expansion of a CGG repeat. Two main types of mutation have been categorised. Clinical expression is associated with the presence of the full mutation, while subjects who carry only a premutation do not have mental retardation. Premutations have a high risk of transition to full mutation when transmitted by a female. We have used direct detection of the mutations to characterise large families who illustrate the wide variation in penetrance which has been observed in different sibships (a feature often called the Sherman paradox). A family originally found to show tight genetic linkage between the factor 9 gene and the fragile X locus was reanalysed, confirming the original genotype assignments and the observed linkage. The size of premutations was measured by Southern blotting and by using a PCR based test in 102 carrier mothers and this was correlated with the type of mutation found in their offspring. The risk of transition to full mutation was found to be very low for premutations with a size increase (delta) of about 100 bp, increasing up to 100% when the size of premutation was larger than about 200 bp, even after taking into account (at least partially) ascertainment bias. These results confirm and extend those reported by Fu et al (1991) and Yu et al (1992) and explain the Sherman paradox.(ABSTRACT TRUNCATED AT 250 WORDS)

Genome analysis and the human X chromosome

A unified genetic, physical, and functional map of the human X chromosome is being built through a concerted, international effort. About 40 percent of the 160 million base pairs of the X chromosome DNA have been cloned in overlapping, ordered contigs derived from yeast artificial chromosomes. This rapid progress toward a physical map is accelerating the identification of inherited disease genes, 26 of which are already cloned and more than 50 others regionally localized by linkage analysis. This article summarizes the mapping strategies now used and the impact of genome research on the understanding of X chromosome inactivation and X-linked diseases.

A 530kb YAC contig tightly linked to the Friedreich ataxia locus contains five CpG clusters and a new highly polymorphic microsatellite

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.

Direct DNA analysis of fragile X syndrome in Spanish pedigrees

Eleven complete Spanish pedigrees with fragile X syndrome were analysed by Southern blotting with the DNA probe StB12.3 previously isolated and described by Oberlé et al. [1991]. This probe allowed the direct detection of affected males and carrier females and was able to distinguish between normal males and normal transmitting males (NTMs). One hundred and twenty three individuals were analyzed, 115 from the pedigrees and 8 from the general population. Five mosaic cases were found (4 males and one female) showing both the premutation and the full mutation. One half of the females with the full mutation were mentally retarded but no female with mental retardation carried the premutated pattern, suggesting that the absence of the full mutation in females is a very good criterion for pre-or postnatal diagnosis of normal mental status.

On some technical aspects of direct DNA diagnosis of the fragile X syndrome

Direct DNA analysis of fragile X [Fra(X)] mutations has already shown its clear superiority for postnatal and prenatal diagnosis of the disorder and for carrier detection. However, it is of great importance to have conditions which guarantee optimal reliability and sensitivity. Some mutations may be more difficult to detect, especially in female carriers: this is the case for small amplifications of the CGG repeat (premutations) or for smears which can be generated by the instability of the full mutation in somatic tissues. We present the various alternatives (probe/enzymes combinations) for Southern blot based diagnosis, the possible artefacts and our detailed experimental protocol, which has given excellent results on a large number of families. While detection of amplification, using for instance EcoRI, appears sufficient for initial testing of mentally retarded patients, once the fra(X) diagnosis has been established, we favor the use of an EcoRI+EagI digest, which detects both amplification and abnormal methylation, for analysis of the family, including carrier detection and prenatal diagnosis. We discuss the place of proposed PCR based techniques for detection of mutations, or for indirect tracking using polymorphic microsatellites in the immediate vicinity of the fra(X) locus.

Moderate instability of the trinucleotide repeat in spino bulbar muscular atrophy

Increased length of a protein-coding CAG repeat within the androgen receptor gene appears to be the only type of mutation responsible for spino-bulbal muscular atrophy (SBMA or Kennedy disease). We have analysed a large 4-generation SBMA family and found that the mutant allele was unstable upon transmission from parent to child, with a documented variation from 46 to 53 repeats and a tendency to increase in size (7 increases and a single decrease in 17 events), which appeared stronger upon transmission from a male than from a female. Our results suggest also limited somatic instability of the abnormal allele, with observable variation of up to 2-3 repeats. This indicates that the behavior of the CAG repeat is similar to that observed for small premutations in the fragile X syndrome, or small abnormal alleles in myotonic dystrophy, two diseases which are caused by expansion of an unstable trinucleotide repeat.

Molecular genetics of the fragile-X syndrome: a novel type of unstable mutation

Fragile-X syndrome, the most common inherited form of mental retardation, has a very unusual mode of inheritance. The disease is caused by a multistep expansion, in successive generations, of a polymorphic CGG repeat localized in a 5' exon of FMR-1, a gene of unknown function. Two main mutation types have been categorized. Premutations are moderate expansions of the repeat and do not cause mental retardation. Full mutations are found in affected individuals and involve larger expansions of the repeat, with abnormal methylation of the neighboring CpG island. The full mutations demonstrate striking somatic instability and extinguish expression of FMR-1. Premutations are changed to full mutation only when transmitted by a female with a frequency that increases up to 100% as a function of the initial size of the premutation. Direct detection of the mutations provides an accurate test for pre- and postnatal diagnosis of the disease, and for carrier detection. A similar unstable expansion of a trinucleotide repeat occurs in myotonic dystrophy.

Analysis of full fragile X mutations in fetal tissues and monozygotic twins indicate that abnormal methylation and somatic heterogeneity are established early in development

The fragile X syndrome, the most common cause of inherited mental retardation, is characterized by unique genetic mechanisms, which include amplification of a CGG repeat and abnormal DNA methylation. We have proposed that 2 main types of mutations exist. Premutations do not cause mental retardation, and are characterized by an elongation of 70 to 500 bp, with little or no somatic heterogeneity and without abnormal methylation. Full mutations are associated with high risk of mental retardation, and consist of an amplification of 600 bp or more, with often extensive somatic heterogeneity, and with abnormal DNA methylation. To analyze whether the latter pattern is already established during fetal life, we have studied chorionic villi from 10 fetuses with a full mutation. In some cases we have compared them to corresponding fetal tissues. Our results indicate that somatic heterogeneity of the full mutation is established during (and possibly limited to) the very early stages of embryogenesis. This is supported by the extraordinary concordance in mutation patterns found in 2 sets of monozygotic twins (9 and 30 years old). While the methylation pattern specific of the inactive X chromosome appears rarely present on chorionic villi of normal females, the abnormal methylation characteristic of the full mutation was present in 8 of 9 male or female chorionic villi analyzed. This suggests that the methylation mechanisms responsible for establishing the inactive X chromosome pattern and the full mutation pattern are, at least in part, distinct. Our results validate the analysis of chorionic villi for direct prenatal diagnosis of the fragile X syndrome.