Fragile X syndrome and deletions in FMR1: new case and review of the literature.

The fragile X syndrome phenotype of mental retardation is almost always caused by abnormal CGG trinucleotide amplification within the FMR1 gene. Occasionally fragile X syndrome results from point mutations or deletions within or around the FMR1 locus. We have identified a mentally retarded African American male with typical fragile X phenotype and a 300-400 base pair intragenic deletion near the CGG repeat segment, present in his peripheral blood lymphocytes with no apparent mosaicism. His mother, who is not retarded, has a full FMR1 CGG expansion mutation with 700-900 repeats. A review of 23 published cases with FMR1 gene deletions shows full FMR1 mutation in the mother of only 1 other propositus, a male with FMR1 full mutation/premutation/deletion mosaicism of his cultured skin fibroblasts and peripheral blood lymphocytes. The various deletions within FMR1 and their clinical significance are reviewed.

Characterization of the full fragile X syndrome mutation in fetal gametes.

Fragile X syndrome results from the expansion of the CGG repeat in the FMR1 gene. Expansion has been suggested to be a postzygotic event with the germline protected. From an analysis of intact ovaries of full mutation fetuses, we now show that only full expansion alleles can be detected in oocytes (but in the unmethylated state). Similarly, the testes of a 13-week full mutation fetus show no evidence of premutations while a 17-week full mutation fetus exhibits some germ cells with attributes of premutations. These data discount the hypothesis that the germline is protected from full expansion and suggest full mutation contraction in the immature testis. Thus, full expansion may already exist in the maternal oocyte, or postzygotic expansion, if it occurs, arises quite early in development prior to germline segregation.

Instability of the FMR2 trinucleotide repeat region associated with expanded FMR1 alleles.

The fragile sites FRAXA and FRAXE, located approximately 600 kb apart on Xq27.3 and Xq28, respectively, are due to a CGG trinucleotide repeat expansion. Although the expansion mechanism for these and other trinucleotide repeat disorders remains unknown, the similarities between the FRAXA and FRAXE regions suggest a possible association between the 2 sites. DNA from 953 individuals was analyzed to determine the distribution of FRAXE repeat sizes in this population and to ascertain potential association between FRAXA and FRAXE repeat sizes. Thirty-four FMR2 alleles ranging from 3-42 repeats were identified. No FRAXE expansions were found in this population, supporting previous findings that FRAXE expansions are rare. However, in the fragile X syndrome affected group, a FMR2 delection, 2 cases of FRAXE repeat instability and a FRAXE mosaic male were identified. Also, a previously identified, rare FMR2 polymorphism was observed. Statistical analysis showed no correlation between normal FRAXA and FRAXE repeat sizes studied, although there was a significant size difference in larger FMR2 alleles that segregated with expanded FMR1 alleles. These findings support the idea of an association between repeat expansion in the FMR1 gene and instability or deletions in the FMR2 gene.

Fragile X phenotype in a patient with a large de novo deletion in Xq27-q28.

A 2-year-old boy with manifestations of the fragile X syndrome was found to have a cytogenetically visible deletion of Xq27-q28 including deletion of FMR-1. Molecular analysis of the patient was recently described in Tarleton et al. [1993: Hum Mol Genet 2(11): 1973-1974] and the deletion was estimated to be at least 3 megabases (Mb). His mother had 2 FMR-1 alleles with normal numbers of CGG repeats, 20 and 32, respectively. Thus, the deletion occurred as a de novo event. The patient does not appear to have clinical or laboratory findings other than those typically associated with fragile X syndrome, suggesting that the deletion does not remove other contiguous genes. This report describes the phenotype of the patient, including psychological studies.

Regulation of Caulobacter crescentus ilvBN gene expression.

As part of an effort to determine the mechanisms employed by Caulobacter crescentus to regulate gene expression, the ilvBN genes encoding the two subunits of an acetohydroxy acid synthase (AHAS) have been characterized. Analysis of the DNA sequences indicated that the C. crescentus AHAS was highly homologous to AHAS isozymes from other organisms. S1 nuclease and primer extension studies demonstrated that transcription initiation occurred 172 bp upstream of the AHAS coding region. The region between the AHAS coding region and the transcription initiation site was shown to have the properties of a transcription attenuator. Deletion analysis of the region containing the stem-loop structure of the proposed attenuator resulted in the derepression of ilvBN expression. Thus, it appears that C. crescentus uses attenuation to regulate the expression of the ilvBN operon.

Molecular genetic advances in fragile X syndrome.

The fragile X syndrome is recognized as the most common heritable condition resulting in mental retardation. The disabilities are substantial, and therefore early detection is mandatory to assist with reproductive counseling of families in which the fragile X syndrome has occurred. Highly accurate, direct DNA diagnostic testing can now be performed to diagnose the fragile X syndrome without the involvement of individual family members, as was the situation with the use of DNA linkage analysis. Such testing is rapidly becoming a standard diagnostic tool for screening of individuals with suspected fragile X syndrome, of potential unaffected carriers, and of patients with undefined mental retardation. Fragile X testing should be considered for all children with developmental delay of unknown cause. Autistic children will occasionally be found to have mutations in FMR-1. Detection of affected individuals will allow early intervention for these individuals and will assist families with their reproductive decisions (including prevention) in subsequent offspring. An understanding of the molecular genetics of fragile X syndrome has resulted in the resolution of the Sherman paradox and is the first molecular characterization of a chromosomal fragile site, a finding that almost certainly will be important in understanding the cause of chromosomal rearrangements involving fragile sites. In addition, molecular details of the fragile X mutations have yielded insight into "heritable unstable elements," of which the fragile X chromosome is one of the first characterized examples. Thus a similar molecular mechanism involving a trinucleotide repeat may explain the genetics of myotonic dystrophy and spinal-bulbar muscular atrophy (Kennedy disease); it seems reasonable to assume that other genetic diseases also may result from disruption of genes by inherited unstable elements.

Isolation and characterization of ilvA, ilvBN, and ilvD mutants of Caulobacter crescentus.

Caulobacter crescentus strains requiring isoleucine and valine (ilv) for growth were shown by transduction and pulsed-field gel electrophoresis to contain mutations at one of two unlinked loci, ilvB and ilvD. Other C. crescentus strains containing mutations at a third locus, ilvA, required either isoleucine or methionine for growth. Biochemical assays for threonine deaminase, acetohydroxyacid synthase, and dihydroxyacid dehydratase demonstrated that the ilvA locus encodes threonine deaminase, the ilvB locus encodes acetohydroxyacid synthase, and the ilvD locus encodes dihydroxyacid dehydratase. C. crescentus strains resistant to the herbicide sulfometuron methyl, which is known to inhibit the action of certain acetohydroxyacid synthases in a variety of bacteria and plants, were shown to contain mutations at the ilvB locus, further suggesting that an acetohydroxyacid synthase gene resides at this locus. Two recombinant plasmids isolated in our laboratory, pPLG389 and pJCT200, were capable of complementing strains containing the ilvB and ilvD mutations, respectively. The DNA in these plasmids hybridized to the corresponding genes of Escherichia coli and Serratia marcescens, confirming the presence of ilvB-like and ilvD-like DNA sequences at the ilvB and ilvD loci, respectively. However, no hybridization was observed between any of the other enteric ilv genes and C. crescentus DNA. These results suggest that C. crescentus contains an isoleucine-valine biosynthetic pathway which is similar to the corresponding pathway in enteric bacteria but that only the ilvB and ilvD genes contain sequences which are highly conserved at the DNA level.